# [EVDL] two BLDC motors off one controller?



## EVDL List (Jul 27, 2007)

A question on BLDC motors/controllers.... Does anyone know if it is possible to drive two smaller motors from one controller. I'm thinking that if only one of the motors has hall sensors feeding back to the controller then if I add another motor in parallel then this second motor should mirror the motion of the sensored one. These motors will be driving 
exactly similar loads at the same speed and direction.

Cheers
DaveW


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## EVDL List (Jul 27, 2007)

I've tried this with RC airplane motors (BLDC type, sensorless). It
depends on the controller. I had an RC controller that could do it.
The problem was sometimes the two motors would lose synchronization
and one would stop and burn up. Almost lost the plane, but the wires
to the stalled motor melted and the one motor kept me going enough to
land .
I wouldn't be inclined to try it again

-- 
-Jon Glauser
http://jonglauser.blogspot.com
http://www.evalbum.com/555

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## EVDL List (Jul 27, 2007)

You would have to mechanically connect them together so that one
couldn't rotate without the other one rotating.

Even if they were turned off, rotating one without the other will mess
it up. The next time you turned it on, the one without hall effect
sensors would be out of phase, which could do anything from field
weakening to field strengthening to reverse/regen, depending on what
the positions were relative to each other.

And left and right wheels of a car aren't exactly similar loads; when
you turn, one wheel will go slightly faster, causing the motors to get
out of phase.

Also, when putting them together to mechanically link them, you would
have to get them electrically lined up just right first.

-Morgan LaMoore

On Thu, Jun 12, 2008 at 2:09 PM, david woolard


> <[email protected]> wrote:
> >
> >
> > A question on BLDC motors/controllers.... Does anyone know if it is possible to drive two smaller motors from one controller. I'm thinking that if only one of the motors has hall sensors feeding back to the controller then if I add another motor in parallel then this second motor should mirror the motion of the sensored one. These motors will be driving
> ...


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## EVDL List (Jul 27, 2007)

Thanks for the useful replies, i'll not be doing that then! That is kinda what I suspected the situation would be - it would work if everything was perfect, but it wouldn't take much to un-synch them and land me in trouble.

The follow on question is do you have any suggestions on how would I go about synching two controllers so they run in parallell? Do you think that applying the same control input would be enough to do it.

Thanks again
DaveW


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## EVDL List (Jul 27, 2007)

The follow on question is do you have any suggestions on how would I go
about synching two controllers so they run in parallell? Do you think that
applying the same control input would be enough to do it.


Depends on what you are doing with them and if you want a constant speed or
a constant load. If the loads are substantially independant (uncoupled) but
need to run the same speed, then running two speed controllers would be
fine. If the loads are coupled, then you would need to have torque control
on each motor. If the controller uses a PID speed control loop (zero error),
then the two motors will fight one another if they are coupled (for example,
two motors coupled into one transmission). If they are uncoupled (for
example, two wheels on a wheelchair), then PID speed control on each will
work fine. If the controllers use only a proportional control loop, or just
a raw current setpoint (such as the Solectria BLDC controllers from the
BRLS-xx series) then you can just isolate the control signals and use them
in parallel if you want. For best efficiency if a coupled system, however,
the torque command from each motor could be modulated to stay in the 'happy
zone'.

About sharing a control signal - please isolate them or ensure the
controller logic parts are isolated. If you forget this then ground currents
can cause lots of explosions. In my earlier days, I blew up a UQM (Uniq
Mobility) BLDC controller by not doing this. The revised design used Analog
Devices analogue isolators.

-Dale

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## EVDL List (Jul 27, 2007)

A question on BLDC motors/controllers.... Does anyone know if it is possible
to drive two smaller motors from one controller. I'm thinking that if only
one of the motors has hall sensors feeding back to the controller then if I
add another motor in parallel then this second motor should mirror the
motion of the sensored one. These motors will be driving
exactly similar loads at the same speed and direction.


I wouldn't do it, even if it 'works', there are too many things that could
cause it to blow up. If the loads are slightly different, the phase shift
between the motors may cause issues. Even if the two motors are
belt-coupled, it is asking too much for them to stay locked in phase. A
phase difference between the motors that does not cause an increase in
torque between the motors will cause missed 'steps' and possibly a blowup of
the controller, or possibly motor mechanical damage (Azure tells you not to
regen brake a free-spinning BLDC motor without inertia backing it up so you
don't wreck it). Also, the controller will be expecting a motor inductance
to accomplish its current control, and you've reduced that with two motors
in parallel.

-Dale

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