Vfd

Hi,

Just a quick question. I have a bank of 12 identical motors that I need to all run at the same speed through a VFD. Can I connect them all to one VFD in parallel, or is this generally not accepted. It seems less expensive to do it this way.

Thank

Richard

I think it's acceptable provided each motor has it's own individual overload unit.

With many motors connected to one VFD, you need an output choke on the VFD.

You can run all 12 motors on one drive but you will need to add up all of the motor FLA's and add 110%. Size the drive to that ampacity. As already mentioned, you will need an overload block in each motor's leads with the control contacts tied in a closed series loop and run back to the drive as a fault loop.

The motors will not match speed perfectly. You can expect about as much error as the motor nameplate slip. That would typically be 2% or so of base speed. If you need more accuracy than that, you will need to use individual drives for each motor.

VFD's

Hi,

The accuracy is not that important, I just need to roughly control the speed. just to clarify you are saying that I need to ties all the overloads together is series, so that a fault in any one motor will trigger the VFD?.

Thanks Richard

Richard:
I did that several years ago and the best advice i can give you It´s to run every motor in a separate conduit.

I agree

Another issue is branch circuit protection after the VFD.

With a single motor to a VFD the input fusing to the VFD will protect the downstream wiring. When you use multiple motors on one VFD, the input fusing to the VFD will be too large to protect the downstream wiring.

widelto said:

Richard:
I did that several years ago and the best advice i can give you It´s to run every motor in a separate conduit.

Click to expand...

plc_noob said:

I agree

Click to expand...

Please elaborate.

Have done this for years:-
CAUTIONS
1. Install a filter rated for the drive current between the output of the drive and the overloads.
2. You can typically run wiring for up to 3 motors in each conduit.
3. Use a single ground for the drive and run the motor grounds ONLY to the drive ground.
4. Your motor speed will vary much more than the slip based on varying motor sizes and load. Expect at least 10-15% fluctuation.
5. Use inverter rated motors.

It will be better if we put the equal length of cables for each motor after the VFD.
But Dear (Bill)Widelto can i know why it is important to run each motor thru separate Conduit
(This is my First Post to Community so plz ignor if there is any Mistake)

-------------------
Pawan Rathi

I would run each motor leads in a separate conduit also because
1. Only one can fail. Much easier to fix. Admittedly if one fails then the whole machine goes down.
2. My lack of full understanding of VFD output problems ie spikes, harmonics etc. Seems to me putting more in one conduit will compound problems.

Re question about branch circuit protection
Technically each of 12 motor feeds from VFD is a separate branch (???).
Therefore the branch would need its own overcurrent device which is also able to interrupt a fault current ie short.

With 12 motors on one VFD it seems to be a good question. As suggested insertion of the overloads would be acting to provide overcurrent protection for the individual motors and would be connected to trip the VFD. If there is a major fault ie short circuit the VFD would trip on overcurrent.

BUT the VFD would provide protection at 12 X the "ideal" branch circuit rating.

Is this an adaptation of NEC tapping a feeder rule?

Below NEC articles address grouped motors
NEC 430.87, -53, -24 seem appropriate. Am I missing one. Dont see VFD in index.

Dan Bentler

Dan Bentler

The electronic motor overload function of the VFD is useless in your application [it is the individual motor overloads are protecting each motor].
Disable the overload function in the VFD and/or set it at to the maximum possible value [depends on manufacturer of VFD].
Wire the external overloads in series to the Overload input of the drive so that if any one motor trips then the drive shuts off. Enable the external overload input parameter of the VFD.

BitsNBytes, I agree with all your comments except motor speed error. There is no reason to believe that the error will exceed normal motor slip which would be about 2%.

Hi,

Thanks for all your help. I tried it with 6 motors today and it seemed to work fine. If you have any further information it would still be greatly appreciated.


Thanks

Richard