Single-Phase Motor Wiring Diagram: Capacitor-Start Capacitor-Run (CSCR)
This is a free printable single phase motor wiring diagram with capacitor start capacitor run: download the diagram as SVG or open it and print to paper or PDF.
A capacitor-start capacitor-run (CSCR) single-phase motor uses a start capacitor in series with the auxiliary winding to provide starting torque, then switches out the start capacitor once running, leaving a run capacitor permanently in circuit for improved running efficiency and power factor.
Single-phase induction motors do not inherently produce a rotating magnetic field — the supply produces a pulsating field that creates no net starting torque. Capacitors are used to create a phase-shifted current in a second auxiliary (start) winding, producing the spatial and temporal phase difference needed to initiate rotation.
The capacitor-start capacitor-run (CSCR) motor is the highest-performing common single-phase motor type, combining the advantages of two separate capacitors optimised for different duties.
Motor terminals: a typical CSCR motor has four accessible terminals — Main winding (M1, M2), Auxiliary/start winding (A1, A2), and may include a centrifugal switch terminal. The main and auxiliary windings share one common terminal in some configurations, giving three terminals externally.
Start capacitor: an electrolytic (aluminium electrolytic) capacitor of relatively high capacitance (typically 50–500 μF) rated for intermittent duty. It is connected in series with the auxiliary winding and remains in circuit only during starting. An AC electrolytic start capacitor is not designed for continuous operation — it will overheat and fail if left permanently in circuit. The centrifugal switch (or solid-state relay) disconnects the start capacitor once the rotor reaches approximately 75–80% of synchronous speed.
Run capacitor: a film capacitor (usually metallised polypropylene) of lower capacitance (typically 5–50 μF) rated for continuous AC duty. It remains permanently in series with the auxiliary winding, providing a continuous phase shift that improves running power factor, efficiency, and smooths torque. Film run capacitors are rated for continuous AC operation at the motor's supply frequency and voltage.
Starting sequence: at rest, both the centrifugal switch and the start capacitor circuit are closed. Both windings carry current with the start capacitor providing a phase shift that creates an approximation of two-phase power, generating starting torque. Once at speed, the centrifugal switch opens, disconnecting the start capacitor. The motor then runs on the main winding with the run capacitor keeping the auxiliary winding energised for improved efficiency.
Applications: compressors, pumps, conveyors, woodworking machinery, and any load requiring high starting torque and good running efficiency from a single-phase supply.
How to wire single phase motor wiring diagram with capacitor start capacitor run
- Isolate the motor supply and verify absence of voltage Switch off and lock out the circuit supplying the motor. Verify absence of voltage at the motor's terminal block with a calibrated voltage tester before opening any covers. Capacitors retain a charge after switch-off — do not touch capacitor terminals until discharged.
- Discharge the start and run capacitors safely Use a discharge resistor (typically 20 kΩ, 5 W) connected across each capacitor terminal pair, or a dedicated capacitor discharge tool. Never short a charged capacitor with a screwdriver or bare wire — the resulting arc can cause burns and damage the capacitor.
- Identify the motor winding terminals from the nameplate or wiring diagram Locate the terminal block on the motor. Common configurations: three terminals (main common, main, auxiliary) or four terminals (M1, M2, A1, A2). Match terminal labels to the motor nameplate wiring diagram — never assume a terminal assignment.
- Connect the main winding to the supply Connect Live (Line) to M1 and Neutral to M2 (or as indicated on the nameplate). For a reversible motor, swapping the auxiliary winding connections (not the main) will reverse rotation direction — check the nameplate for reversal instructions.
- Connect the run capacitor in series with the auxiliary winding Wire the run capacitor (film type, continuous duty) in series with the auxiliary winding terminals. Connect one capacitor terminal to A1 and the other to the Live supply (or as specified by the wiring diagram). The run capacitor stays in circuit permanently.
- Connect the start capacitor and centrifugal switch in series with the auxiliary winding Wire the start capacitor (electrolytic, intermittent duty) in series with the centrifugal switch contacts, then in parallel with the run capacitor and auxiliary winding path (exact configuration per the motor diagram). The centrifugal switch contacts must be verified closed at rest before reassembly.
- Restore power and verify starting and running performance Restore the supply. The motor should start promptly and reach full speed within 2–5 seconds. Listen for the centrifugal switch click as speed rises (indicates the start circuit has been disconnected). Measure running current with a clamp meter — it should be at or below nameplate FLC.
Specifications
| Supply voltage (common) | Single-phase 230 V AC 50 Hz (or 120 V 60 Hz for North America) |
|---|---|
| Start capacitor capacitance range (typical) | 50–500 μF (intermittent duty, AC non-polarised electrolytic) |
| Run capacitor capacitance range (typical) | 5–50 μF (continuous duty, metallised polypropylene film) |
| Centrifugal switch operating speed | Approximately 75–80% of synchronous speed |
| Starting current multiple (typical CSCR motor) | 4–8 × full-load current (FLC) |
| Minimum insulation resistance (winding to frame) | ≥1 MΩ at 500 V DC (new motor; consider replacement if <1 MΩ) |
| Applicable standard | IEC 60034-1 (rotating electrical machines); IEC 60252-1 (AC motor capacitors) |
| Overload protection requirement | Thermal overload relay or internal thermistor/auto-resettable protector per IEC 60364 |
Safety warnings
- Capacitors store electrical charge and remain dangerous even after the supply is switched off. Always discharge both start and run capacitors using an appropriate discharge resistor before touching any capacitor terminals. A charged start capacitor can deliver a severe electric shock.
- Start capacitors are rated for intermittent duty only. If the centrifugal switch fails to open (sticks closed), the start capacitor will remain in circuit, rapidly overheat, and fail — potentially explosively. Investigate any motor that emits a burning smell during or after starting.
- All motor installation and wiring must comply with the applicable national wiring standard (IEC 60364, BS 7671, AS/NZS 3000, SANS 10142, NEC NFPA 70) and must be performed by a qualified and authorised electrician.
- Single-phase motors can have a very high starting current (4–8× FLC). Ensure the supply circuit MCB or fuse is of a motor-start or time-delay type that will not trip on the starting inrush current.
- Never attempt to free a stalled motor by repeatedly cycling the supply. A stalled motor draws locked-rotor current continuously, overheating rapidly. Allow it to cool and investigate the mechanical load before re-attempting a start.
Tools needed
- Calibrated voltage tester or multimeter (CAT III minimum)
- Capacitor discharge tool or 20 kΩ, 5 W resistor
- Capacitor tester (dedicated, or LCR meter)
- Clamp meter (for measuring motor running current)
- Insulation resistance tester (megohmmeter, 500 V)
- Screwdrivers and spanner set (for motor terminal box and mounting)
- Lock-out/tag-out (LOTO) equipment
Common mistakes
- Replacing the start capacitor with a continuous-duty run capacitor — the start capacitor circuit requires an intermittent-rated type, and using the wrong type can cause the replacement to overheat and fail even if capacitance is correct.
- Installing a replacement capacitor with a lower voltage rating than the supply voltage, causing premature dielectric failure and a potentially explosive capacitor failure.
- Failing to verify that the centrifugal switch contacts are closed at rest before reassembling the motor, missing a fault that will prevent starting.
- Testing motor winding resistance with a standard multimeter resistance range and concluding the windings are open because the reading is not near zero — motor winding resistance is often several ohms to tens of ohms; use an appropriate LCR meter or low-resistance ohmmeter.
- Assuming motor rotation direction cannot be changed once installed — on most CSCR motors, swapping the auxiliary winding connections (A1/A2) relative to the run and start capacitors reverses the direction; always consult the motor nameplate reversal diagram.
Troubleshooting
- Motor hums at startup but does not rotate
- Cause: Failed or open-circuit start capacitor, or centrifugal switch is stuck open (start circuit never energised) Fix: Isolate supply and discharge capacitors. Test the start capacitor with a capacitor tester — check for open circuit, short circuit, or capacitance significantly below rating. Manually inspect centrifugal switch contacts for being stuck open. Replace the faulty component.
- Motor starts but runs hot and draws high current
- Cause: Failed run capacitor (reduced capacitance or open circuit), causing the motor to run essentially as a capacitor-start-only motor on the main winding Fix: Isolate supply, discharge capacitors, and test the run capacitor — capacitance should match nameplate value within ±5%. An aged film capacitor often loses capacitance gradually. Replace the run capacitor with the correct value and voltage rating.
- Motor starts but does not accelerate fully — runs slowly
- Cause: Centrifugal switch failing to open (start capacitor remaining in circuit, reducing effective run winding current); or mechanical overload on the driven equipment Fix: After stopping, inspect and test the centrifugal switch mechanism — at rest it should be closed; at running speed it should open. A switch stuck closed keeps the start capacitor in circuit, creating an incorrect phase relationship that reduces running torque. Also check the mechanical load for binding.
- Burning smell from motor after startup
- Cause: Centrifugal switch not opening, leaving start capacitor in continuous operation; or a winding insulation fault Fix: Switch off immediately. Allow to cool. Test the centrifugal switch mechanism as above. Test winding insulation resistance — values below 1 MΩ to earth (measured at 500 V DC with megohmmeter) indicate damaged insulation. A motor with degraded insulation must be rewound or replaced.
Frequently asked questions
What is the difference between the start capacitor and the run capacitor in a CSCR motor?
The start capacitor is a high-capacitance (50–500 μF) electrolytic type rated for intermittent duty only — it is in circuit only during starting and must be disconnected by the centrifugal switch once the motor reaches running speed. The run capacitor is a lower-capacitance (5–50 μF) film type rated for continuous AC operation and stays permanently connected.
What does the centrifugal switch do in a capacitor-start motor?
The centrifugal switch is a speed-sensitive mechanism mounted on the rotor. At rest and during starting, it is closed, keeping the start capacitor circuit active. Once the motor reaches approximately 75–80% of synchronous speed, centrifugal force opens the switch contacts, disconnecting the start capacitor from the auxiliary winding circuit.
Why does my CSCR motor hum but not start?
The most common cause is an open-circuit or failed start capacitor — the motor receives supply to the main winding and hums but has no starting torque. Other causes include a centrifugal switch stuck in the open position (never allowing the start circuit to energise) or a failed auxiliary winding. Disconnect power and test the start capacitor first.
Can I use any capacitor as a replacement start capacitor?
No. The replacement start capacitor must match the original in capacitance (μF) and AC voltage rating. Using a DC electrolytic capacitor is dangerous — start capacitors are non-polarised AC types. The voltage rating must be at or above the motor's supply voltage. A capacitance within ±10% of the original value is generally acceptable.
What causes a run capacitor to fail, and how does it affect motor performance?
Run capacitors fail from voltage transients, overheating (often due to operating voltage near or above the rating), age, or manufacturing defects. A failed or reduced-capacitance run capacitor causes the motor to run on the main winding only, reducing efficiency, increasing current draw and temperature, causing vibration and noise, and eventually overheating the motor.
Full written guides
- Single-Phase Motor Wiring Diagram: Capacitor Start and Run
- Capacitor Wiring Diagram: Start, Run, and Motor Capacitors
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