Level 1, Unit 4, Section 2 – 2.3 Single-Phase Motor Operation

2.3.1 — Electromagnetic Induction — From Voltage to Torque

Every induction motor converts electrical energy into mechanical rotation through a six-step chain of electromagnetic events. Each step follows directly from the previous one — understanding this chain is the key to diagnosing motor faults, because a fault anywhere in the chain produces a predictable and traceable symptom.

1

AC Current in Stator Windings

Alternating current flows through the insulated copper windings wound around the laminated steel stator core. The laminations reduce eddy current losses that would otherwise convert magnetic energy to heat.

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2

Alternating Magnetic Field Created

Current through the windings produces a magnetic field in the stator core (Ampère’s Law). Because the current alternates at 60 Hz, the field also alternates — reversing polarity 120 times per second.

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3

EMF Induced in Rotor Conductors

The changing stator field cuts through the rotor bars (Faraday’s Law). A voltage (EMF) is induced in each bar proportional to the rate of change of the magnetic flux — exactly as in the secondary of a transformer. This is why induction motors are sometimes called “rotating transformers.”

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4

Rotor Current Flows

Because the rotor bars are short-circuited at both ends by the end rings of the squirrel cage, the induced EMF drives current through each bar. The magnitude of this current is determined by the induced voltage and the rotor’s resistance and reactance.

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5

Rotor Magnetic Field Created

The rotor current produces its own magnetic field around each bar. By Lenz’s Law, this rotor field opposes the change that created it — it tries to follow the stator field, always lagging behind it by the slip angle.

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6

Interaction of Fields Produces Torque

The rotor field interacts with the stator field (Fleming’s Left-Hand Rule). A force acts on each current-carrying rotor bar, pushing it perpendicular to both the bar and the magnetic field. The sum of all these forces across all bars produces the rotational torque that turns the shaft and drives the load.

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Why slip is essential — not a defect

Slip is the speed difference between the rotating stator field and the rotor. If the rotor ever reached synchronous speed, there would be no relative motion, no changing flux through the rotor bars, no induced EMF, no rotor current, no rotor field, and no torque. The motor would immediately decelerate back into a slip condition. Slip is therefore not a sign of inefficiency — it is the physical mechanism by which the motor produces torque.

Slip and Speed

% Slip = [(N_s − N_r) ÷ N_s] × 100

N_s = synchronous speed (RPM) | N_r = actual rotor speed (RPM)

A 4-pole, 60 Hz motor has N_s = 1 800 RPM. At full load with a nameplate speed of 1 725 RPM:

% Slip = [(1 800 − 1 725) ÷ 1 800] × 100 = 4.2%

Typical full-load slip is 3–5% for induction motors. Slip increases as load increases (more torque needed → more rotor current needed → more slip required to maintain the induced EMF). Under light loads, slip decreases and the motor runs closer to synchronous speed.

2.3.2 — Creating a Rotating Magnetic Field

A rotating magnetic field is what actually drives the rotor. A single-phase AC supply produces a pulsating field, not a rotating one — this is the fundamental starting problem. To create rotation, two magnetic fields must be displaced both in space (physically offset in the stator) and in time (their currents reach peak values at different instants).

Single-Phase: Pulsating Field

One winding produces one axis of magnetic flux that alternates between north and south at 60 Hz. It has no preferred rotational direction. The rotor experiences equal torque in both directions — net torque at standstill is zero.

The pulsating field can be mathematically split into two equal counter-rotating components. At rest they cancel. Once the rotor is spinning, one component dominates and sustains rotation.

Two-Phase Equivalent: Rotating Field

If a second winding is placed at 90° spatial offset in the stator, and its current is displaced 90° in time (phase), the two fields combine into a resultant that sweeps continuously around the stator bore.

All single-phase motor starting methods approximate this two-phase effect by using resistance, capacitance, or shading coils to produce a time-displaced current in an auxiliary winding.

Rotating Field — Four Snapshots

Each diagram below shows the direction of the resultant magnetic field at 90° intervals through one AC cycle. The field sweeps one full revolution per cycle.

0° (t = 0)

N S

Field points upward. Winding A at peak; Winding B at zero.

90° (t = T/4)

N S

Field points right. Winding B at peak; Winding A at zero.

180° (t = T/2)

N S

Field points downward. Winding A reversed; Winding B at zero.

270° (t = 3T/4)

N S

Field points left. Winding B reversed; Winding A at zero.

How Each Motor Type Achieves the Phase Shift

Motor Type	Method of Time Displacement	Phase Shift Achieved	Resulting Starting Torque
Split-Phase	Higher resistance in start winding (lighter wire) shifts its current relative to the run winding	~30–40°	Low (75–175% FLT)
Capacitor-Start	Series capacitor in start winding causes current to lead voltage, increasing phase separation	~80–90°	High (200–350% FLT)
CSR	Start cap (large) for starting + run cap (small) maintained during running	~80–90° start; ~30–40° run	High start; efficient run
PSC	Permanent run capacitor in auxiliary winding — same value for start and run	~30–60° (compromised)	Moderate (50–150% FLT)
Shaded-Pole	Copper shading coil delays flux in shaded portion of pole by Lenz’s Law	~40–50° (spatial + time)	Very Low (<100% FLT)

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The 90° ideal — why capacitors work best

Maximum starting torque is produced when the two winding currents are exactly 90° apart in time AND the windings are exactly 90° apart in space. A correctly sized capacitor in the start winding achieves close to the ideal 90° time displacement, which is why capacitor-start motors produce roughly twice the starting torque of split-phase motors despite similar physical size.

2.3.3 — Winding Circuits — What’s in Each Motor Type

Before tracing a starting circuit in the field, you need to know which components are present in each motor type. Identifying the components on the motor itself (external capacitor, internal switch, terminal markings) tells you which type you are dealing with.

Always energised Main Winding 〰️ Run Winding Heavy wire, low resistance, high inductance. Carries current during both starting and running in all motor types.

Start only (CS/SP) Auxiliary Winding 〰️ Start Winding Lighter wire, higher resistance. Connected only during starting in split-phase and CS motors; stays in permanently in PSC/CSR.

See motor type Capacitor(s) 🔋 Start / Run Cap Start cap: large, electrolytic, intermittent duty. Run cap: small, oil-filled, continuous duty. Present in CS, CSR, and PSC motors.

CS / Split-Phase Disconnect Device 🔘 Centrifugal Switch / Relay Opens the start circuit at ~75% synchronous speed. Centrifugal switch is internal; starting relays are external and used for hermetic compressors.

Always energised Shaded-Pole Only 🔗 Shading Coil Short-circuited copper band around part of each stator pole. Creates the phase delay that produces the weak starting torque. No external connections.

Optional Protection 🌡️ Thermal Protector Bimetallic or PTC device embedded in the windings. Opens the circuit on overtemperature; resets automatically or manually depending on type.

Component State — Starting vs. Running

Motor Type	Run Winding (Starting)	Aux Winding (Starting)	Capacitor (Starting)	Run Winding (Running)	Aux Winding (Running)	Capacitor (Running)
Split-Phase	✓ ON	✓ ON	— None	✓ ON	✗ OFF	— None
Cap-Start (CS)	✓ ON	✓ ON	✓ Start cap in circuit	✓ ON	✗ OFF	✗ Disconnected
CSR	✓ ON	✓ ON	✓ Start + run cap parallel	✓ ON	✓ ON	✓ Run cap only
PSC	✓ ON	✓ ON	✓ Run cap in circuit	✓ ON	✓ ON	✓ Run cap in circuit
Shaded-Pole	✓ ON	— Shading coil only	— None	✓ ON	— Shading coil only	— None

2.3.4 — The Starting Sequence — Step by Step

The following sequence describes a capacitor-start (CS) motor — the most common type in HVAC/R compressor and pump applications. The same sequence applies to split-phase motors (omit the capacitor) and CSR motors (add the run capacitor remaining after step 5). PSC motors follow steps 1–2 and 6 only — no disconnect event occurs.

1

t = 0 — Power Applied

Both Windings Energised Simultaneously

Line voltage is applied to the run winding directly and to the auxiliary winding through the start capacitor and the closed centrifugal switch contacts. The capacitor creates a phase-shifted current in the auxiliary winding.

Current = LRA (5–8× FLA)

2

t ≈ 0–50 ms — Initial Torque

Rotating Field Established — Rotor Begins to Move

The two winding currents (near 90° apart) create a rotating magnetic field. Torque is produced and the rotor begins accelerating from rest. Starting torque is at its highest value (locked-rotor torque).

Torque ≈ 200–350% of FLT

3

t ≈ 50–300 ms — Acceleration

Motor Accelerates Along the Torque-Speed Curve

The rotor accelerates rapidly. As speed rises, the frequency of the induced rotor currents decreases (lower slip frequency), rotor reactance drops, and rotor current increases efficiency. Torque may dip through the “pull-up torque” valley before rising to breakdown torque. Current begins to fall from LRA toward FLA.

Current decreasing from LRA toward FLA

4

t ≈ 300–500 ms — Cutout Speed

75% Synchronous Speed Reached — Switch Opens

The centrifugal switch weights fly outward under centrifugal force and open the contact, disconnecting both the start winding and the start capacitor from the circuit. For hermetic compressors, a current relay or PTC relay performs this function externally instead of an internal centrifugal switch.

~1 350 RPM for a 1 800 RPM synchronous motor

5

t ≈ 500–800 ms — Post-Cutout Surge

Brief Torque Dip Then Rapid Acceleration to Full Speed

The sudden loss of the auxiliary winding causes a momentary torque reduction. The motor continues to accelerate on the run winding alone, quickly approaching full-load speed. Total elapsed time from energisation to steady-state is typically under one second for motors up to 5 HP.

Current settling toward FLA

6

t > 1 s — Steady State

Motor Running at Full-Load Speed

The motor runs on the main winding only (CS and split-phase) or with the run capacitor in the auxiliary winding (CSR, PSC). Slip settles at the value required to produce the torque demanded by the load. Current equals FLA for rated load. Any increase in load increases slip, increases rotor current, and draws more current from the supply.

Current = FLA | Speed = ~1 725 RPM (4-pole)

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Failed-to-start vs. failed-to-run — different causes

A motor that hums but won’t start has a starting circuit fault (open start capacitor, failed centrifugal switch that won’t close, open auxiliary winding, or severely low supply voltage). A motor that starts but trips on overload has a running circuit problem (high load, low voltage, open run capacitor, worn bearings, or restriction in the driven equipment). The starting sequence makes these two categories easy to separate.

2.3.5 — Torque-Speed Characteristics

The torque-speed curve plots how much torque a motor produces at each shaft speed from standstill to synchronous speed. Four critical points on this curve are labelled on every motor data sheet and are essential for matching a motor to its load.

Torque-Speed Curve — Capacitor-Start Motor (relative to FLT)

~175%

Locked Rotor
Torque
(0 RPM)

~130%

Pull-Up
Torque
(~30% Ns)

~275%

Breakdown
Torque
(~75% Ns)

100%

Full-Load
Torque
(~96% Ns)

0%

No-Load
Torque
(~99% Ns)

LRT — Locked Rotor

PUT — Pull-Up

BDT — Breakdown

FLT — Full-Load

No-Load

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Locked Rotor Torque (LRT)

Torque developed at the instant of start-up with the shaft held stationary. This must exceed the static friction and initial load torque or the motor will not start. Typically 75–350% of FLT depending on motor type.

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Pull-Up Torque (PUT)

The minimum torque produced during acceleration to full speed. If the load torque exceeds the pull-up torque at any point during acceleration, the motor will stall at that speed rather than continuing to accelerate.

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Breakdown Torque (BDT)

The maximum torque the motor can produce at any speed. If load torque momentarily exceeds breakdown torque, the motor stalls. The ratio BDT/FLT is the motor’s overload capacity margin.

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Full-Load Torque (FLT)

The torque produced at the nameplate horsepower and speed. This is the normal continuous operating point. The motor runs stably at any point between no-load speed and the breakdown torque limit.

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Load curve intersection — the operating point

The actual operating speed is where the motor’s torque-speed curve intersects the load’s torque-speed curve. A fan load increases as approximately the square of speed (low torque demand at low speed, high torque at full speed) — this makes fans easy to start. A compressor with a loaded cylinder presents high torque demand immediately at start-up — requiring a high LRT motor such as capacitor-start or CSR.

2.3.6 — Normal Running Operation

Once the starting circuit disconnects and the motor reaches steady-state, three interrelated quantities govern normal operation: slip, current, and temperature. All three respond to changes in load, supply voltage, and ambient conditions.

Effect of Load Changes on Running Parameters

Parameter	Load Increases	Load Decreases	Why
Shaft Speed	▼ Decreases slightly	▲ Increases slightly	More torque needed → more slip → lower rotor speed
Slip	▲ Increases	▼ Decreases	Higher slip required to induce more rotor current and produce more torque
Rotor Current	▲ Increases	▼ Decreases	More torque = more rotor field = more induced rotor current required
Supply Current (FLA)	▲ Increases	▼ Decreases	Stator must supply more real power to match the increased mechanical output
Winding Temperature	▲ Increases	▼ Decreases	Higher current = more I²R losses = more heat generated in windings
Power Factor	▲ Improves	▼ Worsens	Under light load, reactive (magnetising) current dominates; under heavy load, real power component dominates

Heat Generation During Running

The gap between electrical input power and mechanical output power appears as heat. Minimising these losses is the goal of premium-efficiency motor design.

Approximate Loss Distribution — Standard Induction Motor at Full Load

Stator I²R (copper losses)

~35%

Rotor I²R (slip losses)

~22%

Core losses (hysteresis & eddy)

~20%

Friction & windage

~13%

Stray load losses

~10%

Thermal Protection During Running

Built-In Thermal Protector

Bimetallic disc embedded in or on windings
Opens circuit when winding temperature exceeds limit
Automatic reset: resets when cooled (risk of repeated cycling if overload condition persists)
Manual reset: requires technician action before restart
Identified by a small button on the motor end bell

External Overload Relay

Sized to motor FLA, not motor HP
Heater element calibrated to trip at 125% of FLA (typical)
Trip time is intentionally delayed to ride through starting inrush
Protects against sustained overload, not instantaneous short-circuit
Must be reset manually after tripping — then investigate cause

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Never restart a tripped motor without investigating first

An overload trip means the motor reached a temperature that would damage insulation if sustained. Resetting and restarting without finding the cause — excessive load, blocked airflow, low voltage, single-phasing on a three-phase motor, or a failing start component on a single-phase motor — will repeat the thermal event and shorten motor life. Each 10°C above rated temperature approximately halves insulation life.

Power Factor During Running

Induction motors are inductive loads. They draw both real power (to drive the load) and reactive power (to maintain the magnetic field). The ratio of real to apparent power is the power factor.

At Full Load

Power factor typically 0.75–0.90
Real power component is large relative to reactive
Most efficient operating point for the motor and supply circuit
Run capacitors in PSC and CSR motors improve PF further

At Light Load or No Load

Power factor drops to 0.20–0.50
Motor still draws significant reactive (magnetising) current
Apparent current (kVA) greatly exceeds real power (kW)
Oversized motors left lightly loaded are poor for power factor

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Right-sizing motors for efficiency and power factor

A motor running continuously at 40–50% of its rated load draws poor power factor and wastes energy magnetising a larger core than necessary. Replacing an oversized motor with a correctly sized one reduces both energy waste and operating current, benefiting the electrical service and the building’s power factor penalty charges.