VFD help for a ChemEng

[Hippo]

Hello. I recently ran into a problem with the startup of a pair of compressors.

The vendor was asked to provide VFDs to allow the machines to turn-down to half of the design gas rate.

The vendor provided a 1000-rpm 200hp motor. On startup, the compressor could not develop the pressure needed. The reason stated was that the motor was too small. I must admit that I didn't understand this b/c we have a 200hp motor, and I know that the hp required based on the process conditions is 174hp at full load.

It turns out that the motor at turndown speed (450rpm) can only deliver 40hp and 80hp at max speed.

This is where I need a crash course (for dummies) on this limitation. Can someone briefly and simply explain this 200 hp motor can't put up 170hp?

Also, apparently we have an 1800rpm driver and a 1000rpm motor. Is this odd, or does it cause any problems?

Thanks for the help. I just want to educate myself a little on what went wrong and make sure it is avoided next time.

Mark

 

[waross]

1800 rpm is normally associated with 60Hz.
1000 rpm is normally associated with 50 Hz.
What is your supply frequency and the frequency of your motor and drive?
respectfully

 

[jraef]

"Also, apparently we have an 1800rpm driver and a 1000rpm motor."
I don't understand what you are saying here. What do you mean by "driver" vs motor? The VFD? The VFD does not have an "RPM" rating.

Anyway, moving on...

When you lower the speed with a VFD, you are maintaining TORRQUE at the same value, but since HP is an expression of an amount of torque at a given speed, the net HP will go down with speed. So if your final turn-down speed was 1/2 of the design speed, your shaft HP becomes 1/2 as well.

So now let's look at what your vendor supplied. When you said they provided a "1000RPM 200HP motor", I'm a bit puzzled here. Motor base speeds are determined by a number of poles and the design frequency (I'll use synchronous here for a bit of clarity, but realize that slip speeds are slightly less) . A 2 pole motor is 3600RPM at 60Hz, 3500RPM at 50Hz. A 4 pole is 1800/1500. A 6 pole is 1200/1000. So it looks as though they supplied you with a 6 pole motor designed for 50Hz, but if they did, it would be rated in kW not HP, because HP is a term now used only in N. America where we all use 60Hz. So maybe they provided you with a 1200RPM 60Hz 200HP motor knowing that it would be turned down with the VFD, but if they did, then it would only put out 177HP at 1000RPM (adjusted for slip). Cut that in 1/2 now and you are at 88HP.

But this is still a lot more than the 40HP they are telling you that you have at the 1/2 speed (even though 450RPM is less than 1/2 of 1000RPM), so something is definitely amiss here. Either you are not clear on what your equipment is, or someone is trying to pull the wool over your eyes on what they gave you. Try to get some more specific information on what you have installed, i.e. nameplate data, VFD model numbers etc. and we'll see if we can help more.

JRaef.com
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[jraef]

Heh, crossed in the Reply stage there waross! Great minds think alike.

 

[waross]

Hi jraef;
That was my exact thought.
LOL
Respectfully

 

[DickDV]

Hippo, I believe that drive/motor/load behavior becomes more understandable when you leave horsepower out of the analysis and simply focus on torque and speed.

So, for your compressor, there is a level of torque required over the speed range that you intend to operate. That torque may remain constant over the speed range or it may vary. At any rate, you need to pick a motor that can produce not less than this torque at the various speeds. Induction motors magnetically (disregarding thermal limitations) are essentially constant torque machines so, whatever your compressor's peak torque is, that is the minimum torque needed in the motor (I am assuming a direct coupled situation here, no reduction or increase in speed due to gearing, belting, etc.).

To use the various motors mentioned in the above posts, if you have a motor nameplated 200hp at 1800rpm, that motor is capable of 600ft-lbs of torque as it slows down. If the motor is nameplated 200hp at 1200rpm, that motor is capable of 900ft-lbs. If the motor is nameplated 200hp at 1000rpm (unlikely for the reasons cited above) that motor would develop 1050ft-lbs of torque.

A standard induction motor cannot continue to cool itself as it slows down so, at some speed, the torque has to be derated to stay within the thermal limits of the motor. In the 200hp range, a 3 to 1 speed reduction range is common but you need to ask that question for your specific motor to be sure.

In order for an induction motor to produce this constant torque, the VFD has to be able to produce enough amps to fully energize the motor. A VFD can typically output from 0 to 250hz which is way beyond the capability of most motors so it is unlikely that the drive is the limiting factor here. If the motor is adequate for the job by the above rules, then it is likely that the drive is sized to small--it cannot produce enough amps to fully drive the motor.

Hopefully, this will allow you to isolate the area of your problem and deal with the vendor to get it fixed.

Good luck!