380V 3ph Elevator VFD Question

[oldspwr]

I have been asked by a hospital to re-feed a pair of elevators that are each fed with a VFD. Both VFD's together are fed from a 150A breaker in a 480V 3 ph panel.

Here is the info I received on the motors:

33.5KW
67.5A
43.5 Max op Freq
1305 RPM
380V 3 ph.

How do I calcalate the current load on the 480V side? Originally I took 33.5KW / (480V)(sq 3) and came up with 40.29A each. Does this sound right?

Thanks...

Tom M.

 

[rbulsara]

If the VFD is rated for 380V, you will need a stepdown transformer?

What do you mean by 480V side? What side is it and of what?
VFD have only one input rating.

 

[jraef]

No, I think rblusa misunderstood something. You can use a 480V VFD and program it to limit the output voltage at 380V/ 50Hz to match the motor, which sounds as if that is the solution someone came up with to solve the 380V elevator motor fed from a 480V source. You can't go the other way, i.e. make a VFD give 460V when supplied with 380V, because the VFD cannot create potential. But it certainly can limit / reduce it, just make sure you also limit the maximum frequency to 50Hz at the same time.

Back to your original question, you are essentially correct, just left out a few fudge factors. Base it on the kW, that remains constant no matter what. Assume some losses in the VFD, around 3% conservatively, and about a .95 power factor (corrected by the VFD) presented to the line side for this purpose. So it becomes 35kW/(480*sqrt3*.97*.95) = 45.74A.
Then oversize the conductors to cover 125% of that and you are there.

"Our virtues and our failings are inseparable, like force and matter. When they separate, man is no more."
Nikola Tesla

 

[rbulsara]

Thanks, jraef ..it was not just clear from the OP. I took it as nameplate on the VFD.

 

[DickDV]

One caution with running 380V/50hz motors from a 460V vfd.

The output pulses will be about 670VDC high, not 550VDC high as on a 380V power supply.

As a result, the motor insulation will be stressed more than usual due to the higher pulses. The effective resulting fundemental voltage will be limited to 380V but the pulses will be at the higher level.

If the motors are judged to be vulnerable at these higher pulse levels, the situation can be relieved somewhat by adding a 5% motor lead reactor between the drive and the motor. The reactor will reduce the high frequency components and reduce insulation stress in the motors.

 

[jraef]

Good advice.

 

[oldspwr]

Thanks for all the help, I appreciate it.

Tom