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What you will learn from Run Start Capacitors HVAC Motors | Electrical Basics :

  1. Common uses for capacitors in HVAC
  2. The two different types of capacitors used in HVAC
  3. How capacitors are made
  4. Rules for replacing a capacitor
  5. Capacitor safety measures
  6. Lots of valuable related links to give you a closer look so you will learn more about HVAC

Run Start Capacitors HVAC MotorsRun Start Capacitors HVAC Motors | Electrical Basics

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 its 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.

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 its 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.

Use extreme caution!!!

Related Link: HVAC Electric Motor Basics - A Closer Look

start & run motor capcitors

This multimeter has a setting specifically for checking motor capacitors.

Run Start Capacitors HVAC Motors - Inside Capacitors

The inside part of capacitors is 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.

Run Start Capacitors HVAC Motors - 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. With the Microfarad ratings, it is important to follow the 10 percent rule: that it stays within plus or minus 10 percent of the microfarad rating. With the voltage rating, it is okay to go up on the voltage with the replacement but never down. In other words, if you have a capacitor rated at 370 volts that is 25 microfarads if you do not have an exact replacement but one that is 25 microfarads and 440 volts that is an acceptable replacement.

Run Start Capacitors HVAC Motors - Motor Capacitors

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 be done safely with no injuries.

Run Start Capacitors HVAC Motors - Motor Capacitor Replacement

Run & Start Capacitors for HVAC MotorsIf your AC runs or your heat pump runs you can still have a bad capacitor or a capacitor that is getting weak. Whether you have a bad capacitor in the condenser unit or the air handler capacitors can get weak and the system will continue to run. When the bad capacitor gets too weak to help the motor then the motor may not run or it will run but at a reduced speed. This will create more heat and a larger than normal amp draw and eventually system failure.

When HVAC technicians do routine maintenance a part of that service should include checking the capacitor to make sure it is within range of the rate MicroFarads or mFd’s. Anything less than 10% of the mFd rating and replace the capacitor. Capacitor replacement is not difficult but caution is advised because you are dealing with high voltage even when the power is off. HVAC technicians will generally replace the capacitor wire for wire making sure to read the labels, especially on dual capacitors. The labels are Com - Herm - Fan with Com being common, Herm being the hermetic compressor, and of course, the fan being the condenser fan motor.

Related Link: Troubleshooting HVAC Capacitors - A Closer Look

The following Run & Start Capacitors for HVAC Motors 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.

Air Conditioner Run Start Capacitors HVAC Motors | Recap and Conclusion

  • Bad Dual Capacitor

    A bad dual-run capacitor. The top of the capacitor is all swelled up rather than being flat.

    Motor capacitors for HVAC systems are categorized by Run Capacitors and Start Capacitors.

  • Motor capacitors are used on condenser fan motors, blower motors, and compressors.
  • Start capacitors are used only on compressors for start assist while run capacitors are used on all three types of motors including the condenser fan motor, the blower motor, and the compressor.
  • Dual capacitors are run capacitors and used for the condenser fan motor and the compressor. These dual capacitors are found in the condensing unit of the HVAC system.
  • A bad capacitor will be plus or minus (usually minus) 10% the MicroFarad rating listed for its rating.
  • Voltage ratings are applicable when changing a bad capacitor and the rule is you can go up on the voltage rating but never down.
  • There are other types of capacitors but don’t confuse them with the HVAC motor capacitors.

These can be used in AC or air conditioners, heat pumps, and even other heating and cooling appliances so there really are no specific AC run capacitors or heat pump run capacitors. Nor are there and air conditioner run capacitors. These are simply run capacitors used on PSC motors to help them run. The same with start capacitors.
Lastly, other Resources to help you with Capacitors and HVAC Motors:

High Performance HVAC

Run Start Capacitors HVAC Motors

To learn more about heating systems and HVAC use this resource.

See the About the Author page for information about the author of this article.

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