Prepared by:- Chandan pathak ( ) Utpal rathod( ) Hitendra vyas( )

The expression for the actual speed of a 3-phase induction motor is given by:- This show that the actual speed depend on to factors Synchronous speed Ns & Slip s. If we change any one of them so we can vary the speed of an induction motor

 Another way to change the speed is to change the torque produce by the motor which is given by :-  If the motor torque is less than the load torque then the motor speed will be decrease & if the torque is higher then the load torque the motor speed will increase.  Mainly two methods control the speed of induction motor by stator parameters and rotor parameters.  The parameter like frequency, supply voltage, no of poles, external resistance can be control the speed.

 In this type of control the frequency is constant and stator voltage varied to vary the speed of induction motor.  Torque is proportional to the square of stator supply voltage V1 follows :-  But E ₂ is proportional to the supply voltage V ₁

 Torque slip characteristic up the value S is low so that (s X ₂ )² value is neglect  R ₂ and K is also constant hence,  Stator supply voltage V ₁ is reduced from to bV ₁, where b < 1.

 Actual speed N is given :-  The actual speed can be changed by the changing synchronous speed. But synchronous speed Ns can be change by the changing the stator frequency F ₁ so theoretically speed can be control by only F ₁.

 Only changing in F ₁, keeping V ₁ is constant has an adverse effect on air gap flux because air gap flux given by,  So air gap flux g is proportional to ratio of (V ₁ /F ₁ ). Hence the ratio (V ₁ /F ₁ ) is kept constant by changing the stator voltage V ₁ and frequency F ₁ simultaneously.  This is necessary to keep the air gap flux constant. Hence this mathod is called constant (V/F) control.

The ac input is constant voltage and constant frequency is apply to ac to dc converter which is rectifier. At the out put of ac to dc convector a dc voltage is obtain a capacitor bank is used to reduce the ripple contents in the dc voltage. The dc voltage is applied of an inverter. The inverter is an electronic circuit which convert the dc voltage in to the 3-face variable voltage and frequency ac voltage.

 The synchronous speed is given by,  So it is possible to change the synchronous speed by changing the number of poles.  The no of poles can be changed by changing the connection of stator winding with the help of simple switching.  The no of the poles in the ratio of 2:1.so two different speeds are obtained.

 This control is basically the stator voltage control because when the rheostat change which is connected to the stator circuit,a part of supply voltage will drop across the rheostat.

 The actual voltage applied to the stator is reduced the speed change due to variable stator voltage.  In the stator position a minimum stator voltage is applied to the speed of the induction motor is minimum stator voltage is applied an speed of the induction motor is minimum and the reduced towards the run position the stator voltage is increased and the speed also increases.

The set up speed control Using external rotor resistance Is show in figure. The variable rotor resistance Is connected through The slipring and brushes. The values of critical slip Sm Corresponding to the maximum torque increases with the increases In the value of R ₂ and the torque slip characteristics.

 If assume that the load torque Tl is constant then the operating  Point of the introduction motor shifts from point A to point B.  So the motor speed changes from N ₁ to N ₂ where N ₂ is less than N ₁.  Thus the speed decreases with increase in the rotor resistance R ₂.