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Page 64
Chapter 3: Motors
Electrical Theory & Applications for HVACR
THE RUN CAPACITOR
Run capacitors are made of dielectric material and conductors, enclosed in an oval or
round metal case containing a dielectric oil to help dissipate heat. A run capacitor
increases motor running torque by keeping the start winding slightly energized during
the run cycle. Run capacitors are connected in series with start windings, just like start
capacitors; however, run capacitors are not disconnected. They limit the amount of
electrons entering the start winding and many are dual types marked with hermeƟc,
fan, and a common center terminal.
A run capacitor usually has an idenƟfying mark, like a minus sign or red dot, on
one of the terminals, showing which plate is closest to the metal case. The hot
wire should be wired to this terminal. If the capacitor becomes shorted to the
case, the main fuse or circuit breaker will open. This will prevent the motor
from operaƟng without both of its windings, or ground current thru the start
winding limiƟng the potenƟal of overheaƟng and damage.
CAPACITOR RATINGS
Capacitors have two important raƟngs. One is VAC (volts, alternaƟng current) and the other is
microfarads (mfd or µf). Microfarads indicate the capacitor’s energy storage capacity. Start
capacitors have much higher microfarad raƟngs than run capacitors. Run capacitors are
approximately 1.5 mfd to 60 mfd, while start capacitors can be up to 1,600 mfd. Most start
capacitors have a range of mfd raƟngs, such as 88 ‐ 108 mfd.
When capacitors are connected in parallel, their microfarad raƟngs are added together. When
connected in series, the microfarad raƟng is lower than the lowest capacitor raƟng.
The formula for finding total capacitance in series is:
The peak voltage raƟng of a capacitor must be the same or higher than the OEM capacitor. The back
EMF produced in the start winding while the motor is operaƟng is higher than the applied voltage. If
peak voltage is more than the raƟng of the capacitor, the capacitor plates will short or open. When
capacitors are connected in parallel, applied voltage cannot exceed the lowest voltage raƟng. When
capacitors are connected in series, applied voltage cannot exceed the sum voltage raƟng.
SINGLE-PHASE MOTOR STARTING RELAYS
HermeƟc refrigeraƟon compressors cannot use internal relays to disconnect start windings aŌer start
up. Depending on compressor size, there are several opƟons for external motor starƟng relays.
Fig. 3‐23: Run capacitor