Motors & Motor Controls
Motor types, nameplate data, starters, contactors, overload relays, VFDs, and motor circuit conductor sizing.
- Identify common motor types including split-phase, PSC, and three-phase induction motors
- Interpret motor nameplate data including FLA, code letter, and service factor
- Explain the function of contactors, overload relays, and holding contacts
- Describe VFD, soft starter, and wye-delta reduced voltage starting methods
- Size motor circuit conductors at 125% of motor FLA per the CEC
Lesson 1
Motor Types - Single-Phase, Three-Phase & PSC
Single-Phase Motors
A single-phase motor typically requires a starting mechanism because single-phase power does not produce a rotating magnetic field. Unlike three-phase motors that self-start due to the rotating field created by three phases 120 degrees apart, single-phase motors need an auxiliary means to create the initial rotation.
Common single-phase motor types:
Split-Phase Motor
Starting mechanism: Centrifugal switch
Start winding: Higher resistance, lower inductance
Applications: Pumps, fans, power tools
Starting torque: Moderate
PSC Motor
Starting mechanism: Permanent run capacitor
No switch: Capacitor runs continuously
Applications: HVAC fans, compressors
Starting torque: Low
Capacitor-Start Motor
Starting mechanism: Start capacitor + centrifugal switch
Start winding: Disconnected after starting
Applications: Compressors, conveyors
Starting torque: High
A split-phase motor typically uses a centrifugal switch as its starting mechanism. The centrifugal switch disconnects the start winding once the motor reaches approximately 75% of synchronous speed.
The permanent split capacitor (PSC) motor is the most commonly used motor in residential HVAC systems. It uses a run capacitor that remains in the circuit at all times, providing both starting and running characteristics without the need for a centrifugal switch.
Three-Phase Induction Motors
Three-phase induction motors are the workhorses of industrial and commercial applications. They are simple, reliable, and self-starting. The rotating magnetic field is created naturally by the three phases, so no starting mechanism is required.
Single-phase motors need a starting mechanism because single-phase power does not produce a rotating magnetic field. A split-phase motor uses a centrifugal switch. The PSC motor is most common in residential HVAC. Three-phase motors are self-starting.