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Monday 30 November 2015

DC motor brush problems

Problems with DC motor brushes on Treadmills

See photos below for an interesting case study with regards to DC motors that was found by one of my Engineers.

The motor in the photos had passed our motor current test at 3.9A at 220VAC on load at 12kph, suggesting all was well. However, the machine, a European spec Sprint 9 domestic treadmill, showed an intermittent error 3 which we noticed was triggered when using one of the console programs functions which had a fast acceleration at start-up. The problem was not showing when starting the machine slowly on manual mode.

The issue is that the carbon brushes and commutator have been arcing/sparking (burning) as shown in the middle picture below. This is caused when there is a small gap between the face of the carbon brush and the commutator (the copper segmented ring on the rotor of the motor). In this case, the burnt brush in the motor gets stuck in the slideway which you can feel when you push the brush in with your finger! The brush does not freely slide in and out of the slideway. This means it has been prevented from having a good contact with the commutator, which is why it has been arcing.

By nature, arcing is transient (very short) bursts of energy. Because the spark is present for only a very short period of time, the motor does not show as faulty when measuring the current draw on the current clamp – the sparks would have to be much bigger and last for a lot longer to have an effect on the current draw of the motor. This is why it had passed our motor current test.

However, the spark does create a great deal of electrical interference which causes the MCB to shut down (intermittently) which is why we tended to get the error 3 during a rapid acceleration rather than on a slow start (the arcing would have been more during high acceleration). Our motor test box has very high immunity to electrical interference and it is not affected by the arcing which is why it did not cut out under the same circumstances.

This is a fairly rare fault but worth bearing in mind especially if you have reports of the motor intermittently cutting out – check the brushes and commutator for burning, especially if the current readings are good!

The photos show you what a good brush will look like and what a bad brush will look like and what a burnt commutator will look like.

On this job, my Engineer changed the motor for a new one which cured the problem. We decided that it was more cost effective to do this rather than strip down the motor to fix the slideway.

Good brush (shiny surface with only light burning at one edge)

Bad brush showing lots of burning (matt black deposits at surface)

View of motor showing brush and slideway

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