March 21, 2024
Q: If a power tool has overload and thermal protection, is it still possible to burn out the motor?
A: Yes, if the operator doesn’t give the tool a chance to cool by running at no load after the overload trips, there is still a chance to burn out the motor.
At the moment that the overload trips, the tool has been operating at overload for some seconds. This generates an enormous amount of heat, especially in the armature. The best way to reduce this heat is to run the motor at no load so that its cooling fan will have a chance to bring down the temperature in the motor. It’s easy to check how hot the motor is by putting one’s hand in the airstream of the exit cooling vents. You will notice that it typically takes a few minutes to bring the temperature down.
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If the overload and thermal protection were to strictly shut down the motor and then not allow it to run until the temperature came down, you would need to wait 20 minutes or more. That would be exceedingly inconvenient. The fastest way to cool the motor is to allow it to run at no load, and so the electronics are designed to allow the motor to immediately restart after overloading.
If the operator ignores the heat of the motor and doesn’t run at no load to cool, and instead immediately puts full load on the motor, it will trip out again fairly quickly. If he repeats this process and stubbornly continues to push the motor, the temperature will reach a critical level and the motor will fail. Basically, the insulating varnish on the motor windings will melt and it will short circuit.