Roland Jollivet wrote:
> Hi All
>
> I was wondering if anyone had experience with driving a single ballscrew
> with two similar DC motors, one at either end?
>
> So we have;
> - one ballscrew
> - one rotary encoder
> - one servo drive
> - two DC servo motors, one at either end of the shaft
>
> One will obviously turn 'backwards' relative to the other, not that that
> should have any bearing on anything.
>
> I'm trying to figure out if there would be a detriment to connecting the
> motors in parallel to one servo drive. Availabe power is not a problem, but
> possible current-interplay between the motors is.
> Perhaps they should be in series, but peak current would be halved One
> motor would always draw a bit more power, but the servo drive will see a
> single load.
>
> Anyone come across such a system in the real world?
>
>
> Regards
> Roland
>
I haven't actually done this, but I know a bit about motors and how they
behave. (I've been doing motor control and motor drives as my day job
for almost 20 years.)
If you can run the motors in series, you will get pretty good results.
Putting the motors in series ensures that they both have the same
current, and thus approximately the same torque. Having them on the
same shaft ensures that they spin at the same speed, and thus have
approximately the same voltage. As a result, both current/torque and
voltage/speed will be well matched between the motors - each will be
supplying approximately 50% of the load.
The only downside of the series arrangement is that for any particular
speed you will need twice as much voltage from the drive. If you want
to run the motors near the top of their speed range, you will run out of
voltage. (You could use two 90V motors in series on a 180V drive, or
something similar.)
Hooking the motors in parallel is a riskier proposition. Speed and
voltage are directly related in a DC motor, by the motor constant Kv.
(Usually measured in volts per RPM or similar.) The common shaft will
force both motors to turn at exactly the same speed. The parallel
wiring will force them to have exactly the same voltage. If the motors
don't have exactly the same Kv, something has to give. For a minor Kv
mismatch, the result will be mismatched currents, with one motor running
hotter. If the Kv mismatch is more severe, you might actually have one
motor supplying negative torque, increasing the load on the other one
instead of helping it.
Regards,
John Kasunich
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