Design of Stator

Design of Stator


1). Stator winding:-

  • Generally two types of winding are commonly used for stator of three phase induction motor. 
  (a) Single layer much winding is used for small motors with small number of slots. It is housed in semienclosed slots. 
   (B) Double layer lap winding with diamond shaped coils is used for medium size motors. It is housed in open slots.

  • As per the temperature class (class E, B, F etc.) impregnated insulaaon is used.
  • The three phases of stator windings can be connected in either star or delta depending upon the starting method employed. The squirrel cage induction motors are usually started by star delta starter and hence the stator is designed for delta connection, with six leads for connection to stator. 
  • For slip ring induction motor I. e. wound rotor construction motors, starting method used is resistance in rotor circuit, so stator can be designed either for star or delta as per requirement. 

2). Stator slot design:-

  • Once the number of slots and cross section area of stator conductor is found out, next task in the stator design is to select shape of stator slots, detemnine the number of slots and then fix the size of stator slot.

3). Shape of stator slots:-

  • Two types of shape are commonly used in the stator which are (I) open slots, (ii)semienclosed slots. Both have their own advantages, disadvantages and they shall be selected depending upon application as well as performance requirement.

(I) Open slots:-

  • This type of shape of the slot gives easiness in the installation of winding. In this winding coils can be formed and insulated properly before they are inserted in the slots. This slot shape allows good serviceability as windings are more accessible. 
Open slots

  • Thus open slots are advantagous from the point of view of repair and replacement of winding. Open slots reduces excessive slot leakage (as it gives high reluctance path over open surface) which reduces the leakage reactance and thus improves overload capacity of induction motor. However, this shape has less tooth area which increases flux density in tooth, thereby increasing magnetizing current which decreases the power factor of the motor. Also tooth pulsation losses are increased which gives noisy operation.

(ii) Semi enclosed slots:-

  • In this type of slots air gap Contraction factor is small. Also the flux density in tooth is small which gives lower magnetizing current and hence gives better power factor. This slot shape also reduces tooth pulsation loss and so gives quieter operation.
Semienclosed slots

  • However, winding must be taped and insulated after placing the coils in the slot. In addition to it windings are not easily accessible for repair and replacement and slot leakage reactance is more compared to open slots.




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