Single-phase induction motors
Double-revolving-field theory (1-φ field = forward + backward). Starter families: split-phase, capacitor-start, PSC, cap-start-cap-run, shaded-pole. ECMs increasingly replacing them in residential / small commercial.
Step 1 — Why 1-φ can't start: pulsating = forward + backward fields
Reference notes
Single-phase induction motors dominate residential and small commercial electric machines because most service drops are single-phase. Use Next → to walk through why a single-phase winding cannot self-start, the four starter mechanisms (split-phase, capacitor-start, permanent-split-capacitor, capacitor-start-capacitor-run, shaded-pole), and the modern transition to ECMs.
The starting problem
A single-winding 1-φ stator produces a PULSATING magnetic field — magnitude varies sinusoidally with time but direction stays fixed. By the double-revolving-field theorem, this pulsating field decomposes into:
- Forward-rotating component at +ω_s, amplitude ½ of peak.
- Backward-rotating component at −ω_s, amplitude ½ of peak.
At rest (slip s = 1), the rotor sees equal slip from both → equal opposing torques → net starting torque = 0. The motor cannot start by itself.
Once spinning forward, the forward component dominates (low slip) and the backward component (high slip) only weakly opposes. The motor runs.
So the engineering problem is: provide a starting torque mechanism. Different solutions give the families of single-phase motors.
Family of 1-φ induction motors
| Type | Starting T | Efficiency | Typical use |
|---|---|---|---|
| Split-phase | ~150 % FLT | ~65 % | Fans, blowers, light pumps |
| Capacitor-start | 300–450 % FLT | ~65 % | Compressors, hard-start pumps |
| Permanent-split-capacitor (PSC) | 75–150 % FLT | 65–75 % | Ceiling fans, HVAC blowers |
| Cap-start-cap-run | 300–450 % FLT | 70–80 % | Industrial single-phase, best overall |
| Shaded-pole | 40–50 % FLT | 20–30 % | Small fans, hair dryers, clocks |
Split-phase
- Two stator windings, 90° apart in space: main (heavy wire, low R/X) + auxiliary (thin wire, high R/X).
- Different R/X ratios → ~20–30° electrical phase shift between main and aux currents.
- Approximate rotating field → moderate starting torque.
- Centrifugal switch disconnects aux at ~75–80 % sync speed; motor runs on main alone.
Capacitor-start
- Adds an electrolytic capacitor in series with the auxiliary winding.
- Capacitor shifts aux current ~90° relative to main → nearly perfect rotating field at standstill.
- Starting torque 300–450 % FLT.
- Centrifugal switch disconnects both aux winding AND capacitor at ~75 % sync speed.
- Capacitor is electrolytic, short-duty (1–2 s cycles).
Permanent-split-capacitor (PSC)
- Aux winding + capacitor stay connected during running operation. NO centrifugal switch.
- Capacitor is film type, rated for continuous duty.
- Trade-off: modest starting torque (75–150 % FLT) but better running performance because aux contributes to a more rotating-field approximation.
- Dominant motor design for residential HVAC fan/blower and ceiling-fan applications.
Capacitor-start capacitor-run
- TWO capacitors: large electrolytic for starting + smaller film capacitor for running.
- Centrifugal switch disconnects only the starting capacitor at ~75 % sync speed.
- Best overall single-phase performance: 300–450 % starting torque + 70–80 % running efficiency.
- Used for industrial single-phase up to a few HP — air compressors, hydraulic pumps.
Shaded-pole
- Salient stator poles with a heavy copper shading ring around one corner of each pole face.
- Induced current in the ring (by Lenz's law) delays flux rise in the shaded portion → small rotating component drags the rotor.
- Very low starting torque (40–50 % FLT), poor efficiency (20–30 %).
- Simplest, cheapest 1-φ motor. No capacitor, no centrifugal switch. Used in small fans, hair dryers, microwave turntables, clock motors.
Equivalent circuit (double-revolving theory)
Model the 1-φ motor as two equal 3-φ induction motors mechanically coupled — one forward-rotating with slip s, one backward-rotating with slip (2 − s). Equivalent circuit: main winding R-X, plus forward branch with R_2/(2s) and backward branch with R_2/(2(2−s)), each with their leakage reactances. Net torque = forward torque − backward torque. At rest (s = 1) → equal opposing torques → 0.
vs three-phase induction
- Self-start: 3-φ yes (produces a true rotating field directly), 1-φ no (needs starter).
- Efficiency: 3-φ ~30 % higher at same HP due to no backward-component braking.
- Size: 3-φ smaller for same power.
- Torque ripple: 3-φ has nearly constant torque; 1-φ has fundamental ripple at 2× line frequency (120 Hz on 60 Hz).
Modern trend: ECMs replace 1-φ induction
Many small-HP residential and commercial applications are transitioning to Electronically Commutated Motors (ECMs) — small BLDCs driven from rectified single-phase mains by an integrated drive. Advantages:
- Efficiency 80–90 % (vs 65–75 % for 1-φ induction).
- Continuously variable speed.
- Longer life (no brushes, no centrifugal switch, no electrolytic capacitor degradation).
- Smaller for same HP.
- Better control / smart-grid integration.
Already widespread in: variable-speed HVAC blowers, modern ceiling fans, premium refrigerator compressors, washing-machine drum drives, dishwasher pumps. Initial cost premium 30–50 %, paying back through energy and reliability.