in regards to the vfd size, I personally would go much higher than 10%.
I've always added a good safety margin just to be safe
Hmm... might be wise to be more concerned about protecting the customerwe are feeling our way on the best way to protect ourselves.
You can filter all you want, but if you choose to go with long leads then you are not addressing the stray voltage concerns.Since this is a 480V system and long leads
Do you think it is an issue for concern if the leads for the motors are different lengths? I think that when ever I had to run more than one motor from a single drive I was always concerned that motor leads should be about the same length for all motors. Does that make sense?
No, that's not necessary. Decades ago when people used Current Source Inverter (CSI) drives in LV applications, circuit impedance was important to the GTO thyristor firing circuit so in that case, motor lead length for multiple motors WAS important. But nobody has made a Low Voltage CSI drive now in a couple of decades, maybe more, yet things like that have a tendency to live on in folklore well past their truths. Modern PWM Voltage Source Inverters don't care.Do you think it is an issue for concern if the leads for the motors are different lengths? I think that when ever I had to run more than one motor from a single drive I was always concerned that motor leads should be about the same length for all motors. Does that make sense?
Select 230V three phase source power. Install a drive isolation transformer and connect the secondary as grounded wye. Locate the transformer as close to the drive as possible and place the drive in the middle of the various fans to minimize lead length. Size the drive at 15hp normal duty. 10hp is not big enough to start 7 motors simultaneously.
Immediately after the output of the VFD to the motors, install a sine filter. (Expect it to cost about as much as the drive).
Grounding technique is critical to success. Use shielded motor leads and bring all of the motor grounds and shield grounds, the transformer ground, the sine filter ground and the VFD ground back to a single grounding point. Do not mix or bond this grounding system with the rest of the grounding in the facility. (You may get some pushback from the local inspector on this but it is important).
If you want to provide full voltage bypassing in case of VFD failure, connect the bypass contactor from the secondary of the isolation transformer to the input side of the sine filter. Remember this contactor must be interlocked with the drive output contactor so you cannot feed line power backward into the VFD.
I know some of this sounds extreme but dairy cows are extreme. There have been fortunes lost over trying to put VFD's in dairy barns. Good luck and be careful.
DickDV
DickDV,
The only thing I think is important to change (possibly) in your excellent treatise is that the power cable shields and motor grounding conductors should always be routed direct to the VFD ground terminal, not a ground lug or bar. I have seen quite a few issues with Common Mode noise being introduced in that short little circuit conductor from the VFD ground to a panel ground or an MCC ground bus. Remember, Common Mode noise always has to return to its source, and in the case of a VFD, that's the VFD ground plane. Any added connections increase the likelihood of it finding some other unintended path of lower resistance, like a cow's udder...