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Speed control of three phase induction motor:
· V / f control or frequency control.
· Changing the number of stator poles
· Controlling supply voltage.
· Adding rheostat in the stator circuit.
· Adding external resistance on rotor side.
· Cascade control method.
· Injecting slip frequency emf into rotor side.
· 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.
· The speed of an induction motor depends upon the number of poes which the stator is wound.
· If two independent stator windings are used for different number of poles say for four poles and for two poles are made on the stator, definite rotor speeds can be obtained.
· The two windings are to be insulated from one another. When any of the windings is used, other winding should be kept open circuited by the switch.
· For example, a 36-slot stator may have two 3f -windings, one with 4 poles and the other with 6-poles. With supply frequency of 50Hz, 4 pole winding will give
· The limitation of this method is only two definite speeds can be obtained. Smooth control of speed over wide range is not possible.
· Slip can be varied by changing the applied stator voltage i.e. motor speed can be varied by varying the supply voltage, because
Torque µV 2
· If the voltage is reduced as, torque is reduced as square of the voltage
· For example, if the applied voltage is reduced from V to 0.9 V, the torque will be reduced from T to 0.81 T.
· Since the torque is reduced to 81 percent, the rotor cannot continue to rotate at speed N1, it speed will be reduced. i.e. its slip will increase until the increased rotor current will make up for the reduced stator voltage and produce the required load torque at a a lower speed N2.
· This method of speed control is rarely used for industrial three-phase motors because of the requirement of additional costly voltage changing auxiliary equipment.
· For small induction motor used in home appliance, voltage control method of speed changing is often used.
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