Single phase motors will commonly have both a start and run capacitor. The difference between the two is that the start capacitor has a much higher ability to store charge, also known as its capacitance rating, for its size than that of the run capacitor. Essentially, the start cap gives the motor a bolt of energy in order to get it up and running, while the run capacitor keeps the motor going after the start capacitor shuts down.

How do I select the correct Start Capacitor?

Most electric motor start capacitors will come with similar specs.

●Voltage – 110/125, 165, 220/250 or 330 VAC

●Frequency – 50 or 60 Hz

●Capacitance – 50 to 1200 UF

It is important to note that when choosing a replacement capacitor, you should always select one with the same capacitance and frequency as the original. In terms of voltage, you can choose a new capacitor with a voltage that is higher or lower than the original but it must still be higher than the operational voltage of the motor for it to work. The start capacitor can usually be identified by their black phenolic or plastic case and a recessed top where the connections are located.

What else should I know about Start Capacitors before I buy one?

Since a start capacitor is designed to give a quick burst of energy, it can sometimes burn out. There are two easily identifiable signs that the start capacitor is not functioning normally: if your motor is either not turning on at all, or is slow to start.

If the motor is simply slow to start, your start capacitor may have lost its capacitance rating due to wear and age. It is also possible in this case that you may have other issues with your motor unrelated to the capacitor, but replacing a start capacitor is relatively cheap, so it is a good place to start.

In more serious cases, the motor's starting circuit might be engaged too long for the intermittent duty rating of the start capacitor. The top of the start capacitor may become dislodged, and the insides may, in some cases, be partially or fully ejected. If start capacitors are left energized too long due to a faulty starting circuit on a motor they can overheat. Similarly, but not quite as dramatic, a start capacitor may just exhibit a ruptured pressure relief blister. If any of these do occur, and damage the capacitor, it must simply be replaced.