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I recently bought two 12 V DC motors from Amazon. After I received them today, I tried testing them. One of the two motors run fine but the other motor shows erratic behaviour. It gets very hot within 10-15 seconds of running, and then stops. It also doesn't run as fast as the other motor.

During testing I directly connected a 8.6 V LiPo battery to the motor and it had sparks coming out of it. I then soldered wires to the motor's contacts and added another cell to the pack making it 11.98 V. The sparking stopped, and the motor started working, but I noticed this behaviour. There's also very high resistance when I try to rotate the motor shaft by hand, while on the other motor, it turns very easily.

I'm not sure if I damaged my motor, or if it was already damaged when I received it. How can I determine the cause of the problem?

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  • $\begingroup$ "Sparks coming out" is a bad sign to begin with, and the end of sparks usually indicates that either some dirt burned off or some bad insulation burned thru and there's now a partial short. However, the high mechanical resistance definitely indicates a defective motor. Try to return it. $\endgroup$ May 23 '16 at 13:24
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You cannot damage a DC motor just by applying a lower then expected voltage to it.

If the motor shaft is harder to rotate then on similar motor, it probably has a mechanical damage. However, a short circuit in the winding could also be the cause for it.

Try to measure the electrical resistance between the two connectors of the motor and compare the value with the healthy one. If it is the same value then you have a mechanical issue, if not then you most probably have a short circuit somewhere in the windings.

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  • $\begingroup$ Okay I'm getting a 2.1 on the defected motor and 2.7 on the healthy one. What now? $\endgroup$
    – YaddyVirus
    May 22 '16 at 12:53
  • $\begingroup$ Try to rotate is mechanically very slowly and continue measuring the resistance. See if there is a different value on any of the windings (assuming you have more then one...). If you get the same value everywhere the motor is electrically ok. $\endgroup$
    – 50k4
    May 22 '16 at 13:11
  • $\begingroup$ I cannot open the motor its completely sealed, so I am taking the readings from the two power contacts provided. I can see huge deviations in resistance reading when rotating the motor mechanically, whats that supposed to mean? $\endgroup$
    – YaddyVirus
    May 22 '16 at 13:48
  • $\begingroup$ When you rotate you produce voltage and that messes up the measurment. Try rotating and then holding it still then continue. In steady states the resistancr should never be 0 or very close to 0. $\endgroup$
    – 50k4
    May 22 '16 at 21:54
  • $\begingroup$ Again, very high deflections in resistace, in the steady state resistance is 1.9-2.0 $\endgroup$
    – YaddyVirus
    May 23 '16 at 6:01
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It sounds like your bad motor has a mechanical problem. For some reason, the rotor is not spinning as freely as it should. This may be due to a bad bearing, something misaligned, etc. Return it for a new one or get your money back.

As long as you never applied more than the rated value of 12 V to your motor, you did nothing that damaged it. There was something originally wrong with this motor. At worst, there is some cascading damage, but that is all a result of the original problem that was present when you received the motor. Again, return it.

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