The Cross Field Theory Of Single Phase Induction Motor

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The Cross Field Theory of Single Phase Induction Motor
The cross field theory of single phase induction motors looks at the induction motor from a totally different point of view. This theory is connected with the voltages & currents that the stationary stator magnetic field can induce in the bars of the rotor when the rotor is moving. Consider a single phase induction motor with a rotor which has been brought up to speed by some external method. Such a motor. Voltages are induced in the bars of this rotor, with the peak voltage occurring in the win- dings passing directly under the stator windings. These rotor voltages produce a current flow in the rotor, but because of rotor’s high reactance, the current lags the voltage by almost. Since the rotor is rotating at nearly synchronous speed, that time lag in current produces an almost angular shift between the plane of peak rotor voltage and the plane of peak current. The resulting rotor magnetic field. The rotor magnetic field is somewhat smaller than the stator magnetic field, because of the losses in the rotor, but they differ by nearly in both space and time. If these two magnetic fields are added at different times, one sees that that the total magnetic field in the motor is rotating in a counterclockwise direction. With a rotating magnetic field present in the motor, the induction motor will develop a net torque in the direction of motion, and that torque will keep the rotor turning. If the motor’s rotor had originally been turned in a clockwise direction, the resulting torque would be clockwise and would again keep the rotor turning.

Double Revolving Field Theory According to this theory, any alternating quantity can be resolved into two rotating components which rotate in opposite directions and each having magnitude as half of the maximum magnitude of the alternating quantity. In case of single phase induction motors, the stator winding produces an alternating magnetic field

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