Single-phase Induction Motor

Single-phase Induction Motor

  The winding used normally in the stator (Fig. 34.1) of the single-phase induction motor (IM) is a distributed one. The rotor is of squirrel cage type, which is a cheap one, as the rating of this type of motor is low, unlike that for a three-phase IM. As the stator winding is fed from a single-phase supply, the flux in the air gap is alternating only, not a synchronously rotating one produced by a poly-phase (may be two- or three-) winding in the stator of IM.  This type of alternating field cannot produce a torque ((T₀)_st = 0.0), if the rotor is stationery (ω_r = 0.0 ). So, a single-phase IM is not self-starting, unlike a three-phase one. However, as shown later, if the rotor is initially given some torque in either direction (ω_r ≠ 0.0 ), then immediately a torque is produced in the motor. The motor then accelerates to its final speed, which is lower than its synchronous speed. This is now explained using double field revolving theory.