VSD versus motor sizing, oversized VSD 1

[FreddyNurk]

A question came up today regarding VSD sizes versus motor size. Case in context, drive is an existing unit, something like a 150kW, brand name unknown.
For various reasons the consideration of using existing VSD with a much smaller motor, say 11kW.

In theory, this should work, with the possible caveat of ensuring parameters to suit the revised motor are available, but does anyone have any experience in issues that might arise in putting this into practice?

The obvious response is to procure the correct drive size for the motor (not to mention upstream protection and so on), as its quite a waste, but I'd be interested in whether there are issues in the drive being capable of running the motor at a desired speed or other issues if the drive is way oversized.

EDMS Australia

 

[LionelHutz]

Probably depends on the drive. I've had a drive work successfully with that big a gap, but it's possible there are models that won't.

 

[jraef]

As a gross generalization, more modern drives are using faster switching transistor systems that require very precise monitoring of their effects on the motor. That means the current feedback from internal sensors must be very accurate. When you get too far afield of the drive size vs the motor size, that becomes problematic. In general then, most drives have a 50% lower limit, meaning a 150kW drive would be good down to 75kW. It also matters if you are using any kind of vector control, including sensor less vector control (SVC) which also needs accurate current feedback.

Older drives that used only V/Hz control however may not have had as dramatic of a restriction, but there still may be an issue with the drive being able to a accurately determine the overload condition of the motor. When I did things like this in the past, I just used an external overload relay. But even then you have to be careful because not all OL relays can be used with variable frequency.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[EdStainless]

The drives that I worked with had many settings that were based on the full scale value, so maybe current limit was in steps of 2% of full scale. This limited their use on much smaller motors.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy

 

[itsmoked]

I recently had a drive that was for a 5hp motor but I was using it for a 2hp as a simple setup to develop the PLC code for a big installation. The drive absolutely refused to do anything but V/Hz because of the lowered accuracy of the sensing because of the smaller motor.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

If the drive is being relocated the feeder must be sized for the drive rating, not the motor rating.
Running long 150 kW feeders for an 11 kW motor will quickly use up the cost of a new, smaller VFD.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Excellent point Bill!

Keith Cress
kcress -

http://www.flaminsystems.com

 

[FreddyNurk]

Thanks for the feedback folks, it was pretty much what I was expecting.

EDMS Australia