Two induction motors paralleled to one VFD output?

[electromike]

I was wondering if there were any special considerations that should be known befor attempting to parallel 2 identical 3-phase induction motors on one vfd output.The motors will be driving a sled via rails. There will be a motor and gearbox at each side of the cart to drive the cart on the rails. I do know that I will need to upgrade the capicity of the regen brake module and resistors, but what else will be different from just having one motor on the output of the vfd?

 

[Skogsgurra]

This is being done all the time. If you run scalar mode (V/f), then you have no problems at all, other than the need to provide two thermal protections - one for each motor.

Running vector mode may give you a mental problem (what motor ID shall you use?). Some drives can auto-identify a twin motor and some will not. Sometimes it is OK to enter half the Ls value and half the Rs value. There are usually some hints given in the manual. If not, ask the supplier.

Your drive will have to supply both motors with magnetizing current. So you may need to have a larger drive than you expect. It is not just two motors that share one mechanical load.



Gunnar Englund

http://www.gke.org

 

[DickDV]

I find that if you total up the motor FLA's and add an extra 10% for each extra motor and then size the drive output current to equal or exceed that number, then you will not have problems with what typically ends up being nuisance faults on starting.

These faults usually are Output Short Circuit or Output Overcurrent faults.

 

[jraef]

Just a word of experience on driver motor applications however. The wheels may (will) occasionally slip on the rails, or they may not stay exactly the same diameter as they wear. With 2 motors on the same drive, your sled will skew and there will be nothing you can do about it. Better to use 2 drives for things like that, and as time goes on you can correct for skew of your sled.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[ratio]

Perfect situation is to have an encoder on each motor, preferably a linear encoder that will not be effected by slip. Have the inverters controlling the sync between the two in a master/slave type relationship by elaluating speed via position (pulses from home).
Otherwise just use one controller with seperate overloads as above. This is how most gantry cranes are controlled.
Cheers.

 

[waross]

I suspect that the closer you try to control your motors the more problems you will have. With two controllers and encoders, your settup will have to be very accurate or the sled will slew.
Gantry cranes were controlled for decades with an induction motor on each side. Some were wound rotor motors, some were two speed motors, some were single speed motors. They often had brakes and the brakes often induced skew when they were not properly adjusted.
The slip in the induction motors did a pretty good job of running straight and correcting the skew induced by the brakes.
Try running in open-ended Volts/Hertz mode. If you need closer control you can use a speed transmitter and a PI or PID controller to forward the setpoint to the VFD controller.
respectfully