Single-Phase Motor Wiring Diagram: Capacitor Start and Run

A three-phase induction motor starts itself -- the rotating magnetic field from three phases produces starting torque automatically. A single-phase motor does not. With only one AC phase, the stator field pulsates back and forth along one axis, creating no net torque on a stationary rotor. Single-phase motors solve this with an auxiliary start winding and, in most designs, a capacitor that phase-shifts the current in that winding to create a rotating field strong enough to start the rotor spinning. Once it reaches about 75% of synchronous speed, a centrifugal switch disconnects the start circuit.

This guide covers how to wire the four main single-phase motor types: capacitor-start (CS), capacitor-run (CR), capacitor-start/capacitor-run (CSCR), and permanent split-capacitor (PSC).

Winding Basics

Every single-phase induction motor has two sets of stator windings:

The auxiliary winding must carry current that is phase-shifted relative to the main winding. The phase shift creates a rotating magnetic field, which produces starting torque. Once the rotor is up to speed, the field set up by the rotor's rotation helps maintain it, so the auxiliary winding can be removed from the circuit.

Motor terminal markings follow NEMA convention:

Always check the nameplate diagram before wiring -- terminal assignments vary between manufacturers.

Capacitor-Start (CS) Motor

The capacitor-start motor uses a large electrolytic capacitor (typically 75--600 µF) in series with the start winding. Electrolytic capacitors cannot handle continuous AC -- they are rated for short-term intermittent duty only, typically a 3-second maximum start time.

A centrifugal switch mounted on the rotor shaft disconnects the start winding and capacitor once the motor reaches approximately 75% synchronous speed.

Wiring

L1 ─────────────────────────────── T1 (Main winding)
                                   T2 (Main winding) ─── L2
L1 ──── Centrifugal switch ──── Capacitor ──── T3 (Start winding)
                                   T4 (Start winding) ─── L2

In practice on a terminal block:

  1. Connect line voltage L1 to T1 and to one side of the centrifugal switch circuit.
  2. Connect line voltage L2 to T2 and to T4.
  3. The centrifugal switch connects in series with the start capacitor and T3. When the motor reaches speed, the switch opens, disconnecting the capacitor and start winding.

Starting torque: high (150--350% of full-load torque). Common applications: compressors, pumps, machine tools.

Capacitor-Run (CR) Motor

The capacitor-run motor uses a smaller oil-filled (film) capacitor (typically 2--40 µF) that stays in the circuit permanently. Film capacitors handle continuous AC duty. The result is improved power factor and quieter operation, but lower starting torque than a CS motor.

No centrifugal switch is present.

Wiring

L1 ─────────── T1 (Main winding) ─── T2 ─── L2
L1 ─── Capacitor (oil-filled) ─── T3 (Start/Run aux) ─── T4 ─── L2

Applications: fans, blowers, small pumps where starting load is light.

Capacitor-Start/Capacitor-Run (CSCR) Motor

The CSCR motor combines both approaches -- a large start capacitor for high starting torque, plus a small run capacitor that stays connected for improved running efficiency. The start capacitor connects in parallel with the run capacitor during starting, then the centrifugal switch removes the start cap.

Wiring

L1 ─────────────────────────────────────── T1 (Main)
T2 ─── L2

L1 ─── Run cap (oil-filled) ─── T3 (Aux)
             └── Centrifugal switch ─── Start cap (electrolytic) ─┘

T4 ─── L2

During starting: both capacitors in parallel supply a high phase-shift capacitance. After reaching speed: centrifugal switch opens, only the run cap remains in circuit.

Applications: air compressors, large pumps, woodworking machinery.

Permanent Split-Capacitor (PSC) Motor

PSC motors use a single oil-filled run capacitor with no centrifugal switch -- the auxiliary winding and capacitor remain in the circuit permanently. Starting torque is modest (50--100% of full-load torque), but PSC motors are extremely reliable because there is no switch to fail.

Wiring

L1 ─── T1 (Main winding) ─── T2 ─── L2
L1 ─── Capacitor ─── T3 (Aux winding) ─── T4 ─── L2

Applications: HVAC fan motors, refrigerator condenser fans, multi-speed fans. The PSC motor's speed can be varied by switching different taps on the main winding -- standard on ceiling fan and furnace blower motors.

Reversing a Single-Phase Motor

Reversing a capacitor motor requires swapping the auxiliary winding's relationship to the main winding. This is done by reversing the connections to either the main winding or the auxiliary winding -- not both.

On a dual-voltage motor with accessible auxiliary winding terminals:

  1. Swap T5 and T6 (or equivalent auxiliary winding leads) while leaving T1/T2 connections unchanged.
  2. OR: Swap T1 and T2 while leaving the auxiliary winding unchanged.

Some motors have a reverse-terminal arrangement built into the connection diagram on the nameplate. Check before rewiring.

Not all single-phase motors are reversible. Shaded-pole motors, for example, have built-in asymmetry that makes reversal impossible without disassembly.

Capacitor Specifications

Using the wrong capacitor damages the winding and the capacitor. Key specs:

Safety Note

Single-phase motors connected to 120 V or 240 V mains carry potentially lethal voltage. Always disconnect and lock out the power supply before working on the motor terminals or capacitor. Start capacitors store charge and can deliver a painful shock even with power off -- discharge them through a 20 kΩ, 5 W resistor across the terminals before touching. Run capacitors hold less energy but should still be discharged as a precaution.

You can draw and verify the wiring configuration in CircuitDiagramMaker before you connect anything to mains power -- it takes five minutes and eliminates most terminal mistakes.

Common Wiring Faults

Start capacitor wired in permanently: The electrolytic cap overheats within seconds and fails, sometimes violently. Verify that the centrifugal switch is working and wired in series with the start cap.

Wrong capacitor polarity: AC electrolytic start capacitors are non-polarized -- they have no polarity requirement. If you received a polarized DC electrolytic capacitor by mistake, do not use it in this circuit.

Centrifugal switch not opening: The motor hums at full speed and the start winding overheats. Inspect the switch contacts and the centrifugal weights.

Both windings on the same phase, no cap: The motor hums but does not start -- the rotating field is absent.

Create Your Own Single-Phase Motor Wiring Diagram

CircuitDiagramMaker lets you draw and compare wiring configurations side by side:

Create your own single-phase motor wiring diagram -- free

Key Takeaways