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Motor with Gearbox vs. High Efficiency High speed motor
4

Motor with Gearbox vs. High Efficiency High speed motor

Motor with Gearbox vs. High Efficiency High speed motor

(OP)
I have a situation where I replaced an older motor that was running a process along with a gear box and the set up was controlled with an ABB VFD, we have replaced the entire set up with a new high efficiency high speed (5500RPM) motor but the new motor is still controlled by the existing ABB VFD.

Once everything was replaced and set up, we fired everything up and as the result we see the current used is by about 10% higher.

Why is that? Why would the current go up when we are using higher efficiency motor?

Thanks in advance for any replies.

Enver

RE: Motor with Gearbox vs. High Efficiency High speed motor

Where are you measuring the current and how are you measuring it.

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

RE: Motor with Gearbox vs. High Efficiency High speed motor

(OP)
Hi waross,

there is a PLC panel display at the unit we are driving that gives us all the information.
so I would say the load has a smart panel display that gives us that info.
new info has been compared to the old and we see 10% of current increase.

RE: Motor with Gearbox vs. High Efficiency High speed motor

Your VFD is outputting a variable voltage and frequency.
As an example; You may be running at 20% less voltage and 10% greater current. The actual efficiency may be further complicated by the relative power factors of the two motors.
10% current increase with a different motor running at a different gear ratio is meaningless by itself as an indication of efficiency.

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

RE: Motor with Gearbox vs. High Efficiency High speed motor

(OP)
Thanks waross,

I completely agree, wouldn't higher current usage mean more energy used?
I mean if we replace a motor and a gearbox (350HP, 480V, 3phase old and new) with new high speed motor to perform the same task and the new ends up using 10% more current wouldn't that translate in more energy used? And if so what was the point of making this change when our end result was supposed to be more energy efficient setup.

By the way, I am actually helping with this issue and do not have all the specifics, so essentially what I am trying to do is get my bases covered so that I have answers for most questions.

Thanks,

RE: Motor with Gearbox vs. High Efficiency High speed motor

See if the VFD displays Watts. Current alone is meaningless.

A 100 HP motor will draw about 100 Amps at 600 Volts.
A 50 HP motor will draw about 120 Amps at 240 Volts.
How much will the 100 HP motor save???
Which is more efficient???
You don't know what the effective voltage was to the old motor or for the new motor.
and
Maybe the salesman lied when he said that the new motor was high efficiency, but probably not.

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

RE: Motor with Gearbox vs. High Efficiency High speed motor

(OP)
Thanks,

Good point, I'll have a chance to look at some detailed information later this week.

Thanks for the input,

RE: Motor with Gearbox vs. High Efficiency High speed motor

If you bring back more information to share and compare, be sure to bring back also:

1) the base speed or number of poles for old and new motor
2) the speed each ran/runs in either hz from vfd display or rpm from hand tach
3) the old gearbox ratio (2:1?) and the new gearbox ratio (1:1?)
4) Include the old and the new motor part nos also in case anyone here has access to efficiency info on them

THEN the more meaningful comparisons can begin.

www.KilroyWasHere.com

RE: Motor with Gearbox vs. High Efficiency High speed motor

No kidding. This is full of holes and HOPEFULLY, someone retained a qualified EE to investigate this rather than go by what some salesman told you.

One other important piece of information: What is the machine you have it connected to? A VERY VERY common mistake, one that is FREQUENTLY made by uniformed salesmen, is to think that speeding up a motor to make up for a gearbox speed increase is going to be OK. An AC motor can only be 2 poles, it cannot have less than that. 2 poles mean 3600RPM (synch). If you now have a motor that claims to be 5500RPM, and it cannot be any less than 2 poles so that means it is simply getting 92Hz (roughly) from the VFD. Once a motor goes over it's base speed however, the HP remains constant. So if they sold you on a motor that says it is 350HP at 5500RPM, that is not the same as having a motor that provides the same TORQUE as your 350HP 3600RPM motor! Do the torque calcs, you will see what I mean.

HP = Tq. x 5250 / RPM.
and therefore
Tq = HP x 5250 / RPM.

So:
350HP at 3600RPM = 510 ft. lbs. of torque
350HP at 5500RPM = 334 ft. lbs. of torque!

So for you to get the SAME torque as your original motor, 510 ft. lbs AT 5500RPM, that new motor would need to say it was at least 486HP at 5500RPM. Does it?

If it says it is still 350HP at 5500RPM, then you are getting only 65% of the torque you used to get. So if your old gearbox was anything less than a 1.54:1 ratio, you now have LESS shaft torque to work with. Less shaft torque means higher slip in the motor, higher slip mean higher current. So notwithstanding all of the other potential measurement issues mentioned above, this may be your problem, or a contributor to your problem, as well.

"Will work for (the memory of) salami"

RE: Motor with Gearbox vs. High Efficiency High speed motor

(OP)
Thanks jraef,

The reason why this is full of holes is because I am working with insufficient information. Unfortunately, this project has been lingering around for years and the Process and Mechanical Engineers have decided back in the day to do, what they saw as, one for one replacement. However when you compare the two setups, motor w/ gear box vs high speed motor is more like apples and oranges then anything else.

I posed the question in a hurry and without really thinking the entire situation through. I shouldn't be losing sleep over this because like you said this whole thing is just full of holes. Without having complete history and info we as EE's can't make solid recommendations.
Therefore I went back and did tell the engineers (of other disciplines) to get complete info on the old motor, gear box, setup, new motor, energy usage for old and new motors. Just then we can start to evaluate the entire situation.

In the meantime, to a certain extent I do agree with you but I also disagree on some items.
The new motor is a 350HP, 480V, 3phase, Inverter Duty, 5500 RPM Max, Constant HP. Its base speed is 3550 which makes it obviously a 2-pole motor with an approximate 1.4% slip. Because it is a constant HP the torque will get lower with the higher speed.
VFD's vary voltage with frequency in a V/Hz ratio so the if f is less the 60Hz the VFD will adjust the voltage as necessary. However if the higher speed is required then the voltage will tap out at 480V and the frequency will be increased to provide additional speed.
As a matter of fact they are currently operating the new motor and the entire process is working absolutely fine where the output frequency goes up to 130-150Hz and the RPM reach a max of 4500RPM, approx.
Obviously one needs to have the right motor, right VFD and the right controls programmed to have this setup.
The load they are driving is a WFI Still pump for a pharmaceutical process.

In any case, this whole thing seems to me a problem that is blown out of proportion. In reality there is no problem because the process works really well, they have a cleaner set up and just for kicks someone is questioning why the new motor uses 10% higher current without really telling me if the information is showing live feed vs max during the entire process or the actual average current usage. At the end of the day like waross said just having the information about current without knowing what my Watts are I can't tell them if the energy used is less or more (taking into account they haven't given me the Watts that the old setup used).

However, I am trying to get more info about the whole VFD, Motor, high frequency, high RPM setup and operation. I just can't find any good papers on it that will explain how that whole thing works.
I also heard that the higher the frequency, the motor operates on, the windings get smaller, can't say that made clear sense to me.

If anyone has some good papers, explanations, graphs I would love to get some more education on it.

Unfortunately as a consultant I am more into basis of design and management side on every project and I lose touch with the technical aspect so I have to do a lot of digging to get some answers.

Thanks again to all for the feedback and support.

RE: Motor with Gearbox vs. High Efficiency High speed motor

The new motor is a 350HP, 480V, 3phase, Inverter Duty, 5500 RPM Max......Its base speed is 3550

As a matter of fact they are currently operating the new motor and the entire process is working absolutely fine where the output frequency goes up to 130-150Hz and the RPM reach a max of 4500RPM, approx.


Glad to hear you went back and told them more real data needed to explain.

Consider NOT saying the above stuff to them though; it is too wrong: If motor is indeed 2 pole, then 5500rpm max means 90hz max output from vfd - this is VERY typical for max. There are few 350hp motors rated to 120hz and probably none from std mfgrs rated to 150hz. Along same line, 4500rpm & 150hz means a 4 pole motor running 2.5x base speed: again, I doubt they would have found such a motor in 350hp.

www.KilroyWasHere<dot>com

RE: Motor with Gearbox vs. High Efficiency High speed motor

(OP)
Hi Mike, Thanks for the feedback.

Below information is coming from the manufacturer. This is not something I came up with. It is actually information coming from the cut sheet.
The new motor is a 350HP, 480V, 3phase, Inverter Duty, 5500 RPM Max......Its base speed is 3550

This information is actual performance, something that "the field guys" have witnessed out in the field while operating the pump:
As a matter of fact they are currently operating the new motor and the entire process is working absolutely fine where the output frequency goes up to 130-150Hz and the RPM reach a max of 4500RPM, approx.

In the meantime while waiting for more information (if it every comes back to me) I contacted the manufacturer of the motor and this was his answer:
Essentially as the voltage goes from positive to negative, the current flow changes direction, and the associated magnet changes polarity. The higher the frequency, the faster the polarity changes.
The 3600 RPM is for 2 pole pairs changing at 60 hertz.
The formula is RPM = (Hz x 120) / poles
Therefore for 5500 RPM = (X hz x 120) / 4
Hz = 183


I agree with his 3600RPM for 2 poles sentence
I have no problem with the formula to calculate (manipulate) speed, etc..
How the heck do you tell me 3600RPM is for 2pole pairs and in the very last calculation you use a "4"? Doesn't make any sense. I mean is it a 2-pole motor or a 4-pole motor? Note: not asking for calculation just a silly question trying to figure out what these guys purchased smile

I know that vendor's, distributors, even most of manufacturer's engineers are tough to get answers from (for whatever reason), but I mean come on, you can do better than that.
Apparently he will get back to me with more information so I am anxious to see what else I can get out of him.

All the knowledge I have and info I was able to find confirms what you and other members already wrote in your responses and I completely agree with you all but in this case we seem to have a working example of this high speed motor that has some "new" characteristics that I would like to know more about.

It all seems to be confusing and until I get some more information I'm going to stay restless because I just have to know smile

RE: Motor with Gearbox vs. High Efficiency High speed motor

How the heck do you tell me 3600RPM is for 2pole pairs and in the very last calculation you use a "4"? Doesn't make any sense. I mean is it a 2-pole motor or a 4-pole motor? Note: not asking for calculation

I assume the salesman is mixing up the engineering facts. A 2 pole motor has "1 pole pair," a 4 pole motor has "2 pole pairs." Again, unless your customer has a very special (aka $ 35,000.00) motor, he does NOT have one capable of 5500rpm/150hz if 4 pole.

In our world of servos, we DO talk about "pole pairs" ; we generally have 8-32 poles so pole pairs makes sense to use. In the world of AC induction motors, I have never heard anyone refer to their motor as a "1 pole pair motor," but that does not mean there isn't somebody out there saying it.

Bottom line is the information you have is WRONG.

www.KilroyWasHere<dot>com

RE: Motor with Gearbox vs. High Efficiency High speed motor

YOU do the math: if 60hz gives 3600rpm, and speed is directly proportional to hz, then 90hz (90/60)= 5400rpm, 120hz=7200rpm & 150hz (150/60)=9000rpm. no magic here.

www.KilroyWasHere<dot>com

RE: Motor with Gearbox vs. High Efficiency High speed motor

Are you sure the motor is connected for 480V? If you connected a 240V motor to the VFD and programmed the VFD correctly then you could run the motor up to 120Hz while maintaining a constant torque output. This would let you get 700hp out of your 350HP motor.

RE: Motor with Gearbox vs. High Efficiency High speed motor

Ok, so we know a little more, that helps. Most likely a WFI pump of that size is going to be centrifugal, I cannot imagine a PD pump like a gear pump or a progressive cavity pump being made to be able to stay clean enough for pharma Water For Injection processes. I've never seen one anyway.

So assuming it is centrifugal, it is entirely possible that the 350HP rating is so high specifically BECAUSE they intended it to be used at high pump shaft speeds, regardless of how that was attained. So knowing the torque you WANT at the high speed, you work backward to get a HP at a base speed that you need to purchase. I mention this just to get beyond the initial issue of possible mis-sizing. It may be intentional for all we know.

So now addressing only the higher current issue, there are other possibilities here. As you increase above motor base speed, like I said, the loss of torque can result in an increase in slip, which means more current draw. When the speed increase was done with a gearbox, the motor would have been at a fixed speed and rated slip, then the gearbox was increasing the shaft speed at the pump. One thing we dont know yet for sure (do we?) is the base motor speed of the original moror. My suspicion is that maybe what really happened here is that the original motor was 4 pole, to which they added a gearbox to speed it up to run the pump faster. Someone may have looked at that and said, "Why don't we just use a 2 pole motor and start off at a higher speed?" Seems reasonable to me. The problem would be that although you would be eliminating mechanical losses in the gearbox, i.e. friction and heat, you may at the same time be just shifting the looses to the motor now. If I determined I needed 350HP from a 4 pole motor to have enough shaft torque at the pump at a higher speed, I would need to replace that 350HP 4 pole motor with a 700HP 2 pole motor to get the same shaft torque at the pump! This is what's behind LionelHutz comment by the way, it's an old trick, although I've never seen it done at this power level. Without increasing the HP then, the faster motor has LESS shaft torque, therefore more slip and it draws more current. It might have been a LOT more were it not for the elimination of the losses in the gearbox, so all you are seeing is the current delta in the overall SYSTEM EFFICIENCY differences.

If however this WAS all still done correctly, and let's say the original design was a 4 pole 200HP motor that they replaced with a 2 pole 350HP motor (because of removing the gearbox losses), then the current difference may still be explained in other ways:

1) With the added mass of the gearbox, the 4 pole motor was running at closer to full load so it was at a better power factor. Now the 2 pole motor has less load on it, so the PF is technically worse. You will see higher CURRENT as a result, but the actual kW would more likely be LOWER. I find trying to explain PF issues to wrench turners to be less than a satisfying experience, I usually have to finish the conversations with someting like "Because that's the way it is in the electrical world". Good luck with that if this is the case. The infamous "Beer Mug" analog often helps, if for no other reason than to find something else to talk about with them...

2) All other issues are right with the world and all my above hypothesizing is completely in left field. But... They now added in the issue of running over base frequency. The current, if measured from the INPUT side of the VFD, is now slightly higher because you have added losse. Inside of the VFD, running at a higher output frequency adds switching losses that you did not have before. Its not much, but there would also be added heat in the motor, which is also a total throughput efficieny loss, which would also show up as a current increase.

"Will work for (the memory of) salami"

RE: Motor with Gearbox vs. High Efficiency High speed motor

A 240V 350HP motor would be fairly demanding in terms of current and cabling. In Europe around 350HP is where we would make the jump to 3.3kV or 6.6kV (equivalent would be 4.16kV in the US, I think).

The growing number of 690V drives in Europe would be amenable to a reconnecting a 400V delta machine to 690V star, with a corresponding 73% increase in base speed and power output.

RE: Motor with Gearbox vs. High Efficiency High speed motor

Further to Scotty's post; The existing VFD will probably not be able to supply the current that 350 HP at a 240 Volt connection would demand.

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

RE: Motor with Gearbox vs. High Efficiency High speed motor

Further to my own post, leave the 400V machine in delta and increase the frequency by 73% to operate at 690V. Brain in freewheel, fingers in top gear... [blush

RE: Motor with Gearbox vs. High Efficiency High speed motor

An old colleague of mine wrote an interesting and useful piece a few years ago.
( http://www.dpaonthenet.net/article/10084/Lies--dam... )
He'd tried for many years, as I think he still does, to explain the myth that using a 'high efficiency class motor' along with a VFD does not equal savings as a result of the design of the motor. In fact, it can be quite the opposite.

RE: Motor with Gearbox vs. High Efficiency High speed motor

(OP)
Thank you all for your feedback, no new information was provided so far but when it comes I'll submit an update.

RE: Motor with Gearbox vs. High Efficiency High speed motor

I re-read the first post and it is the same VFD so the motor can't be connected at a different voltage. The VFD simply wouldn't support the higher current.

I'd agree it's possible the new motor has more slip at the higher speeds which increases the current.

It's also possible the VFD is simply not programmed correctly for the new motor which is causing the higher current.


Quote:

1) With the added mass of the gearbox, the 4 pole motor was running at closer to full load so it was at a better power factor. Now the 2 pole motor has less load on it, so the PF is technically worse. You will see higher CURRENT as a result, but the actual kW would more likely be LOWER.

Jraef - You're not trying to say the motor current will increase as the motor load is reduced?

RE: Motor with Gearbox vs. High Efficiency High speed motor

(OP)
Curiosity is taking over so I took a trip to visit this new motor.

Actually we have two instances but only one of the new motors has been installed so that gave me the opportunity to see the old setup and the new one.

The old setup is made up of 350HP, 2-pole, 460V, 3phase, 60Hz motor with max speed of 3570. The motor is connected to a gear box with (not confirmed) 1:1.25 ratio so that the 3570 speed can be brought up to approximately 4500RPM which is what is needed for this particular pump and process. The entire setup is controlled by an ABB ACH550 drive.

The new setup has the gear box removed and a different but same type of AC drive (ABB ACH550) is controlling this new fancy motor. I found out that the motor is custom made with Inverter type, 350HP constant HP, laminated frame, 4pole, 460V, with frequency range of 120Hz - 185.4Hz.

See picture attached for the following:
Motor's nameplate shows that at low range speed of 3565RPM the motor functions at 120Hz and it varies up to 185.4RPM it can go up to 5500RPM.

Associated VFD has a frequency input range of 48-63Hz while output (motor connection) frequency can range from -500Hz - 500Hz.

As Mike said in one of his previous posts, there is no magic here:
YOU do the math: if 60hz gives 3600rpm, and speed is directly proportional to hz, then 90hz (90/60)= 5400rpm, 120hz=7200rpm & 150hz (150/60)=9000rpm. no magic here.
So if we follow the same logic at 60Hz we have 3600RPM and at 90Hz we would have 5400RPM.
The set output frequency (at the VFD for new motor is in range of 120Hz to 185.3. Therefore at (120*120)/4pole = 3600RPM and at (120*185)/4 = 5500RPM approximately.

This is starting to make sense now especially having the actual VFD info, parameter settings and knowing characteristics of the motor.

I was told that this to make this product and process work they are running the motors/pumps at approximately 4250RPM for 22 hours at a time.

RE: Motor with Gearbox vs. High Efficiency High speed motor

Well, the VFD did change and they are running the motor up to 120Hz while maintaining a constant V/Hz pattern.

Taking it back to the grid connected ratings, the motor is 175HP, 1800rpm, 60Hz, 240V.

The motor is then operated in a constant torque by applying a constant V/Hz up to 120Hz/480V and the HP output doubles to 350HP @ 480V, 120Hz.

The motor is then running in field weakening above that point maintaining a constant HP output.

The motor doesn't really have a frequency range of 120Hz - 185.4Hz. The first is the frequency where field weakening starts and the second is the maximum frequency.

I don't understand why they wouldn't use a 175HP, 3600rpm, 60Hz, 240V motor and then it could maintain a constant torque output all the way to 7000rpm and there would be no need to use field weakening.

RE: Motor with Gearbox vs. High Efficiency High speed motor

Probably the mechanical limits of the rotor - 7000 rpm will be beyond the capability of a standard rotor.

RE: Motor with Gearbox vs. High Efficiency High speed motor

(OP)
Hi LionelHutz,

The VFD ramps up the motor up to the required speed of above 4000RPM. It never functions at low speeds, less than 120Hz.
So when you say 240V, 60Hz that is the ramping V/Hz ratio to bring it up required level.

When you say:
Taking it back to the grid connected ratings, the motor is 175HP, 1800rpm, 60Hz, 240V.
How did you get 175HP?
I thought that the:
HP2/HP1 = (RPM2/RPM1)^3 so that would make the new HP2 go down from 350HP to about 43.75HP

I don't know if I agree with this, could you please explain?
The motor is then operated in a constant torque by applying a constant V/Hz up to 120Hz/480V and the HP output doubles to 350HP @ 480V, 120Hz.

I agree with this:
The motor is then running in field weakening above that point maintaining a constant HP output.
HP stays constant, Torque decreases, Field Weakening state, Voltage is constant, frequency increases, flux will decrease because of the decreasing torque.

Again, I don't completely agree with the 175HP and it was my understanding that using the field weakening method we are able to achieve high spin speeds without the cost of additional transmissions or oversized motors. Due to momentum little torque is needed to maintain or increase the speed therefore lower torque is an acceptable tradeoff.
I don't understand why they wouldn't use a 175HP, 3600rpm, 60Hz, 240V motor and then it could maintain a constant torque output all the way to 7000rpm and there would be no need to use field weakening.

Please correct me if wrong.

Thanks,

RE: Motor with Gearbox vs. High Efficiency High speed motor

Wow. I would like to see the Baldor Spec sheet on this B1032402 motor - any chance you have it and can post it here?

So now your cookin with details. You should share the old 2 pole motor nameplate jpg also for complete comparison.

Old 2 pole 350hp 3600rpm 2 pole motor means they had 350*5252/3600= 510#-ft rated torque @ 3600 base speed, which should be about same 390-400amps. Since they geared it backwards, the 4500rpm torque is/was 510/1.25= 408#-ft <-there is the comparison point for the new motor running same 4500rpm....

New motor obviously is a 4 pole 1800rpm rpm motor but they are running it up to 2x base speed for 120hz....This is a 175hp motor!

If you doubt this, go look at weight of 175 & 350hp motors - your nametag shows 175hp model....

This is actually a 175hp, 4 pole, 1800rpm, 230v motor being run in constant v/hz to 120hz, then constant hp to 185hz; actually, to get the 350hp to 550rpm it probably is more like a 200v rated motor to allow the 185hz but that is a fine tweak you probably don't care about.

Since you now have a new motor rated 350hp upto 5500rpm @ 403 amps, you can compare both old and new at 4500rpm in amps as you did. Most of the time a motor like this can run to 120hz w/o requiring extra current; but to go to 3x base speed generally does mean reduced torque output rating - so to show same 350hp here, it will require motor current - hence the 403a vs 386a. You would be safe without the real speed torque curve on it, to say the 385 goes upto 403 linearly from 2x to 3x base speed, so with 4500rpm being about middle between those, you can assume it produces 350hp with 395 amps. Since 350hp*5252/4500=408#-ft, just like old motor.

So comparing amps of old vs new in this case should be valid. So back to OP the long way: Unless you show some other info on the original motor nameplate you give us, I would say the new motor pulling 10% higher current is indeed quite reasonable for the same load since you are running into the less efficient area at 2.5x base speed. I could understand - and expect - the new motor, running 4500rpm (2.5x base speed) to be less efficient and require this 10% higher current for same torque output.

www.KilroyWasHere<dot>com

RE: Motor with Gearbox vs. High Efficiency High speed motor

Well, the VFD did change and they are running the motor up to 120Hz while maintaining a constant V/Hz pattern.


Drive did not change: it is the same as it should be.

I don't understand why they wouldn't use a 175HP, 3600rpm, 60Hz, 240V motor and then it could maintain a constant torque output all the way to 7000rpm and there would be no need to use field weakening.

Then it would be 2x too small (175*5252/3600= only 255#-ft - and it goes down from there....

Probably the mechanical limits of the rotor - 7000 rpm will be beyond the capability of a standard rotor.

nameplate says mechanical limit is 5650rpm above which it can explode; I am suprised they could get to even this speed on this size motor.

www.KilroyWasHere<dot>com

RE: Motor with Gearbox vs. High Efficiency High speed motor

lhoash,

To help understand what happens at higher than base speeds and how motor mfgr can play games with the ratings, check out this real life curve.....

This too is a 4 pole motor; but notice we run the v/hz constant not to 1800rpm, but rather to around 2500rpm; since this is a "230v" motor, at old base speed of 1800rpm we actually only supply about 180v. That way we can continue to increase the voltage 'artificially' all the way to 230v @ 2500rpm...

You can see that as we get over 2x 'new' base speed of 2500rpm, it is same power, we peak up a bit more, but as we continue ABOVE that, the power is dropping for the same current input (efficiency going down). I think THIS is why your new "hi efficiency" motor is so much LESS efficient than the old motor and its gearbox losses! Someone did NOT get speed torque curve on the NEW motor and compare it to the OLD motor; hence they missed the efficiency loss. If the object was to use less power, they missed the boat: if the object was to reduce complexity by removing the gearbox (KISS), they the won.

As an aside, why Baldor can call this motor 350hp at 5500rpm with HIGHER current than the lower 3600rpm current, look at my curve again. This motor is called an 8kw motor @ 30amp input..... You can see that since it actually produces MORE than 8kw at 3600rpm (like yours), it will draw LESS current than the 'rated' 30 amps..... so your new motor is probably REALLY rated 403amps continuous and can output that much more torque at 3600rpm also - but that would confuse people so they don't list it that way. Baldor just tweaked the voltage on THEIR 4 pole motor to make it meet YOUR requirements and labeled it thus.

www.KilroyWasHere<dot>com

RE: Motor with Gearbox vs. High Efficiency High speed motor

sorry for my apparent duplication of info; I wrote my first reply at 1345 hours but got interrupted by a customer visit and did not send it until 1533. Since it was in the compose window all this time, I saw no other replies yet. Sorry for duplication of thoughts.

www.KilroyWasHere<dot>com

RE: Motor with Gearbox vs. High Efficiency High speed motor

(OP)
Just when I thought all this makes sense, Mike comes along and kills my dream smile

Just kidding, thanks for the feedback and the breakdown. See attached picture that has side by side info, cut sheet of new motor (all I got for now) and the nameplate of old.

New motor obviously is a 4 pole 1800rpm rpm motor but they are running it up to 2x base speed for 120hz....This is a 175hp motor!
Can you explain this a little bit? Why are we ok to call this 175HP when running at 2x the base speed.

Are you using: HP = VI(eff)/746 to calculate Amps that you mentioned?

Thanks,

P.S. still digesting the rest smile

RE: Motor with Gearbox vs. High Efficiency High speed motor

This might be going in a tangent, but the type of VFD you are using is a dedicated HVAC drive. You state you are using a ACH550.
What is your U/F ratio set to? You have a choice of linear or squared and my guess is that the default is squared. I appreciate this is typically only applicable below field weakening range but if operating above this, you could run into issues. Might be worth checking with ABB.

RE: Motor with Gearbox vs. High Efficiency High speed motor

(OP)
Mike,

thanks for the graph.
Dwelling on this a little more but in the meantime, are you saying that in this case we the VFD is varying the voltage and the frequency starting at 60Hz and 230V and ramps it up to 120Hz, 480V?

Generally, for a typical motor the VFD would vary/control V/Hz ratio starting 0-60Hz and whatever the Voltage calculates out to be, is that correct?

I am sorry, I am still trying to wrap my head around varying V/Hz and where each starts and stops?
Also, in this discussion we actually have a 175HP motor at 230V but we are somehow running it at 480V and getting 350HP.

I can understand that the V/Hz ratio but if a motor is classified as 480V how do we have a 230V motor but run it at 480V?

Sorry for dwelling on this I just need to dummyfy it down so that there is no loose ends.

Thanks,

RE: Motor with Gearbox vs. High Efficiency High speed motor

lhoash; The manufacturer takes a 230V or 200V motor off the standard production line and re-plates it to 460V. The modern insulation systems generally are 600V systems so it isn't much of a stretch. The 460V VFD then lets you keep the V/Hz thing going to way up yonder frequencies. It also allows the torque not to flat-line.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Motor with Gearbox vs. High Efficiency High speed motor

ihoash,

Let's try starting from an accepted premise. You already accept that a 4 pole (1800rpm) 60hz 230v motor rated 175hp will produce 175hp*5252/1800rpm= 510#-ft of torque at 1800rpm, right? If not, there is no sense in reading further.

Assuming you accept that, ask yourself 'what is the v/hz ratio on this motor at 1800rpm?' Can you accept it is 230v/60hz= 3.83v/hz? If not, there is no sense in reading further.

I think you already accept that this same motor will produce 510#-ft or torque from 0 to 1800rpm. Why? Because the v/hz ratio is the same at any of those speeds! Agreed? If not, you need to stop here and go google ac induction motor basics some more.

This means that same v/hz ratio is CONSTANT at any speed from 0 thru 1800rpm. Ie., 180rpm is 6hz so voltage applied to motor is 3.83*6= 23v, and as such, motor will still produce 510#-ft of torque. Etc any other speed from 0-1800rpm (ignoring too slow speeds under about 6 hz)

Continuing, since HP=NT/5252, as hz (speed) goes up -- and T remains CONSTANT -- HP INCREASES

OK, accepting that, you can see that if you only had 230v available, you cannot continue on that 3.83v/hz curve above 1800rpm, right? So NOW, YOU ARE OUT OF VOLTS! What happens if you still increase Hz ANYWAY? The v/hz ratio goes down, SO THE TORQUE PRODUCED GOES DOWN. SIMPLE. So as hz goes up, accepting that torque goes down while speed goes up, what happens to the HP output available? IT REMAINS CONSTANT. So upto some speed above 60hz, the HP is now constant since torque is going down. So by the time you hit 120hz, your v/hz ratio is now only 1.91, so you produce 1/2 the torque but are going 2x the speed - aka, 175hp still. If you keep going, the motor will begin to stop being a good motor and will produce less and less torque due to efficiency roll off and another thingy called breakdown torque rating - but you needn't worry about that right now.

If you are still with me, lets put this same motor now on a 460v supply. AHA! Now we do not have to reduce the v/hz ratio (meaning constant hp) but can continue increasing voltage along same perfect 3.83v/hz ratio until we again run out of voltages - where? 460v. What freq? 120hz. So - stay with me here - since the torque remained constant due to the constant v/hz curve, is this not identical to the 230v case above from 0-1800rpm? But now we ran 0-3600rpm on same v/hz curve, so torque is STILL 510#-ft, yet speed is now 3600rpm. What is the HP at 3600rpm? You do the math. See how we got you 350HP FOR FREE?

Could your customer have simply bought a 350hp, 460v, 60hz 2 pole motor again - capable of 4500rpm max speed? Sure. Could they have just reused the old motor? NO, because it was not rated to the required 4500rpm max speed - if it WAS, then they could have just replaced the gearbox and ran the vfd up.

Head spinning?

www.KilroyWasHere<dot>com

RE: Motor with Gearbox vs. High Efficiency High speed motor

(OP)
Mike,

That was the breakdown I needed, I knew 95% of all of what you said. As a matter of fact I was just explaining the V/Hz ratio and the variance between these two parameters to keep that Torque constant. Theoretically I did a lot of research in the past on the motors and not quite that much on the VFDs so on theory I am OK and follow all what you are saying but these "small" facts that you can take a 230V motor and run it at 460V I can't say that I knew that. I am also assuming that it's not just that easy and that you have to prep a motor to accept different parameters and settings.
Like "itsmoked" mentioned in his post:
The manufacturer takes a 230V or 200V motor off the standard production line and re-plates it to 460V. The modern insulation systems generally are 600V systems so it isn't much of a stretch. The 460V VFD then lets you keep the V/Hz thing going to way up yonder frequencies. It also allows the torque not to flat-line. I didn't know that and unless you are dealing with motors all day and you have quite a bit of exposure with the manufacture of the motors, how can you know that (unless you have all day to sit around and do research smile )

In any case these little holes were making it hard to piece everything together.

Thank you all again (especially you Mike) for taking the time to do a thorough explanation and for sharing your experience and knowledge.

Enver,

RE: Motor with Gearbox vs. High Efficiency High speed motor

Thanks for acknowledgment Ihoash. Know that there are general 'rules of thumb' on these things like all things in life..... one is MOST ac induction motors can run upto 1.5x base speed without issue but not more. Has to do with mechanical balancing and bearings. Some say "since a 2 pole motor can go 3600rpm, so can its equiv 4 pole model rated 1800." Unfortunately, motor mfgrs seldom use the same bearings in those two motors; hence the 90hz max (1.5x 1800rpm) limit. But that is a detail the mfgr must give for a given model. So bottom line is it is not a 'no brainer' to simply run a 4pole motor up to 5500rpm without specific design being done - like this one. Glad you are gettin it!

www.KilroyWasHere<dot>com

RE: Motor with Gearbox vs. High Efficiency High speed motor

Quote (mikekilroy):

Drive did not change: it is the same as it should be.

Quote (lhoash):

The new setup has the gear box removed and a different but same type of AC drive (ABB ACH550)

It sure sounds like the drive changed....


Quote (mikekilroy):

Then it would be 2x too small (175*5252/3600= only 255#-ft - and it goes down from there....


Actually, 175HP would likely be too small but I highly doubt it would be 2X too small. If the pump needs 350hp at the maximum speed of 5500rpm then it certainly won't need 350hp at 3600rpm. My guess would be a 229HP @ 3600rpm would work fine. Might get away with a 225HP, 240V, 3-phase motor and it could* run up to 7000rpm and produce 450HP.

* for the nitpickers I didn't say it would or that the motor is suitable for this, just that it could.
 

RE: Motor with Gearbox vs. High Efficiency High speed motor

Don't forget that a 230 Volt motor at 1750 RPM has twice the torque of the same HP motor at 3500 RPM. If you double the frequency to 120 Hz, you may also double the voltage to 460 Volts. At 120 RPM and 120 Hz, you have the same torque and double the speed and HP.
RE voltage rating of the windings: 230/460 Volt windings must have insulation suitable for 460 Volt operation. For inverter duty the voltage rating will be higher.
The voltage limit of a motor winding has more to do with magnetic saturation of the magnetic circuit than with insulation breakdown.

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

RE: Motor with Gearbox vs. High Efficiency High speed motor

Quote (lhoash)

these "small" facts that you can take a 230V motor and run it at 460V I can't say that I knew that.

Never ignore the frequency when talking about a VFD and running the motor at some voltage. You need to talk V/Hz ratio or voltage @ frequency. This makes it much clearer.


Quote (mikekilroy)

So by the time you hit 120hz, your v/hz ratio is now only 1.91, so you produce 1/2 the torque but are going 2x the speed - aka, 175hp still.

Good explanation but you made a mistake here. It's much more complex then this. The torque drops by the square of the voltage reduction. So, when supply 1/2 of the motors rated V/Hz ratio you can only get 1/4 of the torque. This is why posts earlier mentioned the motor will have a higher slip when operating above the base frequency which could be hurting the efficiency of the motor and causing a little higher current.

There is an easy way to visually look at this. Get a motor speed vs torque curve which shows the motor characteristics operating at rated voltage and frequency. Now, to produce a motor curve when you are running the motor at double the frequency you will double the frequency or speed scale and quarter the torque scale. Notice how the torque at double the rated motor speed has dropped to 1/4 of what it was before? Well, to maintain a constant HP you need the motor to produce 1/2 the toque, so the motor has to run slip more to do so.




RE: Motor with Gearbox vs. High Efficiency High speed motor

Quote (mikekilroy)
Drive did not change: it is the same as it should be.
Quote (lhoash)
The new setup has the gear box removed and a different but same type of AC drive (ABB ACH550)
It sure sounds like the drive changed....


Sorry for the confusion about vfd model, I must have misread lhoash's 3 references:

1) new high efficiency high speed (5500RPM) motor but the new motor is still controlled by the existing ABB VFD
2) old setup is....an ABB ACH550 drive.
3) new setup has....same type of AC drive (ABB ACH550

www.KilroyWasHere<dot>com

RE: Motor with Gearbox vs. High Efficiency High speed motor

Good explanation but you made a mistake here. It's much more complex then this. The torque drops by the square of the voltage reduction. So, when supply 1/2 of the motors rated V/Hz ratio you can only get 1/4 of the torque.

Good point that the MAXIMUM available torque decreases at square of velocity above base speed (we of course still have same voltage). Since this MAXIMUM did not dip under the AVAILABLE rated torque at this 120hz yet, I chose to not confuse the issue with breakdown torque, simply mentioning it in passing as it had no impact on the 175 vs 350hp discussion at that point.

www.KilroyWasHere<dot>com

RE: Motor with Gearbox vs. High Efficiency High speed motor

I'm just pointing out that the torque curve of the motor does not drop linearly with speed as you enter field weakening. I'd hate for someone to go away from this thread and think that with 1/2 the rated voltage the motor will produce 1/2 rated torque.

And yes, the motor rated voltage is 460V for a 3.38V/Hz ratio when running at 120Hz so it is running at 1/2 of the rated voltage when supplied with 230VAC, 120Hz.

RE: Motor with Gearbox vs. High Efficiency High speed motor

(OP)
Thank you Lionel and Mike.
Great points and like Lionel said these threads should be dwell-ed on to eliminate the confusion and any wrong information.

Just to clear some confusion that I created.

The VFD drives didn't change at all.
Both drives are ABB ACH550 which used to feed old setups and will eventually feed both new setups.
(At this time we still have one old and one new installation)

This website is honestly the best one so far and all of the younger and upcoming engineers should feel lucky to have experienced and knowledgeable engineers like you guys giving feedback and actually finding time to answer all these random questions.

RE: Motor with Gearbox vs. High Efficiency High speed motor

Lhoash, I did not want to get a discussion going on breakdown torque since it was not really relevant to your questions, but since it has been brought up and is being discussed as if it is a limit for your case, it probably should be examined more....

I did a quick google search for a chart showing both breakdown and rated torque above base speed on a vfd but did not find one; so I made one up. A typical breakdown torque rating can be around 250% of rated.

First a description of rated, available, and breakdown (Max) torques to be clear....

- Rated is what the motor can put out all day long and as you know can be dependent on speed (constant below 'base' speed, drop linearly above) - this is a thermal limit.
- breakdown is the absolute max torque you can get out of the motor at its rated voltage - again, changes with speed, but in this case, it drops as the SQUARE of the speed increase above 'base.' - this is a physical design limit
- available is the lower of continuous or breakdown.

So attached is my chart (first one) showing these on a typical 10hp motor, as well as my spreadsheet (second one) that generated it.

You can see that breakdown torque has no effect on the motors rated output torque, since it is well above the rated torque to over 4500rpm: this is the reason I did not bring up breakdown torque or torque dropping at the square of anything rating.

Lionelhutz' discussion about why your motor is less efficient at these higher speeds (due to more slip) is a good one; I simply showed previously that the less efficiency was due to 'stuff' going on at these higher speeds - including more slip, more eddy current losses, more dynamic friction losses, windage, and POSSIBLY breakdown torque effects.

Feel free to modify the spreadsheet attached and put in YOUR new motor data as you know it! Unlike this typical motor, you will find YOUR motor's BD torque is WAY above its rated torque well out past its 5500rpm rating due to their new 'base' speed of 3600rpm! LOTS & LOTS of tricks can be played with motors.....



www.KilroyWasHere<dot>com

RE: Motor with Gearbox vs. High Efficiency High speed motor

(OP)
Great stuff Mike,

Thanks,

RE: Motor with Gearbox vs. High Efficiency High speed motor

[quote]but since it has been brought up and is being discussed as if it is a limit for your case{/quote]

I never posted anything about it being a limit, but it always is one of the limits when running in field weakening.

But since you bring it up and want to expand on the curves, you shouldn't call the first curve the rated torque. That is nothing more than the theoretical constant HP curve. There are a number of possible reasons the speed and/or torque can be limited besides hitting the breakdown torque.

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