Chapter4-Induction machine 《Electric Machinery 电机学（英汉双语）》课件.ppt

* Fig. 4-38 Skin Effect in a deep bar Fig. 4-39 Typical torque-speed curves for different rotor designs 1－The common cage rotor induction motor; 2－The deep bar cage rotor induction motor * 4. 10 Speed control of the induction motor So we can adjust its speed from three aspects: change the pole number of the stator winding; change the power frequency f1; change the slip of the motor s. * 4.10.1 Speed Control by Changing Pole number There are two major approaches to change the number of poles in an induction motor: the method of consequent poles the multiple stator windings. * Fig. 4-40 Pole-changing Schematic Diagram (a)Forward Series Connection (b) Reversed Series Connection (c) Reversed Parallel Connection * 4.10.2 Speed control by changing frequency Speed control below base speed constant * * When the stator frequency rise from the rated value, as the stator voltage can not increase, the main flux of the motor will be reduced with the rise of the stator frequency, so the maximum torque will also decrease. So the speed control above base speed is constant voltage and weak flux. This is nearly constant power. Speed control above base speed * Fig. 4-36 control characteristics curve under variable frequency speed control 1-voltage frequency ratio is constant 2- with the stator voltage compensation * 4.10.3 Speed control by changing the stator voltage The torque developed by an induction motor is proportional to the square of the applied voltage. * 4.10.4 Speed control by changing rotor resistance This method is only applicable for the wound rotor induction motor. * 4.10.5 Wound-rotor Induction Motor Cascade Speed Control To compensate the disadvantages of speed control by changing the rotor resistance, an additional electromotive force can be cascaded in the rotor circuit, where the frequency is equal to the rotor electromotive force. This is so-called cascade speed control. * The mechanical characteristics of low cascade synchronous speed control The m