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Speed controller for brushless electric bike motor? Answered

I have obtained a 400W 24V 2800RPM Monophase BLDC motor with related power supply and a 24V 40A-rated pulse width modulator unit, however from what i understand the PWM won't give me full control (if any at all) of the actual speed of a BLDC motor.
I want my motor's torque to remain relatively intact and for it to cover a range of about 50-300 RPM, with no need for exact control.
Are my only choices to build a pulley mechanism or to buy a mechanical gearbox? I'd like to control my motor electronically.



Best Answer 3 years ago

Pulse Width Modulation (PWM) cannot produce more torque than the motor at full its DC SUPPLY.

You already have your 2800rpm 1/2 HP with a single lb-ft torque motor,

and now need a 1/9 speed reduction that will also give you 9 lb-ft of torque.

Belts are lighter the a gear box but you need two belts for a 9x reduction


Sorry were did I say that PWM produces more torque? I could have sworn I said the torque was multiplied by the gear box, not the PWM driver he was looking at.

And pardon me, You would be looking for a 9:1 to a 10:1 gear ratio. Then reduce the RMP through the controller.

Khalim and his intact torque was the why for the inclusion of PWM which does maintain the best of the motor torque during each pulse.

Pardoned you are, a ratio of 10:1 is 20 short of 300 while 9:1 is 11over and closer to 300.

Big thanks to you both. I feel Iceng's answer, didactic link and PWM torque clarification helped me grasp exactly how to go about it with confidence.

For an electric Bike, Either one sounds close enough to me :) Definitely agree that PWM maintains Higher torque vs Reducing Voltage. Good luck, And I hope we see the final Product !

In truth, i'm only making a potter's wheel with the scooter's motor. I'd like to make something as glamorous as an electric bike, but i enjoy commuting by pedaling! Regardless, the principle is the same. If i'm successful, i'll be sure to provide an instructable!


3 years ago

I have wanted to make a potters wheel myself !

lets keep exchanging information.

Are your mains 120VAC 60Hertz or 220VAC 50Hertz ?

220 VAC 50Hz
It's not a complex project after everyone's kind replies here!
The important part to make a serious pottery wheel is to preserve motor torque and avoid wobbling.
I've gotten a 24V 400W DC power supply for my motor ordered, together with a beefed up 24V 40A 2000W PWM. I'm going to gear down the 2800 RPM to the 300-280 with a 10:1 to 9:1 pulley ratio or so. I am planning to use the motor itself as a lathe, by mounting cheap circular cutting boards on it and digging a ridge in them with a heavy broad screwdriver. Rubber belts are easy to find and they work with wood pulleys, i've been told.
Finally, i'll probably use a pillow bearing or two to stabilize a metal shaft to mount the pulley on and yet another round cutting board as head. The PWM knob (as i don't like to use pedals for pottery) will then provide quite acceptable control from 5% (14-15 RPM) upwards . I'll be using a discarded piece of furniture i found on the street as a base, since to be manageable the finished thing must weight at least 20Kg.
The price i calculated for the entire thing is under 200$, albeit cheaper is possible by sourcing more parts. If it's noisy, i'll use the excess soundproofing foam left from my recording room project to somewhat soundproof its interior.
If i'm successful at making this one, the next and definitely more exciting step is to make a portable one.
It's going to take some time before i begin in the earnest, since i just ordered a number of aforementioned items online.
That's my current plan.

Good as far as I understand your plan.

While I do not know about wood pulleys,

I DO know about cooling PWM driven motors at low RPM.

When a motor is running slow it has a high RMS current heating due to the narrow Pulse Widths, So plan on using a quiet squirrel cage fan to HELP cool the metal motor parts.


Either a gearbox or a pulley system could work just fine; I would lean towards a gearbox if possible. You are looking at a ratio of almost 100:1. at that Ratio you will be multiplying you torque !

As per control, Second rickharris

Technically you Could run it with PWM, with one channel and matching controller PER COIL. So you probably want a proper controller for it Check back were you bought the motor. or Hit Ebay, their atleast $60 each before shipping.

Thanks! I would definitely go for a gearbox myself, but ones like the zeromax brand are almost unsourceable where i reside. I am still not sure of which electronic controller to get. I don't mind spending $60 for one, but when i search for brushless 400w controllers only these cheap ones come up. (Image added)

Could you or anyone else point me to a specific line or type of products?


Well your stuck with somthing like that, Or go for Industrial Control; Really Pricey. How much did you spend on this motor Again?

I got it for 40$ from a guy who broke his scooter.

Yhea I can second spending $100 ish on somthing cool like that. Don't suppose you know the Scooters Model? There should be a Direct replacement for the controller. You may need to get creative to bypass the Safety lockouts. (The OEM controller will have all sorts of precautions to keep them from getting sued) Ask someone here (and post a wiring diagram) before you order it, just to make sure.

Exactly because of the closed nature of the OEM controllers i am reluctant to do use them. The motor is sensorless and has only a positive and negative, so i'm definitely going to use the occasion to learn how to adapt motors for my necessities by grasping the theory behind motor control.

Your brushless motor is effectively a 3 phase permanent magnet motor. The rotation is controlled by switching each phase in turn on and off. The speed of this switching controls the speed of the motor. Thus the current through the motor at any one time is constant giving you good low power performance. One good reason for using them for electric vehicles.

PWM isn't the right control for this motor.


Thanks for the informative reply! I had completely missed the existence of sensorless BLDCs and was getting frustrated with mine as consequence. Admittedly, i'm still not sure which controller to look for in my specific case!