Dc brushless motor

good afternoon
does anyone know anything about the dc motors put on e-bikes as I have been refurbishing one and I put 12 volts into it and the wheel locked up and wouldn’t spin. I have read online that you need a bldc motor controller. does anyone know how these work or why the wheel won’t spin.

Yeah, basically think of a BLDC motor as similar in construction to a poly-phase permanent magnet motor.

The stator has field windings for the different phases (often 3) and the rotor has permanent magnets on it.

Applying 12V to one of the coils will make the rotor align itself with the stator field. To keep the rotor moving, you have to keep moving the stator’s magnetic field around. That’s what the BLDC motor controller is doing. It switches the current between the different phases to achieve a rotating magnetic field to make the rotor go around.


Hah, I think that explanation is probably still a bit too low-level…

Think of it like turning a crank. If you just put a heavy weight on the crank it will drop to the bottom point and not move any further. When you turn it yourself you use different muscles to pull up and push down on the crank at different points as it turns in a circle. BLDC motors are like that - they need computer control to power different coils (=pulling in different directions) at different points in the cycle to keep it moving around.

Normal DC motors have a commutator that does this switching around mechanically, but it limits what the motor can do, and requires brushes that wear over time and need replacing. AC motors use line frequency to do the switching, but are locked at one speed (unless you have a device called a VFD which can produce different frequencies… but then we’re back to computer control again.)

guys brilliant really appreciate your help

You can tell if it’s a brushless motor usually by looking at the wires coming out of it. If there are 3 heavy wires it’s probably a BLDC. Shorting any two of them should make the motor very hard to turn by hand. If the motor has an additional skinny cable coming out, then it has Hall sensors - these tell the electronics what the rotor position is at any given moment (and hence which coils to switch on in order to produce forward torque)

… and if it doesn’t have those skinny wires, you’re in for some fun and games that we call: “sensorless motor control”.

Some motor controllers I think even do both (e.g. Tritium’s controllers… you might’ve seen them hanging around Team Arrow’s shop a few months back), they start off using sensor motor control to get the motor spinning, then switch to sensorless using the back-EMF when the RPMs get up high enough. Sensor-based motor control is far easier to do then sensorless, so I’d definitely be looking for those additional hall sensors as life will be much simpler.

It’s worth noting that shorting the coil on a conventional DC motor also makes it hard to turn.

Sensor less control is solved, even for low speed high torque applications.


Give me a poke next you see me at the space and I show you how my skateboard accelerates.

Controllers for BLDC ebike motors are plentiful and cheap on eBay or Aliexpress. Can’t say for quality of course.