Capacitor Education
Capacitors as used in Heating and Air Conditioning.
A capacitor is defined as- An electrical device consisting of two conducting plates separated by an electrical insulator (the dielectric), designed to hold an electric charge. Charge builds up when a voltage is applied across the plates, creating an electric field between them. Current can flow through a capacitor only as the voltage across it is changing, not when it is constant.
Start Capacitors are used in some Heat Pumps and Air Conditioners. These capacitors DO start the compressor but have nothing to do with the FAN. They can be identified by the fact that they are round and are in a black plastic can. They work in conjunction with a separate black relay to start the compressor.
There are also Hard Start Kits which are usually added to systems that are having trouble starting a stuck or failing compressor. These units incorporate a built in relay within the body of their enclosure.
Important to note: Not ALL Condensers, Furnaces or Air Handlers use a run capacitor. Run Capacitors are only used on a P.S.C. motor (Permanent Split Capacitor).
There are round or oval run capacitors available. The specifications are more important than the configuration, as long as it will fit in the cabinet safely, either could be used.
If your motor won't start on it's own accord, it could be a bad capacitor since run capacitors "split" the phase so that the motor can start. It could also be a bad start winding in the motor, or sometimes a flat spot within the motor. The starting of the motor takes place through the "Start" winding within the motor. If the motor is determined to be bad, you should always change the run capacitor with the correct size. If the motor does start and runs, but runs hot or at a slow speed then suspect the run capacitor. Inspect the capacitor for leakage, bulging of it's can or bad terminals. Remember to discharge the capacitor before touching any of it's terminals by shutting the main power off and grounding any terminals to each other and bare metal with an insulated screwdriver. Failing to do this will result in a nasty shock!
Below is a shot of (2) typical oval run capacitors found in Heating and Air Conditioning equipment.
The capacitor on the left is a Dual Rated Run Capacitor. What this means is, it is actually (2) capacitors sharing (1) body. Note the left terminal is marked "FAN". This is where the (usually brown wire, read the wiring diagram) goes that comes from the condenser fan motor. The right terminal is marked "HERM" which stands for compressor and accepts the run wire from the compressor. The common or center terminal is the common wire to both the other terminals (follow your wiring diagram on your particular unit).
The capacitor on the right is a single Run Capacitor and can only operate one device. Single run capacitors are ALWAYS used for Furnace and Air Handler indoor P.S.C. blower motors, you won't ever find a Dual Rated Capacitor on an indoor unit. If you have bought a motor for your Heat Pump or Air Conditioning condenser, you should always change the capacitor with the one recommended for the motor you purchased. If your Air Conditioner uses a Dual Rated capacitor, you simply install the new single run capacitor using the (usually brown wires or brown with a stripe) (1 wire on left terminal, the other on the right terminal) just disregard your present FAN terminal on your existing Dual Rated Capacitor.
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To use your existing Dual Rated Capacitor (assuming it is the right one for the motor you purchased) just install the brown wire from the motor to the FAN terminal and wrap some electrical tape around the end of the other brown wire from the motor as it won't be used. See more about the specs of capacitors lower on this page. |
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Here we are showing the box of a typical Dual Rated Run Capacitor. 30 stands for 30 MFD (Microfarad) also 30 UF terminology is used but it means the same for practical purposes. The larger of the numbers is for the compressor (HERM terminal). The smaller number is for the (FAN terminal). 5 stands for 5 MFD (Microfarad) also 5 UF terminology is used but it means the same for practical purposes. |
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Here is a closeup of the terminals. The center terminal is "C" which stands for common. Read your wiring diagram. |
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Here are the specifications you should find on the side of the capacitor. Usually you will see, as in this case, 30/5/440. The 440 stands for the voltage that the capacitor can withstand. On residential units a 370 volt is most often used as home voltage is never over 240 volts. There is no advantage to using a 440 over a 370 volt, even so, some manufacturers of equipment want a 440 volt used- always go by their recommendations. |
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