Capacitors: Better than a mousetrap

Sept. 1, 2009
I recently read the article, Dc motor selection: Part 1 of 2 by George Hunt, in the July 2009 issue of MSD. The article mentions a problem with dc motor

I recently read the article, “Dc motor selection: Part 1 of 2” by George Hunt, in the July 2009 issue of MSD. The article mentions a problem with dc motor brushes — they spark when rotating from one connection to the other, causing wear due to reverse EMF. This problem has a simple, low-cost fix that other electronic devices (such as dc relays) have always incorporated. I am surprised that motor manufacturers never use it: Simply install a large-value capacitor across the brushes. The capacitor absorbs and quenches the sparks.

I place capacitors everywhere, including in my car; the window motors last much longer, as do all the switches. The capacitors should have been there from the beginning. In fact, early gasoline engines, built in the days before electronic ignition, had a capacitor across the point gap. Take this capacitor out and the points burn out. Put it in, and it extends life for 20,000 miles.

A few years back, I put a capacitor across the battery in our electric Yale pallet truck that burnt out a relay every year. The dc motor brushes still show no wear, and I have not replaced a relay for more than six years.

For successful installation, calculate what value of capacitor should be used, and then connect the capacitor as close as possible to the spark source. One can visually see sparks disappear when a capacitor is placed in the circuit. See for yourself; they make a huge difference. It is unfortunate that this technology has not been used in dc motors, because it significantly extends life.
Todd Grigsby
Portland, Ore.

Author response: Thanks for your input. Faulhaber/MicroMo has been putting cap rings on our brushed motors for years. We offer them on our coreless dc brushed motors to reduce, but not eliminate, arcing and brush wear at high voltages. It is true that this is a great solution — but not without cost. In addition, when a motor with a cap ring is in a closed loop and driven with pulse width modulation, the time constant is very low. So, as current surges into the cap ring, the controller gets “tricked” (by the cap ring) into “thinking” that there is too much current being applied to the motor. At that point, the controller's current-limiting circuit clamps down to deny the motor its rated current. The solution here is to increase the current limiting on the controller, but this leaves the motor unprotected and defeats the purpose of current limiting. It is in these cases where a brushed motor must be left out of the design, and a brushless solution implemented.
George Hunt, Faulhaber Group/MicroMo

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