9.2 Electric Motor Types, Capacitors & Starting Components
Key Takeaways
- Single-phase HVAC motors include Shaded Pole (low starting torque, ~30% efficiency), PSC (medium torque, ~65% efficiency), CSIR (high starting torque with starting relay), CSCR (highest single-phase torque and efficiency), and ECM (brushless DC, up to 85% efficiency across variable static pressures).
- Run capacitors (oil-filled aluminum cans, 1.5 to 80 µF, rated 370V/440VAC) remain continuously in circuit to create an auxiliary winding phase shift; start capacitors (dry electrolytic plastic, 50 to 600+ µF, rated 125V-330VAC with bleed resistors) operate only for 0.5 to 1.0 second during startup.
- Capacitor capacitance tolerance is typically ±5% to ±10% and can be tested under running load using the formula: Capacitance (µF) = (Capacitor Current in Amps * 2652) / Voltage Drop across Capacitor.
- Potential starting relays use Back-EMF (Counter-Electromotive Force) induced in the start winding to energize a high-resistance coil (terminals 2-5) at ~75-80% rotor speed, opening normally closed contacts (terminals 1-2) to drop out the start capacitor.
- Current starting relays utilize a low-resistance, heavy-gauge coil in series with the run winding to pull normally open start contacts closed on high inrush current, opening by spring/gravity as running current drops.
Electric Motor Types, Capacitors & Starting Components
Electric motors convert electrical energy into mechanical rotational power to drive compressors, condenser fans, indoor air handlers, draft inducers, and hydronic circulator pumps. In the HVAC industry, electric motors represent the primary active mechanical devices in both residential and commercial equipment.
For the Kentucky Master HVAC Contractor exam, technicians must understand alternating current (AC) motor physics, single-phase starting methods, capacitor design, and the electromechanical operation of potential and current starting relays.
1. AC Induction Motor Physics & Synchronous Speed
AC induction motors operate on the principle of electromagnetic induction. Alternating current flowing through the stationary stator windings creates a rotating magnetic field. This field induces an electric current in the squirrel-cage rotor bars, creating an opposing magnetic field that forces the rotor to follow the stator's rotating field.
+-----------------------------------------------------------------------------+
| MOTOR SPEED & SLIP FORMULAS |
| |
| Synchronous Speed (RPM) = (120 * Frequency in Hz) / Number of Poles |
| |
| Standard Synchronous Speeds at 60 Hz (North America): |
| - 2-Pole Motor: (120 * 60) / 2 = 3,600 RPM |
| - 4-Pole Motor: (120 * 60) / 4 = 1,800 RPM |
| - 6-Pole Motor: (120 * 60) / 6 = 1,200 RPM |
| - 8-Pole Motor: (120 * 60) / 8 = 900 RPM |
| |
| Rotor Slip: |
| Induction motors must operate at slightly less than synchronous speed |
| to cut magnetic lines of force and generate torque. |
| % Slip = [(Synchronous RPM - Actual RPM) / Synchronous RPM] * 100 |
| Example: A 4-pole motor operating at 1,725 RPM has a slip of: |
| [(1,800 - 1,725) / 1,800] * 100 = 4.17% Slip |
+-----------------------------------------------------------------------------+
2. Single-Phase & Variable-Speed Motor Classifications
Because single-phase alternating current produces a pulsating rather than a rotating magnetic field, single-phase induction motors require auxiliary stator windings and phase-shifting devices (capacitors or shading poles) to establish initial rotational torque.
| Motor Type | Starting Torque | Running Efficiency | Starting Mechanism | Typical HVAC Applications |
|---|---|---|---|---|
| Shaded Pole | Very Low (50%–100% of full load) | Low (30%–35%) | Copper shading coil on stator pole face delays magnetic flux | Combustion draft inducers, small refrigeration reach-in fans, small bath exhausters. |
| Permanent Split Capacitor (PSC) | Moderate (100%–150%) | Medium (60%–70%) | Single run capacitor permanently wired in series with start winding | Residential indoor blower motors, outdoor condenser fan motors. |
| Capacitor Start Induction Run (CSIR) | High (300%–400%) | Moderate (60%–65%) | Start capacitor in series with start winding, disconnected at 75% speed by relay/switch | Commercial refrigeration compressors, belt-drive industrial blowers, farm duty equipment. |
| Capacitor Start Capacitor Run (CSCR) | Very High (350%–450%) | High (70%–75%) | Start capacitor + run capacitor in parallel at start; start cap drops out at 75% speed | High-tonnage residential heat pump compressors, commercial air conditioning compressors. |
| Electronically Commutated Motor (ECM) | Extremely High (Constant programmable torque) | Highest (80%–85%+) | Integrated microprocessor inverter converting AC to pulsed 3-phase DC (Brushless DC) | Variable-speed indoor air handlers, high-efficiency modulating condensing units, ECM circulator pumps. |
| Three-Phase Induction | High (200%–300%) | Very High (85%–95%) | Inherent rotating magnetic field from 3 out-of-phase AC sine waves (no caps/relays) | Commercial chillers, rooftop units (RTUs), large package units (3-phase 208V/480V). |
Electronically Commutated Motors (ECM)
ECMs utilize permanent magnet rotors and stator windings driven by solid-state electronic control modules. Key advantages include:
- Constant CFM Modulating Capability: The onboard microprocessor monitors motor current and back-EMF to detect duct static pressure changes, automatically modulating RPM to maintain design airflow (CFM).
- Energy Efficiency: ECMs consume up to 60% to 75% less electrical power than traditional PSC motors, particularly when operating at continuous low-speed fan circulation.
- Diagnostic Safety: Never test an ECM motor by applying direct 120V/240V power to the low-voltage communication harness. Diagnostics require verifying line voltage at the power plug (120V/240V) and proper DC PWM (pulse-width modulation) or 24VAC tap call signals at the control harness.
3. Dielectric Capacitors: Run vs. Start Engineering
Capacitors store electrical charge and introduce a leading phase shift (up to 90 electrical degrees) between current and voltage in the motor's auxiliary (start) winding relative to the main (run) winding. This phase angle differential creates the synthetic two-phase rotating magnetic field needed for motor operation.
+-----------------------------------------------------------------------------+
| RUN VS. START CAPACITORS |
| |
| RUN CAPACITORS: |
| - Construction: Metal/aluminum oval or round casing, oil-filled dielectric|
| fluid, internal pressure-sensitive interrupter for burst protection. |
| - Capacitance: 1.5 µF to 80 µF (Microfarads). |
| - Duty Rating: CONTINUOUS DUTY (100% duty cycle). |
| - Voltage Rating: 370 VAC or 440 VAC. |
| - Function: Stays in circuit 100% of runtime; improves running efficiency,|
| lowers operational current draw, and improves motor power factor. |
| |
| START CAPACITORS: |
| - Construction: Black phenolic plastic or bakelite round shell, dry |
| electrolytic design (paper soaked in conductive electrolyte). |
| - Capacitance: 50 µF to 600+ µF (High capacitance for starting torque). |
| - Duty Rating: INTERMITTENT DUTY (Maximum 0.5 to 1.0 second per start; |
| rated for no more than 20 starts per hour). |
| - Voltage Rating: 125 VAC, 250 VAC, or 330 VAC. |
| - Safety Feature: Bleed Resistor (15kΩ to 20kΩ, 2W) soldered across |
| terminals to safely bleed residual DC charge and protect relay contacts.|
+-----------------------------------------------------------------------------+
Dual-Section Run Capacitors
Residential split systems commonly utilize a single dual-section run capacitor combining compressor and fan run capacitors in one metal container. Terminals are labeled:
- C (Common): Receives line voltage from the contactor.
- HERM (Hermetic Compressor): Connects to the compressor start winding (e.g., 45 µF).
- FAN (Outdoor Condenser Fan): Connects to the fan motor start winding (e.g., 5 µF).
Capacitor Tolerance & In-Situ Field Testing Formula
Capacitors have a rated tolerance of ±5% to ±10%. On an unenergized bench, a digital multimeter with capacitance function measures microfarads (µF). While the motor is actively running under full load, capacitance can be verified with precision using voltage and clamp-meter current:
+-----------------------------------------------------------------------------+
| RUNNING CAPACITANCE CALCULATION FORMULA |
| |
| Capacitance (µF) = (Auxiliary Winding Amps * 2,652) / Capacitor Volts |
| |
| Field Procedure: |
| 1. Measure AC current (Amps) on the wire connected to the HERM or FAN |
| terminal using a clamp-on ammeter. |
| 2. Measure AC voltage across the capacitor terminals (between C and HERM).|
| 3. Multiply measured current by constant 2,652, then divide by voltage. |
| |
| Example Calculation: |
| - Measured Start Winding Current = 5.2 Amps |
| - Measured Voltage across C to HERM = 310 VAC |
| - Capacitance = (5.2 * 2,652) / 310 = 13,790.4 / 310 = 44.48 µF |
| - On a 45 µF rated capacitor, 44.48 µF is within ±5% tolerance (42.75–47.25 µF)|
+-----------------------------------------------------------------------------+
4. Motor Starting Relays & Hard Start Kits
When a high-torque single-phase compressor starts against unbalanced refrigerant head pressure (such as after a brief power interruption or on non-bleed TXV systems), a start capacitor must be introduced into the circuit at the moment of start and rapidly disconnected as the motor approaches running speed.
+-----------------------------------------------------------------------------+
| POTENTIAL STARTING RELAY ARCHITECTURE |
| |
| Terminal 5 --------[ HIGH-RESISTANCE RELAY COIL ]-------- Terminal 2 |
| | |
| Terminal 1 --------[ NORMALLY CLOSED (NC) CONTACTS ]----------+ |
| |
| Wiring Connections: |
| - Terminal 1: Connects to Start Capacitor output. |
| - Terminal 2: Connects to Compressor Start (S) terminal & Run Capacitor. |
| - Terminal 5: Connects to Compressor Common (C) / Line Neutral. |
| |
| Operating Sequence: |
| 1. At Standstill: Contacts 1-2 are CLOSED. Start capacitor is in parallel |
| with run capacitor across start winding. |
| 2. Power Applied: High starting torque is generated; rotor accelerates. |
| 3. Back-EMF Generation: As rotor speeds up, the auxiliary winding acts as |
| a generator, producing Back-EMF (CEMF) voltage across Start & Common. |
| 4. Coil Pickup: At ~75% to 80% speed, Back-EMF reaches relay pickup |
| voltage (300V–400V). Coil 2-5 energizes, snapping contacts 1-2 OPEN. |
| 5. Continuous Operation: High Back-EMF keeps coil energized and contacts |
| 1-2 open until line power is removed. |
+-----------------------------------------------------------------------------+
Potential Relay vs. Current Relay vs. PTC Thermistor
| Starting Device | Operating Physical Mechanism | Contact State at Rest | Coil Location | Application Domain |
|---|---|---|---|---|
| Potential Relay | Back-EMF (CEMF) generated across start winding proportional to rotor speed | Normally Closed (NC) (opens at 75%–80% speed) | High resistance (thousands of ohms) across Start & Common (Terminals 2 to 5) | Commercial refrigeration, heat pump compressors, high-tonnage AC (CSCR systems). |
| Current Relay | Inrush Current flowing through run winding | Normally Open (NO) (closes instantly on high LRA, drops open as current falls) | Low resistance (heavy copper wire) in series with Run winding | Fractional-HP refrigeration compressors (1/12 HP to 1/3 HP refrigerators/freezers). |
| PTC Thermistor | Joule Heating increases internal resistance from <50Ω to >10,000Ω in milliseconds | Solid-state ceramic (no mechanical contacts) | In series with auxiliary start circuit | Economy hard start kits for residential air conditioners. |
What electrical physical phenomenon energizes the coil of a potential starting relay to open its normally closed contacts and remove the start capacitor from the circuit as the motor reaches operating speed?
During a routine maintenance diagnostic check on an operating compressor, a technician measures 4.5 Amps of AC current on the start winding conductor and 280 VAC across the run capacitor terminals (C to HERM). What is the calculated operating capacitance under load?
Which of the following single-phase electric motor topologies provides the highest running efficiency and starting torque while dynamically modulating airflow (CFM) across variable duct static pressures?