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

  1. Common uses for motor capacitors heating and air systems
  2. Capacitor Types for HVAC Motor Applications. The two different types of capacitors used for HVAC - Motor Run Capacitor and the Start Capacitor
  3. How capacitors are made
  4. Rules for replacing a motor capacitor
  5. Motor capacitor safety measures
  6. Lots of valuable related links to give you a closer look so you will learn more about HVAC

Run Start HVAC CapacitorsRun Start Capacitors for HVAC | Electrical Basics

There are two basic types of motor capacitors most commonly used in HVAC applications today. The motor air conditioner run capacitor and the motor 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.

HVAC 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 in ac units and heat pumps, AC compressors, and circulator pumps for hot water and chilled water.

Common applications for the start capacitor are generally limited to air conditioner compressors (heat pumps also) 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 capacitors (starting capacitors) are also referenced when discussing HVAC run capacitors. Three phase motors do not use any type of capacitors while single phase motors do use them.

A motor AC run capacitor (or start capacitor) can cause serious injury or worse so caution is advised whenever working around capacitors. HVAC run capacitors can be discharged using a resistor across its contacts. A 20,000-ohm resistor should be used to discharge the 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 for HVAC Systems - Inside AC Run 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 at the disconnect and breaker. Capacitors are hermetically sealed and should not be leaking any fluid(s).

Run Start Capacitors for Motors - Capacitor Specifics

Capacitors are measured or rated according to a microfarad rating. Generally, AC 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.

How to Test a Capacitor for Your Air Conditioning System

To check the air conditioner capacitor to see if it 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 When you check the capacitor you check it for its Mfd or microfarad rating and 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.

High amp draws could mean you have a capacitor going bad. You may also need a hard start kit which hooks to the capacitor. In many cases, compressors and the condenser fan motor share a dual capacitor. Each side should be checked separately on the capacitor. The HVAC industry uses two different types of capacitors (or shapes) one is round and the other is oval. When you check a dual capacitor you need to check both sides. The fan and the compressor (typically marked as HERM). If you do not feel comfortable doing this call for air conditioning repair. They will know exactly what to do and add a compressor hard start if necessary.

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 Heating & Cooling 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. A good sign of a bad capacitor is a bulging or leaking capacitor in your heating and air conditioning unit.

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 AC Run & Start Capacitors 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 | Recap and Conclusion

  • Bad Dual AC Run Capacitor

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

    Motor capacitors for heating and cooling systems are categorized as Run Capacitors and Start Capacitors. They are only used on single-phase systems. Three-phase systems do not require capacitors.
  • 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. Some of these capacitors are oval and some are round.
  • A bad faulty capacitor will be more than plus or minus (usually minus) 10% the MicroFarad rating listed for its rating. A bad capacitor can cause your air conditioner unit circuit breaker to trip and/or the motor to overheat. Electrical symbols include mfd uf.
  • A capacitor changes the magnetic field in the windings of the motor. It reduces the amperage
  • 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 AC and heat pump capacitors.
  • If you have a motor with no capacitor you either have a shaded pole motor or a variable speed ECM motor.

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.

Hard Start Capacitor - Compressor and Fan Capacitors for Air Conditioners and Heat Pumps

If you get a failed capacitor for a compressor sometimes the AC guy will add a hard start capacitor to the replacement capacitor. These are not used for condenser fans but only for compressors. When your compressor stops the refrigerant flow stops in your system. The system is still pressurized and when the compressor starts again it needs a little boost to overcome that pressure. The compressor goes from 0 RPM’s to full speed in less than a second. This causes an amp surge in the compressor circuit. Technically, it is referred to as locked rotor amps or LRA. A hard start capacitor (also referred to as a hard start kit) is simply used to help the compressor on startup to overcome the increased pressures in the refrigeration circuit. These are not used on compressors that utilize start capacitors. These hard start capacitors can be used on heat pumps & air conditioners.

Central Air Conditioner Capacitor Replacement

Fans do not use hard start kits because they do not have high pressure to overcome on startup. A fan will use a run capacitor when the fan is running to help it run more efficiently. It gives the fan in your central air conditioner system a boost by changing the phase angle inside the motor. Compressors and fans often share a round dual capacitor or an oval dual capacitor. As noted above, when it goes bad you either replace it will the same type and rating capacitor (in MFD’s or microfarad’s) or you can replace it with the rated type for the compressor and fan with two capacitors. For example, if the compressor uses a 45 Mfd capacitor and the fan needs a 5 Mfd capacitor you can use two capacitors instead of one dual 45/5 Mfd. That is provided you put a 5 Mfd on the fan and a 45 Mfd on the compressor. The wiring sometimes confuses people so if you are not sure call an HVAC technician to wire it. Miswiring can cause serious damage to components in your air conditioner or heat pump.

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.

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