Construction and Working Principle of 3-Phase Induction Motor

     An induction motor consists of two main parts  

(i)  Stator and  

(ii)  Rotor  

     Stator is a stationary part and rotor is the rotating part of an induction motor

 Stator :

     Stator construction is similar to the stator of a synchronous machine. The stator consists of Stator core, Stator frame and Stator winding. The stator core is laminated and is slotted on its inner cylindrical surface shown in figure .These slots house the 3-phase stator winding. The stator frame provides mechanical support to the stator core.

    The large size motors are of open type slots, so that form-wound and coils can be insulated properly. But small size motors are provided with semi-closed type slots to make the air-gap, reluctance minimum and to prevent excessive tooth loss. 

Construction of 3-Phase Induction Motor

     The 3-phase stator winding connected either in star or delta. It is wound for a definite number of poles depends upon the speed requirement, Greater the number of poles lesser the speed and vice versa. This winding when excited by a 3-phase supply produces a rotating magnetic field. The ventilating ducts are provided for the cooling purpose.

    In some cases, all the six terminals of 3-phase stator winding are brought out so that user can connect them either in star or delta. Almost all the stator windings of induction motors are distributed type.

Rotor :

     The rotor is present inside the stator. The rotor is laminated with cylindrical shaped structure. The armature winding is placed on the rotor with rotor slots.

   The two types of rotor construction motors namely  

(i)  Squirrel cage rotor,

(ii)  Slip ring or wound rotor.

Squirrel Cage Rotor : 

     In this type, rotor consists of un-insulated copper or Aluminum bars placed in the slots. These bars are permanently short circuited at both ends by conducting end rings. It is called as Squirrel cage rotor because the entire structure looks like a cage is shown in figure. The cage winding is adaptable to any number of poles

Construction of 3-Phase Induction Motor

     The squirrel cage motor is cheap and robust. However its starting torque is low because the resistance of the rotor is very small and there is no way to insert external resistance as the winding is permanently short circuited ends of the rotor core for necessary cooling. 

Skewed Rotor Slots : 

     The rotor slots are not always made parallel to the rotor shaft axis but are slightly skewed with certain angle as shown in Figure. This arrangement gives following advantages.  

1. It reduces the magnetic noise during operation of the motor.

2. It helps in producing uniform torque.

3. It reduces cogging,which is nothing but magnetic locking of the rotor with the stator.

Slip Ring or Wound Rotor : 

     A wound rotor has a laminated core with slots on its outer surface. These slots consists of a 3-phase rotor winding, which is similar to the stator winding. Both the stator and rotor windings are designed for the same number of poles. 

    The 3-phase rotor winding is usually star connected.The three ends of the rotor winding are connected to the slip rings mounted on the motor shaft. So in this type of rotor, the external resistance can be added with the help of brushes and slip ring arrangement in series with each phase of the rotor winding.

Working Principle of 3-Phase Induction Motor  :

Basically induction motor works on the principle of induction.

     When 3-phase supply is given to the stator winding of the motor, the field winding setups a rotating magnetic field rotating at synchronous speed. Let direction of rotation of this field is clockwise. This rotating flux cuts the stationary rotor conductors. So their exists a relative motion or speed between the rotating magnetic field and the rotor conductors, an emf is induced in the rotor according to the Faraday's laws of electromagnetic induction.

     The induced emf in the rotor drives a current through rotor conductors and its direction can be determined by the Fleming's right hand rule. For the case considered.direction of rotor current in a conductor is towards the observer. The current carrying rotor conductors produce their own flux called as rotor flux. Hence by applying right hand thumb rule, the direction of rotor flux alone is as shown in figure.

Construction of 3-Phase Induction Motor

     The interaction of stator and rotor fluxes, the rotor conductors ( according to the left hand rule ) experience a force, the overall rotor experiences a torque. Hence rotor starts rotating in the same direction as that of the stator rotating field and tries to catch up the speed of rotating magnetic field.

Construction of 3-Phase Induction Motor

Advantages of Three Phase Induction Motor :  

(i) It is very simple in construction.

(ii) Reliable operation. 

(iii) It is self starting. 

(iv) It is very rugged, especially cage type. It requires minimum maintenance.  

(v) Easy operation. 

(viii) Its efficiency is high and p.f is reasonably good.

(vii) It is quite cheap in cost. 

Disadvantages of Three Phase induction Motor : 


(i) Speed decreases with increase in load. 

(ii) Starting torque is inferior to that of dc shunt motor.  

(iii) Methods of its speed control involve more cost.

Reference -   

     Electrical Machines - II By U.A.Bakshi, M.V.Bakshi

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