Can a single VFD control two motors of equal horse power? 2

[DFLewis]

We are working on a setup where we have two 3/4HP motors carrying/sharing similar loads (The system would be difficult to describe so for the sake of brevity imagine two cranes lifting a steel beam simultaneously from opposite ends). The original design incorporated one VFD for each motor with a signal isolator, relays, etc. to control the speeds between the two motors. They need to run somewhat in sync but can be off a couple of rotation for every 100 or so.

Someone proposed eliminating one of the drives to simplify the circuit (eliminate the relays, signal isolator, etc.), but I am not sure if this is possible or if we would get a similar performance from both motors.

Anyone have any experience or thoughts on the matter? What questions or concerns would you have?

Thanks,

DFLewis

http://www.phoenix-engineer.com

 

[DickDV]

You can certainly operate two motors from one drive. To protect the motors from overload, you will need to provide overload blocks in each motor circuit with the block's output terminals going back to a fault loop on the drive.

As to control performance, the motors will be free to drift around with respect to speed with their slip speed range, typically 3% on small motors like that. So, if one motor sees a much larger load than the other, it could run up to about 3% slower as long as the load persists.

Otherwise, the motor speeds will track together within that range of error. You should size the drive by amps---add up the FLA's of the two motors and add 10%. That's the continuous output amp rating the drive will need to have. If you are lifting, I suspect that a heavy-duty drive (sized for 150% shortterm overload) would be best.

 

[benta]

It should work if it's a pure V/Hz open loop control.
If it's a vector drive, that's a completely different proposition.

Benta.

 

[jraef]

A few other things to consider:
1) If the mechanical transmission system has any possibility of slippage, i.e. belts or clutches, then you must have separate VFDs. In other words if you need to compensate even in the slightest amount for those differences in position, you will not be able to do it with one drive on two motors. Any error that does occur will also be cumulative; a "couple of rotations every hundred or so" adds up VERY quickly! 1800RPM means you can theoretically be off 18 rotations every minute of operation, even if you only operated an hour per day that's 1080 rotations per day, 7560/week etc. etc. Do you have any way to reset the error mechanically?

2) If it is in any way a hoist application, I would NOT use V/Hz unless you have a serious enough gear ratio that if not under power, the load will not drop on it's own. You cannot rely on a brake for this because with V/Hz, as soon as you release the brake the load may drop before the VFD can make the motor create enough torque to overcome the movement.

3) The motors must be EXACTLY the same, which means if one goes out, you must have an IDENTICAL spare or replace them both with motors that are identical to each other.




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[DickDV]

I am in complete agreement with all the additional cautions mentioned by benta and jraef.

 

[Compositepro]

Additionally, you must have some way to synchronize the position of the two hoist periodically. This is usually done by having limit switches that create a "home" position and running both hoists till they stop automatically at home. This can also be done manually by lowering each hoist individually to a reference level and then switching the motors to tandem operation.

 

[DFLewis]

All good thoughts and suggestions. I think this answers my main questions and brings up a few good points while we are finalizing the design.

Thanks all!

DFLewis

http://www.phoenix-engineer.com