AC Motors
Induction motors: synchronous speed, slip, full-load current, shaft torque, and power-factor correction.
The three-phase induction motor runs most of the industrial world. Three windings create a magnetic field that ROTATES at a speed fixed by the line frequency; the rotor chases it, always lagging slightly — that lag is what makes torque.
- 01 Synchronous speed The field rotates at n_sync = 120·f / p, where p is the number of magnetic poles. More poles = slower field. At 60 Hz: 2 poles → 3600 rpm, 4 → 1800, 6 → 1200.
- 02 Slip — why the rotor lags If the rotor ever caught up with the field, no field lines would be cut, no current induced, no torque made. So it settles just below: the shortfall, as a…
- 03 Power, efficiency & full-load current The nameplate gives SHAFT power (what the load receives). The line must supply more, to cover losses (efficiency η) and the magnetizing current that does no…
- 04 Shaft torque Power is torque times rotational speed: P = T·ω. Same power at half the speed means TWICE the torque — why low-rpm motors are physically bigger.
- 05 Power-factor correction That 0.86 power factor means the line carries current that does no work. A capacitor bank supplies the reactive part locally, unloading the wiring upstream.…
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