# BLDC motor open circuit effects

#### JoelTee

Joined Jun 12, 2022
4
Hi
I am trying to use a BLDC motor as a generator using the regenerative braking function. I have a situation where I need to be able to spin it while the e-lock is on, so I am thinking of using relays to open circuit the field windings while I do this. When I close the relays, I want to be sure that I won't damage anything with excessive voltage.

If I spin the motor with the field windings (U, V, W) open circuit will the voltage between field winding keep increasing? Or will the voltage be dependant on the rpm? And when the motor stops spinning, will that voltage on the field windings drop back to 0?

Thanks!

#### nsaspook

Joined Aug 27, 2009
11,755
Yes, there is no voltage when not spinning. One spin by hand.

A drill driving a open circuit BLDC motor.

Slow and fast turns output voltages and frequencies.
https://support.maxongroup.com/hc/en-us/articles/360004496254-maxon-Motors-as-Generators
The general formulation of the law of induction states that the induced voltage is proportional to the speed of magnetic flux variation. Therefore, the amplitude of the induced voltage is proportional to the motor speed. Again, it can be obtained from the EC motor speed constant.

#### JoelTee

Joined Jun 12, 2022
4
Thanks!
Do you know if it's also true that the e-brake or anti-theft system on the motor controller will short 2 or more coils together, creating maximum current and maximum torque?

#### nsaspook

Joined Aug 27, 2009
11,755
Thanks!
Do you know if it's also true that the e-brake or anti-theft system on the motor controller will short 2 or more coils together, creating maximum current and maximum torque?
Yes, shorted motor coils acts as a brake. Per Lenz's law.

Lenz's Law - seeing that the magnetic field induced by a current induced by a change in magnetic flux (Faraday's Law) counteracts the change in flux.

#### JoelTee

Joined Jun 12, 2022
4
Awesome. Thanks!
Do you know then, if it could damage the Motor Controller by spinning the motor while the 'E-Brake' is on?
I guess at a slow rpm you would get a low voltage but a very high current as there's no resistance in the circuit. But I would have thought the Motor Controller would allow for this

#### nsaspook

Joined Aug 27, 2009
11,755
Most of the resistive dissipation with be in the motor winding. Your motor controller should have specifications for this if electronic e-brake is an option.

#### JoelTee

Joined Jun 12, 2022
4
Most of the resistive dissipation with be in the motor winding. Your motor controller should have specifications for this if electronic e-brake is an option.
Ok. Good to know.
Because I recently cooked the motor controller and I thought it was maybe because I ran it with the e-brake on.
I did notice a power spike when I connected a phase wire to the motor controller. This was my series of events,
1. I spun the motor while open circuit. Then stopped it spinning.
2. Powered up the motor controller & e-lock (e-lock off)
3. Connected the phase wires to the motor controller. (I noticed a power spike when I connected the 1st phase wire but the motor controller appeared fine after)
4. Spun the motor with the e-lock on, & motor control unit connected to battery and motor (This is when the motor controller stopped working)

This is a picture of the motor controller after the incident.
Do you think I could have damaged it by connecting the phase wire while the motor controller was powered?
Or do you think it could have been because of running it with the e-lock on?
Or any other ideas?

Note that the throttle has been left unconnected during the testing