Induction Motor

How an induction motor works

Three coils sit at 120° around the stator core. Feed them three sinusoidal currents 120° out of phase — three-phase mains. The vector sum of their magnetic fields rotates at a steady speed around the air gap, like a pair of sliding bar magnets dragging an invisible field with them.

Drop a copper-bar rotor into that rotating field. Each bar sees a changing flux, so by Faraday's law a voltage appears along it. The bars are short-circuited by end rings, so current flows. That current sits in the rotating field, so by F = BIL it feels a force. The forces around the rotor add up to a torque that drags the rotor along, chasing the field.

The rotor never quite catches the field. If it did, the relative motion would vanish, induced voltage would drop to zero, and the torque would disappear. Equilibrium is a small lag — the slip — at whatever value gives just enough torque to balance the load.

     stator (3 windings, 120° apart)
        ╲   |   ╱
         ╲  |  ╱
   ───────┐│┌───────       rotating field n_s = 120 f / p
          │││         ──→  field rotates this way
   ╔══════╪╪═══════╗
   ║ rotor (cage)  ║       rotor follows at n = n_s · (1 − s)
   ║    bars + end ║
   ║    rings      ║       slip s = (n_s − n) / n_s
   ╚═══════════════╝
   induced rotor current → torque → rotation

Worked example: 60 Hz, 4-pole motor at 4% slip

The synchronous speed is set by the mains frequency f and the number of magnetic poles p the stator winding produces:

           120 · f
   n_s  =  ─────────         (rpm, with f in Hz, p = pole count)
              p

For a 4-pole motor on 60 Hz mains:

   n_s = 120 × 60 / 4 = 1800 rpm

That's the speed of the rotating stator field. The rotor itself never reaches it. With a typical full-load slip of 4%:

   slip   = s · n_s   = 0.04 × 1800 = 72 rpm
   actual = n_s − slip = 1800 − 72  = 1728 rpm

This is why nameplates on common 60 Hz industrial motors read 3450 / 1740 / 1160 / 870 rpm — those are 2-pole, 4-pole, 6-pole, 8-pole machines at typical loaded slip, not exact synchronous speeds.

For 50 Hz mains the same 4-pole motor runs at n_s = 1500 rpm and a loaded speed near 1440 rpm. The same calculation gives a 2-pole 50 Hz motor 3000 rpm synchronous, often labelled 2880 rpm at the nameplate.

Construction details

• Stator. A laminated-steel cylinder with axial slots. Copper or aluminium windings sit in the slots, distributed to give a smooth sinusoidal MMF. Three winding sets are connected in star or delta.
• Squirrel-cage rotor. Most common. A laminated steel cylinder with axial slots holds aluminium or copper bars short-circuited by end rings — visually a "squirrel cage" if you stripped the iron away. Cast aluminium rotors are mass-produced; copper-bar rotors raise efficiency by 1–2 percentage points.
• Wound rotor. Less common. The rotor carries a three-phase winding brought out through slip rings. External resistors can be added during start-up to limit inrush current and increase starting torque, then shorted out for normal running.
• Air gap. Typically 0.3–2 mm. A bigger gap reduces magnetising-current efficiency; a smaller gap risks rub if bearings or shaft are out of tolerance.
• Skewed bars. Rotor bars are skewed by one stator slot-pitch to suppress cogging torque and acoustic whine.

Induction vs other motor types

Motor type	Power supply	Speed control	Brushes?	Typical efficiency	Common applications
Squirrel-cage induction	3-phase or 1-phase AC	VFD or pole-changing	No	85–97% (size-dependent)	Pumps, fans, conveyors, HVAC
Synchronous (wound-field)	3-phase AC + DC field	None at constant frequency	Brushed or brushless exciter	96–98%	Large generators, paper-mill drives
Brushed DC	DC	Voltage	Yes (mechanical commutator)	75–85%	Toys, cordless tools, automotive starters
Brushless DC (BLDC)	DC + electronic controller	Controller	No (Hall sensors or sensorless)	85–95%	Drones, EVs, computer fans, e-bikes
Stepper	DC + step driver	Step count (open loop)	No	30–60%	3D printers, CNC, lab automation
Servo (PMSM + encoder)	3-phase from drive	Closed-loop position/torque	No	85–95%	Robotics, machine tools, packaging
Universal (series)	AC or DC	Voltage / chopper	Yes	30–70%	Vacuums, blenders, hand drills

The induction motor wins on cost, ruggedness, and life expectancy. With a modern variable-frequency drive in front it also competes on efficiency and controllability with the more glamorous BLDC and PMSM machines, while costing roughly half as much per kilowatt.

Real-world specs and where you'll find them

• Tesla Model S front motor (2012 launch). A 320 kW three-phase induction motor — Tesla famously named the company after Nikola Tesla and used induction (not permanent-magnet) rotors in early cars to avoid rare-earth dependence. The peak shaft speed was about 18,000 rpm.
• Industrial centrifugal pump motor. 75 kW (100 hp), 4-pole, 1480 rpm at 50 Hz, 95% efficiency under IEC IE3 standard. Frame size 280M, totally enclosed fan-cooled (TEFC), about 600 kg.
• Domestic ceiling fan. A single-phase induction motor with a capacitor-start auxiliary winding. 60–80 W, 4 or 6 poles, 300–400 rpm at typical loaded slip.
• NYC subway car traction motors. Modern R-211 series cars use 4× 175 kW three-phase induction motors per car, fed from VFDs that allow regenerative braking back into the third rail.
• Large industrial blower. 4 MW, 6-pole, 6.6 kV three-phase induction motor for blast-furnace air supply. Efficiency 96.7% at full load. About 25 tonnes.

Common failure modes

• Bearing failure (40–50% of all induction-motor failures). By far the leading cause. Bearings die from contamination, lubrication starvation, misalignment, or — increasingly common with VFDs — shaft currents that pit the races (so-called fluting or EDM damage). Typical symptom: rising vibration at characteristic bearing-defect frequencies. Cure: insulated bearings or shaft-grounding rings on VFD-fed motors.
• Stator winding insulation breakdown (about 30%). Heat-aging, voltage spikes from inverters, contamination, or moisture ingress eventually degrade enamel and slot-paper insulation. A turn-to-turn short releases enormous local current and can carbonise a coil in seconds. Cure: keep operating temperature in the Class F range (155°C max), use Class H insulation for VFD-fed motors, install dV/dt filters between drive and motor.
• Broken rotor bar (5–10%). Cast-aluminium rotor bars can fracture from thermal cycling during repeated starts, or porosity from a bad casting. A broken bar shows up as 2·s·f sidebands around the line frequency in the stator current spectrum (motor current signature analysis, MCSA). Severely broken rotors lose torque and overheat the surviving bars.
• Rotor eccentricity. If shaft, bearings, or end-shields shift, the rotor sits closer to one side of the stator. Magnetic pull pulls it harder on the closer side, accelerating bearing wear and creating audible 2·f noise.
• Single-phasing. If one of three supply phases is lost (blown fuse, broken wire), a running motor keeps spinning on the remaining two phases but draws roughly 1.7× normal current. Without a phase-loss relay, the windings cook within minutes.

Historical context

The induction motor was invented in parallel: Galileo Ferraris demonstrated a working two-phase model in Turin in 1885, and Nikola Tesla filed his US patents on the polyphase induction motor and the three-phase system in 1888. Westinghouse bought Tesla's patents and built the first 100 hp industrial induction motor in 1893 to power Forest Hill, Pennsylvania. The 1893 World's Columbian Exposition in Chicago showcased a 100,000-light AC system — the public, decisive demonstration that AC could distribute power over distances that DC couldn't.

The 20th century industrialised this design relentlessly. Improved steel laminations (CRGO silicon steel, 1934), better insulation (Class B mica/epoxy, then Class F polyimide), die-cast aluminium rotors, and finally variable-frequency drives in the 1980s turned the induction motor from a fixed-speed brute into a precision drive. Modern IE5 "ultra-premium efficiency" motors push beyond 96% on midsize ratings. Roughly 70% of all electrical energy used in industry today drives an induction motor — making any 1% efficiency improvement worth tens of terawatt-hours globally.