power ratio VFD-motor 3

[fab1961]

Dear all,

given a VFD sized for 4 kW motors is there any problem driving two motors 0.75 kW
in parallel? In other words, how much smaller can be the load compared to the VFD?
The driving mode is a simple V/f.

Thank you

 

[LionelHutz]

That should work fine. I would expect the VFD would work at least down to a 10:1 ratio or so.

 

[itsmoked]

Lately, I have NOT found that to be true. Typically, trying to run motor(s) at two sizes smaller than the drive rating one starts having issues. Often with settings you want not being allowed.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[fab1961]

Thank you. What worried me most is some potential issues during working, when motors run.

 

[Tanweer Ahmed]

For running two motors in parallel is technically possible but not a good idea. The reason is that the VFD's built-in protection features will not work. It's better and cheaper to use two 0.75kW VFDs.

 

[itsmoked]

Tanweer Ahmed; Welcome to Eng-Tips.

I agree with Tanweer's point. 0.75kW VFDs are very inexpensive. Two drives are usually simpler for maintenance to deal with and allows some redundancy.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Welcome to Eng-Tips Hussel
By the way, have you checked our posting guidelines?
You should see the posting guidelines in the left hand panel.
Bill
--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[jraef]

A VFD must be capable of protecting a motor even if it is being run at very low speeds ad low load, so it's not a factor of the VFD's ability to protect, it's more a factor of how accurate you need the control to be. As load gets lower and lower, a VFD's output gets less and less capable of maintaining the correct V/Hz pattern the motor needs as the motor's fixed internal losses become more and more of the total load. That is one of the advantages of Vector Control in a VFD, it allows the VFD to maintain that precision down to very low speeds and loads.

But you can't use Vector Control on two motors connected to the same drive, the drive must be in V/Hz control. So having only 1.5kW on a 4kW drive may result in the two 0.75kW motors being "sloppy" in their speed control response. For this reason, the general rule-of-thumb is that the motor should not be less than 50% of the VFD size. However that is subjective, I have recently found some that are good for a 10:1 size range.

As far as protecting the motors, the VFD cannot do that either, because it cannot differentiate one motor from the other, it only sees the total current. So you must put additional separate Over Load Relays down stream of the VFD to protect the motors. Here in North America, that is required by our electrical codes. Then here you must be careful too because not all OL relays are suitable for being used on the output of a VFD, you have to check with the relay mfrs.


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