Connecting two BLDC motors in series?

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fechter   100 GW

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Re: Connecting two BLDC motors in series?

Post by fechter » Oct 27 2008 9:07pm

It wouldn't be too hard to connect a pot or hall sensor to the steering and make the controllers drive the outside wheel harder than the inside wheel. It would almost be like tank steering. This would make U-turns a breeze. I think they have stuff like this for RC models. I think a simple proportioning circuit would do it.

AFA the disembodied power stage for a controller, this has been suggested several times. If you isolate the PWM from the power stage with opto couplers, there would be almost no way to blow up the PWM part. An old audo amplifier usually has a really nice looking case and a very ample heat sink.
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cerewa   100 W

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Re: Connecting two BLDC motors in series?

Post by cerewa » Nov 02 2008 9:20am

I see no problem with running two BLDC motors together, but they would need separate controllers at least. The turn radius and tire scrub wouldn't be an issue at all IMO.
This is correct. What Papa is missing is the fact that there's very little reason why the motors would want to maintain the same RPM at different speeds. Going around a very tight u-turn at very low speed, the inside wheel will receive slightly more torque than the outside wheel. But the outside wheel will also be going so slow that the influence of speed that reduces amps (and torque) will be almost nonexistent.

So then you have two wheels being driven with about the same amount of torque/force. But this won't stop them from turning at different speeds, any more than they are unable to turn at different speeds when the power is off. (i.e. you can spin the wheels with your hand and yes there is some resistance from the magnets, but so what.

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rkosiorek   1 MW

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Re: Connecting two BLDC motors in series?

Post by rkosiorek » Nov 02 2008 11:45am

that little schematic i posted earlier in this thread allows one throttle to control 2 separate BLDC motors and controlers. i have used it with a pair of Crystalyte V1 40A controllers and a pair of X5302 rear motors in an experiment.

only thing i might add is a small op amp in series with the throttle to shift the level of the throttle up slightly to compensate for the loss in the balance resistor.

the resistors are only used to adjust the sensitivity of the throttle input to the controllers to ensure that both thart turning at the same time.

rick
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Papa   1 kW

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Re: Connecting two BLDC motors in series?

Post by Papa » Nov 02 2008 12:53pm

cerewa wrote:
Going around a very tight u-turn at very low speed, the inside wheel will receive slightly more torque than the outside wheel. But the outside wheel will also be going so slow that the influence of speed that reduces amps (and torque) will be almost nonexistent.
The absent of quantitative data leaves me skeptical, and I'm constantly reminded of the additional losses associated with driving two controllers - not to mention the added cost. Theory is nice, and warmly appreciated, but I'd feel more comfortable with hard data. Sadly, my pocket just isn't deep enought to freely experiment

Further,.. the apparent advantage (or option) with brush, is the ability to electronically shift between series and parallel... hopefully, on-the-fly.
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rkosiorek   1 MW

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Re: Connecting two BLDC motors in series?

Post by rkosiorek » Nov 02 2008 2:38pm

Papate wrote:
Further,.. the apparent advantage (or option) with brush, is the ability to electronically shift between series and parallel... hopefully, on-the-fly.
why would that be hopefully. the Kill-A-Cycle has been doing that on the fly for years. at full power no less. it uses a couple of series wound DC motors (no magnets) to generate some impressive torque.

http://www.killacycle.com

i tried my circuit over the last summer with 2 X 5302 motors on a wooden frame. this was a mockup of the drivetrain for a 3 wheeled Electrathon vehicle i wanted to make. the lack of a differential was noticable, especially at low speeds in turns. but adding a touch of rear wheel steering (front wheel +/- 45deg. rear proportionaly linked to front limited to +/- 7deg) mostly solved that issue. i decided against the dual rear motors because of the weight. those motors weigh about 25lbs each.

rick
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