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Unread 19-06-2003, 15:30
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Al Skierkiewicz Al Skierkiewicz is offline
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Patrick,
Where PWM frequency comes into play in this application is that there is a distinct correlation with the amount of time that current is being delivered by the brush and how often the brush is moving to a different commutator segment. If the PWM frequency is such that the controller goes through two or more transitions per segment then duty cycle is close to average power delivered. If the PWM frequency is such that less than one transition occurs per commutator segment then the duty cycle and average power differ greatly. I think this is what is happening to your motors. With the higher(than rated) applied voltage your are sending very high currents to each segment that are enough to get the motor turning but because the the next commutator in line (or the next two or three) may not receive any current at all, all power delivered to the motor occurs over a very few windings. You can experiment to verify this. Measure the electrical power input (volts times amps) and measure the motor mechanical power out and I am sure you will see a big difference.
The lowering of PWM frequency and the raising of the voltage both contributed to the motor failing.
Now you did not mention what the commutator looks like in the failed motors, but I am willing to bet it is all dark and burned, and probably has lot's of pitting as well. Let us know what you find.
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Good Luck All. Learn something new, everyday!
Al
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www.wildstang.org
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Storming the Tower since 1996.
 


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