Start Capacitors for HVAC Compressors - Start Capacitor with Potential Relay
Start Capacitors for HVAC Compressors - HVAC single phase compressors always come hooked up to a run capacitor. This helps the compressor when it is running, however, very few HVAC compressors come with a start capacitor. Most HVAC technicians know HVAC compressors pull a lot of amps on startup. This large amperage draw on startup is referred to technically as locked rotor amps or inrush current. It can be as much as 40,000 watts or more of pure power to get an HVAC compressor started.

Start Capacitors for HVAC Compressors - Potential Relay

Start Assist HVAC Compressors

This multimeter has a special setting for checking the Microfarad ratings on capacitors. Rule of thumb is +/- 10% of capacitors listed rating.

Start Capacitors for HVAC Compressors - That large amount of amperage on the compressor windings continuously can slowly degrade the life of the compressor. The compressor starts over and over again during the summer for cooling and year round for a heat pump.

The run capacitor does little to help the compressor on startup. In certain circumstances, the manufacturer will recommend installing a start capacitor on new installations. It is a prudent thing to add a start capacitor to older installations. Many older systems need a slight boost on start-up.

There are two ways we can do this job of helping the compressor start. The first method for older installations on compressors that are having issues starting - most technicians use this method when they encounter a compressor with problems. Technicians use a hard start kit which is usually available at the local HVAC supply house. It is easily hooked up to the compressor wiring following the directions on the hard start kit wiring diagram. Of course, every HVAC tech knows it is important for safety to turn off the power to the unit. Additionally, to discharge any capacitors before proceeding otherwise serious injury or worse can occur.

Start Capacitors for HVAC Compressors - Potential Relay

The next method to help the compressor on startup is to add a start capacitor. This is added to the HVAC compressor circuit along with a potential relay. The potential relay is necessary. Necessary because we do not want to leave the start capacitor in the compressor circuit after the compressor starts. The potential relay allows the start capacitor to assist the compressor to start. It then opens the circuit between the compressor and the start capacitor after the compressor starts and is running.

The potential relay does this using back EMF or back electromotive force. Otherwise known as back voltage coming from the compressor. As the compressor starts and voltage goes through the system it sends out a back EMF voltage. Furthermore, this voltage energizes the coil in the potential relay for the start capacitor.

Start Capacitors for HVAC Compressors | Back EMF

The back EMF is constant while the compressor is running. The relay will hold the start capacitor out of the compressor circuit because of the Back EMF. Until the compressor stops. Then the coil in the relay is de-energized and start capacitor awaits another startup. Furthermore, the start capacitor can assist the compressor on the next startup.

Other methods of taking the start capacitor out of the circuit include a current relay. The current relay opens the circuit between the compressor windings and the start capacitor. Lastly, the current relay works off a certain amount of current.

Related Link: Learn more about compressors and compressor troubleshooting. Delve deeper and learn more about HVAC.

Why a Start Capacitor is Needed for the Compressor

Some of the reasons why a start capacitor is needed for a compressor are the following reasons:

  • A compressor that continues to trip a circuit breaker or blow fuses.
  • If the startup amps or locked rotor amps exceed the rated L.R.A. on the compressor data plate.
  • When the line set between the condensing unit and the evaporator coil meets or exceeds the prescribed horizontal length as recommended by the manufacturer.
  • If the line set between the condensing unit and the evaporator coil meets or exceeds the prescribed vertical lift as recommended by the manufacturer.
  • When the voltage in the area is lower than normal.
  • Older compressors that have received trouble calls concerning startups.
  • For air conditioner or heat pump systems that dim the lights when the units turn on.
  • Lastly, systems with TXVs that are slow to equalize on shut down.

MicroFarads (MFD)

Always consult the manufacturer or an engineer for proper MFD ratings. This is for the start capacitor and the proper voltage on the coil in the potential relay. Additionally, only properly trained HVAC technicians should install the start capacitor. Follow all safety procedures when working around high voltage systems. Serious damage can occur if the start capacitor is not hooked up properly. Furthermore, damage to the compressor and other components in the HVAC system.

Lastly, other Resources to help you with Capacitors and HVAC Motors:

High Performance HVACStart Capacitors for HVAC Compressors

Technical Resource: Refrigeration and Air Conditioning Technology