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
- 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.
- Synchronous speed
- N_s = 120f / P (rev/min)
- Slip
- slip % = ((N_s − N_r) / N_s) × 100
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
AC Motors
AC motors quiz
AC Motors, quiz
1. To reverse the direction of a three-phase induction motor you should:
Swap any two of the three supply leadsSwap all three supply leadsReverse the neutral connection2. A 4-pole induction motor on a 400 Hz supply has a synchronous speed of:
12 000 rpm6000 rpm24 000 rpm3. An induction motor never reaches synchronous speed because:
Relative motion (slip) is needed to induce rotor currentFriction always prevents itThe supply frequency varies4. A motor with a synchronous speed of 12 000 rpm runs at 11 400 rpm. The slip is:
5 %0.5 %50 %5. A three-phase supply applied to three stator windings produces:
A rotating magnetic fieldA stationary pulsating fieldA steady DC field6. Which is NOT a starting method for a single-phase induction motor?
Three-phase excitationCapacitor-startShaded-pole7. A synchronous motor runs at:
Exactly the synchronous speed under all loadsSlightly below synchronous speedA speed that varies with load8. Slip in an induction motor is defined as:
The difference between synchronous and rotor speedThe difference between supply and rotor frequencyThe mechanical backlash in the gearbox9. If the supply frequency is doubled, the synchronous speed of a motor:
DoublesHalvesIs unchanged10. At the instant of starting, the slip of an induction motor is:
100 %0 %5 %