What About Capacitors?

By: Peter Kirtschej, Service Coordinator, ABR Wholesalers, Inc.

April 8, 2014

What are capacitors? And why do we use them in our industry? A capacitor is an electronic component that stores charge. There are two conductors inside.  The conductors are usually plates: one positive and one negative. The plates accumulate electric charge when connected to a power source – positive plates accumulate positive charge and negative plates accumulate negative charge.  These plates are separated with a non-conductive substance. The energy that is stored in the plates is called capacitance. Capacitance is the amount of electric charge stored in the capacitor. We measure capacitors in MFD - micro farads. 

The HVAC industry uses run capacitors and start capacitors. Run capacitors are used on condenser fan motors, inducer motors and blower motors.  A run capacitor creates an even flow of energy to an electric motor. The constant flow of energy eliminates dead spots and makes the motor more efficient. Start capacitors are used in heat pumps and A/C condensers. They are only energized for a short period of time on startup when they give the motor extra starting torque. Start capacitors are usually wired through a start relay, and are energized for 10 to 20 seconds. Many of the start capacitors also have a resistor wired in between the terminals. This is called a bleed down resistor because its function is to remove any charge that may still be stored inside the start capacitor after it is disengaged from start relay.  

When diagnosing a capacitor, set your UEI DL379 multi meter to MFD. Disconnect all wires from the capacitor and hook one lead of the multi meter to one side of the capacitor and the other lead to the other side. The multi meter will show the charge.  For instance, if the capacitor is rated for 5 micro farads, the multi meter should read 5 MFD. If we see any other MFD reading on a capacitor rated for 5 MFD, we know we need to replace the capacitor. The process of testing a dual capacitor is similar.  For example, let’s say you have a 20+5 5% @370 volt capacitor. The MFD reading on the multi meter between the C terminal and the HERM terminal should be 20 MFD, and the reading between the C terminal and the FAN terminal should be 5 MFD. Most dual capacitors have a 5-6% tolerance.  In HVAC, capacitors are marked either 370v or 440v. If a 440 volt cap is removed, a 370 volt cap should not be installed. However if a 370 volt cap is removed, a 440 volt cap can be installed because it can handle up to 440 volts.

For more information, check out our service tip blog:  www.abrwholesalers.com/servicetips  To purchase a UEI DL379 multi meter to read MFD, please call or visit one of our branches.