VFD controlled Motor

[kkp]

50HP, 460V, 3PH motor controlled by VFD. What will be the feeder and breaker size to VFD? What will be the branch circuit wire size from VFD to the motor? What will be the non-fused disconnect size at the motor. VFD and motor located within 15ft. VFD provided with integral disconnect and bypass.

Thanks.

 

[Len_B]

Re: VFD controlled Motor

kkp,

I believe this is correct, perhaps another member will check this post. Because there is a bypass, "across the line", starter in the VFD, I believe the calculations should simply ignore the VFD presence.

50hp = 65amps@460vac per table (art 430)

65amps x 115% = min 75amps or 50hp rated motor disconnect
(Note that this motor disconnect may not be required provided the main VFD / controller disconnecting means is lockable and otherwise suitable as the motor disconnect)

Conductor size is dependent on temperature and wiring method. (art 310)
example: 65amps x 125%(continuous load) = 81.25amps = # 4's @ 75 C

Breaker sizing (inverse time) 250% x FLA (65amps) = 162.5amps = 175amp(next std size) (art 240) Notes: breaker must be sized for motor starting "across the line" with VFD in bypass mode; breaker could be sized as large as 400% (300amp,next std size) if necessary due to motor starting current.

Len

 

[Nick]

Re: VFD controlled Motor

This is covered in 430.2. The last few VFD?s I have hooked up have not required anything more than if you sized it like a regular squirrel cage motor. The biggest concern is usually keeping the conductors from the VFD to the motor as short as possible.

 

[Nick]

Re: VFD controlled Motor

Len, looks good to me. Just one comment. We need to know what type of circuit breaker he is using. A lot of motor control centers I have seen lately have been specified with motor circuit protectors. These are magnetic only circuit breakers designed for motor loads. The manufacturers provide literature for sizing and setting the adjustable trips.

 

[pwhite]

Re: VFD controlled Motor

check with the manufacturer of the vfd to see what type of circuit protection is required. the trip characteristics for a circuit breaker are different from that of fuses.

i had to replace circuit breakers with fuses when a customer questioned the fuse protection. we had used a type of breaker for years with no problems. the customer felt the paper fuses would be better protection due to the required trip characteristics.

 

[kkp]

Re: VFD controlled Motor

Thanks for the help, guys.