Very interesting post John. Could I ask, what year did you design and build the circuit and would you approach the solution with the same components. Would you use different "hardware" to perform the same task if given the same problem today? I'm a mechanical type and the first though I had was using a stepper to solve the same problem. That's probably overkill but just wondered how you would approach the solution today. Excellent post John.
In the controls area we would be more likely to call this an "unintended autorepeat" situation. While it would probhably keep right on going, it does not fit the common understanding of perpetual motion, which is to remain moving without external power. Quite a large difference there. And I am still trying to imagine the circuit that would have that capability.
I built a circuit to delay the release of a solenoid. I found that the solenoid packed enough latent magetism in the plunger to produce a spike in the coil of >140 volts. This causeed the circuit to go into oscillation. A diode on the emitter of the driving transistor and a small bypass capacitor after the diode got the offending surge to ground and all has been well since then.
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.
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!
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.
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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