Years ago, my plant had a large water tank with a hand-cranked valve on the drain. Since it took considerable time to manually operate the valve, a motor-driven actuator was procured, and I was asked to get it installed.
The actuator had a small, reversible, capacitor-run, single-phase AC motor. It also had integral limit switches to stop motion when fully open or fully closed. The gear reduction ratio was so large that it took about a minute to cycle from open to closed.
So an operator would not have to stand and hold a push button, I sketched up a relay circuit to latch the motor on until the appropriate limit switch opened. Since the motor power was low, both the motor and the relays were powered from the same 120V AC circuit.
Our electricians built the control box and installed it. I came out to test it, and when I pushed the open button, the motor took off, opening the valve. After about a minute, when the valve was fully opened, the motor didn't stop. Instead, it reversed and closed the valve. When fully closed, the motor again reversed to open it. It refused to stop cycling.
I rechecked the circuit and found everything wired per my sketch. It didn't make sense at first. Then I thought about the stored energy in the system. When the limit switch interrupted AC input to the motor, considerable energy was left in the still-spinning motor and its run capacitor. The relay configuration was such that the EMF was applied through a normally closed contact on the relay that had just de-energized to another relay, which started motion in the opposite direction.
A quick modification of the circuit fixed the problem, but, intrigued by the incident, I took another small induction motor (0.06 HP/3,300 RPM) and chucked the shaft into a drill press. After a little experimentation with run capacitor values, I was able to make the motor operate as a generator.
After inducing some initial magnetism in the rotor by applying a DC pulse, a considerable voltage built up when the rotor was spun. I measured over 120 V AC at 64 Hz. There was over 200 volts across the eight-microfarad run capacitor, indicating 2/3 of an amp through the motor windings. After a couple of minutes, the motor was warm.
The lesson learned was that energy doesn't instantaneously disappear just because we flip a switch. It's got to go somewhere.
This entry was submitted by John Reed and edited by Lauren Muskett.
John Reed is an electrical engineer.
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