Motors, Contactors
& Starters

Electric Motors in HVAC

HVAC systems use electric motors for compressors, condenser fans, evaporator/air handler blowers, pump motors, and damper actuators. Understanding motor types and their electrical characteristics is essential for diagnosis and replacement.

Single-Phase Motor Types

Motor Type	Starting Method	HVAC Application	Capacitor?
PSC (Permanent Split Capacitor)	Run capacitor - always in circuit	Condenser fans, evaporator fans, small compressors	Run capacitor only
CSR (Capacitor Start Run)	Start + run capacitors; start cap disconnects after starting	Larger compressors	Both start and run caps
Split-phase	Auxiliary winding provides starting torque; disconnects after starting	Small fans, pumps	No capacitor
Shaded pole	Copper shading ring creates phase shift	Draft inducers, small fans	No capacitor

ECM Motors (Electronically Commutated Motors)

ECM motors are DC brushless motors controlled by an integrated electronic module. They are used in modern variable-speed air handlers and furnace blowers because they:

Are 60-70% more efficient than standard PSC motors
Automatically adjust airflow to maintain constant CFM against varying static pressure
Operate very quietly at low speeds
Have soft-start capability - reduce startup current surge

? ECM Diagnosis Warning

ECM motors require different diagnostic procedures than PSC motors. Never attempt to measure ECM motor winding resistance with a standard ohmmeter - you will get incorrect readings due to the integrated electronics. Use the motor manufacturer's diagnostic procedure and check control board outputs instead of motor windings directly.

Capacitors

Capacitors are among the most commonly failed HVAC components. Rated in microfarads (?F or MFD):

Type	Purpose	When It Fails
Run capacitor	Stays connected during operation; improves motor efficiency and power factor	Motor hums but won't start, or runs but draws excess current and overheats
Start capacitor	Provides extra torque during startup then disconnects via start relay	Motor will not start; may hum loudly. Check start relay too.
Dual-run capacitor	Single capacitor with two sections - one for compressor, one for fan motor	Whichever motor's section fails shows symptoms; other may still run

? Discharge Before Testing

Capacitors store charge after power is disconnected. Always discharge with a 20,000? resistor or discharge tool before handling. Use a capacitor tester (capacitance meter) to test - a capacitor reading more than �6% from its rated value should be replaced.

Contactors and Motor Starters

Contactors

A contactor is a high-current electromagnetic switch. A 24V control signal energizes the contactor coil, which closes the power contacts supplying 240V to the compressor and condenser fan. Key diagnostics:

24V present at coil but contactor doesn't pull in: Bad coil - measure coil resistance (typically 8-20?)
Contactor pulled in but compressor won't run: Check power contacts for pitting or failure - measure voltage drop across closed contacts (should be nearly zero)
Contacts welded closed: Compressor runs continuously even when thermostat opens - replace contactor immediately

Motor Starters (Three-Phase)

Commercial HVAC uses three-phase motor starters which add overload relays to contactor function. Overload relays protect motors from sustained overcurrent that would overheat motor windings. If overloads trip, find and correct the cause before resetting.

? Module 3 Module 5: Control Circuits ?