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Sherlock Ohms

Unexpected Perpetual Motion

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William K.
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Platinum
Re: Motor over-run effects
William K.   1/23/2014 3:24:37 PM
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For one project I did include a connection to short-circuit the motor when it was supposed to be stopped. But the short was implemented through the "back" contacts of the driver relays, and the motor as a 72RPM synchronous type, without much rotating mass. And no hardware failure could result in the application of power across the short circuit.

But it would be very interesting to see the circuit that produced the results described in the posting. It would be a good lesson in ideas to avoid.

akili
User Rank
Iron
Motor over-run effects
akili   1/23/2014 11:55:56 AM
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I have seen this kind of problem before, in one case on some really tiny precision motors used to raise a small work platform into an oven for baking false teeth.  For that project we arranged for the motor to be shorted out when stopped.  OK for a small motor but be very careful when using this trick on bigger motors.  Avoid the failure mode where the power is applied to the still-shorted motor!

Battar
User Rank
Platinum
The real perpetual motion
Battar   1/23/2014 1:55:48 AM
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If you had indeed created a perpeutal motion machine, as the headline suggests, you might be infringing US patents US7095126 or US5804948. The patent office say they will not grant patents for perpetual motion machines, but they have.

William K.
User Rank
Platinum
Unintended reversing function.
William K.   1/22/2014 11:04:53 AM
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This is an interesting tale, and I spent a few moments atempting to guess what circuit arrangement led to the problem. It appears that the source of the unintended relay operation was caused by using the same contact to latch the relay and power the motor. 

If the valve did include a limit switch to open the power to the motor feed for each direction, then it would be possible to use a simple switch to select which way to run the motor, and leave the switch on, since the limit switch would open the feed and stop the motor from traveling in that direction, which is how many motorized valves are intended to function. Unfortunately the failure mode is for the switch to not open, leaving power applied to a stalled motor.

 

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