One VSD on 2 motors

  • Thread starter Alexander Paulsen
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Alexander Paulsen

The client installed one ABB ACS400 20HP drive to run two 7.5 HP drive motors.

The system appears to unstable at speed. It makes fiberboard, and there is a cutting saw on the end. The saw is placed at an angle and an encoder drives the saw. The saw cuts the material while it is moving. The cut appears wavy after installing the new drive. They claim the cut was laser-straight using an old SquareD OmegaPac drive.

I amp checked the motors and one draws 6 amps while the other draws 4 amps. They say ganging the drives together is necessary to keep the upper and lower belts syncronized.

I suggested that it is impossible for a single drive to supply the correct voltage to each motor, since the motors have different load characteristics. If, for example the upper drive has a momemtary need for more current to maintain the speed, the drive will detect the need but and supply it by extending the pulse width, but both motors will see this adjustment.

They theorize that the motor that does not need the additional current will not draw more current or change its slip characteristics. I am not convinced that this is a valid configuration.

Does anyone here have any experience with the problems of driving 2 motors from a single drive?


Alex Paulsen
Vector drives (which I think this drive is) try to determine what the rotor is doing based on voltage and current vectors. There is some really ugly math that goes with running a modern drive so that the the performance of the motors over a wide speed range is consistent. Putting two motors on the same drive means that the vector information will be scrambled since both motors probably won't be turning at the exact same speed and acceleration.

If it is possible to defeat the speed and torque control portions of the control it may be possible to run the 2 motors in an open loop mode but it may cause some performance problems such as overheating, the drive thinking it is overheating the motor, and much lower torque output.

One last note: when a drive is configured, the motor parameters are dialed into the drive. This usually eliminates the need to have an external motor starter relay and protection circuit. Driving 2 motors with a single drive means that if one motor stalls, the drive may not detect the problem and damage the motor in the process. Each motor should have its own protection circuit.

Alexander Paulsen

Thanks Tom;
I did install dual overload blocks when I first saw the application. My tech looked at it today and the speed variations are very bad in the linear v/f mode, better in the square mode but still unacceptable.
I am going to try a constant torque drive like the ACS600, it may behave better.
I explained to the customer that I do not like one drive running 2 motors, but they are convinced it is the only way to syncronize the two conveyors.
Thanks for the feedback. I'll post the results of trying the new drive.

It is just a thought ... but how about a pair of servos. I think that it might be the "correct" way to go. I know they cost more.


Alexander Paulsen

I mentioned the servo idea, scared the crap out of them, plus they abhor spending money if they don't think they have to.
I am confident this will work for them, an ACS600 constant torque drive, 20HP for the 2 7.5 HP motors, they usually run less than 40HZ anyway.
I tried to get them to replace the gear head so they can run the motors at a higher freq, but since they had spares in stock they did not want to spend the money.
Anyway, thanks again for the feedback, I have the drive on order, It will go in Monday or Tuesday.