Feeding multiple VFDs

[collins]

I have a portable pump that has 2 VFD controlled motors, but with a single input plug. Can I size my cable to this plug as:
125% of Rated Input of largest VFD plus the FLC of the smaller motor?
or do I need to add 125% of both VFD's Rated Inputs?

 

[jim dungar]

Conductors supplying VFDs must be sized based on the full load input amps of the VFD, the motor has nothing to do with it.

So, I would go with 125% of the largest VFD input + 100% of the remaining VFD inputs.

 

[weressl]

I have a portable pump that has 2 VFD controlled motors, but with a single input plug. Can I size my cable to this plug as:
125% of Rated Input of largest VFD plus the FLC of the smaller motor?
or do I need to add 125% of both VFD's Rated Inputs?

Even though that was not your question you should NOT connect different size motors to an ASD.

 

[petersonra]

Even though that was not your question you should NOT connect different size motors to an ASD.

I am curious as to why? The ASD just creates a different frequency. How is this any different than connecting the two motors to the same branch circuit?

You do need to make sure you have proper SC and OL protection, but that can be handled.

 

[weressl]

I am curious as to why? The ASD just creates a different frequency. How is this any different than connecting the two motors to the same branch circuit?

You do need to make sure you have proper SC and OL protection, but that can be handled.

Because the impedance difference between the two devices would result in imballanced voltage delivery at the motor terminal. Short ciruit 'protectiion is inherently provided by the drives own overload protection. Usually 250% at 1 sec. or something like that, depends on the drive manufacturer..

What kind of short circuit can an ASD deliver? Puny......

As far as overload protection, it is pretty much meaningless since the amperage changes as the speed changes - assuming variuable torque application - but it could still burn up the motor at lower speeds if it is sized for the nameplate FLA. That is also why you want to make sure that the motors are equal size, even brand and model number and that their feeders are equal length. If you want to do it right, you employ thermistors as motor protectors, embedded in the motor winding.

 

[petersonra]

Because the impedance difference between the two devices would result in imballanced voltage delivery at the motor terminal. Short ciruit 'protectiion is inherently provided by the drives own overload protection. Usually 250% at 1 sec. or something like that, depends on the drive manufacturer..

What kind of short circuit can an ASD deliver? Puny......

As far as overload protection, it is pretty much meaningless since the amperage changes as the speed changes - assuming variuable torque application - but it could still burn up the motor at lower speeds if it is sized for the nameplate FLA. That is also why you want to make sure that the motors are equal size, even brand and model number and that their feeders are equal length. If you want to do it right, you employ thermistors as motor protectors, embedded in the motor winding.

Note that in my post I noted the need for dealing with OL and SC. I agree it is usually impractical for larger motors, but for smaller ones, I don't see the problem. Most of the time, the SC situation is nor an issue for smaller motors, and it is easy to add external overloads for each motor tied into the enable circuit.

I don't see how making the motors the same size would permit you to skip the external overloads in any case, as both motors have to be protected and you can't do that with a single setting in the drive.