Typical Turndown for Motor with VFD

[Ashereng]

Hello everyone,

I have a quick question regarding control of a motor with VFD.

The motor:
- 350 hp
- 3600 RPM
- TEFC enclosure
- 449TS frame size
- 3/60/460 phase/freq/volt

A VFD has been installed, not sure of the make.

What is the typical turndown for a motor this size with a typical VFD (to match the motor).

I have been told that 5:1 is typical, and 20:1 is possible. Does this sound right?

"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
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[DickDV]

The answer to your question is entirely dependent on how much torque the motor must deliver at those lower speeds.

For example, if the torque is constant as the motor slows down, about 3/1 is all you can expect in a 440 frame motor.

On the other hand, if the load torque goes down as a function of the square of the speed as in centrifugal pumps and fans, that same motor would be good for around 10/1.

Your motor manufacturer usually can provide torque-speed curves for there motors. You should probably get one.

 

[Skogsgurra]

With an external fan and a good space-vector control, you should be able to have your 20:1. Add an encoder and you can have rated RPM - zero RPM. And more than rated RPM at a reduced torque.

Gunnar Englund

http://www.gke.org

 

[DickDV]

skogsgurra, this is a TEFC motor. That's totally enclosed fan cooled. That would be a shaft fan.

I agree with you with an aux blower but this motor has only a shaft fan.

 

[jraef]

DickDV,
Really, only 3:1 CT on a 440 frame? That surprises me.
Or is that because it's TEFC as well as being a 440 frame? I have gone 5:1 or even 6:1, even with V/Hz drives. But to be honest I wasn't taking torque measurements so I don't really know if I was really needing full torque or not (conveyors and crushers). My applications worked, but I may have had more motor than necessary to start with since I didn't select the motors.


Ashereng, you need to find out more about the VFD and attached load either way.

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[waross]

If it is an issue, it may be easy to add an external fan to a TEFC motor. The designs of TEFC that I am familiar with would easily take an add on cooling air source blowing through the shaft mounted fan.
respectfully

 

[Ashereng]

Thank you everyone for your responses.

I asked my questions as I do not have information on either the motor or VFD, and needed a ball park feel type of answer. Also, I don't have flow rates required of my centrifugal pump, so I am pretty much groping in the dark so to speak.

What I am trying to do is to give an "envelope" of what may be expected of the equipment, in the absence of specific information, to determine if anything should be pursued.

I too suspect that I have a lot more pump, and motor, for what I need to do, and the turndown needed should be well within what we will ultimately ask of the motor and VFD.

I have thought about the possibility of added an auxiliary fan (and yes, I think it should be relatively possible to mount it on the motor casing or nearby support), but was reserving it for higher turndowns. I am hoping a 3:1 or 4:1 will be sufficient, and also that an additional would not be need. Above these turndowns, then I would need to take another look at making sure everything stays cool.

From the responses, it seems that 5:1 is typical, and 20:1 is possible.

Again, thanks for the responses.



"Do not worry about your problems with mathematics, I assure you mine are far greater."
Albert Einstein
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?

 

[jraef]

Ah, well if it's a centrifugal pump you may not need the fan anyway. 2 things about applying a VFD to a centrifugal load that apply to your question:
[tab]1) The load on the motor decreases with the cube of the speed, so the motor is significantly less loaded and therefore not creating as much heat.
[tab]2) You probably will not be turning speed down that much anyway because at some point your pump ceases to pump anyway.

You should be fine.

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Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[Skogsgurra]

Yes. Seldom see centrifugal pumps operating as low as 33 percent. Might as well shut them down below that speed.

Gunnar Englund

http://www.gke.org

 

[DickDV]

jraef, you are right on the extra turndown range but that is due, as you suspect, to the motor being less than fully loaded.

Most of my motor experience is with Reliance, Marathon, and ABB and they all end up around 3/1 in the larger NEMA frames, assuming full rated torque output over the range of speeds. And, of course, that would be assuming 40 degrees C ambient, also.