Everything You Ever Wanted to Know About Capacitors

What is a capacitor and why are they needed in a system and how do I check them out?

First of all, capacitors are only installed on single phase motors and compressors  Three phase motors and compressors do not use capacitors.  A capacitor is a device capable of storing and releasing an electrical charge. There are 2 types of capacitors, the RUN capacitor and the START capacitor.  These are used on PSC (permanent split capacitor), and CSR  / CSCR (Capacitor start capacitor run) motors and compressors.  The CSR/CSCR motors need a potential relay or start relay which will drop out the start capacitor once the motor “comes up to speed”. Capacitors should always be sized based on the motor /compressor manufacturer’s recommended capacitor size.

The RUN capacitor is wired in series with the start winding of the motor and stays in the circuit all of the time.  They are designed to dissipate heat associated with continuous operation of the motor.  The whole purpose of the RUN capacitor is to bring the start winding back in phase with the run winding. The start winding is slightly out of phase with the run winding to provide starting torque for the motor. The RUN capacitor also provides “running torque” once the motor is up and running.

The START capacitor is always used with a start relay or potential relay.  Because it is designed to ONLY stay in the circuit while the motor is starting, the relay is necessary to “drop” the capacitor out of the circuit. Unlike the RUN capacitor, it is NOT designed to dissipate heat associated with staying in the circuit for prolonged periods.  The purpose of the START capacitor is to increase the phase angle between the start and run windings to create GREATER STARTING TORQUE. Because this is changing the phase angle, the start relay is installed to drop it out once motor comes “up to speed”.  It is also wired in series with the start winding.

Potential or “voltage” starting relays are used with single-phase capacitor-start/capacitor-run motors, which need relatively high starting torque. Their main function is to assist in starting the motor.

These starting relays consist of a high-resistance coil and a set of normally closed contacts. The coil is wired between terminals 2 and 5, with the contacts between terminal 1 and 2.

The operation of the potential starting relay is based on the increase in back electromotive force (back EMF) or a bucking voltage that is generated across the start winding as the motor increases in speed.

The large metal mass of the motor’s rotor turning at high speeds has a voltage generating effect. This generated back EMF opposes line voltage and can be measured across the start winding . The back EMF is usually a higher voltage than the line voltage and can be in the 400-V range. All motors have different magnitudes of back EMF.

The back EMF voltage generated across the start winding causes a small current to flow in the start winding and in the potential relay coil since they are in the same circuit. When the back EMF has built up to a high enough value, referred to as pick-up voltage, the contacts between terminals 1 and 2 will be picked-up opened. This will take the start capacitor out of the circuit. The pickup voltage usually occurs when the motor has reached about 3/4 speed.

When the power is applied through the cycling control, both the run and start windings are energized. The start and run capacitors provide the phase shift for starting torque because of their capacitance adding when wired in parallel. In fact, both capacitors are wired in series with the start winding.

The combination of a start capacitor and relay is commonly known as a hard start kit and are typically used when a system has a TXV installed or when the system has low voltage (208 VAC). 

Capacitors are rated in microfarads and also have a voltage rating on the case..  Microfarads are usually identified on the capacitor with the Greek symbol “μ” for “micro” and an F for farad.  The voltage rating on the capacitor does not represent the line voltage applied to the equipment; this voltage rating is the maximum amount of back electromotive force (EMF) the capacitor can have applied to it during normal operation without damage occurring.  You can always go higher on the voltage rating of a capacitor but you should NEVER go lower as this could cause the capacitor to be damaged.

TROUBLESHOOTING:  (see attachment at the end of this post for detailed use of meters to check out capacitors) When a run capacitor is tested with a μF meter, the capacitor should test within the μF % listed on the capacitor.  Start capacitors should test equal to or up to 20 % greater than the μF rating on the capacitor.  If the test indicates that the start capacitor has less than the rated μF, the capacitor should be replaced.

The capacitor should also be checked with an ohm meter from each terminal to the case of the  capacitor to make sure the capacitor is not grounded.

If the voltage rating is considered to be the problem, this can be measured by carefully placing the probe of a volt meter on the terminal coming from the start winding of the compressor to the capacitor and the other probe to “ground”.  This will give you the back EMF voltage the motor is generating.  If the back EMF is greater than the voltage rating on the capacitor, the capacitor should be replaced with a higher voltage rating above the back EMF voltage that was read.  NOTE: Be sure to be careful when taking this measurement as the back EMF voltage can be over 400VAC.

A note of safety: you should be aware that the capacitor may have “stored energy” even though the electrical disconnect has been locked out and the line voltage has been removed from the system.  A resistor should be used to “bleed” the stored energy from the capacitor.  The recommended resistor is a 20,000 ohm 2 watt resistor.  You should not use a screw driver to bleed or short the capacitor as that could cause damage to the capacitor or motor itself.

Keep in mind, if you do not have a properly operating blower motor, condenser fan motor or even the compressor, the capacitor should always be checked to make sure it is providing the proper phasing and starting torque for the motor in question. A capacitor that is out of the μF %,  can cause the motor to draw higher amperage and eventually drop out on its overload.  This can then lead to coils freezing, high pressure trips, and even compressor failures.

It may be a “small component” but it does a “big” job in keeping single phase motors operating properly.  Don’t overlook it!

Capacitor Testing & Troubleshooting

(Thanks to the  UPG Training Group for the Capacitor Testing & Troubleshooting attachment)