Title Unit-3.pdf ✅

Speed Control of Three Phase Induction Motor A three phase induction motor is basically a constant speed motor so it’s somewhat difficult to control its speed. The speed control of induction motor is done at the cost of decrease in efficiency and low electrical power factor. Before discussing the methods to control the speed of three phase induction motor one should know the basic formulas of speed and torque of three phase induction motor as the methods of speed control depends upon these formulas. Synchronous Speed Where, f = frequency and P is the number of poles The speed of induction motor is given by, Where, N is the speed of the rotor of an induction motor, Ns is the synchronous speed, S is the slip. The torque produced by three phase induction motor is given by, When the rotor is at standstill slip, s is one. So the equation of torque is, Where, E2 is the rotor emf Ns is the synchronous speed R2 is the rotor resistance X2 is the rotor inductive reactance The Speed of Induction Motor is changed from Both Stator and Rotor Side. The speed control of three phase induction motor from stator side are further classified as :  V / f control or frequency control.  Changing the number of stator poles.
 Controlling supply voltage.  Adding rheostat in the stator circuit. The speed controls of three phase induction motor from rotor side are further classified as:  Adding external resistance on rotor side.  Cascade control method.  Injecting slip frequency emf into rotor side. Speed Control from Stator Side   V / f Control or Frequency Control Whenever three phase supply is given to three phase induction motor rotating magnetic field is produced which rotates at synchronous speed given by In three phase induction motor emf is induced by induction similar to that of transformer which is given by Where, K is the winding constant, T is the number of turns per phase and f is frequency. Now if we change frequency synchronous speed changes but with decrease in frequency flux will increase and this change in value of flux causes saturation of rotor and stator cores which will further cause increase in no load current of the motor . So, its important to maintain flux , φ constant and it is only possible if we change voltage. i.e if we decrease frequency flux increases but at the same time if we decrease voltage flux will also decease causing no change in flux and hence it remains constant. So, here we are keeping the ratio of V/f as constant. Hence its name is V/ f method. For controlling the speed of three phase induction motor by V/f method we have to supply variable voltage and frequency which is easily obtained by using converter and inverter set.  Controlling Supply Voltage