Soft-start or Variable Speed Drive? 2

[sry110]

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.

 

[jraef]

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"

 

[jraef]

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"

 

[mikekilroy]

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.

http://www.KilroyWasHere<dot>com

 

[Compositepro]

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

 

[jraef]

...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"

 

[waross]

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

 

[ozmosis]

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.

 

[jraef]

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"

 

[Strong]

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

 

[sry110]

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.

 

[sry110]

@ 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.

 

[ScottyUK]

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.

 

[sry110]

@ 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!

 

[LionelHutz]

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.

 

[Compositepro]

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.

 

[ScottyUK]

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.

 

[mikekilroy]

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?

http://www.KilroyWasHere<dot>com

 

[davidbeach]

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

 

[ScottyUK]

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.