Checking Capacitors


The following is based on two of the books in my wiring series, ‘Wiring & Fuel Burning Equipment, Fundamentals, Volume One” andWiring & Fuel Burning Equipment, Meters, Volume Two.” The article also features our 88E multimeter, my favorite personal meter.

The capacitor motor works just like the split-phase motor, but the capacitor provides an extra boost to the circuit. That boost allows the capacitor motor to start with more torque. This extra torque can be used to power larger loads, or it can allow a smaller motor to operate a load similar to a split-phase motor. With the capacitor motor you can do the job and use less current to do it with. With a Permanent Split Capacitor (PSC) motor there is no starting switch, which many found to be very unreliable and has almost disappeared from our lives, good riddance.

A capacitor is an electrical device that can store energy in an electrical field between a pair of closely-spaced conductors called ‘plates.” When voltage is applied to the capacitor, electric charges of equal amount, but of opposite polarity, build up on each plate. Capacitors are used in electrical circuits, such as motors, as energy-storage devices and act as a booster. In the PSC motor design they also eliminate the need for a starter switch as in the split-phase motor. Capacitors were also referred to as condensers, an antiquated term.

The ability to test a capacitor has become important for the heating service technician since today’s coal, oil, gas and wood heating installations use more capacitor motors than ever before. You can find capacitor motors in circulators, residential oilburner and gasburner motors, commercial burner motors and both belt-driven, and direct-drive warm air furnace blowers.

Capacitance Troubleshooting


In Figure 1 we have a typical capacitor found on a powerburner today. This capacitor is for use with a PSC motor. Using the 88E we will test this capacitor to see if it is within the manufacturer’s specifications.

1.      Discharge any capacitor to be tested by shorting the terminals of the capacitor with an insulated device such as a screwdriver or a jumper wire. Many also recommend the use of a resistor, but it in fact can make the process more hazardous.

2.      Turn the selector knob to the ‘capacitance” range.

3.      Insert the black test lead into the COM terminal and the red lead into the V/Ω/Hz terminal.

4.      If the LCD doesn’t display 0, press the REL button to clear the reading.

5.      Place the test leads in parallel with the capacitor being tested, Figure 1. If the capacitor is marked, the red test lead is for the positive pole. The LCD displays the capacitance value.

The value shown should be as marked on the capacitor and within the tolerance range also required by the capacitor’s manufacturer.

Today’s oil and gas heating installations use more capacitor motors than ever before. You can find capacitor motors in circulators, Figure 2, residential powerburner motors, Figure 3, commercial burner motors, and direct-drive warm air furnace blowers, Figure 4. It’s sad to note that many of today’s circulators may be replaced simply for an inexpensive capacitor, so use your meter and do it right.

See ya.

*George Lanthier is the owner of Firedragon Enterprises, a teaching, publishing and consulting firm. He can be reached at 608 Moose Hill Road, Leicester, MA, 01524. His phone is 508-421-3490, fax at 508-421-3477 and his website can be found at


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