Parallel motorS run by one VFD

[Ali-rashid]

We have two 30kw motors connected in parallel to a 75kw vfd. The motors are mechanically coipled to the load. In these cases, my knowledge says that the VFD can only work in V/F mode and vector control should not be used. But surprisingly, The VFD has only a vector mode and is set on Closed loop vector control and an encoder is used too.

Evey VFD manufacturer manual I have seen says it is not possible to drive multiple motors from one VFD in vector mode. Now I want to replace the drive, and I dont know if I should choose vector or v/f mode.

 

[itsmoked]

What's the load?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[Ali-rashid]

It's crane winch.

 

[FacEngrPE]

A crane hoist - two motors connected to the same shaft?
I have not replaced a VFD for a crane having a two motor hoist, so the answer below needs to be considered in that light. I have been involved in Flux Vector hoist drives with encoder feedback, single motor. That arrangement works well.

If I needed to do this I would first consider only manufacturers selling drives rated for hoisting duty. Then I would discuss the installation with the (or perhaps several) factory engineering department(s). Local sales agents are usually not much help with this sort of thing.

I can see this two coupled motor scenario working, with the drive being tuned to the parameters of the two combined motors, and having encoder feedback. I can also see that the normal tuning process will likely not work. The factory engineering department likely will need to be involved in the drive tuning.
The factory may also need to be involved in setting up the motor protection scheme.

 

[jraef]

OK, here's the thing...
The VFD does not KNOW that there are two separate motors on the output. When you want to use any kind of Vector Control, you must do an "Autotune" (or sometimes called an "ID Run") procedure wherein the drive learns the motor circuit parameters. The VFD will just test the circuit connected to it and report back to the drive's vector algorithm what the required values are. It does not know if those are for one motor, two motors, 17 motors, 283 motors etc. It just starts off with the motor values entered into the drive earlier, then tests and learns the value of voltage drop across the resistance of the (implied) motor stator at rated motor current, the value of voltage drop across the leakage inductance of the (implied) motor at rated motor current, the amps required for full motor flux and (if done with a rotating autotune) the time in seconds for a motor coupled to a load to accelerate from zero to base speed at rated motor torque, all referenced to the motor nameplate data entered into initial programming. If you have more than one motor connected to the drive and take the motor nameplate data and double it (other than speed), it will still calculate all of these other values, but the results will be incorrect for the individual motors. It will likely still operate, but not as it is supposed to.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[EdStainless]

"consider only manufacturers selling drives rated for hoisting duty"

Consider this your pro tip of the day.
This is super critical.
And if this system has been around a while there be other things that need to be updated on your hoist also.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[Gr8blu]

Ali The reason all drive manuals tell you to only run one machine per drive is that being able to load-share in either tandem (two or more machine driving one input shaft to produce one output) or parallel (two or more machines driving multiple input shafts to produce one output) REQUIRES one drive per motor. A single drive CANNOT adjust for the idiosyncrasies of a multi-unit "string". And believe me - there WILL BE idiosyncrasies and unbalances that you probably know nothing about. It's only going to get worse if you must repair or replace one (or both) of the motors later on.

That being said - you CAN operate more than one motor per drive IF you're not all that worried about being able to share the mechanical load equally between the motoring units. You just have to tune the drive for the "system", not a single motor.

It takes a lot of discussion between FACTORY motor guys, FACTORY drive guys, and end users to get this correct.

Converting energy to motion for more than half a century