1-phase IM principle; air gap effect

Working principle (5 marks)

A single-phase induction motor has a stator with a single (main) winding on a laminated core, and a squirrel-cage rotor.

1. When the winding is connected to a single-phase a.c. supply, it produces a pulsating field along a fixed axis, φ = φ_m cos ωt. This field alternates but does not rotate.
2. By the double revolving field theory, this pulsating field is equal to two fields of magnitude φ_m/2 rotating in opposite directions at N_s = 120f/P.
3. At standstill the two fields give equal and opposite torques on the rotor, so the net starting torque is zero. The motor is not self-starting.
4. With a starting arrangement (an auxiliary winding in series with a capacitor or a resistance, or a shading coil) a rotating field is produced at starting, and the rotor accelerates.
5. Once running in one direction, say forward, the slip with respect to the forward field is s and with respect to the backward field is 2 − s. The forward torque is larger than the backward torque, and the net torque is T = T_f − T_b. The rotor accelerates toward N_s but stays below it. The auxiliary winding is then cut out by a centrifugal switch, and the motor runs on its main winding alone.
6. Compared with a three-phase motor of the same size, the output is lower (about 60 %), because the backward field reduces the torque, and the efficiency and power factor are also lower.

Effect of air gap (3 marks)

The air gap of an induction motor is the non-magnetic part of its magnetic circuit.

• Nearly all the magnetising current is needed to set up flux across the air gap, whose reluctance is far larger than that of the iron. So a large air gap needs a large magnetising current, which lowers the power factor, increases the copper loss and so lowers the efficiency.
• The single-phase motor already has a low pf because of the backward field, so the air gap is made as small as mechanically possible (about 0.3 to 0.8 mm for small motors) to keep the magnetising current low and the pf high.
• A very small air gap increases noise, tooth (harmonic) flux pulsations and mechanical problems (rotor rubbing, effect of bearing wear). A larger gap reduces the zig-zag leakage reactance (improving the overload capacity) and allows greater mechanical tolerance.
• The air gap is therefore a compromise: small to improve the pf, but large enough for mechanical safety and lower noise and harmonics.