Stator of 3 phase induction motor

Construction of induction motor

It contains stationary part stator and rotating part rotor with least possible air gap in between them.
(0.3 - 0.5 mm for small machine) , (1.0 - 1.5 mm for large machine)

Stator Design

Stator design

It contains slots made up of silicon Steel lamination. (Thickness 0.35 - 0.65 mm).
The function of stator core is to carry three phase winding.

Slot can be of three types:-

  1. Open slots
  2. Semi closed slots
  3. Closed slots

For open slots:- easy to design and for regular maintenance and repair involves less cost.

For close slots:- it is difficult to design.

For semi closed slots :- it is also quite difficult.

Here I want to tell you some key point:-

  • The purpose of core to contains the winding.
  • The purpose of Im is to produce magnetic flux.
  • If magnetizing current requirement is high the power factor is more lagging.
  • Insulation is the weakest link in the rotating machine for insulation purpose replacement of coil open winding is best.
  • In closed slot machine air gap is uniform and least therefore current required is less power factor is more as compared to open slot winding.
  • The flux which is linked to both winding is taking part in energy transfer is called mutual flux.
  • When the flux not linked to other winding and not taking part in energy transfer is called leakage flux. the leakage flux have its own inductance.
  • Leakage flux is maximum in closed slots.
  • As air gap increase leakage flux also increase.
  • As a gap decrease leakage flux becomes linkage flux.
  • Reluctance is uniform for closed slots.
  • Reluctance is more for open slots.
  • If reluctance is uniform the flux distribution is also uniform.
  • Open slots makes the flux distribution is non uniform and this phenomena is called slots harmonics or teeth harmonics. Harmonics are produced due to the slots of stator so its name a slots harmonics or teeth harmonics.
  • In open slots winding the leakage flux is minimum so flux is more powerful and motor is more powerful.

  • By keeping all views semi closed winding is better option.
  • For DC machine open slots is used .
  • Battery DC less commutation requirement.
  • Reactance should be as low as possible due to low reactance all synchronous machines are open slot winding.
  • Squirrel cage induction motor is closed slots winding


Generally semi closed slots are preferred large rating induction machines or synchronous generators open slots are preferred.

Reason :- 

  1. Maintenance consideration
  2. Leakage reactant is to be less.

Generally closed slots are not prefered.
The operating power factor depends on the air gap. if we consider a transformer without a cut in core and we can see the required magnetising current is less compared to the magnetizing current required for core with a cut.

If the air gap between stator and rotor increase 

  1. Power factor reduce
  2. Leakage flux and leakage reactant will increase.   


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