2.2 Electric Motors, Capacitors, Relays, Contactors, and Starters

2.2 Electric Motors, Capacitors, Relays, Contactors, and Starters

Electric motors convert electrical energy into mechanical energy to drive compressors, circulating fans, blowers, and pumps throughout HVAC systems. Because electric motors represent the largest electrical loads in heating and cooling equipment, technicians must master motor operating principles, starting circuitry, capacitor applications, and heavy-duty switching controls.

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Electric Motor Principles and HVAC Motor Types

An electric motor operates via electromagnetic induction. When alternating current flows through stationary stator windings, it creates a rotating magnetic field. This magnetic field induces an electrical current in the rotor (squirt-cage assembly), creating an opposing magnetic field that forces the rotor to turn.

Synchronous Speed and Motor Slip

The speed of the rotating magnetic field in the stator is known as synchronous speed ($N_s$):

$N_s = \frac{120 \times f}{p}$

Where $f$ is frequency (60 Hz) and $p$ is the number of magnetic poles in the stator.

• 2-Pole Motor: $N_s = \frac{120 \times 60}{2} = 3,600\text{ RPM}$ (Actual running speed under load: ~3,450 RPM)
• 4-Pole Motor: $N_s = \frac{120 \times 60}{4} = 1,800\text{ RPM}$ (Actual running speed under load: ~1,725 RPM)
• 6-Pole Motor: $N_s = \frac{120 \times 60}{6} = 1,200\text{ RPM}$ (Actual running speed under load: ~1,075 RPM)

The percentage difference between synchronous speed and actual rotor speed is called motor slip, typically ranging from 2% to 5% in induction motors.

Single-Phase Motor Types in HVAC

Single-phase AC power does not naturally generate a rotating magnetic field; it creates a pulsating field. Single-phase motors require auxiliary start windings and phase-shifting components (capacitors) to establish a directional starting torque:

1. Shaded-Pole Motors: Feature a heavy copper shading strap around a portion of each pole piece. Provides low starting torque, low operating efficiency (30% to 40%), and is restricted to small fractional-horsepower applications such as flue draft inducers and small unit heater fans.
2. Permanent Split Capacitor (PSC) Motors: Utilize a run capacitor permanently connected in series with the auxiliary (start) winding. The run capacitor creates a 30-degree phase shift between the main (run) and auxiliary windings. Delivers medium starting torque and high running efficiency (60% to 70%). Universally used in residential condenser fan motors and standard blower motors.
3. Capacitor-Start, Induction-Run (CSIR) Motors: Incorporate a high-capacity start capacitor in series with the start winding and a centrifugal switch or start relay. The start capacitor provides a large phase shift (up to 80 degrees) for high starting torque. Once the motor reaches 75% to 80% of synchronous speed, the switch or relay opens, disconnecting the start winding and start capacitor. Used on commercial refrigeration compressors.
4. Capacitor-Start, Capacitor-Run (CSR / CSCR) Motors: Combine the high starting torque of a start capacitor with the high running efficiency of a run capacitor. A potential relay disconnects the start capacitor at 75-80% speed while keeping the run capacitor energized across the auxiliary winding. Standard configuration for single-phase residential and light commercial air conditioning compressors.
5. Electronically Commutated Motors (ECM): Brushless DC / 3-phase permanent magnet motors driven by an integrated microprocessor inverter module. ECMs convert single-phase AC input into DC, then chop it into 3-phase variable frequency output. ECMs achieve up to 80-90% efficiency and automatically adjust motor RPM to maintain constant airflow (CFM) across varying duct static pressures.

Three-Phase Motors

Three-phase motors operate on three distinct AC voltage phase lines separated by 120 electrical degrees. The incoming power naturally generates a smooth, rotating magnetic field across the stator. Three-phase motors produce high starting torque, operate at maximum efficiency, and require no start windings, capacitors, centrifugal switches, or start relays. To reverse the rotational direction of a three-phase motor, a technician simply interchanges any two incoming line power leads ($L1, L2, \text{or } L3$).

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Motor Capacitors: Selection, Sizing, and Testing

Capacitors store electrical energy on conductive plates separated by an insulating dielectric material. In HVAC motor circuits, capacitors shift the current sine wave ahead of the voltage sine wave to generate starting and running torque.

Run Capacitors vs. Start Capacitors

Dual-Value Run Capacitors

Dual run capacitors combine two independent capacitors into one metal canister sharing a common terminal:

• C (Common): Receives line power from the run winding / contactor.
• HERM (Hermetic): Connects to the compressor motor start winding.
• FAN: Connects to the outdoor condenser fan motor start winding.

Capacitor Formulas and Testing

• Microfarad Rating Formula: If a multimeter lacks an MFD function, capacitance can be verified under operating load using voltage and current measurements: $\text{Capacitance (}\mu\text{F)} = \frac{2,652 \times \text{Start Winding Current (Amps)}}{\text{Voltage Across Capacitor (Volts)}}$
• Bleed Resistors on Start Capacitors: Start capacitors must feature a 15,000 to 20,000 Ohm, 2-Watt resistor soldered across their terminals. The bleed resistor dissipates residual voltage after the start relay opens, preventing severe contact arcing when the relay re-closes.

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Relays, Contactors, and Magnetic Starters

Potential Relays

Potential relays drop start capacitors out of CSR compressor circuits. A potential relay operates on the principle of back-EMF (Electromotive Force) generated across the motor's start winding as the rotor accelerates.

• Terminal 5: Connected to 240V Common.
• Terminal 2: Connected to Compressor Start Winding and Start/Run Capacitors.
• Terminal 1: Connected to Start Capacitor.
• Operation: Terminals 1 and 2 are normally closed (NC). The relay coil is connected across terminals 2 and 5 (parallel to the start winding). As the compressor accelerates, back-EMF across the start winding increases. When back-EMF reaches the relay's rated pickup voltage, the coil energizes and opens contacts 1-2, taking the start capacitor out of the circuit. When power is removed and motor speed drops, back-EMF collapses below drop-out voltage, allowing contacts 1-2 to re-close.

Current Relays

Current relays are used on small fractional-horsepower compressors (e.g., refrigeration units). Their contacts are normally open (NO) and wired in series with the start winding, while the relay coil is wired in series with the main run winding. Upon initial startup, high locked-rotor inrush current through the coil closes the contacts, energizing the start winding. As the motor speeds up, current drops, allowing gravity or a spring to open the contacts.

Contactors and Magnetic Starters

• Contactors: Electromechanical switches rated for heavy inductive loads (20A to 90A+). A 24VAC, 120VAC, or 208/240VAC control coil energizes an electromagnet, pulling down a movable armature to close high-voltage contacts. Contact pitting increases contact resistance; measuring > 0.2V AC drop across closed contactor contacts indicates severe pitting requiring contactor replacement.
• Magnetic Starters: Combine a contactor with a thermal or electronic overload relay assembly. Used primarily on three-phase motors to protect against single-phasing (loss of one power leg), sustained overcurrent, and locked-rotor stalls using bimetallic heater elements or solid-state current sensors.