Three-Phase Induction Motor
AC Motor
The industry workhorse: a rotating magnetic field from three-phase supply drags a squirrel-cage rotor around by induction — no brushes, no magnets, nearly no maintenance. Runs everything from pumps to conveyor lines.
Schematic symbol
Motor
How three-phase induction motor appears in a circuit diagram.
Types & variants
Key specs
Power rating
kW / HPContinuous shaft output. 1 HP ≈ 0.746 kW.
Rated speed
RPMSynchronous speed Ns = 120f/P minus slip. A 4-pole 50 Hz motor: Ns = 1500, runs ~1440 RPM.
Slip
%(Ns − N)/Ns. Typically 2–5% at full load; torque is produced only because the rotor lags the field.
Full-load current
ANameplate FLC — sizes the contactor, overload relay and cable. Starting current is 6–8× FLC direct-on-line.
Power factor
—0.8–0.9 at full load but poor at light load — why industries add capacitor banks.
Efficiency class
IE codeIE1 (standard) to IE4 (super premium). Higher class pays for itself in energy within a year or two of continuous running.
Markings
Nameplate: kW/HP, voltage (415 V Δ or 415 V Y), FLC, RPM, frequency, frame size, insulation class (usually F), IP rating, duty (S1 continuous), and connection diagram inside the terminal box lid.
Standard values
0.37, 0.75, 1.5, 2.2, 3.7, 5.5, 7.5, 11, 15, 18.5, 22, 30, 37, 45, 55, 75, 90 kW; 2-pole (2880 RPM), 4-pole (1440 RPM), 6-pole (960 RPM), 8-pole (720 RPM) at 50 Hz.
How to choose
1) Match kW to the load with ~15–20% margin. 2) Pick pole count for the speed the load needs (or plan a VFD). 3) Choose the starting method: DOL below ~7.5 kW, star-delta or soft starter above, slip-ring for very high starting torque. 4) TEFC for dusty sites, flame-proof for hazardous areas. 5) Check torque type: pumps/fans need low starting torque, crushers and lifts need high.
Pinout & package
Terminal box with six studs U1 V1 W1 / W2 U2 V2 — link them in star (three bars parallel) or delta (bars vertical) per the lid diagram. Foot-mounted (B3) or flange (B5/B14) frames, IEC frame sizes 63–355.
Example circuits
- DOL starter: MCB → contactor → overload relay → motor
- Star-delta starter reducing starting current to one-third
- VFD driving a pump for energy-saving flow control
- Two-speed operation by pole changing (Dahlander winding)
Common failures
Bearing wear (most common — listen for growl), winding insulation failure from overheating or single-phasing (one fuse blown → motor hums and burns), moisture ingress, broken rotor bars (torque pulsation), and misalignment stressing the shaft.
How to test
Megger each winding to earth (>1 MΩ minimum, ideally >100 MΩ), compare the three winding resistances with a multimeter (should match within a few %), spin the shaft by hand for bearing roughness, then run uncoupled and check the three line currents are balanced.
Substitutes
Same frame size, kW, speed and mounting swaps directly — frame sizes are standardised. A squirrel cage + VFD replaces most slip-ring applications. For single-phase-only sites, either a single-phase motor (small sizes) or a VFD with 1-phase input / 3-phase output.