How-To Quick Tips

How to test Brushless Motors for short circuits – no tools necessary!

Troubleshooting a motor can be quite a hassle unless you understand a bit about how brushless DC motors work.

A brushless motor uses electromagnets to spin the motor by pushing and pulling the magnets attached the bell of the motor.
Electromagnets can also be used to generate power. When you spin a motor by hand it generates power. Many modern generators use this same process to create power.

  • If you short any two of the wires together you can create resistance within the motor when you try to spin the motor by hand.
  • A brushless motor should spin freely when all the wires are separated as there is not a complete circuit.
  • If the motor resists your rotation regardless of the wire connections, it’s likely that your motor has an internal short circuit.

 

On the left, I have shorted two of the motor wires.
On the right, the motor wires are not touching.
This test should be done without the ESC connected.
  • Matt Shimmin

    Treat it as a three phase motor. All leads should have continuity between them. If any of the leads has continuity with ground (case) the motor is bad. Like wise if you don’t have continuity between all three motor leads then its also bad. Simple test. The ESC is basically a VFD with a DC bus that chops voltage and superimposes it on a sin wave. Amplitude and frequency are varied to manipulate the motor speed.

    • Tim S.

      It’s great to see Pilots pitching in to help other pilots. Thanks for the detailed breakdown!

  • Karol Busuttil

    I had a crash this week and one of the motor bell came off the motor, while nothing was bent the motor is not turning freely (like image above) I have disconnected the battery and the motor from the esc’s however the problem still persists. Is there a fix for this or should I buy a new motor ?