Running a stepper from 2 drives?

[Art99]

I'm a newbie to CNC BUT am starting a cnc conversion of my minilathe. I have acquired an 8 lead stepper that I plan on running with the coils in parallel. I have also acquired a G540 drive. My stepper requires 2.2amps per winding or 4.4 amps when run in parallel.

Since the G540 is limited to 3.5 amperes per driver, I will not be able to get full power out of this eight lead stepper.

Since I won't need all 4 drives in the G540, I thought about running the two windings off two separate drives on the G540 and paralleling the step and direction inputs of those two drives. I thought that if the circuitry in the drives was the same this would work well. I could run each drive with a 2.2 ampere current limit and thus get full power from the stepper. I was told however that this wouldn't work.

My questions are "Why?" AND "Would it damage the G540?"

As an "almost electrical engineer" it seems to me that it would work and be ok.

Art

[george4657]

The problem is that the drives would have to be phase synced so they were both turning on the drive at the same time. Even the slightest difference in timing would mean that one would be trying to turn off a coil and and the other would be trying to turn on a coil at the same time.

George

[Art99]

George

I agree BUT if the drives are identical and driven from the same pulse input and direction input then they should be in phase or at least very close (less than) 10% since they are 10x microstepping).

At any rate, I don't see how it would harm the G540 if they were slightly off exact phase.

Also I don't know what the bad consequences to the driving might be if they were perhaps 5% off exact phase.

Art

[doorknob]

This is just a guess, but the stepper motor likely has bifilar windings, which means that the two windings for each phase will be tightly coupled to each other inductively, and so out-of-phase currents between the two windings will result in induced voltages and currents that may lead to improper functioning of the current chopping circuitry in each drive.

My guess is that the switching regulators in each drive employ feedback loops of some sort to modulate the current, and the signals that they will be sensing will differ from their design parameters. So such a hookup might result in the release of magic smoke. Or maybe you would get unexpected shaft motions. Or both. But that's just a WAG, so YMMV...

[H500]

It might work, since the current in each winding is controlled separately.

[Mariss Freimanis]

Do not attempt to run one motor with 2 drives. The drives switching sequences are not synchronized. Connecting the drive outputs together will result in short-circuit currents when one drive's output connects to 50VDC while the other drive's output connects to ground.

Mariss

[Art99]

Not clear at all to me.

The drives are NOT connected together at their outputs. Each one is driving a SEPARATE set of UNCONNECTED windings on the motor.

Other than the inductive coupling between the windings, it would be like connecting the shafts of two 4-wire motors and running them from two separate drives for which both drives had exactly the same step and direction inputs.

Art

[LeeWay]

Not sure if you are aware of who Mariss is, but I think I would take his advice on this. Kinda right from the Horse's mouth, so to speak.

[Mariss Freimanis]

I replied with the following in a PM to a member here. I thought the reply was important enough to put up as a post here. It refers to the idea of using two drives to run a single motor by using each of the normally paralleled windings going to two separate drives instead.

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Sorry but that idea won't work and it has the potential to damage both drives if you were to try it.

The paralleled windings on an 8-wire motor are are bifilar. That means they act as a transformer primary and secondary. Any attempt to drive these windings individually from separate drives would cause high shorted-turn currents to flow in each drive and this has a high potential for damaging or destroying both drives.

Each drive has its own 20kHz oscillator which drives the switching sequence for the motor windings. There is no way for these oscillators to phase-lock with each other. This means the switching frequency for each drive will be slightly different. One drive will apply the opposite polarity to one winding compared to the other drive and this constitutes a shorted transformer condition.
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Mariss

[Art99]

Mariss

Thanks!!!!!!!!!!

Your explanation is clear to me and I WILL take heed your advice and NOT try this although I might have been in the marketplace for another G540 after I did the experiment.

Thanks again.

Art