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Page 66
Chapter 3: Motors
Electrical Theory & Applications for HVACR
SOLID STATE AND PTC
There are two solid‐state or electronic starƟng devices: posiƟve
temperature co‐efficient thermistors (PTCs) and electronic
relays. Either of these can be wired in series with a start
winding, with or without a start capacitor (See fig. 3‐14).
A PTC has a resistance of approximately 5 to 15 ohms cold. As
current passes through, the PTC heats up and resistance
increases to 10,000 ohms or higher. This high resistance
virtually stops current flow to the start winding and/or
capacitor. A PTC can be checked by wiring it in series with a 100
waƩ, 120 Vac incandescent light bulb. When power is applied,
the light should be bright, and then fade out in about 60 seconds.
An electronic start relay relies on either voltage sensing or Ɵme delay to disconnect the starƟng
circuit. It is wired with only two leads in series with the start capacitor, connected to the run and
start terminals of the motor.
CALCULATING MOTOR HORSEPOWER
Motors are rated according to the amount of torque they can produce. This turning power is
measured in horsepower. The electrical consumpƟon is measured in waƩs. A one horsepower motor
consumes approximately 746 waƩs. Thus, a five horsepower motor will use approximately 3,730
waƩs (5 X 746=3730). This formula assumes 100 percent efficiency. When waƩage and voltage are
known, Ohm’s Law allows us to determine amperage draw at full load condiƟons. True horsepower
must take into account the power factor, which is approximately 0.95, motor slippage and fricƟon.
Field calculated waƩage will be higher as much as 30% or more depending on type of motor.
SERVICE FACTOR
Service factor (SF) is a mulƟplier marked on data plates that indicates the total permissible
horsepower loading when the motor is operaƟng at rated voltage and frequency.
Example:
A motor rated at 0.75 Hp with a service factor of 1.2 is rated at 0.9 Hp.
(0.75 HP X 1.2 SF = 0.9Hp)
TIP:
When replacing a motor, the replacement motor’s horsepower mulƟplied by its service factor
must be equal to or greater than the original motor’s horsepower mulƟplied by service factor.
LOCKED ROTOR AMPS (LRA)
At start‐up, before a rotor starts to rotate or if it is locked, current flow in the motor is determined by
the resistance of the windings. StarƟng current (inrush current) is about six Ɵmes higher than normal
running amperage. This high current flow is called locked rotor amps (LRA). As the motor picks up
speed, counter‐EMF is generated and reduces current flow. At operaƟonal speed, current flow is
determined by resistance of the run winding, counter‐EMF generated by the motor, and the load
placed on the motor.
Fig. 3‐27: PTC relay with capacitor