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Chapter 3: Motors
Electrical Theory & Applications for HVACR
A PTC is a thermistor with very low resistance at room temperature and
very high resistance when hot. It is connected in series with a supply
power and start winding. As current flows through the PTC, it heats up,
creaƟng a large voltage drop and removing nearly all current to the start
winding.
CS OR CSIR MOTORS
To improve starƟng torque, a start capacitor is connected to the start
winding of an inducƟon start motor. Single‐phase inducƟon motors
with start capacitors are used when a motor must start under loaded
condiƟons. Extra torque is required for starƟng purposes only.
Once started, the capacitor and the
start winding must be disconnected
from the circuit. In HVACR systems,
these motors are commonly found on
belt drive fans, pumps, and fracƟonal
horsepower refrigeraƟon compressors.
PSC MOTORS
A permanent split capacitor (PSC) motor is oŌen used to operate fans,
blowers and refrigeraƟon compressors. A PSC motor uses a run
capacitor in series with a start winding while in operaƟon. The run
capacitor has two funcƟons: keeping the start winding 90 degrees out
of phase, and limiƟng current flow through the start winding to keep it
energized. In this way, the start winding assists the run winding,
making the motor operate more efficiently.
Run capacitors have low microfarad raƟngs and act as throƩling devices
to limit the number of electrons flowing through the start winding.
Adding capacitance improves power factor and motor efficiency. The
run capacitor is connected into the start circuit permanently, hence the
name permanent split capacitor.
CSR MOTORS
Single‐phase motors with run and start capacitors are
called capacitor start‐capacitor run (CSR) motors.
CSR motors are used when a motor must start under
loaded condiƟons and run with efficiency. They have
the highest starƟng torque of all split‐phase motors.
Both the start and run capacitors are used to start
the rotor turning. Once the motor has started, the
start capacitor is disconnected from the circuit while
the run capacitor remains connected.
Fig. 3‐15: CSIR motor wiring
Fig. 3‐16: PSC motor wiring
Fig. 3‐17: Motor with potenƟal relay wiring
Fig. 3‐14: PTC wiring