I found myself troubleshooting the windsheild wiper system on a Plymouth Voyager. The variable speed wiper system would not work. Turns out after buying a $185 multifunction switch, that the root cause was the wipers would not park properly. Unparked, the variable speed would not start.
Why did they not park? Because Chrysler used the spare contact on the power switch relay to ground the back EMF of the motor to ground to brake the motor.
A relay used in this manner will fail eventually because the relay contacts have very little current to burn through surface corrosion.
John R, it is certainly reasonable that spinning one of the PM brushless motors would generate a serious voltage, and that certainly could be a shocking hazard, no doubt. In fact, I know that I have seen manufacturers warnings on drive systems that cautioned against back-driving the motors, since it would generate voltages that would damage the control system. Some VS and servo drives do specify a resistor ro handle that power, since without some place for the power to go the voltage certainly will rise.
And there is definitely no reason to consider a stepper, since, in addition to the reasons that you described, qucikly closing a fluid valve can lead to very large pressure spikes that can do quite a bit of damage.
I hve found that applying DC to a spinning induction motor brakes it rapidly. Applying a stationary magnetic field will produce current in the squirrel cage and perhaps some hysteresis losses in the iron as well. It should be possible with the ECM motors having permanent magnet rotors to simply switch in a resistor load to brake the motor. Perhaps some manufacturer will make this an option, or one might modify the motor to make the windings accessible to external cuircuitry.
I did this job about 12 years ago. A few relays sufficed. In fact I could have simply put in a three position switch (open-off-close). As the water in the tank was critical in case of a plant fire I wanted some additional protections though. The circuit also included an emergency stop switch and the motor/actuator had overtravel limit switches which were wired to kill power in case the first limits failed.
A stepper would not have been a good match for this application as speed control wasn't required. The gear reduction ratio was tremendous, thousands of motor revolutions to completely open or close the valve.
A few years later I had another stored energy problem. A customer had designed a machine for conditioning torque rods. These were steel and came in diameters of 1/2 to 3/4 inches. Conditioning consisted of twisting each rod somewht past its elastic yield point. The machine had a 5 hp motor and needed speed control, so a Hitachi variable speed drive was added. The VFD had no problem controlling the motor as the rods were twisted, but when motion was reversed energy was fed back to the VFD from the rod. This showed up as a voltage spike within the VFD which tripped it out. The sudden release of torque from the motor allowed the machine to spin backward out of control. The solution was to add an energy dissipating power resistor.
Recently I was given a 1 hp Genteq X13 ECM motor which had failed. I was unfamiliar with this type and opened it up. Inside was an electronics package which connected to the motor windings. The rotor had a permanent magnet with a number of poles. The windings comprise a three-phase motor or generator. Spinning the shaft produced nearly 400 volts at 360 Hz. from the windings.
It seems wise to take some time to understand the capabilities of new types of motors so that potential hazards can be avoided. For instance a system which could spin an ECM motor with failed electronic components or internal shorts might make electrical connections hazardous even if there was no source of electrical power connected.
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.
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