Typical Turndown for Motor with VFD
Typical Turndown for Motor with VFD
(OP)
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?
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
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?





RE: Typical Turndown for Motor with VFD
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.
RE: Typical Turndown for Motor with VFD
Gunnar Englund
www.gke.org
RE: Typical Turndown for Motor with VFD
I agree with you with an aux blower but this motor has only a shaft fan.
RE: Typical Turndown for Motor with VFD
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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RE: Typical Turndown for Motor with VFD
respectfully
RE: Typical Turndown for Motor with VFD
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?
RE: Typical Turndown for Motor with VFD
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.
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.
http:/www.jraef.com
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: Typical Turndown for Motor with VFD
Gunnar Englund
www.gke.org
RE: Typical Turndown for Motor with VFD
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.