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Soft-start or Variable Speed Drive?
2

Soft-start or Variable Speed Drive?

Soft-start or Variable Speed Drive?

(OP)
I am a mechanical guy in need of electrical expertise.
I have a motor-driven gear system that has a considerable about of backlash to be taken up before the motor+gear sees the load. When it sees the load, there is a large impact that can damage mechanical components in our system. The impact is due to the motor (3-phase, 1800 RPM) hitting the stationary load at full speed.

I need a way to softly and/or slowly run the motor+gear up against the load to reduce the impact effect. Keep in mind that the motor+gear is completely unloaded (except for the weights and inertias of their own components which is negligible) from the moment it is energized until the driven load appears. Also, once the load is engaged and we are past that point of impact, I need the motor to produce its full locked rotor torque (typically ~300% of Full Load Torque for the motors we use) in order to start driving the load.

Some specific questions:
1) Will a basic soft starter keep the current reduced when there is no load on the motor, such that when our unloaded motor suddenly encounters the load the electrical torque will still be reduced to the set value? For example, let's say I set the soft starter to ramp from 20% to 100% current over 8 seconds (10% per second), and let's say the motor will run unloaded for 1 second until it hits the driven load. Will the motor be at 20% current (and correspondingly reduced torque) when the load suddenly appears, or will the soft starter have already decided the motor is at full speed / no load and therefore bypassed the soft start function altogether?

2) How much torque will a typical VSD allow the motor to produce? I could use a VSD to slowly run the unit up against the driven load, but once the load is engaged I need the motor to produce high starting torque to get it moving. Does the VSD allow the motor to generate 150% torque, 200% torque, higher, lower? I would typically count on ~300% locked rotor torque to get the load moving, but if the VSD only allows %150-200 then I would need to increase the motor size accordingly.

Thanks in advance for any insight.

RE: Soft-start or Variable Speed Drive?

A) Soft Starters:
1) "Better" soft starters will be able to provide what is called a "dwell" function wherein the initial torque value can be held at an artificially lower level for a short time. It is specifically for the situation you describe, albeit more often seen on chain driven systems where you must protect against chain slap. Do not expect to find this capability in a low-end soft starter.

2) To get 300% torque out of the motor, I imagine you are using a Design A or D? In that case, you would need the flexibility in the soft starter to immediately go Across-The-Line (full voltage) as soon as you are done with the dwell feature. Be careful here, many are not designed to handle that. When evaluating that, you will also need to know how long it takes your load to accelerate with that LRT.

B) VFDs.
1) Any VFD can handle the "dwell" concept of course.
2) Again assuming a motor capable of delivering 300% peak torque, the trick would be to just make sure the VFD can deliver the required current for the necessary time to do this. In other words you would over size the DRIVE, not necessarily the motor.

"Will work for (the memory of) salami"

RE: Soft-start or Variable Speed Drive?

Re: my description of a "dwell" setting:
Some manufacturers refer to this as a "Kick Start" or "Pedestal Start" function, but read the descriptions. Some do not allow for setting LOWER than some relatively high levels of initial torque, which might defeat your purpose.

"Will work for (the memory of) salami"

RE: Soft-start or Variable Speed Drive?

I began writing a whole reply about how you would no longer require 300% torque to accel the unloaded inertia, then realized you said you need 300% motor rated torque after reaching base speed to I need the motor to produce its full locked rotor torque (typically ~300% of Full Load Torque for the motors we use) in order to start driving the load.

Why?

A clutch kick in? 300% instantaneous load changes on vfd output is typically a no-no. If this is indeed the case, your 300% torque instant change will require like 600% rated current spike; and so unless you oversize the drive by a factor of 10-15x, you will not get there.

Can you eliminate the instant load change by better utilizing the vfd instead?

If you cannot change this instant 600% current increase requirement, it seems to me your only choice will be a soft start controller that relays itself out of the circuit when up to speed.

www.KilroyWasHere<dot>com

RE: Soft-start or Variable Speed Drive?

If you need to reduce impact use a torsionally elastic drive shaft to act as a spring.

RE: Soft-start or Variable Speed Drive?

Quote (MikeKilroy)

...If this is indeed the case, your 300% torque instant change will require like 600% rated current spike; and so unless you oversize the drive by a factor of 10-15x, you will not get there.
Well, not THAT much!

A good Sensorless Vector Drive can pump out about 200-250% torque fairly easily for a few seconds, so 2X sizing would give you that 300% value for at least that long. Then if you need it for longer, a typical heavy duty rated VFD can handle 150% over current for 60 seconds, so that equates to needing around 4X sizing. Still, if it is a 200HP motor, that's not insignificant! So I agree, a soft starter might be a better choice here, especially if there are no other benefits to being able to change the speed.

"Will work for (the memory of) salami"

RE: Soft-start or Variable Speed Drive?

I had a similar issue but without the high torque demand. We used the Ramp up function of the VFD and it worked fine.
Please tell us about the application. There may be techniques to start your load with less than a greatly over-sized drive.
A couple of 'for instances';
If you can take a DOL start except for the backlash issue. you may be able to use a small contactor to energize the motor through some impedances. Give it a few seconds to take up the slack and then go DOL.
If you are using a clutch you may be able to reduce the torque demand by eliminating the clutch and ramping up the speed.
The more information you can provide, the better solutions may be provided.
Still guessing but running a motor up to speed at no load is no guarantee that the drive train will not develop some backlash while running unloaded. If the load is removed there is almost sure to be some backlash develop for the next load cycle.
How about keeping the drive train lightly loaded at all times? A fan driven by the end of the gear train comes to mind. It may have to be geared up or belted up. An eddy current brake is another possibility.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Soft-start or Variable Speed Drive?

What is the application? I'm trying to picture various scenarios but it would be helpful to know what is actually happening. Reason: sometimes, looking at an application that has no control on speed/frequency of the driven motor, can have very different load characteristics when applying variable speed/frequency onto the application (positive or negative aspects). E.g. fitting a VFD onto conveyors feeding crushers will control the load on the crushers compared to a conveyor running at full speed every time it is started and thus immediately loading the crusher on start-up.
It might be that you do not actually need 300% LRT or equivalent.

RE: Soft-start or Variable Speed Drive?

And another thought, being that you are an ME (no offense)...

Generally current and torque are synonymous in an AC motor, but only AFTER it is at speed. If, in your existing starting situation, you are seeing 300% current and assuming that means 300% torque, that may be an issue here. Until the motor is at speed, much of the current is not really producing torque at the same rate (because the power factor is low). So if you are making this determination based on observing 300% current as the load accelerates, that is likely just the result of it putting out Break-Down Torque, which on a Design B motor is about 200-220% of FLT. Against, any SVC drive can deliver that now, which is what is behind Ozmoisi' response. People not used to applying VFDs are often surprised by their perceived ability to accelerate a load faster, but it is because almost all of the applied energy goes into acceleration as opposed to reactive power and associated heating losses.

"Will work for (the memory of) salami"

RE: Soft-start or Variable Speed Drive?

What is the motor's HP rating for 1800 rpm nominal speed? What is actual peak torque requirement? Machine size would be important when considering various options.

Walt

RE: Soft-start or Variable Speed Drive?

(OP)
Thanks for the input, gents.
The application is a turning gear system, comprised of an AC motor, worm gear reducer and self-synchronizing clutch that engages/disengages our system from the driven shaft. The tricky part here is the clutch, which is an SSS clutch- essentially it is a gear coupling that screws itself into engagement. Here is a video demonstrating the operating principle using a model of the clutch:
http://www.youtube.com/watch?v=bETbJLqqE1g

The nature of the clutch is that it goes from unloaded to loaded as a step function, not a gradually applied load like a plate-type clutch (like the clutch in your car's manual transmission). Unfortunately changing the type of clutch is not an option, so I need to find a way to make it work for me.

The purpose of the turning gear is to start the Customer's drive train from rest ("Breakaway from rest") and get it turning continuously at a relatively low speed. The operating scenario of the turning gear is this: When the driven equipment is at rest, energize the AC motor. The motor drives through the worm gear speed reducer and rotates the input section of the clutch. The input section of the clutch goes through approx. 20 degrees of rotation before engaging the output section, at which point the load is seen by our system. When the clutch engages it connects our rotating output shaft to the Customer's stationary shaft.

The high impact torque occurs at the moment the clutch engages. Leading up to that moment our motor is at full speed and no load. Then in an instant, like a step function, the motor sees the inertia of the Customer's drivetrain and high torque required to break it away from rest. So the issue is that the impact torque spike resulting from this activity can sometimes cause damage to the mechanical components in our system. My goal is to find a way to get through the clutch engagement process either slowly (VFD) or softly (soft start) to minimize/eliminate impact at engagement, but then once the clutch is engaged and the backlash is effectively removed from the system I need the motor to receive full rated voltage in order to generate enough torque to break the Customer's drive train away from rest. Note, the resistance torque in the Customer's drive train drops off drastically once the shaft is broken away due to the shaft getting up on an oil film in its bearings.

Let me know if you all need any more info to paint a picture of what's happening here.

RE: Soft-start or Variable Speed Drive?

(OP)
@ jraef:
The motors we are using tend to have both LRT and BDT in the 270-350% range. Being a mechanical system designer, I'm only really concerned with torque output of the motor. I know from looking at the Torque and Current vs. Speed plots that the highest current will occur at Zero speed (locked rotor current), and the current approaches Zero as the motor approaches full synchronous speed. In our application we are observing anywhere from 500%-700% current momentarily at breakaway, and as soon as the shaft train breaks away and starts turning the resistance torque becomes so low relative to the full load rating of the motor that the motor will run nearly unloaded.

@ Strong:
In this application the motor rating is 5HP, 460V/3-ph/60Hz, 1800 RPM. This particular motor has 310% locked rotor torque which works out to 46.5 lb-ft. The torque required from the motor to achieve breakaway of the Customer's shaft is approx. 44 lb-ft. In our application, because the high resistance torque drops off immediately following breakaway, we select the motor based on its peak torque output (usually Locked Rotor Torque) exceeding the required breakaway torque.

RE: Soft-start or Variable Speed Drive?

Is this a generator barring gear application? The description sounds very similar to a system I worked on a few years ago with a SSS clutch and a cyclodrive reduction box.

RE: Soft-start or Variable Speed Drive?

(OP)
@ ScottyUK: This one is a motor-driven compressor drive train. We do a lot of turbine + generator turning gears which also use SSS clutches, but we use worm gearing exclusively. That Cyclodrive gearbox is quite an interesting piece of engineering, but man are there a lot of moving parts to worry about!

RE: Soft-start or Variable Speed Drive?

How long does the motor run and how many rotations does the motor make before the clutch engages?

I don't believe a soft-starter will be any use. The soft-starter gives the motor a reduced voltage and it either makes enough torque to accelerate or it doesn't. There is no speed control. So, if your clutch engagement happens some time after the start the soft-starter will have been accelerating the motor without load the whole time leading up to that engagement. It might be possible, but I suspect not without creating some kind of control that "bumps" the starter on and off to maintain a low speed while unloaded.

As for the VFD, post the breakdown torque of the motor and post the current required at the breakdown torque if you have it. The VFD always works the motor in the speed range between the breakdown torque and full speed. Knowing the rated locked rotor torque and current doesn't help size a VFD.

My guess would be that a quality 10hp heavy duty rated sensorless vector drive would work but that would be the bare minimum. The same but in a 15hp size would be a safer bet. Using typical ratings, it would have the ability of 30A for a minute which is 4 times the motor FLA rating and that should be enough current to develop the breakdown torque of the motor.

RE: Soft-start or Variable Speed Drive?

Your clutch is very similar to how starter motors engage with the flywheel on car engines. Usually there is a very stiff spring to cushion the pinion gear at the end of its slide down the spiral splines.

RE: Soft-start or Variable Speed Drive?

The cyclodrive was one of the most reliable boxes I can recall, superbly engineered and dead smooth. Expensive though!

Lionel - would it matter that the motor wasn't producing much torque and essentially drove into a stall condition at the start of the current ramp, then gradually increased torque until breakaway was achieved? I think it would be possible but would be a fairly severe duty for the soft start. The cyclodrive design I'm familiar with had very little backlash in the gearbox so we didn't have the same problem that sry110 is seeing.

RE: Soft-start or Variable Speed Drive?

The input section of the clutch goes through approx. 20 degrees of rotation before engaging the output section, at which point the load is seen by our system. When the clutch engages it connects our rotating output shaft to the Customer's stationary shaft.

So if you engage the load at 20 degrees of 5hp motor shaft rotation - basically nothing; as far as a vfd is concerned, might as well consider with say a 3 sec accel ramp 0 to 1800rpm, this 20 degree motion happens at 11 msec after turn on, and thus 1.1rpm. We need the gear ratio to see how far this actually is on the motor shaft to do same calculations to see what speed the motor will be going when it hits the load (what you appear to call breakaway).

As I initially posted stating you probably need 600% CURRENT to get the 300% TORQUE, you confirmed this with In our application we are observing anywhere from 500%-700% current momentarily at breakaway: again please confirm by the word breakaway you mean when your clutch engages. That is why I said you would need a way oversized vfd. BUT, depending on your gear ratio, you MAY be engaging at such a low rpm that your motor will never try to even pull this 600% x FLA anymore since it may be in the nice linear slip section of the motor curve for the speed it engages at.... need that gear ratio.

But I question again why the SSS at all? Is THAT the device YOU sell and so it must stay? Every description you give so far makes the SSS engage into a stopped load. Now I could see needing the SSS if sometimes you want to start this motor while the load is already running for whatever reason - THEN it makes sense to keep it in the picture. OR if the load side of SSS sometimes needs to run FASTER than the 5hp motor side, you need it or need to change your controls. But so far, if you ALWAYS start this 5hp motor into a no rotating load, why the SSS at all?

The SSS is a tad different design, but the same thing we use on our old farm tractors called an overiding clutch - it is required on old tractors so we can stop without the bushhog's rotating mass driving us forward.... but I see no reason you have an SSS yet?

www.KilroyWasHere<dot>com

RE: Soft-start or Variable Speed Drive?

The clutch is after the speed reducer, could be many rotations of the motor shaft to get the 20 degrees of clutch rotation.

RE: Soft-start or Variable Speed Drive?

David is correct. The ratio of the system I'm familiar with was about 270:1 on the box plus a further (approx) 10:1 in the chain drive to the SSS clutch.

Mike - the clutch allows the driven shaft to run at a much higher speed than the motor drive can provide. The motor drive is designed to keep the driven shaft rotating slowly while it cools in order to prevent the bend which would occur if the shaft was left stationary after a period of operation at high temperature. Normally the shaft coasts down until it engages onto barring gear - it does not come to a full stop then go on to barring. This type of clutch is fairly common for the application.

RE: Soft-start or Variable Speed Drive?

How far the 5hp motor shaft turns is indeed important; that is why I said we needed to know the gear ratio to consider how the vfd would react to the sudden engagement; we still need that ratio.

Yes, worm boxes are typically less than 400:1 max; again, we need that ratio for further calcs. My direct coupled example was just to show that on a vfd with 1:1 ratio, the engagement of the clutch would have no detrimental effect what so ever since the speed at engagement would be only 1rpm different than commanded. Likewise, with the ratio, we can look at what speed it will engage; it may be well less than 60 hz and again cause the motor to not want anywhere near that 600% current. The current spike wanted can be shown once the ratio is given; then the vfd size can be properly determined instead of guessed at. As I said before, we need that gear ratio.

SkottyUK, thanks for providing the missing data showing WHY the sss is there; that is why I asked if the load shaft might need to go faster than this motor at some point. If that was said earlier & I missed it, I apologize to everyone for questioning the need for the sss in the first place.

Sosry110, what is the gear ratio? I believe if you share that with us we can show you how to totally eliminate that 600% current spike you have today - with a simple vfd drive that is not even oversized one bit.

www.KilroyWasHere<dot>com

RE: Soft-start or Variable Speed Drive?

(OP)
@ mikekilroy:
Thanks for the input, and all valid questions. Here are some additional details:
There is a 20.5:1 gear reduction between the motor output and the clutch, so 20 degrees at the clutch refers as 20 deg. x 20.5 = 410 degrees at the motor, or 1.14 revolutions.

The term "breakaway" refers to the level of torque required to start the Customer's shaft train turning from rest. Typically the "bumping" effect of the impact that occurs at clutch engagement generates enough torque to achieve breakaway. Once the shaft train breaks away, the torque required to accelerate the shaft to full turning gear speed is greatly diminished so the load on the motor drops off proportionally.

Since we only need to generate the breakaway torque momentarily, we do not need a motor with full load rating equal to the required breakaway torque (referred to the motor). Instead, we choose a motor whose locked rotor torque exceeds the required breakaway torque referred to the motor. This allows us to minimize the motor size. Looking at this application specifically:

Breakaway Torque required (at output shaft) = 550 lb-ft
Gear Ratio = 20.5:1
Gear Efficiency = 85%
Torque required from motor = 550 lb-ft / 20.5 / 0.85 = 31.6 lb-ft

Assuming my motor will generate 300% locked rotor torque, and it is an 1800 RPM motor, I can solve for the power rating that is required:
Power required (hp) = 31.6 lb-ft / 3.0 / 3 lb-ft/hp = 3.5 hp
NOTE: the "3 lb-ft/hp" term comes from the equation relating power (in horsepower) to torque (in lb-ft) and speed (in RPM), specifically:
Power (hp) = Torque (lb-ft) x Speed (RPM) / 5252
Plugging in 1800 RPM for speed and rearranging the equation we can say that for an 1800 RPM motor we get 3 lb-ft of torque for each 1 hp worth of rating.

Anyway....based on utilizing the motor's full locked rotor torque, theoretically I need a 3.5 hp motor. Of course this rating does not exist in NEMA world, so I would choose the next higher rating which is 5 hp. Now working through the math to figure out how much torque I could generate:

Max. allowable breakaway = 5 hp x 3 lb-ft/hp x 3.0 x 20.5 x 0.85 = 784 lb-ft

I only need 550 lb-ft, so my 5 hp motor could have a minimum of 211% LRT:
Min. breakaway = 5 hp x 3 lb-ft/hp x 2.11 x 20.5 x 0.85 = 552 lb-ft


So using my 5 hp, 1800 RPM motor with > 211% LRT, I need to be able to "creep" the clutch into engagement via the 20.5:1 gear reduction, and then once the clutch is engaged and the backlash is effectively removed from the system I need the motor to receive full line voltage so that it can provide its full Locked Rotor Torque and overcome the required breakaway torque of the Customer's shaft train.

RE: Soft-start or Variable Speed Drive?

Since you bring up accelerating the load, can you tell us what the load inertia is? If you have never calculated it, can you tell us the size and materials or size and weights of the load? It sounds like it is just a long shaft of steel so should be easy for us to calculate the inertia?

Your added description helps; I will do my math when I get some free time later today, but in the meantime, I think what you consider the breakaway torque being the most significant and inertia to accel not very significant is maybe wrong. Have you ever measured this breakaway with a torque wrench? I suspect you will find it is only slightly higher than actual friction and your real current draw is accelerating that huge inertia in the 100msec that across the line motor starting causes. Remember that T=jw/t and your t time across the line starting is VERY short. Just the fact of putting a vfd on and increasing that t by orders of magnitude should solve you issue.

Just as last verification, you have no requirement to go from 0 to 1800rpm on this motor in a short period of time, right? It can be set to 60 seconds if you wished?

www.KilroyWasHere<dot>com

RE: Soft-start or Variable Speed Drive?

I think that we may be trying to solve the wrong problem here.
We have a motor running unloaded at almost synchronous sped, Then we hit it with an instantaneous load.
The effect of the motor rotor inertia hitting a stationary load may far outweigh any concerns with peak torque and peak current.
The load hits the spinning motor and the gear train sees a very sharp torque transient. Probably the most severe torque effect of the starting cycle. Then the motor slows down. As the motor speed drops past rated speed the current will rise above rated current. But at this point the damage may have already been done by the initial torque transient. Given the method of engagement the issue may not be resolved by reducing back lash. The greatest backlash may well be in the clutch itself and that is fixed.
My understanding is that in most applications an SSS clutch is not intended to start a load, it is intended to allow a load to run above the speed of the clutch drive without over-driving the clutch and clutch drive mechanism.
As I understand it, the motor is up to speed unloaded and then an overload is almost instantly applied to the motor. A VFD or soft start won't do much good in that case. The damage may be done even before the motor drops below rated speed and before the current rises above rated full load current.
Possible solutions:
1: If this is for cool down duty, start the motor and engage the clutch while the load is still turning at above barring speed. Then the clutch and drive should take up the load gracefully and easily.
2: If this is for starting from rest, to establish the oil film, then engage the clutch while the motor is at rest. I understand that the clutch needs 20 degrees of rotation to engage but with a 20.5:1 reduction that is just over one revolution of an 1800 RPM motor. That should be no problem. Now the initial torque transient of engaging the clutch at speed has been avoided.
a> The motor may start easily now DOL.
b> The motor may still benefit from a ramped start with a VFD. I'll defer to jraef on the specifics of the VFD settings for this application.
DOL vs VFD:
If the backlash in the gearbox and in the drive from the motor to the gearbox is still an issue, You may benefit from the VFD.
Consider a design D motor. These motors are designed for impact loads such as shears and punch presses.
Look at the torque curve here:
http://www.baldor.com/pdf/manuals/PR2525.pdf
BUT WHATEVER.- GET THE CLUTCH ENGAGED IN THE FIRST TURN FROM REST OF THE DRIVE MOTOR SHAFT.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Soft-start or Variable Speed Drive?

BUT WHATEVER.- GET THE CLUTCH ENGAGED IN THE FIRST TURN FROM REST OF THE DRIVE MOTOR SHAFT.

Waross, that is indeed what is happening now that we have the gear ratio; THAT was indeed my whole point - without ALL the data, we did not know that the sss engages the load at only 1.14rev of the motor - from a stop. So doing that DOL was BAD but doing that with any ol vfd is piece of cake and there will be NO drastic BANG or spike.

www.KilroyWasHere<dot>com

RE: Soft-start or Variable Speed Drive?

ScottyUK - the motor would need a major pull-up torque dip to be able to "stall" it and maintain a slow speed for any significant time. Even then, I doubt 40%-50% speed reduction is what is really desired here. The point is there is no speed control ability.

mikekilroy - The current at clutch engagement with ATL operation really has no bearing on the current using a VFD. Using ATL operation, at the point of clutch engagement, the motor either stops or comes close to stopping and then it is drawing locked rotor current. With a VFD, the operational idea is to either hold the speed very low or run with a low torque limit until the clutch engages at which point you have the load connected and everything at 0 speed. Then, the VFD basically starts the motor without any torque limit. It will be operating the motor above the breakdown torque speed where the current will be much less then locked-rotor current. There is absolutely no need or point in forcing the equivalent of an ATL start when using a VFD.

Many VFD's have a programming capability where you can enter operation steps. It might be possible to program the VFD to run fast until right before the coupling and then slow down to softly couple before accelerating again.

RE: Soft-start or Variable Speed Drive?

We're on the same page Mike.
I understood that the motor was run up to speed before the clutch was engaged. Starting from stop, a small starter with some resistors may be cheaper than a VFD and will allow the slack to be taken up at low torque before going DOL.
I would consider a design D motor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Soft-start or Variable Speed Drive?

An aspect of this is now revealed in the size. At that size, there is probably no economic benefit to using a soft starter, you will likely pay as much or more for it compared to a VFD now, it's just the way the market has gone. But see below... *

Given such a short engagement time, this is looking like a simple application of what is called an "S-curve" ramp profile in a VFD. Most good ones will provide this. A heavy duty Sensorless Vector Control (SVC) drive would be the ticket because of its ability to make the motor produce its peak torque at any point, and again given the size and relative minor difference in cost, over sizing it per LionelHutz's recommendations would be prudent.

* The only possible consideration for continuing to pursue a soft starter would be if the system is going to run for long periods of time and energy losses are of concern, but speed control has no other value. A VFD, when run at constant speed, withh have about 3-5% throughput losses (in spite of what some salesmen will claim). If you change speed in some other fashion, this is always less than the losses in the other speed change method, so it's not usually an issue. But if you are NEVER changing speed and thus have no associated losses, it becomes one. A soft starter is bypassed once it is at full voltage, eliminating any losses. There are soft starters that have a "slow speed" feature (essentially a cycloconverter) that can be incorporated into the ramp profile. So you would hit the start button, the soft starter would turn at something like 15% speed for a second or two to engage your clutch, then you can have it go Across-The-Line immediately after that. But like I said, it will likely cost the same or maybe more than an equivalent VFD at this HP, so only consider it if the energy losses are significant to your application. To be honest, at 3.5HP loaded I would not get excited about 3% additional losses... The VFD gives you more flexibility.

"Will work for (the memory of) salami"

RE: Soft-start or Variable Speed Drive?

LOL, that's what happens when I get distracted and delay my response.

Only having 1.14 revolutions of the motor before clutch engagement should allow a soft-starter to work for the application. Your ratio at 20.5:1 is much less then I expected. With the description of the gear I was expecting more like 20-40 rotations of the motor.

A good soft-starter will have a dual ramp setup. You probably need to program a very soft ramp and a very aggressive ramp and then have a short delay timer operated off the start command which switches from the soft ramp to the aggressive ramp. The aggressive ramp would basically be "go to full voltage".

RE: Soft-start or Variable Speed Drive?

Finally made a few minutes spare time, so here is my conclusion that states any old vfd drive rated 5hp will do the job as long a reasonable accel RAMP is acceptable.

Using d=.5at^2 & v=at, I ran 4 scenarios and chart them here for the point where the sss engages. RAMP is time from 0-1800rpm programmed into the vfd.

RAMP(sec)....d(rev)....t(sec)....v(rpm)
ATL(DOL).....1.14......0.100.....1800
1.0..........1.14......0.276......500
10.0.........1.14......0.871......156
100.0........1.14......2.760.......50

We can assume worse case that on sss engagement, the motor will instantly stall to 0rpm. So ATL has 60hz going into it yet runs 0rpm momemtarily and hence smacks the engagement with full locked rotor torque and amps as proven empirically so far. Not good.

sry110, if you set the vfd to 1 sec ramp, it will be commanded to run only 500rpm with 16.6hz & 127v going to it. I need to get back to work so cannot think thru the rest right now, but Bill & Scotty sure could pick up and show you how to calculate, from your speed-torque-amps curve, the reduced current that will happen here. I would not recommend setting the ramp to 1 sec though.

10 sec prob is ok but look at the numbers for 100 sec ramp. 50rpm engagement speed - this is at normal slip freq of the motor! And so the motor will try to draw normal nameplate rated current to plow through it - IF you leave it a nice cheap skalar (non vector) drive! You will get a nice soft torque build up to your required 30-40#-ft, no bang, no current spike AT ALL, and it will soft start that sss engagement in my opinion. This is why I asked is you could stand a 100sec accel ramp earlier, before I ran the numbers to confirm this. No oversized drive required at all. Hope I did not go out into left field and miss something - I am sure the others here will let all know immediately if so :)

www.KilroyWasHere<dot>com

RE: Soft-start or Variable Speed Drive?

sry110;
I am still not completely sure;
Do you engage the clutch at zero speed or at full speed???

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Soft-start or Variable Speed Drive?

The design D motor has maximum torque at zero speed and the torque curve from zero speed to full speed is not too many percent from linear. They accept and recover from shock loading very well. They accelerate difficult and high inertia loads well.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Soft-start or Variable Speed Drive?

Anecdotal because I no longer have access to the actual data, but from memory:

A gas turbine generator used an 11kW DOL motor to drive the barring gear. This was through the cyclodrive arrangement I previously mentioned. In the event of mains failure the system was designed to transfer to an alternate supply which was arranged as follows: the 110V station battery fed an inverter which produced 415V 3-ph, which in turn fed the input of a Brush 'Falcon' scalar VFD. The output of the VFD was connected to the barring motor.

The installation never once managed to break the shaft away on the inverter supply, regardless of what we tried with the drive configuration. We actually blew one drive up in the process of trying, during a black start trial witnessed by the TSO: Brush had a included marvellous option to disable all the on-board protection, which my former boss instructed us to do because we were going to fail the witnessed trial and he was under a lot of pressure to pass. You can guess the rest. smile The dead drive was later replaced with a half-decent Eurotherm 584S, and that too failed to break the shaft away, although from memory the drive would have had a better chance if the inverter feeding it had been more capable.

In the end the system was abandoned and replaced by a DC motor controlled by a resistor timestarter, and that scheme never failed to break the shaft away, except for the time when the bearing lift oil failed and the 60T rotor was left sitting on the bearing metal.

I am sceptical about a VFD solution for this application, but wait with interest.

RE: Soft-start or Variable Speed Drive?

sry110: With Scotty's experience that breakaway may be WAY more than you thought, maybe you need to do my suggestion of:

Have you ever measured this breakaway with a torque wrench?

It could be your 'measured' 30-40#-ft breakaway is in fact 300-400#-ft as Scotty's experience suggests! Keep in mind with your DOL present method you are ASSUMING that locked rotor torque is what is doing the breakaway! It may be the KE=.5mv^2 kenetic energy in the rotating rotor that is supplying 300#-ft and the locked rotor only adding a measely 40#-ft more! You GOTTA get a torque wrench on that sss input or output shaft and MEASURE real life stiction!

Obviously if the stiction ('breakaway') torque is way high then that changes everything.

www.KilroyWasHere<dot>com

RE: Soft-start or Variable Speed Drive?

Being a mech engr, you should be able to measure the striction with a torque wrench.

Also, back to inertia; as I asked before, if you tell us what the load inertia is, or the mechanics of the load so we can calculate it, we can then calculate the % that KE involvement is helping breakaway. With both these last pieces of data, you can be comfortable with trying a vfd or softstart I think.

Based on Scotty's experience in similar breakaway case, I would not be comfortable going the vfd route yet since the above 2 facts are MIA.

www.KilroyWasHere<dot>com

RE: Soft-start or Variable Speed Drive?

With your calculation of the motor being required to develop 211% torque minimum you run a very high risk of failure using any old vfd drive rated for 5hp.

My previous VFD sizing recommendation stands. There are 2 ways to do it.

1. Use a VFD with sequencing or step logic and program it to run slow long enough to engage the clutch before accelerating to full speed.
2. Use a timer to hold a slow speed on the VFD long enough for the clutch to engage before "releasing" the VFD to run at full speed.

You clearly posted the numbers and information for the mechanical side so taking your numbers the above is what you need to use so the motor will start how you asked.

Sadly, it's reached the point in this thread that the background noise made me seriously considered not even bothering to respond....

RE: Soft-start or Variable Speed Drive?

The problem still seems to be the inerrant characteristics of the SSS clutch, and it's inclusion.
Coming from an electrical perspective and if someone had an application whereby a motor going DOL was causing issues with the high current demand and the same question was posed "soft start of VFD?", then you would have a similar discussion to above. However, if the request was to retain the contactor between soft start/VFD, then the general recommendation would be to remove the contactor, it adds no value and only adds problems. This is looking from an electrical analogy.
The 'solution', could be a change of design rather than a modification to what you already have, if the core issue is the impact when the clutch engages.
However, this is what happens when you ask electricals for a solution to a mech problem...

RE: Soft-start or Variable Speed Drive?

(OP)
I apologize for the radio silence, gentlemen. I was out of the office at our assembly shop all day and did not want to put myself through the pain of typing anything out on my phone. And I have to say a big THANK YOU to everyone chiming in...this has turned out to be a very educational discussion (for me, at least).

Here are some additional data / comments regarding the application:

* On this particular project, the inertia (WR^2) of the drive train referred to the Turning Gear output shaft is 152 lb-ft^2. Referring this to the motor output shaft via the 20.5:1 speed reduction gives us 0.36 lb-ft^2 at the motor output shaft.

* This particular application has 20.5:1 speed reduction between the motor and the driven equipment, but this is actually toward the lower end of our typical turning gear ratios. The vast majority of our turning gear systems have total gear ratio falling between 76:1 and 300:1, and have either 1500RPM (for 50Hz jobs) or 1800RPM (for 60Hz jobs) motors. That being said, I'm using this project as a means to learn about VFD's and/or soft starters as they apply to our system, but I would like the option to also employ this method on higher ratio (lower speed) turning gears. So assuming my 300:1 ratio is the lowest speed turning gear I would design, and using an 1800RPM motor, this would be 16.7 revolutions of the motor shaft before the clutch completes its 20 degrees of free rotation to engage.

* Regarding verification of required breakaway torque by manual turning test (torque wrench): unfortunately we never have the opportunity to get this information on a new project because the turning gear is being designed and manufactured in parallel with the Customer's drivetrain. Therefore we do not have the opportunity to get a manual torque check of the specific drive train before designing the turning gear....in fact, in most cases the turning gear is installed very early in the drive train assembly process so that it can be used as the manual turning (indexing) tool!
That being said, for every design we do, we get the rotor weights, bearing diameters, bearing types, rotor inertias, etc., from the customer so that we can calculate the required breakaway torque for the drive train. Of course this is just an estimate, but we build in plenty of margin by assuming artificially high friction coefficients for the bearings....like 0.37 for a journal bearing that realistically is in the 0.15-0.22 range.

* Bill (waross) asked for clarification of the speed at clutch engagement. The scenario is this: The customer's shaft, to which the Output Component of the clutch is connected, is at rest. Therefore the clutch output component is also at rest. The turning gear motor is energized, which quickly gets the motor+gearbox+clutch input component up to full speed (I'm assuming that a nearly unloaded 4-pole AC motor will get up to near synchronous speed within a fraction of a revolution). So as my chief engineer would say, the motor is sitting there "twiddling its thumbs" at full speed as the clutch input component is sliding into mesh with the output component and then BANG, the clutch is engage and the motor+gear sees static friction torque and inertia of the customer's drive train.
So to answer your question maybe more concisely, when the clutch engages the input is at full turning gear speed, and the output is at Zero.

* No doubt there is a kinetic energy component at play here. The problem is that it's a tricky animal to calculate, especially when each drive train is different from the one before (we are talking about one-off, custom designs here, not mass-produced commodities). So what I'm trying to do is dial out the kinetic energy aspect altogether by eliminating the impact effect at clutch engagement and then relying on the motor's electrical torque, multiplied by the gear ratio and efficiency, to overcome the calculated required breakaway torque at the turning gear output shaft.

* I agree the problem is inherent of the SSS clutch. But the company I work for was built around, and has become very successful at, the implementation of SSS clutches for automatic turning gear systems. So changing to a different type of clutch is not an option, and I need to find a way to cope with its natural behavior.

* Bill suggested that I get the clutch engaged in the first turn from rest of the drive motor shaft....and I agree! That's exactly what I need to do, but I need a way to do it that either creeps it into engagement at a slow speed to dial out the kinetic energy impact effect (VFD), or engages the clutch at full speed but at a very low electrical torque (Soft start).
In my estimation, neither the kinetic energy "flywheel" effect nor the electrical torque output of the motor independently can cause damage to the mechanical components in our system. I believe it is the combination of the full speed impact and full breakdown torque of the motor occuring in that moment of impact that will exceed the strength of the components.

* Someone mentioned starting the turning gear when the customer's shaft is already turning (catch-on-the-fly) to gently engage the clutch. I agree, this is a beautiful functionality of the clutch, but I am tasked with designing a system that will not only catch-on-the-fly (during drive train coast down) but must also be able to start the train from rest (breakaway). This is non-negotiable.


General comment: In my perfect world, yet still assuming I must use an SSS clutch, I would size my motor and all mechanical components downstream such that the system could achieve breakaway using only the motor's electrical torque and the mechanical advantage of the gear ratio. I would employ a millwright to manually turn the input shaft of turning gear to pre-engage the SSS clutch before each turning gear start-up. I would use a DOL starter with my appropriately-sized motor to achieve breakaway and accelerate to full turning speed.
BUT....since I am in the business of supplying "automatic" turning gear systems, I can't require the customer to provide manual labor to pre-engage the clutch each time they want to use the turning gear.


Thanks again for all the input. If I have left any questions unanswered please let me know and I will post up any more info necessary to further the discussion.

RE: Soft-start or Variable Speed Drive?

(OP)
I can't figure out how to edit my previous post, but I want to clarify one point. I wrote:
"* I agree the problem is inherent of the SSS clutch. But the company I work for was built around, and has become very successful at, the implementation of SSS clutches for automatic turning gear systems. So changing to a different type of clutch is not an option, and I need to find a way to cope with its natural behavior."

You probably ask "well if the company is so successful at this, why are you having so much trouble figuring this out???"

The issue here is that this particular turning gear is required to run at a much higher output speed than our typical design, complicated by the fact that we need to have a torque-release coupling on the output shaft to protect us from the customer's shaft trying to back-drive the turning gear (just trust me on this one, and like the SSS clutch it's non-negotiable), so this makes our system very sensitive to how much torque is being transmitted through it and I'm worried about the impact torque due to engaging at such a high relative speed. Not enough torque, we fail to break away the customer's shaft train. Too much torque, we trip our torque limiting coupling and fail to break away the customer's shaft train.
So instead of brute-forcing it like we normally do, i.e. just increasing the mechanical component size to the point where it's strong enough to absorb the impact torque spike without breaking, I want to learn how to be more finesse about it and use a readily available electronic device to do the job "right" and minimize my mechanical component sizes.

RE: Soft-start or Variable Speed Drive?

LionelHutz says: Sadly, it's reached the point in this thread that the background noise made me seriously considered not even bothering to respond....

sry110, I apologize for making useless background noise. Good luck with your investigation; you were close to separating the details into proper columns so you can do more than guess in the future. But at least big L gave you the solution.

www.KilroyWasHere<dot>com

RE: Soft-start or Variable Speed Drive?

(OP)

Quote (mikekilroy)

sry110, I apologize for making useless background noise. Good luck with your investigation; you were close to separating the details into proper columns so you can do more than guess in the future. But at least big L gave you the solution.

Wait...I must have missed something here. I guess I'll offer the following:

1) I thought LionelHutz's posts were informative and helpful
2) I thought mikekilroy's posts were informative and helpful
3) If there was a way for me to edit my original post to include all of the relevant details (which I did not think were relevant at the time), I would surely do so.
4) If people are getting frustrated at me because I come off as a person who is ignorant to VFDs and other 'smart' motor controllers....well, that's because I don't know much about them and I'm trying to learn.
5) Just because I haven't responded directly to someone's recommendation does not mean I have discounted it....it simply means I am reviewing/studying/digesting it, as this is the first time I've really encountered any of this subject matter.
6) I have not really noticed any "background noise" here, just people trying to give me an education.

RE: Soft-start or Variable Speed Drive?

You posted a fairly clear question wanting to know how capable a VFD and SS are to take slack out of a drive system. There was already another post with more details when I got here. So, I tried to answer your question of the VFD and SS. My answer would have just been more general if you hadn't posted the extra details. Frankly, you posted you are a mechanical guy so why should I be questioning your mechanical calculations or your ability to properly design the mechanical side of the system or your inability to solve your issue mechanically. Sometime, it can be helpful to hear different ideas but from the start your questions struck me as very direct. I didn't see anything indicating you were lost or needed any help except for answering those questions. However, that is only my opinion.

One thing I should maybe elaborate on a bit more. The VFD being a frequency controller "shifts" the speed torque curve of the motor so the motor always operates above the breakdown torque peak. That is why my comments said the locked rotor current and torque mean nothing when sizing a VFD. Same with not requiring the locked-rotor current from the VFD. You are usually better off finding a motor which has a worse locked rotor torque to get a higher breakdown torque when using a motor and VFD combination on an application which can have high peak torque demands. Motor current and motor torque remain somewhat relative to each other above breakdown toque. 200% torque means you need current in maybe the 200% to 250% range. You could look at the motor curves, but generally they are hard to read in that range. I've also seen many curves where the motor current wasn't anywhere close to 100% at the speed where the motor torque was 100%. When I see this it tells me the curves are useless to use near full speed. No matter what the curves say, in real life the motor won't run at rated speed and rated torque while drawing 50% of it's rated current.

A good point made by jraef that might have been missed is that some soft-starters have a jog function so that could be used to run the motor slowly until your clutch engages. This would keep the speed down on the higher ratio systems. But, his other comment about VFD's being so cheap at 5HP brings says why bother with the solution requiring extra wiring for timing the switch from jog to run when you can find a VFD with sequencing logic built in and just tell it to go and it takes care of everything after that.

To give a totally off-topic example of sequencing logic. A customer had a drum application that wouldn't start due to material sitting in the bottom of the drum. We sold a VFD and used the sequencing logic to rock the drum to get it started. He could have used external logic to do it with timers and a set of reversing contactors and such, but the VFD gives an all-in-one solution. Similarly, you can use a VFD, connect power, connect motor and connect 2 wires from a run contact and the physical installation is done.

And now I'm adding even more noise to your thread....

RE: Soft-start or Variable Speed Drive?

I think a VFD with torque control is good enough to do this job. Set a torque limit of about 5-10% above no load motor torque and a ramp torque after reaching this value.
Motor start, accelerate, engage clutch and hit load train; when VFD sense that motor reach this torque initiate a ramp (need to find best equation) torque to keep speed (SSS engaged) and accelerate load at desired speed.

RE: Soft-start or Variable Speed Drive?

(OP)

Quote (LionelHutz)

A good point made by jraef that might have been missed is that some soft-starters have a jog function so that could be used to run the motor slowly until your clutch engages. This would keep the speed down on the higher ratio systems. But, his other comment about VFD's being so cheap at 5HP brings says why bother with the solution requiring extra wiring for timing the switch from jog to run when you can find a VFD with sequencing logic built in and just tell it to go and it takes care of everything after that.

I agree that a single electronic component to do the entire job would be ideal. Let me know if I am on the right track:

* Connect VFD with sequencing logic in front of 5HP, 1800 RPM AC motor
* Set first 'mode' of VFD to run motor at 300 RPM for 1 second
My math is: t (sec.) = 2*Theta (rad.) / omega (rad/sec)
where Theta is the total rotation of the motor in radians from starting to clutch engagement
and omega is the speed of the motor in radians/sec.
Theta = 20 deg. x (pi rad./360 deg.) x 20.5 (gear ratio) = 7.16 radians at the motor shaft
Omega = 300 rev/min x (2*pi rad/rev) x (1 min / 60 sec.) = 31.4 rad/sec at the motor shaft
t = 2*7.16 rad / 31.4 rad/sec = 0.46 seconds

So within the first second, the clutch has engaged at low input speed and the motor is trying to rotate at 300 RPM

* Set second 'mode' of VFD to run motor at 1800 RPM (full speed) for the duration of continuous operation. When this second 'mode' is engaged, the motor can generate 200% of its rated torque which we utilize for breaking away the customer's shaft train.


RE: Soft-start or Variable Speed Drive?

Looks good except you need to be aware that the VFD always ramps from one speed to another. So, after a start it doesn't just jump from 0 speed to 300rpm. There is a ramp time which defines how quickly it accelerates. For a VFD, the ramp time is typically the acceleration from 0 speed to full speed or 0 to 1800rpm. If you use a 10-second ramp time then accelerating from 0 to 300rpm would then take 1.7 seconds (300/1800*10). But, if the ramp was 2 seconds then you're talking 0.34 seconds to 300rpm which basically doesn't affect your plan.

Sizing the VFD can be done a few ways.

You can first pick a motor and then contact the manufacturer to get solid current numbers for the peak torque you need. Then, send that data to the VFD manufacturer and get them to pick a VFD. You spend time going back and forth between manufacturers asking for number guarantees and to make sure everything is nailed down exactly to match what you calculate.

Or, you assume the current is mostly linear with torque and use best judgements to work through the numbers. You need 210% torque minimum. Add a safety factor to ensure you get enough torque so maybe bump the required torque by 25% to 263% torque. Next take your torque and convert to current, so 263% torque means a minimum of 263% current. Finally, know the current could be a little higher so add another safety factor for this, say 25% again. The result is you're needing 328% current. A heavy duty (or constant torque) VFD is typically rated 150% for 1 minute. So, 328%/150% = 2.2X. This means A 10hp VFD would be marginal and a 15HP VFD should be more then capable of running the load. I know most VFD's can also produce 175% to 200% current for short periods of time so I could use that fact to justify trying the 10HP VFD. Overall though, once you weigh the couple of $100 extra for the 15HP vs the risk of failure do you really want to bother?

Now, if you do 100's to 1000' of these systems a year then sizing the VFD tighter could make a few extra $$ for you. In this case, I would recommend testing a smaller VFD at a site where you can get away with testing to validate it will work before placing ones in the field where failure gives you a "black eye".



RE: Soft-start or Variable Speed Drive?

And if you believe in the KISS principal, you do not even need the two step turn on; as I showed earlier, a 5-10 sec ramp will get you to engagement point around 300 rpm anyway - why add the second step when a single ramp does the job?


You have never said how long you run at 1800rpm; it souinded like only short time. You may not be aware that all these vfd's will run your 5hp motor upto 1800rpm at constant torque, but then happily continue it up to 3600rpm in constant 5hp mode. Maybe 50% (most of the lower cost motors) are speced to limit top speed at 90hz (2500rpm), and most lower cost worm gearboxes are limited to about 2500rpm (better ones like Stober easily go to 3-4000rpm). So Even if you use lower cost components, if it were my design I would reduce my peak torque requirement by another at least 50% by increasing the 20:1 ratio (in this case) to 30:1 so you do not have to oversize that vfd so much and get exactly the same torque results for engagement.

www.KilroyWasHere<dot>com

RE: Soft-start or Variable Speed Drive?

Yes Mike, a straight ramp should work fine on the application outlined in this thread. It may not work very well on the next application with a much higher gear box ratio.

RE: Soft-start or Variable Speed Drive?

Food for thought sry110.... Some others here may consider this background noise, but the fact is, you have not separated out how your stored 5hp motor's rotational kenetic energy is assisting at sss engagement. May I suggest that it is not insignificant in the total picture, and if you really want to understand the relationships of torques, speeds, and times on your sss at engagement, you calculate the breakaway vs accel torque requirements.

Here is why I say this:

Typical 5hp motor inertia: .012 #-ft-sec^2
Your reflected 152#-ft^2 load inertia @ motor: .012 #-ft sec^2

So torque required (ignoring breakaway) to accel 0-1800rpm in 100msec DOL 5hp motor is about 50#-ft - close to what you have seen. That is 25#-ft required to accel the motor plus 25#-ft to accel the 152#-ft^2 load.

Then when 'breakaway' happens, you see - about same.

If we assume sss engagement droops DOL motor to 1/2 speed, 900rpm, and it again recovers in another 100msec, which seems reasonable with the values given, it pulls the same 50#-ft during this recovery per your tests. I suggest without scoping speed and current during this DOL start and then engagement of sss, which all basically happens at the same time, you cannot tell that 1/2 the torque load you see is simply torque to accel the load. So I wonkder if you really only need 1/2 the torque you suggest for breakaway. This would bring your peak torque required on a vfd AT SLOW SPEED down to just breakaway torque since you extend the accel out and basically make that 25#-ft portion (accel torque) go away. This would mean your 211% max torque required MAY be closer to 100%.

Looking further, you can calculate that the stored kinetic energy in the motor that is running 1800 rpm before engagement, is .5Iw^2= 260watt-sec. You can further calculate that the energy used during the re-accel portion (after engagement, assuming speed drop from 1800rpm to 900rpm) ALONE is 25#-ft*900rpm/5252= 4.2hp or 5.7kw. Since the recovery was another 100msec, that is 570 watt-sec used just to reaccel the load. But your motor only had 260 watt-sec of energy stored, and we know SOME of that was used for the breakaway, so what does it mean? Seems to me it means most of your present 40 amps measured at 'breakaway' is used to re accel the drooped speed, not for the breakaway itself. If there is any validity in this statement, then a vfd will not require much extra torque for 'breakway' since most of the torque was used in re acceling that load. Means no vfd oversizing required yet again.

The good thing about trying a vfd is you will then likely have software available with it that allows you to scope these things finally. Then you can take that data and analyze it and fine tune how you react to your sss engagement under your terms.

I would certainlyi go with Lionelhutz' recommendation and buy a 15hp vfd for this first one. I would make sure I bought one with a really good USB/ethernet software communications package that does scope functions - at least for this first one.

Future vfd's could be brought back down in oversize capacity as proved by this first one. Future systems could be designed to AT LEAST go 2500rpm on the motor to reduce torque requirements another 50%.

www.KilroyWasHere<dot>com

RE: Soft-start or Variable Speed Drive?

did not get to edit my post; dam cat sat on keyboard and hit send. sorry.

www.KilroyWasHere<dot>com

RE: Soft-start or Variable Speed Drive?

One last nonsense background noise comment.... Please tune out if you consider this more noise - there certainly is no requirement you read it.

I recently had a consulting job on a device called a "barrel cam" (you can google it) used a lot in the glass industry. Its effective operation is close to your sss. I DID use kenetic energy calcs as well as accel/decel/breakaway calcs - to solve a multi year problem where the customer was continually increasing the motor HP to try to get control. They went from 5hp to 40hp and still had similar issues to you. Simple KE calcs proved the solution in their case was adding an inertial flywheel at the motor and all of a sudden they were back to a 5hp motor and with perfect control. So this in depth analysis CAN be useful sometimes....

sry110, you are the mfgr of this equipment; THAT is the reason I thought all this in depth analysis was worthwhile; if I mfgred a system using this sss thing, I certainly would make sure I fully understood its total operation in detail. I do not operate my company without proper analysis, and I am assumed you do not either. Although I am a servo engineer at heart, I make no claims that my free calcs are perfect or don't need tweaking by you, the mfgring engineer - I am sure they do: my 'noise' inputs here were just to give you some potential direction if you wanted it. But, on the other hand, for someone else, just using one of these gidgets, sure, the guess to just oversize the vfd 2.5x would be sufficient.

If that guess was all the info you wanted to gain from your OP, I certainly apologize for all my background noise!

www.KilroyWasHere<dot>com

RE: Soft-start or Variable Speed Drive?

Yes Mike, a straight ramp should work fine on the application outlined in this thread. It may not work very well on the next application with a much higher gear box ratio.

But wouldn't you consider the adjustable RAMP parameter equivelant to the adjustable extra step JOG parameter for the next slightly different system?

www.KilroyWasHere<dot>com

RE: Soft-start or Variable Speed Drive?

It could very easily be that the ramp time required to accelerate up to X speed in the time it takes the clutch to engage is in conflict with the desired acceleration rate after the clutch has engaged.

RE: Soft-start or Variable Speed Drive?

First off, I'm not a VFD guy so maybe I'm way off base, but couldn't you program the VFD to run at some very low speed and highly limited torque, then she it stalls change the operational mode to accelerate with a high torque limit (above break away) and the run at nominal speed? Maybe it takes an external mini-PLC to control it, but somehow this seems like a really simple problem to solve. Perhaps, though, that just means I don't understand it.

RE: Soft-start or Variable Speed Drive?

Torque limit of the breakaway clutch vs the torque required to start the load.
If the setting of the anti-back-drive device is too low, you just won't be able to start the load.
Motor torque: This is more a reflection of the load on the motor than on the electrical supply to the motor.
Why do you need a torque limit?? The torque will be limited by the load. Absent other factors the motor torque will increase until the load breaks away. An artificial torque limit on the motor will, if set higher than the required starting torque, probably never be active. If the artificial torque limit is set lower than the required starting torque then the load may never be started.
Motor torque; This is related to slip frequency. If you look at any torque curve you will see a speed base. At the point of maximum torque the speed may have dropped 40%. The slip frequency has now increased from the normal 2% or 3% to about 40%. The current is well into the overload range. The motor is heating up. VFDs are used to allow a motor to operate at this speed while avoiding the factors that put the motor into overload.
Maybe you should back-drive the motor to the limit of the backlash of s aloppy gear train and give the motor a running start so as to take advantage of the rotor inertia to break away the load. This is not a serious suggestion but more an attempt to step back and look at all possible effects on this application.
I see two possibilities for a low torque start.
1: Enough torque to take up the slack in the gear train.
2: Enough torque to take up the slack in the gear train and engage the SSS clutch.
Consider this:
A VFD to provide enough torque to engage the SSS. Then bypass the VFD and go DOL to get the maximum torque to start the load. Remember that torque limit may not be needed here as the motor will not develop more torque than the load needs and if it does not develop enough torque the load will not start.
Consider a design D motor which has maximum torque at locked rotor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Soft-start or Variable Speed Drive?

(OP)
Sorry again for the radio silence. I'm putting out some other fires at work and I am trying to get back to this ASAP. I have been reading the replies but have not had the time to write my own. Hope to get back to this tonight, thanks for your patience..

RE: Soft-start or Variable Speed Drive?

(OP)
Annnnnnd I'm back.
Many good questions were asked, and I will try to address all points:

Acceleration time to full speed:
We do not have a limit, but typically for this type of turning gear the time elapsing from energizing the motor until the driven shaft train gets to full continuous turning speed is less than 5 seconds.

Inertia/flywheel effect of motor:
I acknowledge and agree that the torque overload problem here is not due to the electrical torque of the motor, but instead due to the inertia/flywheel effect of the motor rotor having to rapidly decelerate when the SSS clutch engages. The inertia/flywheel effect isn't so easy to calculate, nor do most of our Customers want to believe it's a "real" contribution that we can count on to achieve breakaway, versus the motor electrical torque and gearbox efficiency which are easily calculable and therefore reliable from a design standpoint. So the goal here is to remove the inertia/flywheel effect altogether (by slowly engaging the SSS clutch) and select my motor size based on locked rotor torque x gear ratio x efficiency.


Quote (waross)

Consider this:
A VFD to provide enough torque to engage the SSS. Then bypass the VFD and go DOL to get the maximum torque to start the load. Remember that torque limit may not be needed here as the motor will not develop more torque than the load needs and if it does not develop enough torque the load will not start.
Consider a design D motor which has maximum torque at locked rotor.

YES! You're onto it. From a mechanical standpoint, this is exactly what I have in mind - creep the clutch into engagement, and then once it's engaged let the motor go DOL so it can develop its full output torque and speed.
Now getting into the fine details: Can a VFD be had with a built-in bypass and a timer so that the VFD and DOL starter could be wired in series? I'm picturing the VFD is set to run the motor at 300 RPM for 1 second. When 1 second passes the VFD times out and the bypass contactor closes allowing full line voltage to go to the DOL starter. Then the DOL start contact closes and sends full line voltage to the motor. Right/wrong?

RE: Soft-start or Variable Speed Drive?

Back to another suggestion I made. Not as sophisticated but possibly cheaper and simpler. Use three resistors to limit the current and torque of the motor. Energize the motor with the resistors in series for a second to let the motor take up the slack at low torque and then stall. Then hit it DOL.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Soft-start or Variable Speed Drive?

A small soft start operating in current limit would achieve the same thing. It wouldn't need to be rated for the full motor current, just enough to develop sufficient torque to take up the 'slack'. Obviously the motor would stall, but if it is in current limit then that wouldn't matter. Once stalled, go for a standard DOL start.

RE: Soft-start or Variable Speed Drive?

By series resistor, as soon as clutch engage, motor speed down, lose full engaging and motor DOL will lead to a high strike in SSS mechanism, maybe not so good. That, I think by VFD may reach slowly and keep clutch engaged and then increase torque to speed-up load.

RE: Soft-start or Variable Speed Drive?

I think you're mistaken iop.

The motor would stall when the clutch engages, but it will maintain torque to keep the coupling engaged. It just won't be able to provide sufficient torque to break the load away. The motor slip will be 100% and it won't be able to sustain this indefinitely because of rotor heating and lack of cooling, but it would handle it for the few seconds to bring the clutch into engagement.

RE: Soft-start or Variable Speed Drive?

If I understood corectly, SSS is a synchronous clutch, that mean it engage when drive shaft speed is equal with driven (load) shaft speed. Load is not at zero speed, so to engage need that drive shaft to reach load speed, so motor speed can't be zero when engage. Big problem is that motor and SSS will "see" a step torque when clutch engage and may be bad effects in both of them (mechanicaly mainly). If step torque value need to be below a limit, that need to close slowly motor speed to load speed and after full engaging, increase load speed at desired value by proper motor torque control. If step torque is allowed, both above mentioned methods by warros and ScottyUK may be used.

RE: Soft-start or Variable Speed Drive?

I still think there is benefit to a vfd alone without the other equipment. It gives almost infinite choices on start/run methods. An S curve accel ramp even allows really slow engagement of sss then faster accel to speed. Since there is no accel ramp limit, using 10 seconds isn't an issue, even 20 sec is prob ok starting up such a big gadget. It has been shown around a 6-8 sec accel ramp will cause engagement around 300rpm even without the S curve ramp.

Look at its benefits over the softstart or resistors then relay DOL:

- similar hardware cost for much more capability
- reduced torque starting by adjusting the ramp to slow enough speed - easily adjustable
- no extra parts required (KISS)
- REDUCE YOUR TORQUE REQUIREMENT BY 39% or MORE

I think OP must understand very clearly what torque a vfd can produce compared to DOL: THE SAME. This is critical to understanding how the vfd can solve this. If one thinks the vfd cannot produce the SAME EXACT maximum torque as the DOL approach, then the resulting decision will be tainted. With about 250% of nameplate current, the vfd will produce the 225% or so of max motor torque that requires 600% current in DOL. Actually the vfd will produce that 225% torque at the 300rpm while I believe DOL will still be down at 150% at such a low speed (I am sure others will correct me if I am wrong on this statement).

OP calculates the max breakaway torque required, so KNOWS what is required max. In this case he uses a 20:1 worm gearbox in order to get maximum DOL torque from a 5hp motor at DOL speed of 1800rpm. But using the VFD he can run to 2500rpm or more with the same motor and now use a 30:1 worm gearbox. He effectively increased his max available torque again by 40-50% or more. This allows him to reduce the size of the motor or vfd to pay for the cost of the vfd. Or just add the vfd with its 200% max output capability and effectively get around 300% capability withi the higher gear ratio.

Until OP applies soft torque (no 1800rpm crash) to engaged SSS via a vfd or torque wrench, he will not know empirically how good his breakaway calculations really are and how much the stored rotating kenetic energy in his motor rotor really effects the equation. This vfd method allows him to gather that empirical data once and for all. With a good vfd's reporting capability on speeds, amps, torque, it can tell once and for all if a vfd is even required on future jobs or if a reduced voltage scheme is applicable.

I think the main benefits of the vfd over all other methods is allowing OP to run to 2500 or higher rpm on same motor and get more low end torque out of the system for SSS engagement, adjust speed of engagement, and adjust the torque available at that engagement.


www.KilroyWasHere<dot>com

RE: Soft-start or Variable Speed Drive?

Something that should have been asked and could be very important in picking a solution - What kind of redundancy is required in case this new solution fails?

A smaller VFD with the controls and extra contactors to bypass it once the drive is engaged will cost more money and be more complex then just picking the correct size of VFD.

Resistors could work and might be cheaper, but there is no speed control of the motor using resistors. Use too much resistance and it may not accelerate while using too little resistance means it may still reach a higher speed and "bang" the clutch into engagement.

I don't believe worrying about the losses of a VFD is worthwhile. Say the motor takes 1000W input to keep turning (around 1hp on the shaft) and the VFD losses are absolutely terrible at 50% of the output power. That is 500W per hour - 0.5kW x 24h x 365d * $0.2/kWh = $876 to run continuously for a year. Then, I would hope the load is actually running greatly reducing this number. So, is it worth worrying about or not?

iop995 - you should re-read some of the posts. Load is at 0 speed.

RE: Soft-start or Variable Speed Drive?

VFDs: Lots of features and advantages and possibly a few maybes.
Resistors: Cheap and simple.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Soft-start or Variable Speed Drive?

If use SSS to start-up load from rest only, is somewhat strange. In this case, to keep clutch engaged add a small DC motor on same shaft and put DOL IM motor after power-up DC motor. May be a solution even it's more complicate (mechanicaly) than a good programmed VFD.

RE: Soft-start or Variable Speed Drive?

(OP)
@ iop995: Our turning gear system is used both to start-from-rest (breakaway), as well as to catch on the fly during coast down.

RE: Soft-start or Variable Speed Drive?

(OP)
@ mikekilroy: Thanks for the additional thoughts. I understand your concept of over-speeding the motor and increasing the gear ratio accordingly, but our gearbox manufacturer limits us to 1800 RPM maximum input speed (due to the type of seals used on the input shaft). Sorry for the piece-mealed info, but again this was something I didn't mention previously because I did not think it would be relevant.

RE: Soft-start or Variable Speed Drive?

fair enough sry110 on your PRESENT gearbox mfgr. I understand you have a desire to stick with your PRESENT worm gearbox at its much lower efficiency than a spur gear box with same ratio, same price, and 97% efficiency. Sometimes the expertise of the supplier or supplier's sales engineer is well worth sticking with such a certain product at other expenses. Lionelhutz clearly stated that efficiency may not be a decision factor; indeed, you never answered HOW LONG this gidget runs once upto speed so we cannot even guess at efficiency costs.

Just so you realize there are other equally as good or better worm or spur boxes that CAN go 2500 or 3600rpm input speeds all day long; then you've made a proper value judgement call.

So in lieu of higher speed, I would go with the recommendation to THIS TIME go with a 2-3x oversized vfd to be COVERED and gather real life data from it to see how to maximize your company's profits on future jobs. If you do go this route, I hope you pick a vfd capable of recording real time current & speed and downloadable to a computer for analysis.

Glad you came here and posted; it has been a fun, & hopefully, educational, thread!


www.KilroyWasHere<dot>com

RE: Soft-start or Variable Speed Drive?

Remember though - these things are typically sold to very conservative {small 'c') customers who just might not embrace a VFD on this equipment.

Power gen and oil & gas users tend to value reliability above pretty much anything else, and in my experience VFDs don't last as long as DOL starters and definitely are not as easy for a maintenance tech to repair, especially when that repair is probably needed at night, when all the suppliers are closed and when there's little or no engineering support. Sometimes the question is not whether the solution is technically sound, but whether the customer will accept the solution.

RE: Soft-start or Variable Speed Drive?

To Scotty's point, I have experienced similar attitudes toward VFDs in the power gen industry (which to me carries a certain amount of irony), but mainly on LARGE systems. I'm working on a project at a gas fired plant right now on retrofitting the 6 x 2500HP cooling water pumps with VFDs, but only because recent environmental regulatory changes are forcing them to curtail their water use. After 50 years of operating with full flow and bypass-to-waste of what they don't need, they are having to actually LOOK at matching their flow to their needs. The concept of doing this with VFDs is abhorrent to most of the people holding the purse strings, it's been very frustrating. Energy cost is nothing to them, but VFDs are big bad voodoo boxes. They want to do it with valves, but they would have to shut down completely for up to 6 months to install them because the way the piping was originally configured (having never thought this would be an issue). Somehow, that is still preferable to them compared to the voodoo boxes (which could be retrofitted one at a time) at this point.

But on this application, don't forget this is a 5HP motor. Carrying an identical VFD with identical programming as a spare, even if it is a 15HP VFD, would be relatively cheap redundancy if you ask me. I apply a lot of drives in that industry at the small end of the spectrum, even for critical equipment. There are also drives that can separate the controls from the power module, so swapping out a failed power section could involve zero programming and be done in 5 minutes or less.


Quote (sry110)

Now getting into the fine details: Can a VFD be had with a built-in bypass and a timer so that the VFD and DOL starter could be wired in series? I'm picturing the VFD is set to run the motor at 300 RPM for 1 second. When 1 second passes the VFD times out and the bypass contactor closes allowing full line voltage to go to the DOL starter. Then the DOL start contact closes and sends full line voltage to the motor. Right/wrong?
To this issue, "bypassing" a VFD is not as simple as just closing a contactor around it when you no longer need it, like on a soft starter. A VFD must have it's output section completely isolated from line power. So you end up with an open transition and that open transition may result in the load slowing briefly, which may have very negative consequences for the clutch and motor. There is a concept called "Synchronous Transfer", but that is not something you would do on a 5HP motor drive, it would cost more than another drive, maybe two or three more drives... But as I said earlier and Mike has said again, there is no need to do that. A good quality VFD with Sensorless Vector Control can make the motor deliver its maximum rated torque at any point, on demand. So just engage the gears slowly, then tell the drive to punch it. Done.

"Will work for (the memory of) salami"

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