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:
- Main winding (run winding): Heavier wire, fewer turns, lower resistance. Remains energized whenever the motor is running.
- Auxiliary winding (start winding): Lighter wire, more turns, higher resistance. Physically displaced 90 electrical degrees from the main winding.
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:
- T1, T2: Main winding (line terminals)
- T3, T4: (or T5, T6 on dual-voltage motors) Auxiliary winding terminals
- Older motors may use M1/M2 for main and A1/A2 or S1/S2 for auxiliary/start.
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:
- Connect line voltage L1 to T1 and to one side of the centrifugal switch circuit.
- Connect line voltage L2 to T2 and to T4.
- 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:
- Swap T5 and T6 (or equivalent auxiliary winding leads) while leaving T1/T2 connections unchanged.
- 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:
- Capacitance (µF): Must match nameplate rating. Incorrect capacitance causes overheating of the winding and reduced torque.
- Voltage rating: Oil-filled run capacitors are typically rated 370 V AC or 440 V AC. Use 370 V AC for 120 V/240 V applications; 440 V AC is acceptable as an upgrade. Never use a lower voltage rating than specified.
- Type: Start capacitors are electrolytic (AC-rated, intermittent duty). Run capacitors are oil-filled film (continuous duty). Do not substitute one for the other.
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:
- Draw CS, CR, CSCR, and PSC motor connections with correct capacitor symbols
- Show centrifugal switch placement in the start circuit
- Annotate capacitor values (µF) and voltage ratings
- Map reversal wiring for the auxiliary winding
- Export for motor replacement or service documentation
Create your own single-phase motor wiring diagram -- free
Key Takeaways
- Single-phase motors require an auxiliary (start) winding displaced 90° from the main winding to produce starting torque.
- Capacitor-start (CS) motors use a large electrolytic capacitor and centrifugal switch for high starting torque; the capacitor is only in the circuit during starting.
- Capacitor-run (CR) motors use a permanent oil-filled capacitor for improved power factor but lower starting torque.
- CSCR motors combine both capacitors for maximum starting torque and efficient running -- the centrifugal switch removes only the start cap at speed.
- PSC motors have no centrifugal switch; the run cap stays in permanently, making them reliable and speed-variable.
- Reversing rotation means swapping the auxiliary winding connections, not both windings.
- Always discharge start and run capacitors before handling -- they store charge even after power is removed.
Originally published at https://circuitdiagrammaker.app/blog/single-phase-motor-wiring-diagram.
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