Tom Bishop, P.E.
EASA Senior Technical Support Specialist

In this article, we will discuss testing of capacitors for electric motors in general and tests associated with specific uses of capacitors such as for power factor correction, and for electric motor starting (see Figures 1 and 2). For information on sizing power factor correction capacitors see Subsection 2.10 of the EASA Technical Manual, and for determining the correct size capacitor for a motor, see Subsection 2.11 of the EASA Technical Manual.

Preparation for Testing
Note: Make certain to follow all safety-related procedures when performing this and any other electrical work. If the capacitor is rated 600 volts or less, connect a 15-20 kilohms resistor rated 5 watts or greater across the two terminals of the capacitor for at least 10 seconds to discharge any residual capacitance. Check to verify that the voltage has decayed to zero using a DC voltmeter connected to the two terminals of the capacitor. If there is a residual charge there will be a voltage that gradually decays as the meter discharges the capacitor. If the capacitor is rated above 600 volts, check with the capacitor manufacturer for the proper test procedure.  

Before proceeding to test a capacitor, disconnect any leads connected to its terminals.  

Note: If there are multiple capacitors, disconnect them one at a time to test the individual capacitor; then reconnect the tested capacitor before testing another individual capacitor.  

Resistance Meter Testing
A simple pass/fail test for a capacitor is to check it with a multimeter to determine if it develops a capacitive charge. If using an analog multimeter (preferred), set the meter to the kilohms scale and connect the leads to the two terminals of the capacitor. The internal battery of the ohmmeter will apply a voltage across the capacitor; the current will initially be relatively high and then drop off rapidly as the capacitor charges.  

The resistance reading on the meter will behave inversely to the change in current. That is, it will increase rapidly until it is higher than the meter can read, essentially “infinity.” A digital multimeter (DMM) can be used similarly by setting it to the ohms scale. The drawback to the DMM compared to the analog meter is that it may be difficult to detect the rapid increase in measured resistance. If the capacitor passes this test, that indicates that it is functional. However, this test does not indicate whether the capacitor has its rated capacitance.

Either type of multimeter, analog or digital, can be used to detect a shorted capacitor by setting the meter on the ohms scale and connecting it to the two terminals of the capacitor. An essentially “zero” ohm reading indicates a shorted capacitor. Also, visually inspect the capacitor for any obvious defects such as oil leakage, cracks, or a bulging case. If the capacitor is a larger power capacitor protected by a fuse, check the fuse. If it is open, that is an indication that the capacitor is shorted.

Capacitance Meter Testing
Capacitance is typically expressed in microfarads. To determine the microfarad value, use a capacitance tester and connect it to the two terminals of the capacitor. The microfarad value is important for applications such as the start and run capacitors because these values affect single phase capacitor motor starting torque, as well as running current if the motor has a run capacitor. 

In general, the microfarad value should be within about 20 percent of its rated value (which may be a range). An open-circuited capacitor typically charges up quickly and has an extremely high microfarad (usually off-scale) value. Conversely, a short-circuited capacitor will have a very low, or “zero,” microfarad value. If the microfarad reading is unsteady, going up and down, the capacitor is probably arcing internally and should be replaced.  

A capacitor that has not been used for a long time may need to be “re-formed” to restore its capacitive charge. There are no set values for what constitutes a long time, but a capacitor that has not been used in over a year probably needs to be re-formed. Check the equipment manufacturer manual for the period before re-forming is needed, and for the procedure for re-forming the capacitor. 

If there are multiple capacitors, such as for the start winding of an integral hp (kW) single phase motor, and the rated capacitance is known, test the capacitance of the complete circuit and compare it to the rated capacitance. This test verifies that the interconnection of the multiple capacitors is correct. The microfarad value of capacitors varies inversely to the resistance values of resistors in series or parallel. That is, the microfarad value of capacitors in parallel is the sum of the individual capacitor microfarad values (Total capacitance = C1 + C2 ...+Cn); and the microfarad value of capacitors in series is determined mathematically in the same method as for resistors in parallel (Total capacitance = 1/(1/C1 + 1/C2 ...+1/Cn).  

For example, 2 capacitors rated 400 microfarads connected in parallel result in 800 microfarads (400 + 400). Those 2 capacitors in series result in 200 microfarads [1/(1/400) + (1/400)].

Failures
In general, capacitor failures early in their lifetime are usually due to short circuits, and later life failures usually are open circuit. The latter is prevalent with electrolytic (e.g., motor start) capacitors because the electrolyte has dried up.

Capacitor life is affected by factors such as applied voltage, which can be greater than equipment line voltage, and operating and ambient temperature. Power capacitors such as those used in drives and power factor correction applications have a lifetime that varies as an inverse exponential with overvoltage.  

For example, a 10% overvoltage reduces capacitor life by about half. Similar to motor windings, power capacitor life is halved for every 10°C increase in case temperature. The converse of these two rules applies. Capacitor life increases exponentially with reduced voltage and with reduced temperature.

A motor start capacitor can be thermally damaged or destroyed by overheating if it is energized for more than a few seconds. A repetitive overvoltage condition with a start capacitor can result in a reduction in capacitance (microfarads). Also, a pressure vent that has opened indicates internal overpressure most commonly associated with deterioration with time. A run capacitor may be damaged by overvoltage that is continuous or from a transient overvoltage.

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Categories: Technical topics, AC motors, DC motors, Miscellaneous, Testing, Motor testing

Tags: Capacitors

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