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AC Motors
Construction, principles of operation and characteristics of synchronous and induction AC motors, both single and polyphase; methods of speed control and direction of rotation; methods of producing a rotating field.
AC motors — summary notes
FreeMain ideas
- A three-phase supply applied to three stator windings produces a rotating magnetic field automatically.
- Synchronous motors run exactly at synchronous speed N_s = 120f / P.
- Induction motors run slightly slower than synchronous speed. The difference is called slip, and it is essential — without relative motion no EMF would be induced in the rotor and no torque would be produced.
- Single-phase motors are not self-starting and need a starting method: capacitor-start, split-phase, or shaded-pole.
- To reverse a three-phase motor, swap any TWO of the three supply leads.
- ⚠ Exam trap: an induction motor can never reach synchronous speed, because slip is what induces the rotor current. Reverse by swapping two phases, not all three.
Key formulas
- Synchronous speed
- N_s = 120f / P (rev/min)
- Slip
- slip % = ((N_s − N_r) / N_s) × 100
Solved examples
A 4-pole induction motor runs from a 400 Hz supply. Find the synchronous speed.
N_s = 120f/P = (120 × 400)/4 = 12 000 rev/min.
That motor's rotor actually turns at 11 400 rev/min. What is the slip?
slip = ((12 000 − 11 400)/12 000) × 100 = (600/12 000) × 100 = 5 %.
AC motors concept map
FreeAC Motors
AC motors quiz
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