Motor with Gearbox vs. High Efficiency High speed motor 4

[enverd]

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

 

[enverd]

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

 

[LionelHutz]

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.

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?

 

[enverd]

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.

 

[LionelHutz]

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.

 

[ScottyUK]

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

 

[enverd]

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,

 

[mikekilroy]

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.

http://www.KilroyWasHere<dot>com

 

[mikekilroy]

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.

http://www.KilroyWasHere<dot>com

 

[mikekilroy]

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

 

[mikekilroy]

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.

http://www.KilroyWasHere<dot>com

 

[enverd]

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

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

 

[itsmoked]

Nice sleuthing Mike.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ozmosis]

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.

 

[enverd]

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,

 

[itsmoked]

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

 

[mikekilroy]

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?

http://www.KilroyWasHere<dot>com

 

[enverd]

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 )

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,

 

[mikekilroy]

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!

http://www.KilroyWasHere<dot>com

 

[LionelHutz]

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

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....

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.

 

[waross]

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