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:
- 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
Terminal and Lead Identification Table
Single-phase motor terminal markings follow NEMA convention, though exact numbering depends on motor design and voltage configuration. Use this table alongside the nameplate connection diagram, since the nameplate always takes precedence over general convention.
| Terminal | Winding | Typical Role |
|---|---|---|
| T1, T2 | Main (run) winding | Line connections for a single-voltage motor, or one half of a dual-voltage main winding |
| T3, T4 | Auxiliary (start) winding | Connect to the start circuit, often through a capacitor and/or centrifugal switch |
| T5, T6 | Second half of main winding (dual-voltage motors) | Combined with T1/T2 in series for high voltage or parallel for low voltage |
| T7, T8 | Second half of auxiliary winding (some dual-voltage motors) | Paired with T3/T4 depending on voltage configuration |
| M1, M2 | Main winding (older/alternate marking) | Equivalent to T1, T2 on some manufacturers' motors |
| S1, S2 or A1, A2 | Start/auxiliary winding (older/alternate marking) | Equivalent to T3, T4 on some manufacturers' motors |
On dual-voltage motors, the nameplate diagram shows how to combine the winding sections in series (higher voltage) or parallel (lower voltage). Never guess at this configuration -- connecting a motor wired for parallel (low-voltage) operation to the higher line voltage will damage the windings immediately.
Testing Motor Windings and a Capacitor with a Multimeter
Before troubleshooting further or replacing parts, isolate the motor from power and test its windings and capacitor with an ohmmeter (resistance mode):
- Disconnect all power to the motor and discharge any capacitor before touching motor leads.
- Test the main winding: measure resistance between T1 and T2 (or the equivalent main winding leads). A healthy winding reads a low, stable resistance value specific to that motor -- consult the nameplate or motor documentation for the expected value. An open circuit (infinite resistance) means a broken winding.
- Test the auxiliary winding: measure resistance between T3 and T4. The auxiliary winding reads a higher resistance than the main winding because it uses thinner wire with more turns. Infinite resistance again indicates an open winding.
- Check winding-to-frame insulation: measure resistance between each winding terminal and the motor's metal frame or ground lug. This should read as an open circuit (very high resistance, ideally infinite on a standard ohmmeter). Any continuity here indicates a grounded winding and the motor should not be energized.
- Test the capacitor: with the capacitor disconnected and discharged, set the meter to capacitance mode if available and compare the reading to the µF value printed on the capacitor case. On an analog ohmmeter without capacitance mode, touch the leads to the terminals -- the needle should swing toward zero and slowly climb back as the capacitor charges. A capacitor that shows a dead short or no needle movement at all has failed.
- Test the centrifugal switch (CS and CSCR motors only): with the motor at rest, the switch contacts should show continuity. Spin the shaft by hand -- on some designs you can hear or feel the switch open as it passes the trip speed, though a full functional test usually requires running the motor.
Troubleshooting Table: Won't Start, Runs Hot, or Trips the Breaker
| Symptom | Likely Cause | What to Check |
|---|---|---|
| Motor hums but the shaft does not turn | Open start winding, failed start capacitor, or a mechanical bind | Ohmmeter test on the auxiliary winding and capacitor; try turning the shaft by hand with power off |
| Motor starts but runs hot | Run capacitor failing (weak but not dead), voltage mismatch, or a partially blocked winding | Compare capacitor's measured µF to its rated value; confirm supply voltage matches the nameplate |
| Motor trips the breaker on startup | Locked rotor, shorted winding, or a shorted start capacitor drawing excessive inrush current | Check for mechanical binding first, then test winding resistance for a short to frame or between windings |
| Motor trips the breaker after running for a while | Overload from a mechanical load increase, undersized wire causing overheating, or a failing run capacitor | Verify the driven load is not binding, confirm wire gauge matches the breaker size, retest the run capacitor |
| Motor runs but vibrates or sounds rough | Worn bearings, an unbalanced load, or one winding not fully energized | Check bearing play by hand, verify both windings show correct resistance and are properly connected |
| Motor runs backward from expected direction | Auxiliary winding leads reversed relative to the main winding | Swap only the auxiliary winding leads (or only the main winding leads), never both |
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.
Frequently asked questions
Can I run a single-phase motor without a capacitor?
A capacitor-start, capacitor-run, or PSC motor will not start reliably without its capacitor because the capacitor creates the phase shift needed for starting torque. Some motors, such as shaded-pole or split-phase designs, do not use a capacitor at all and rely on a different starting method built into the winding layout, so the answer depends on the specific motor type.
What happens if I use a capacitor with the wrong microfarad rating?
Using a capacitor with too low a µF rating reduces starting torque and can prevent the motor from reaching full speed, causing the start winding to overheat. Too high a µF rating causes excessive starting current and torque, which can also overheat the winding and prematurely wear the centrifugal switch. Always match the capacitor's µF rating to the nameplate specification.
Why does my single-phase motor need a centrifugal switch?
The centrifugal switch removes the start winding and start capacitor from the circuit once the motor reaches about 75% of synchronous speed. The start winding and electrolytic start capacitor are only rated for a few seconds of duty, so leaving them connected continuously causes them to overheat and fail, sometimes within seconds of a stuck switch.
Can a single-phase motor run on a variable frequency drive (VFD)?
Standard capacitor-start or capacitor-run single-phase motors are generally not designed for VFD control, because the VFD changes frequency and voltage in ways that disrupt the phase relationship the capacitor is tuned to produce. Applications needing variable speed typically use a three-phase motor with a VFD, or a PSC motor specifically designed for speed control via voltage tapping rather than frequency change.
How do I know which single-phase motor type I have?
Check the nameplate first -- it usually states the motor type directly (capacitor-start, PSC, etc.) along with the wiring diagram. If the nameplate is missing, a capacitor-start motor has a large, cylindrical electrolytic capacitor and typically an audible centrifugal switch click at startup, while a PSC motor has a smaller oil-filled capacitor with no switch and no click.
Is it safe to replace a start capacitor with one rated for a higher voltage?
Yes, using a capacitor with a higher voltage rating than specified is generally safe and is a common practice when the exact original value is unavailable, since it simply provides more voltage headroom. The microfarad (µF) rating still needs to match the original specification closely, and you should never use a capacitor with a lower voltage rating than the nameplate calls for.