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Cheap electric bike drive - brushless motor and controller for R/C airplanes?

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Dr. Dark Current

Wed Nov 23 2011, 02:16PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Hello,
the other day I found out about those China brushless outrunner motors and controllers (ESC). I'm wondering whether those could be used to make a really cheap bicycle drive.
I'm considering about 200 Watt continuous drive, and these are the parts I think could work:
Motor:

Controller:

Power source: 12V 18 Ah SLA battery (for free from a routine replacement of UPS batteries, should be less than 1 year old).

This is my first encounter with those things, so there are some questions/thoughts for the more experienced people:
1. Is this drive suitable for an electric bike?
2. The motor looks ridiculously small for the power rating. Is this a real continuous rating? I guess it will need forced air cooling?
3. There are 3 small wires coming from the controller. I guess one is ground, the other one is +5V from the regulator, and the third one is for speed control? How does the control work, does it limit current or only sets maximum output voltage (so the maximum current is a parameter set in the controller)?
4. Do you think the controller/motor will work well together?
5. I heard the controllers have some kind of an automatic undervoltage shut-down feature. Will it work well with my SLA battery?

Steve Conner

Wed Nov 23 2011, 02:42PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Bear in mind that E-bikes are hugely popular in China, so the Chinese also make cheap motors and controllers for them

All Chinese bikes and scooters that I've seen use a hub motor system like this:

This is a somewhat high-end one, but it seems like good value given that it has a built-in controller, and comes laced into a wheel ready to use. You may be able to get cheaper ones.

I think RC airplane motor controllers have some sort of limitation that makes them bad for vehicles. For instance, they might not be able to generate high starting torque at low speeds, because a propeller never needs it.

AndrewM

Wed Nov 23 2011, 02:45PM

Registered Member #49 Joined: Thu Feb 09 2006, 04:05AM
Location: Bigass Pile of Penguins
Posts: 362

1. Depends on your drive ratio. You will likely vastly exceed the power rating of that motor during acceleration, but I would expect it can safely hold you at any reasonable speed.

2. I believe the ratings do assume some amount of airflow, yes. Most of the outrunner cases have angled slots to act as a sort of terrible fan.

3. Each coil gets a high freq. square wave.

4. Sure.

5. Undervoltage for a LiPo will be too low to ever save an SLA from damage, but it won't cause you problems.

Dr. Dark Current

Wed Nov 23 2011, 03:02PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Steve,
that motor is around 10 times over my price range, I just wanted to see if I can get something really cheap to work. The motor together with the controller come to around $19.
The low starting torque doesn't concern me as long as it's not lower than the running torque at rated RPM.

Andrew,
I know how does the inverter work, I was just wondering how the control works - does it also control the current and torque? Or only limits maximum RPM (so the motor could fry during extended low-speed max-power sessions)?

As for the undervoltage, I heard the controllers automatically detect battery voltage and based on this they set the undervoltage threshold. It doesn't matter if the threshold is low (I'll install a voltage meter on the bicycle and safely shut down the power before it drops too low), but if the controller sets it too high, I won't be able to get all the energy from the battery.

Marko

Wed Nov 23 2011, 04:53PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Afaik, those RC plane motors only attain their rated power at ridiculously high RPM, at very high frequencies which is what allows them to be so small in first place.
So you would likely need a lossy, high ratio reduction to make them usable for a bike.

Those motors are basically 3 phase synchronous motors, and I'm not sure whether they even have any feedback to the controller or just run open loop.

In any case to drive a bike efficiently you need one of those hub motors like steve mentioned :)

Marko

Dr. Dark Current

Wed Nov 23 2011, 04:59PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Well, if those controllers do indeed run open loop, then they are not usable for a bike drive... maybe if I built an additional circuit that would sense the motor phase current and regulate the controller's input based on this.

Patrick

Wed Nov 23 2011, 07:14PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

i would rather use a more conventional motor. these RC motors i kinda wimpy in high power for long periods, they over heat.

They are meant to run in pulses, IE the length of time between battery swaps, and not reallly full power all the time, your bike may exceed one or more of these condidtions often.

hsieh

Tue Nov 29 2011, 05:39AM

Registered Member #1412 Joined: Thu Mar 27 2008, 04:07PM
Location: Taipei Taiwan
Posts: 278

I am sure these RC plane motor run close loop.They use back emf as feedback signal.

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