There are two basic types of motor capacitors most commonly used in HVAC applications today. The run capacitor and the start capacitor. Motor capacitors are used to either increase run efficiency of the motor or to give it an initial start boost in torque to a motor. They are sometimes referred to as motor starter capacitors or power capacitors. Motor capacitors are only used for single-phase motors that generally see heavy use throughout the HVAC industry. Common applications of single-phase motors which use commonly use run capacitors are air handler blower motors, induced draft or forced draft fan motors on high efficiency furnaces, condenser fan motors, compressors, and pumps for hot water and chilled water. Common applications for the start capacitor are generally limited to compressors and only to start the compressor. Typically, when a start capacitor is used, a special relay accompanies it in the circuit to disengage the start capacitor after the compressor motor gets it’s initial boost in torque from the start capacitor. The most common relay for use in disengaging the start capacitor is the potential relay and the least common is the current relay. Power capacitors or motor start capacitor are also referenced when discussing HVAC capacitors.
Inside Capacitors
The inside part of capacitors are made up of an insulator between two metal plates. The properties of this metal allow the capacitor to store electrons and the insulator prevents the electrons from flowing from one plate to another. The capacitor therefore stores energy in the form of electricity just as a battery would hold a charge of electricity. It is therefore necessary to take great care whenever working around capacitors even when the power to the unit has been turned off. A motor capacitor can cause serious injury or worse so caution is advised whenever working around capacitors. Motor capacitors can be discharged using a resistor across it’s contacts. A 20,000 ohm resistor should be used to discharge the motor capacitor before working near or on a circuit with a capacitor in it. Remember, any motor capacitor, whether it be a motor start capacitor or a power capacitor can cause great injury or worse so handle with care.
Capacitor Specifics
Capacitors are measured or rated according to a microfarad rating. Generally, run capacitors are rated for lower microfarad ratings from 3 microfarads to 50 microfarads. Start capacitors have much higher ranges and can be purchased in sizes all the way up to 800 microfarads. Capacitors also have a voltage rating on them with voltage ranging from 240 volts to 440 volts. It is good practice whenever replacing any electrical components to use an exact replacement. There are rules of thumbs that allow one to use a part that is not an exact replacement which is in a certain range but it is best to use an exact replacement. To check to see if a capacitor is bad use a capacitance tester to measure the microfarad rating on the capacitor. Compare it with the manufacturers rating. If it is more than ten percent off the microfarad rating on the capacitor replace it.
Motor Capacitors Conclusion
Additionally, if the motor or compressor fails to start for whatever reason part of the troubleshooting list of things to check will be the capacitor. A bad capacitor can cause a motor to fail and burn up especially when heavy load conditions exist. It is difficult to tell unless you are familiar with capacitors but a capacitor that is bulged out on the top or sides is a bad capacitor. It needs to be replaced. There is always an inherent danger when working on electrical components and especially on capacitors. They will shock you even when the power to the equipment is off. Always have a trained expert HVAC technician check out the equipment. They can diagnose the problem quickly and efficiently and have the system back up in no time. They are also very familiar with all the dangers involved so it will done safely with no injuries.
The following video offers information for HVAC run capacitors that is helpful for someone wanting to understand the basics about capacitors. One can probably skip the first third of the video.
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When testing a 3 prong capacitor with a digital multimeter what setting should the multimeter be set on? and what number will be displayed if the capacitor is bad?
You are talking about a dual capacitor that is typically installed in a condenser. The capacitor must be discharged before any testing is done so follow the instructions here at High Performance HVAC about discharging a capacitor. On the dual capacitor you will have three prongs as mentioned. One should be stamper C and this is the common terminal. Another should be mark HERM and that is the compressor terminal. Lastly the other will say FAN and of course that is for the condenser fan motor. To test this you need a special multi-meter that can read microfarads. If you have this meter (and not all multi-meters have this) then you would go from common to herm. Whatever the capacitor is rated for the number you will read on the meter should be plus or minus 10% of that rating. If it is a 40 microfarad rating for the compressor then you should be in the range somewhere from 36 microfarads to 44 microfards. The same goes for the fan. The leads from the meter should go from the common terminal to HERM terminal for testing the compressor side and then from the common terminal to the FAN terminal when testing the fan side of the capacitor. Good luck!