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Testing an Electric Motor

Over the years we’ve seen a number of controllers that have been damaged by faults in the motor they are driving, so we thought we’d write a short article on what can go wrong, why it causes problems for controllers, and how to go about testing an electric motor.

Electric motors are assemblies of mechanical parts, as such there are things that can degrade and wear out over time. Older motors can cause problems for  controllers mainly through the following mechanisms…..
  1. Worn brushes. This leads to low brush spring pressure which in turn causes more arcing, which in turn causes radio frequency interference, which in turn can blow mosfets. See this page  and watch the video here for a more detailed explanation.
  2. Individual windings on the armature going open circuit.    This can happen due to vibration causing fatigue of the copper winding wire. If this happens large voltage spikes will occur as the armature rotates. To test for this put a resistance meter across the motor terminals [you should see around 1 ohm] then rotate the armature very slowly by hand through one revolution and watch carefully for any change in the resistance reading as you do so.
  3. Windings going short circuit. This can happen either..
    • a] to the armature and consequently to the motor case. This can happen if the insulation between the winding wire and the armature breaks down, either because of overheating or of old age. If the battery negative is connected to the chassis then that’s a recipe for a damaged controller. To test for this use a resistance meter to check for any continuity between the motor terminals and the motor case, there should be none [i.e. a very high number of ohms]. Again, rotate the armature by hand through one revolution.
    • b] to another part of the winding. If this happens there can be large current and associated voltage spikes as the armature rotates. The test is the same as for 2.
  1. Bearings failing. Apart from the obvious noise this causes, a failed bearing can allow the armature to move around relative to the brushes, in extreme cases this can result in the brush losing contact with the armature. This can result in more arcing [see 1.] and even temporary disconnections resulting in large voltage spikes as the armature rotates [see 2.]. To test for a worn bearing 1] listen to the motor run and 2] rotate the shaft by hand and feel for any roughness.

Here’s a video showing how to test a motor.