Understanding and Handling Capacitors

Always discharge a capacitor by holding onto the insulated handle of a screwdriver and using the metal blade to touch both terminals of the capacitor at the same time before unhooking it from the circuit or handling it. Capacitors can retain a charge for extended periods of time and can discharge through you if you inadvertently touch both terminals.

Capacitors are rated by their capacitance in microfarads and by the maximum voltage the capacitor is designed to tolerate. Our part number C216E250 has the capacitance in microfarads after the letter C (216) and the voltage after the letter E (250). Microfarads on your capacitor’s label can be designated by MFD, uF, or µF after the number, and the voltage will normally be followed by the letter V or VAC (volts alternating current), or a symbol that looks like an S lying on its side.

In electric circuits, the voltages 110,115,120, and 125 are the same, as are the voltages 220,230,240, and 250. 

Electric motor type capacitors are divided into two major categories, start capacitors and run capacitors.

Start capacitors are almost always in a round black plastic case with the exception of some foreign brands, and are designed to only be in the circuit for a few seconds as voltage is first applied to the motor. Many have a range of capacitance on them, example 216-259 MFD. When the capacitor was manufactured, the actual value of the capacitor fell somewhere within that range. Some manufactures only list one value, sometimes the middle of the range and sometimes the lower value. Our part # is the lower number in that range, but all of our listings show the range. If your capacitor has only one value, as long as it is within the range of our capacitor, it can be replaced by that capacitor. The voltage is the maximum voltage the capacitor can tolerate. Always use a replacement capacitor that has at least the voltage rating of your old capacitor, but if size is not a limiting factor, it does not hurt to use a new capacitor with a voltage rating higher than the old capacitor.

Run capacitors are usually encased in metal cases, with the exception of WEG brand, who manufacture theirs in gray plastic cases. Run capacitors come in both round cases and oval cases, and there is no difference in their values, just the shape and size. If space is not a consideration, round and oval run capacitors of the same capacitance and voltage are interchangeable. They only have one MFD value on them with a + or – value after it. Example 30 MFD +/- 10%. When this capacitor was manufactured, it read somewhere between 27 and 33 MFD. Some are +/- 5%, some +/- 6% and some +/- 10%, but they are interchangeable as long as the MFD value is the same. The voltage of a run capacitor is usually 370 VAC or 440 VAC in most motors and 250 VAC or 400 VAC in WEG Brand or some other foreign made motors.  Due to voltage spikes seen by the motor in normal operation, 125 volt rated motors will normally have a run capacitor rated for 370 VAC or, in the case of WEG, 250 VAC. If the motor is dual rated, 115/230 for example, the start winding runs on 115 VAC even when the motor is hooked for 230 VAC, so the start capacitor will be 125 VAC and if it has a run capacitor it will be 370 VAC.  WEG capacitors in the same situation will be 110 VAC starts and 250 VAC runs. Larger, single voltage motors will use 250 VAC rated start capacitors and 370 VAC run capacitors, and larger WEGs will have 250 VAC start capacitors and 400 VAC run capacitors. Always replace the capacitor with the same MFD value and a Voltage at least as high as the original capacitor had.

Most electronic meters, even ones that have a microfarad testing range, are not designed to test motor capacitors as their ranges are not high enough to read over 1 MFD. . Any time the capacitor is tested, and the value is outside the listed range, higher or lower, it is bad and should be replaced. If your capacitor is leaking oil, or has a pushed out circle on the top, it is bad and should be replaced. Run capacitors are designed to expand the top to break the circuit to the terminals when they fail, so if the top of a run capacitor is domed rather than flat, it is bad.

Capacitors are sized using an electrical property called inductance in the motor winding circuit containing the capacitor. The capacitance offsets the inductance of the winding electrically. If the capacitor is replaced by using one with a higher or lower capacitance, it will not offset the inductance and will make the motor less efficient and less powerful.

If you have any questions, please send them to [email protected], and we will try our best to get them answered.