Multiple Motors on One VFD

[cvirgil467]

Question on conductor sizing and OCPD sizing to a VFD with bypass that is operating more than 1 motor.

For conductor sizing to VFD - is this based on 430.24 (125% of largest motor plus sum of the other motors) or 430.122(B)(2) (125% of motor full load rating - I assume this means the sum of all motors combined x 125%)

For overcurrent protection to the VFD - based on size per 430.53(C)?

Thanks.
b

 

[Jraef]

Question on conductor sizing and OCPD sizing to a VFD with bypass that is operating more than 1 motor.

For conductor sizing to VFD - is this based on 430.24 (125% of largest motor plus sum of the other motors) or 430.122(B)(2) (125% of motor full load rating - I assume this means the sum of all motors combined x 125%)

For overcurrent protection to the VFD - based on size per 430.53(C)?

Thanks.
b

No, VFD supply conductors are to be 125% of the VFD input current rating. OCPD is often dictated by the VFD mfr based on what it was UL listed with, but usually follows the NEC rules, again, based on the VFD size. In multi motor applications that gets trickier though, because the rating for the VFD may be too high for the smaller individual motors.

Ammended:
Sorry, I missed that you said "with a bypass". So technically although 430.122 is what would apply, the rule on conductor sizing feeding the VFD rarely results in different conductor sizing based on if you use 430.122 A or B anyway. If for example you go with (B), total motor FLA x 1.25 for the input conductors and have 3 x 7.5HP 460V motors with an FLA of 11A each, 33 x 1.25 = 41.25A for the input conductors. But if you go with (A) and select the VFD first then size the conductors for that based on the input amps or the VFD, you end up in the same place anyway. That's because if you read the suggestions from most VFD mfrs, they will tell you to slightly over size the VFD for multiple motor applications due to the added conductor lengths for multiple circuits increasing the cable capacitance charging current. So I always recommend +10% of current if possible. In the example above then, 33A x 110% = 36.3A VFD, then the next size up is rated 40A output. But the INPUT amps for that drive is 36.7A, so x 1.25 = 45.9A for conductor sizing. That difference between 41.25A and 45.9A is not going to make a difference because it's #8 either way.

 

[petersonra]

No, VFD supply conductors are to be 125% of the VFD input current rating. OCPD is often dictated by the VFD mfr based on what it was UL listed with, but usually follows the NEC rules, again, based on the VFD size. In multi motor applications that gets trickier though, because the rating for the VFD may be too high for the smaller individual motors.

does the line side OCPD actually provide any protection at all to the load side of a VFD?

 

[Jraef]

does the line side OCPD actually provide any protection at all to the load side of a VFD?

In my opinion, no, and I used to go by that. But others have convinced me that as far as the code goes, there is no allowance made.

One drive to one motor, it's kind of a moot point because long ago, UL made the drive mfrs provide Short Circuit Protection for the motor within the drive, so when you program the motor size into the drive, you are also setting up the proper SCPD for it, not just the OL protection. But once you have multiple motors, that no longer applies, for OL and for SCP. So technically, if you buy a 100HP drive and run 20 x 5HP motors from it, not only does each motor need a separate OL relay, but the SCPD for that 10HP drive is now no longer going to be small enough for those 5HP motors.

That's why the best approach to protection on the output going to multiple motors is to use the little IEC Manual Motor Starters / Motor Protection Switches that are both an adjustable OL and a SCPD in one. One problem with that has been that most of those devices have, up until now, only been listed for use at 50/60Hz. Lately mfrs have been releasing versions that are listed to be usable up to 400Hz.

 

[cvirgil467]

There are 4 motors connected to the VFD and each motor is protected by a motor circuit protector that is integral with the VFD. I am clear on the conductor sizing. For the feeder overcurrent protection to the line side of the VFD I assume that can be sized based on the same rules that apply to multiple motors on a circuit as per 430.53(C). I would see why it would a VFD would be any different.

 

[petersonra]

There are 4 motors connected to the VFD and each motor is protected by a motor circuit protector that is integral with the VFD. I am clear on the conductor sizing. For the feeder overcurrent protection to the line side of the VFD I assume that can be sized based on the same rules that apply to multiple motors on a circuit as per 430.53(C). I would see why it would a VFD would be any different.

I have never seen such an arrangement. What brand of VFD has built in MCPs?

 

[cvirgil467]

My bad. They have individual circuit breakers, not MCPs. The VFD is Danfoss. I assume it's the VLT model.

 

[Jraef]

You can use the same rules for sizing the OCPD feeding the drive, as long as you do not exceed the maximum stated by the drive mfr, or if the drive is listed only with a specific fuse. Many are, you must RTFM.

 

[ptonsparky]

You can use the same rules for sizing the OCPD feeding the drive, as long as you do not exceed the maximum stated by the drive mfr, or if the drive is listed only with a specific fuse. Many are, you must RTFM.

I suggest you take a chair, drink, snacks and a blanket when you do RTFM.

 

[Besoeker]

In my opinion, no, and I used to go by that. But others have convinced me that as far as the code goes, there is no allowance made.

One drive to one motor, it's kind of a moot point because long ago, UL made the drive mfrs provide Short Circuit Protection for the motor within the drive, so when you program the motor size into the drive, you are also setting up the proper SCPD for it, not just the OL protection. But once you have multiple motors, that no longer applies, for OL and for SCP. So technically, if you buy a 100HP drive and run 20 x 5HP motors from it, not only does each motor need a separate OL relay, but the SCPD for that 10HP drive is now no longer going to be small enough for those 5HP motors.

That's why the best approach to protection on the output going to multiple motors is to use the little IEC Manual Motor Starters / Motor Protection Switches that are both an adjustable OL and a SCPD in one. One problem with that has been that most of those devices have, up until now, only been listed for use at 50/60Hz. Lately mfrs have been releasing versions that are listed to be usable up to 400Hz.

View attachment 15632

It's what I would do/have done.