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Unit 4 — Electrical Fundamentals
Section 2 — Introduction to Motors

2.2 — Single-Phase Motors

Single-phase motors power most residential and light-commercial HVAC/R equipment. This lesson covers motor construction, why single-phase motors cannot self-start, and the five motor types used to solve that problem.

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2.2.1 — Motor Construction

All single-phase induction motors share the same fundamental mechanical structure. Understanding the role of each component is essential for diagnosis — a symptom like excessive noise, locked shaft, or failure to run always traces back to a specific part.

End Bell
Motor Cross-Section (simplified)
STATOR — Stationary Windings & Laminated Steel Core
Air Gap
ROTOR — Squirrel-Cage Conductors or Bars
Shaft
End Bell
Stator (Stationary) Laminated steel core with insulated copper windings. Current through the windings creates the magnetic field that drives the rotor.
Rotor (Rotating) Sits inside the stator. In squirrel-cage induction motors, aluminum or copper bars embedded in a laminated core carry induced currents that produce torque.
Bearings Support the rotor shaft and allow smooth, low-friction rotation. Either sleeve (oil-lubricated) or ball/roller types. Most common failure point in aging motors.
Frame / Housing Provides structural support and protection for internal components. Defines the enclosure type (ODP, TEFC, etc.) and mounts to the driven equipment.
End Bells Close the motor housing at each end. Provide mounting points for bearings and, where used, the centrifugal starting switch mechanism.
Shaft Extends beyond the housing to transfer mechanical energy to the load via couplings, pulleys, belts, or direct mounting to a fan blade or pump impeller.

Synchronous Speed & Slip

Synchronous speed is the speed of the rotating magnetic field, determined by the supply frequency and the number of magnetic poles. The actual rotor speed is always slightly lower due to slip — the difference required to induce current in the rotor bars and produce torque. Without slip, there is no induced current, no torque, and no rotation.

Ns = (120 × f) ÷ P

Ns = synchronous speed (RPM)  |  f = frequency (Hz)  |  P = number of poles

2-Pole
3 600
Sync RPM @ 60 Hz
4-Pole
1 800
Sync RPM @ 60 Hz
6-Pole
1 200
Sync RPM @ 60 Hz
8-Pole
900
Sync RPM @ 60 Hz

Typical full-load slip is 3–5%. A 4-pole motor has a synchronous speed of 1 800 RPM but a nameplate speed of approximately 1 725 RPM. Under lighter loads, actual speed rises slightly toward synchronous speed but never reaches it.

2.2.2 — The Single-Phase Starting Problem

Three-phase motors self-start because three voltages displaced 120° apart naturally create a rotating magnetic field. Single-phase motors do not have this advantage and require a special starting mechanism.

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Why a Single-Phase Supply Cannot Self-Start a Motor

A single-phase AC supply produces a pulsating magnetic field that alternates in polarity but does not rotate. Physically, this pulsating field can be decomposed into two counter-rotating fields of equal magnitude spinning in opposite directions.

When the rotor is stationary, these two counter-rotating fields produce equal and opposite torques on the rotor. The net torque is zero — the motor will not start on its own.

However, if the rotor is given an initial spin in either direction, one field component slips backward relative to the rotor faster than the other, the torques become unequal, and the motor accelerates in the direction it was spun. This proves that self-starting requires creating an initial phase shift between two magnetic fields — which is exactly what every starting method achieves.

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The 75% rule — when starting circuits disconnect

All starting mechanisms (except PSC and shaded-pole) are only needed until the motor reaches approximately 75% of synchronous speed (around 1 300–1 400 RPM for a 1 725 RPM motor). At that point, a centrifugal switch or relay disconnects the start winding and/or start capacitor. The motor then runs on the main winding alone.

Starting Current (LRA)

At the instant of start-up, with the rotor stationary, the motor draws Locked Rotor Amperage (LRA) — typically 5–8× FLA for single-phase motors. This inrush lasts a fraction of a second but must be considered when sizing:

2.2.3 — Split-Phase Motor (Resistance-Start)

The split-phase motor is the simplest and most economical single-phase motor design. It “splits” the single-phase supply into two currents with a small phase difference using winding resistance alone — no capacitor required.

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Split-Phase — Resistance-Start
Basic Design
How it Works

Two stator windings are energised simultaneously at start-up:

  • Main (run) winding: heavy wire, low resistance, high inductance
  • Auxiliary (start) winding: lighter wire, higher resistance, lower inductance

The resistance difference causes start-winding current to lead run-winding current by about 30–40 electrical degrees, creating a weak rotating field and enough starting torque for low-inertia loads.

A centrifugal switch on the rotor shaft disconnects the start winding at approximately 75% of synchronous speed. The motor continues on the run winding alone.

Performance Profile
  • Starting Torque (75–175% FLT)
    Low
  • Starting Current (6–8× FLA)
    Moderate–High
  • Running Efficiency
    Moderate
  • Cost
    Low

Max HP: Typically ⅓ HP (250 W) for this type

Key component: Centrifugal switch — most common failure point; contacts can weld closed (keeps start winding energised → overheats & fails) or fail open (motor hums but won’t start)

Typical Applications: Small fans Blowers Centrifugal pumps Light-duty machinery

2.2.4 — Capacitor-Start (CS) Motor

The capacitor-start motor improves on the split-phase design by adding an electrolytic capacitor in series with the start winding. This increases the phase shift between run and start winding currents from ~35° to nearly 90°, producing dramatically higher starting torque.

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Capacitor-Start (CS)
High Start Torque
How it Works

A large electrolytic start capacitor (75–1 200 µF, intermittent duty) is wired in series with the start winding. The capacitor causes start-winding current to lead run-winding current by nearly 90°, creating a strong rotating field and high starting torque.

A centrifugal switch or relay disconnects both the start winding and capacitor when the motor reaches ~75% speed. The motor then runs on the main winding alone (same as split-phase at run).

⚠️
Start capacitor — intermittent duty only

Designed for 1–3 seconds of use. If the starting switch fails open and leaves the capacitor in circuit, it will overheat and fail within seconds.

Performance Profile
  • Starting Torque (200–350% FLT)
    High
  • Starting Current (5–9× FLA)
    Moderate–High
  • Running Efficiency
    Moderate (run winding only)
  • Size Range
    ⅛–5 HP (100 W–3.7 kW)
Typical Applications: Compressors Air compressors Pumps with check valves Conveyors Reciprocating equipment
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Diagnosing a failed start capacitor

Shorted capacitor: Motor fails to start; fuse or breaker trips immediately. Open capacitor: Motor hums but won’t start (or starts only with a manual spin) — reduced torque similar to a split-phase motor. Always discharge capacitors before testing or handling them.

2.2.5 — CSR & PSC Motors

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Capacitor-Start Capacitor-Run (CSR)
Best Overall Performance
How it Works

Uses two capacitors to optimise both starting and running:

  • Start capacitor (large, electrolytic, intermittent duty): wired in parallel with the run capacitor during starting for maximum starting torque
  • Run capacitor (small, oil-filled, continuous duty, 2–50 µF): remains in circuit with the auxiliary winding throughout operation

At ~75% speed, a centrifugal switch or relay disconnects the start capacitor, leaving the run capacitor permanently in the auxiliary winding circuit. The run capacitor maintains a near-rotating field during operation, improving efficiency, power factor, and smoothing torque output.

Performance Profile
  • Starting Torque (200–350% FLT)
    High
  • Running Efficiency
    High (both windings energised)
  • Power Factor
    Improved
  • Cost
    Moderate–High (two capacitors)
Typical Applications: Heavy-duty compressors Large air handlers Demanding pumps Industrial applications
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Permanent Split Capacitor (PSC)
Most Common in HVAC/R
How it Works

A single run capacitor is permanently wired in series with the auxiliary winding. There is no centrifugal switch, no starting relay, and no start capacitor — both windings and the capacitor remain energised during both starting and running.

The capacitor value is a compromise between starting and running performance, resulting in lower starting torque than CS or CSR motors but excellent running characteristics, quiet operation, and simplified, highly reliable construction.

PSC motors are easily adapted for multi-speed operation by tapping the main winding at different points or by varying supply voltage through a solid-state speed controller.

Performance Profile
  • Starting Torque (50–150% FLT)
    Low–Moderate
  • Starting Current (3–5× FLA)
    Low
  • Running Efficiency
    Excellent
  • Reliability
    Very High (no switch)

Multi-speed: 2–5 discrete speeds (or continuous control) via tapped windings or solid-state controller — standard for direct-drive fan motors

Typical Applications: Direct-drive fans & blowers Indoor fan motors Condenser fans Low-torque pumps Air handlers
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Diagnosing a failed PSC run capacitor

A failed run capacitor causes the motor to draw excessive current, run hot, lose torque, and often hum or vibrate. Unlike a failed start capacitor (which prevents starting entirely), a weak or failed run capacitor allows the motor to start and run, but inefficiently. Measure capacitance with a capacitor tester — a value more than ±6% from the nameplate rating indicates replacement is required.

2.2.6 — Shaded-Pole Motor

The shaded-pole motor is the simplest single-phase motor design. There is no start winding, no capacitor, and no switch — only a main winding and copper shading coils (short-circuited copper bands) wrapped around a portion of each stator pole.

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Shaded-Pole Motor
Simplest Design
How it Works

When the stator magnetic field builds, it induces a current in the shading coil. Per Lenz’s law, this induced current opposes the change, delaying the magnetic flux in the shaded portion of the pole relative to the unshaded portion.

The result is a sweeping effect of the magnetic field from the unshaded to the shaded side of each pole, producing a weak rotating field and low starting torque.

The shading coils continuously dissipate energy as heat because they remain in circuit at all times, making shaded-pole motors the least efficient of all single-phase types (15–35%).

Rotation direction is always from the unshaded to the shaded pole and cannot be reversed in the field without physically swapping the stator.

Performance Profile
  • Starting Torque (<100% FLT)
    Very Low
  • Efficiency (15–35%)
    Very Low
  • Reliability
    Very High (no moving parts except rotor)
  • Cost
    Very Low

Max HP: Typically under ¼ HP (185 W)

Fixed rotation: Cannot be reversed in the field

Typical Applications: Small exhaust fans Appliance fans Timers Small pumps Evaporator fans (small units)

2.2.7 — Motor Type Comparison

Use this table when selecting or identifying a motor type from its nameplate and circuit components. The starting mechanism column is the fastest way to identify the motor type during service.

Type Starting Mechanism Start Torque Run Efficiency Typical HP HVAC/R Use
Split-Phase Centrifugal switch only Low Moderate Under ⅓ HP Small fans, light loads
Capacitor-Start (CS) Start capacitor + centrifugal switch High Moderate ⅛–5 HP Compressors, pumps
CSR Start + run capacitor + centrifugal switch High High ¼–5 HP Heavy compressors, air handlers
PSC Run capacitor only (no switch) Moderate Excellent ⅙–1 HP+ Direct-drive fans, condenser fans
Shaded-Pole Copper shading coils (no switch, no cap) Very Low Very Low Under ¼ HP Small exhaust fans, appliances
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Rotation direction — a critical check during replacement

When replacing a single-phase motor, verify the rotation direction before commissioning. Compressors and pumps operated in reverse can be damaged within seconds. PSC and capacitor-start motors can often be reversed by swapping two leads on the auxiliary winding — but always verify with the replacement motor’s wiring diagram. Shaded-pole motors cannot be reversed in the field.

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