8.3 Single-Phase Motors: Split-Phase, Capacitor-Start, PSC & Shaded-Pole

8.3 Single-Phase Motors: Split-Phase, Capacitor-Start, PSC & Shaded-Pole

Quick Answer: A single-phase winding cannot start itself. Energized at standstill it produces a pulsating field, which resolves mathematically into two equal fields rotating in opposite directions — the torques cancel and the rotor sits and hums. Every single-phase motor design is therefore an answer to one question: how do we manufacture a second, phase-displaced field just long enough to get the rotor moving in a chosen direction? Resistance split-phase does it with winding resistance, capacitor-start does it with a capacitor, PSC does it with a permanently connected run capacitor, and shaded-pole does it with a shorted copper ring. Once the rotor is turning, the reverse field becomes negligible and the motor runs on the main winding alone.

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1. Why There Is No Starting Torque

In a three-phase motor, three windings displaced 120° in space and fed by currents displaced 120° in time produce a genuinely rotating magnetic field. Put a rotor in it and the rotor chases the field.

A single-phase winding fed by a single-phase supply produces a field that grows, collapses, reverses and grows again along one axis — it pulses, it does not rotate. Double revolving field theory describes what the rotor experiences: the pulsating field is exactly equivalent to two fields of half the magnitude rotating in opposite directions at synchronous speed.

• At standstill, the two fields produce equal and opposite torques. Net torque = zero. The rotor buzzes at 120 Hz, draws locked-rotor current, and heats.
• Once the rotor is turning, the field rotating with the rotor sees small slip and produces strong torque, while the field rotating against the rotor sees very large slip, produces little torque, and mostly contributes losses and the characteristic 120 Hz hum.

That asymmetry is why a single-phase motor will keep running in whichever direction you spin it — including the wrong one — if the starting circuit has failed. Spinning a humming motor by hand and watching it take off is a classic diagnostic, and a classic way to lose a finger. Do it with the power off and a strap wrench, or better, diagnose the starting circuit electrically.

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2. Creating an Artificial Second Phase

Every design adds an auxiliary (start) winding physically displaced 90 electrical degrees from the main (run) winding in the stator slots, and then arranges for the current in that winding to be displaced in time from the main winding current. Space displacement plus time displacement equals a rotating field.

Why the auxiliary winding must come out

On split-phase, CSIR and CSCR machines, the start winding is wound with fine wire for intermittent duty only. Leave it energized and it burns — typically within a minute. Something must disconnect it once the rotor reaches about 75 to 80% of synchronous speed:

A PSC motor has no disconnecting device, because the run capacitor limits auxiliary winding current to a value the winding can carry continuously. That is why PSC motors are quiet, restart freely, reverse easily and last a long time — and why their starting torque is too low for a loaded compressor.

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3. Start Capacitors versus Run Capacitors

This distinction is asked about constantly and mixed up constantly in the field.

The substitution error that destroys motors: fitting a start capacitor where a run capacitor belongs. The electrolytic start capacitor is not built for continuous AC service; left in circuit it overheats within minutes, vents its electrolyte and fails — sometimes energetically.

Testing a capacitor safely:

1. De-energize, lock out, and discharge the capacitor through a 20 kΩ / 5 W resistor across the terminals. A charged motor-run capacitor holds a genuinely dangerous charge. Never short it with a screwdriver: the current spike welds the screwdriver, damages the capacitor and throws metal.
2. Disconnect at least one lead so the windings do not parallel the measurement.
3. Measure capacitance and compare with the nameplate. Replace anything outside about ±10% of rated value (some manufacturers specify −0/+10% on run capacitors).
4. Check for a bulged case, oil weeping, or a vented top. Any of those is an immediate replacement, whatever the meter says.
5. Replace with the same µF rating and the same or higher voltage rating. Increasing µF changes the phase angle and the winding current; it does not "give the motor more starting power".

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4. Connections, Voltage and Rotation

Dual-voltage motors

Many single-phase industrial motors have two run winding halves and can be connected for 120 V (halves in parallel) or 240 V (halves in series). The start winding is rated for the lower voltage and is connected across one run winding half in the series configuration, which is why the connection diagram on the nameplate must be followed exactly rather than reasoned out at the terminal board.

Reversing rotation

Interchange the two leads of the start (auxiliary) winding relative to the run winding. Reversing the supply leads does nothing — the pulsating main field has no direction of its own, so swapping line and neutral changes nothing about which way the motor goes.

Practical consequences:

• On a split-phase or capacitor-start motor, reversal is done at rest, because the start winding is only in circuit below about 75% speed.
• On a PSC motor, the auxiliary winding is always in circuit, so a PSC motor can be reversed with a changeover contactor even while running — which is why PSC is the standard choice for reversible damper and valve actuators.
• A conventional shaded-pole motor is not reversible, because the shading rings are fixed on one side of each pole. Reversible shaded-pole designs exist with two sets of rings and a switchable arrangement, but they are a specialty item.

Identifying windings with an ohmmeter

• The run winding has the lower resistance (heavier wire, more copper cross-section).
• The start winding has the higher resistance (fine wire, higher resistance by design on split-phase).
• Resistance from either winding to the frame should be effectively infinite; any reading is a ground fault, confirmed with an insulation resistance test.

Code data

Full-load current for single-phase motors is taken from CEC Table 45, and motor branch-circuit protection, conductor sizing and overload rules in Section 28 apply exactly as they do to three-phase machines. A common field error is to size conductors from the nameplate amperes when the code requires the table value for branch-circuit calculations.

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5. Troubleshooting Single-Phase Motors

+---------------------------------------------------------------------------------+
|                 SINGLE-PHASE MOTOR TROUBLESHOOTING DECISION TREE                |
|                                                                                 |
|  SYMPTOM: Motor hums, does not turn, draws locked-rotor current                 |
|     |                                                                           |
|     +-- Does the shaft turn freely by hand (power OFF, locked out)?             |
|     |      NO  -> Seized bearing, or the driven load is jammed. Mechanical.     |
|     |      YES -> The starting circuit is dead. Continue.                       |
|     |                                                                           |
|     +-- Measure start capacitor: bulged, vented, or > 10% off rating?           |
|     |      YES -> Replace capacitor (discharge first).                          |
|     |                                                                           |
|     +-- Measure start winding resistance: open circuit?                         |
|     |      YES -> Burned start winding. Rewind or replace motor.                |
|     |                                                                           |
|     +-- Centrifugal switch / starting relay contacts closed at rest?            |
|            NO  -> Stuck switch, worn flyweights, or failed relay.               |
|                                                                                 |
|  SYMPTOM: Motor starts, runs a short time, then trips on overload               |
|     +-- Centrifugal switch failing to OPEN -> start winding stays energized     |
|     +-- Low supply voltage -> excessive running current                          |
|     +-- Overloaded / mechanically binding driven equipment                       |
|                                                                                 |
|  SYMPTOM: Motor runs but in the wrong direction                                 |
|     +-- Interchange the START winding leads (NOT the line leads)                |
|                                                                                 |
|  SYMPTOM: PSC fan motor runs slow and hot, low airflow                          |
|     +-- Run capacitor has lost capacitance -> weak auxiliary field               |
+---------------------------------------------------------------------------------+

The two failures worth internalizing

1. Hums but will not start = the starting circuit is gone. In order of probability: failed start capacitor, stuck-open centrifugal switch or failed starting relay, open start winding, then a mechanical seizure.
2. Starts and then trips = the starting circuit never left. A centrifugal switch that fails to open keeps the fine-wire start winding energized at full voltage. It will draw heavily, smell of hot varnish within a minute, and burn the start winding open — converting a $20 switch repair into a motor replacement. Anyone who replaces a burned start winding without inspecting the centrifugal switch will be back.

Efficiency reality check

Single-phase motors are less efficient than three-phase machines of the same rating, and a shaded-pole motor is often below 30% efficient. On a plant running dozens of small shaded-pole fans continuously, replacing them with electronically commutated or PSC motors is one of the most reliable energy projects an industrial electrician can recommend — the payback comes from motors nobody ever thinks about.