sizing variable frequency drives

[sparky4barleans]

I was wondering if someone could tell me how to size a frequency drive. I have a 40hp motor with a nameplate rating of 44amps at 480 and constant torque. I'm getting conflicting information from the wholesale house.

 

[Jraef]

I was wondering if someone could tell me how to size a frequency drive. I have a 40hp motor with a nameplate rating of 44amps at 480 and constant torque. I'm getting conflicting information from the wholesale house.

You need a drive rated for 44A output or more. Shouldn't be that hard really.

The issues of "Variable Torque" or "Constant Torque" ratings, now also known as "Normal Duty" or "Heavy Duty", have to do with the type of load connected to the drive and the possible overload conditions you might expect to see. If you know for CERTAIN that it is a CENTRIFUGAL pump or fan, then you can use the Normal Duty / Variable Torque ratings, because it's virtually impossible to overload those machines, so the VFD output current is rated for that scenario. But just because it is a pump (if it is a pump...) does not mean it is a centrifugal pump. If you don't know, sizing for Heavy Duty / Constant Torque is always going to work.

So being that you already SAY it is a Constant Torque load, then there should be nothing conflicting in what the supply houses could tell you, unless they are incompetent. Sounds like maybe some are. What are they saying?

PS: if you are in Ferndale CA (not WA or MI), and the conflicting info is coming from an A-B distributor, I want to know!

 

[kwired]

What is the conflicting information?

What kind of load is being driven and any other characteristics of the load.

I have seen cases where the motor is reversed frequently or otherwise accelerated/decelerated frequently and they maybe want to derate the drive.

 

[sparky4barleans]

sizing a vfd

sizing a vfd

I tried to tell the wholesale house that the drive should be sized to the FLA. They said their drive specialists would not recomend going off the FLA of the motor. They wanted to sell me one that was rated for 75 amps when I only have a 44amp load. It is a constant torque and it is an AB drive but I'm in Wa not Ca. Thanks for your help. Just wanted to make sure I was right by sizing the drive to the FLA. Simple enough, why make it complicated.

 

[Jraef]

I tried to tell the wholesale house that the drive should be sized to the FLA. They said their drive specialists would not recomend going off the FLA of the motor. They wanted to sell me one that was rated for 75 amps when I only have a 44amp load. It is a constant torque and it is an AB drive but I'm in Wa not Ca. Thanks for your help. Just wanted to make sure I was right by sizing the drive to the FLA. Simple enough, why make it complicated.

Huh, that's odd. We don't have a drive rated 75A.

NEC chart for a 40HP 460V motor is 52A. Is your nameplate FLA only 44A on a 40HP motor? Must be a high efficiency motor. A PowerFlex 753 40HP Heavy Duty, aka "Constant Torque", drive is rated 52A Heavy Duty, 65A Normal Duty. You would be fine with that. But the next size down is rated for 40A HD, that would be too small. The next size up is rated 65A HD / 77A ND, that would be over kill a little, but if you have a high ambient or a sealed box w/o an AC unit, maybe they are protecting you by de-rating it for a higher operating temperature?

Like I said though, there is no PowerFlex drive rated for 75A.