their working principles

their working principles

1. What "Their Working Principles" Means

This topic elaborates how single-phase and three-phase induction motors actually produce torque from the rotating magnetic field (RMF).

2. Three-Phase Induction Motor — Working

Sequence:

1. 3-phase AC applied to stator windings 120° apart ⇒ creates RMF rotating at synchronous speed

(e.g., 4-pole 50Hz ⇒1500 rpm) with constant magnitude.
2. RMF sweeps past rotor conductors (squirrel cage bars) ⇒ relative speed $N_s - N$ (where $N$ = rotor speed) ⇒ Faraday:

induced in rotor bars.
3. Rotor bars short-circuited (end rings) ⇒ rotor currents $I_2$ flow (frequency $f_2 = s·f$, where slip

).
4. Lorentz force: rotor current $I_2$ in RMF $B$ experiences force

⇒ torque

in direction of RMF ⇒ rotor follows RMF, accelerating.
5. Slip: rotor can never reach $N_s$ (if $N=N_s$, relative speed zero ⇒ $e=0$, $I_2=0$, $T=0$ ⇒ friction slows). So $0< s ≤1$ (motoring: $s$ small $0.01$–$0.05$ at rated load, $s=1$ at standstill/start).

Why self-starting: RMF exists at start ($s=1$), so torque at $s=1$ >0 ⇒ starts.

3. Single-Phase Induction Motor — Working and Why Needs Help

• With single winding, field is pulsating (not rotating):

= two counter-rotating RMFs of half amplitude. At standstill, torques from both RMFs cancel ⇒ no starting torque (double-revolving field theory).

• Starting aid: auxiliary winding + capacitor creates 90° phase shift ⇒ elliptical RMF ⇒ net torque >0 to start. Once started, auxiliary may be cut off (centrifugal switch), motor runs with pulsating field but now rotor sees unbalanced torque, continues.
• Alternative: shaded-pole (copper ring on pole tip delays phase).

4. Worked Checks

(a) 3-phase 4-pole 50Hz Ns=1500, rotor N=1440 ⇒ s=0.04, rotor frequency f2= s·f=2Hz ⇒ rotor currents 2Hz.

(b) Need to reverse direction? Swap any two phases ⇒ RMF reverses ⇒ torque reverses.

(c) Single-phase at standstill: no torque, push rotor slightly ⇒ it picks direction of push (bistable) ⇒ needs starter.

5. Exam Points

• Explain 3-phase RMF at $N_s=120f/P$ and $s=(N_s-N)/N_s$ with $e=Blv$ → $I_2$ → $F=BIl$ → torque — 7-10 marks, very frequent (the most common working principle question).
• Explain single-phase double-revolving field and why no starting torque, need capacitor/shaded pole — 5-7 marks, very frequent (almost certain to ask why single-phase not self-starting).
• Mention $f_2=s·f$ — 3 marks, frequent.