VFD Sizing and Specification

[dpc]

Looking for opinions on best way to specify VFD output current rating.

As an example, if we have a 40 hp, 460 V motor with FLA = 51 and Service Factor =1.15 on a centrifugal pump, what would you consider an acceptable continuous current rating for a VFD? Do you use something like 115% of the motor FLA?

Also, is there an advantage in specifying 1 min and 3 min overload ratings for the drive? I see a wide range of standard overload current capability from the major drive suppliers, ranging from 150% for 1 minute, down to 110% for one minute.

TIA,

Dave

 

[jraef]

I only specified VFD sizes using the motor's SF if it was absolutely necessary for the application, otherwise I just leave it as a safety factor in the design. So to that end, I would not specify that the VFD needs to cover the SFA in most cases.

The same concept holds true for the time rating. I use 150% for 1 minute. Anything more that a supplier wants to give me is a bonus as long as I didn't have to pay extra for it. My reasoning was that a 3 minute 150% OL should be indicative of a design error in the motor selection in the first place and will most likely exceed the OL curve anyway. There are of course special applications where you might want and need that extra capacity, such as on a centrifuge perhaps, but those are the exception rather than the rule.

JRaef.com
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[dpc]

Thanks. So if a motor FLA was 50 A and the drive max continuous output was 50 A, you'd be OK with that? Not questioning, just trying to clarify.

dpc

 

[jraef]

Yes, I would as long as I was not needing to run the motor into the service factor for any longer than a minute or so.

JRaef.com
"Engineers like to solve problems. If there are no problems handily available, they will create their own problems." Scott Adams
For the best use of Eng-Tips, please click here -> faq731-376

 

[DickDV]

dpc, you might be missing something important here. You have to start with the load, not the motor. What does the load require for continuous and short-term torque. Then, translate that back into motor current both continuous and short-term. Finally choose a drive that covers that current requirement.

Just for example, you may have a 50hp motor with 1.15 SF but if its on a pump that requires only 46hp and max speed and acceleration is slow, you don't need anything over the 50hp nameplate amps on the motor. If you could buy a drive right at that output current or even an amp or two less, that would be enough.

 

[thewellguy]

I would also suggest checking with the motor manufacture. Different motors require different overload times. Also when specing a vfd make sure that the pump is not designed to operating in the sf which could cause problems if the pump runns above sfa.

 

[dpc]

Dick,

I appreciate that the pump curve may show that the motor will never be required to run at its FLA, but there is no way I would ever specify a VFD with less current output than the motor FLA. I don't trust the mechanical engineers or the pump curves that much. If something is going to be too small, I don't want it to be the drive.

But I agree that often so many factors of safety get applied to these sizing calculations that we end up with motor and drives that never get close to their max ratings. I suppose if I were working for Toyota and designing drives for the Prius, it might be worth fine tuning the sizing of the drive as you suggest, since they are buying them by the millions, but when specifying a drive for an existing irrigation pump and motor, I'm not sure it would be worth the effort. And I'm basically lazy.

 

[Marke]

Hello dpc

It is common practice to specify the VSD rating on the load characteristics and this may often mean that the VSD is smaller than the motor capabilities.
Modern VSDs can protect themselves pretty well and if an overload occurs, the VSD will slow or trip.
Pumps are often over motored but you don't know it. You commonly find that a pump assembly is rated in the pump load rather than the motor size and as a result we get all sorts of unusual ratings for pumps.
With pumps and fans, it is normal to select a VFD with an overload capacity of 120%, whereas for most other applications it is common to select a VFD with an overload capacity of 150%.
In your case, I would use a 30KW VFD with an overload capacity of 120%.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[dpc]

Thanks, all.

I agree that ideally one could size the VFD based on the driven equipment and maybe that occurs when the VFD, pump, and motor are all supplied by one entity who will be responsible for successful operation. But if you are asked to specify a VFD for an existing installation, it's a little different.

But since there is no standard for sizing, each manuf has a different size breakpoint that pushes them up to their next model. So a few more amps may not cost anything from vendor X, but be a big adder from Vendor Y. I just wanted to get a sense for how others might approach specifying a VFD size.

Cheers,

Dave