Which of the following motors can be used for the unity power factor?
Right Answer is:
Schrage motor can be used for the unity power factor.
The Schrage motor is virtually an induction motor and a frequency changer combined into one machine. The Schrage motor has characteristics similar to those of an induction motor with constant injected voltage. Only there is an increase in speed drop due to the addition of the resistance of the brush contact and regulating winding. At low speeds particularly, this drop is more than that of an induction motor but less than that of a shunt commutator motor.
In a Schrage motor with a 3:1 speed range, the full load speed is about 20 to 30% below the no-load speed. The figure shows the speed-torque curve of a Schrage motor working at super synchronous speed, normal speed, and subsynchronous speed.
The Schrage type motor is useful for drives needing shunt Speed-Torque characteristics and a continuously variable speed over a wide range.
The power factor of the Schrage motor approaches unity at speeds above synchronous as the negative slip results in a capacitive effect. At subsynchronous speeds the power factor falls; however, it can be raised in non-reversing motors by arranging that the brush movement is asymmetric with respect to the `in-line’ position, the axis bisecting a corresponding pair of brushes being progressively displaced in a direction opposite to that of the rotor rotation.
Note:- By varying the brush position the speed of the scharge motor can be obtained below and above synchronous speed and scharge motor can run on positive, negative, and unity slip therefore scharge motor can work on all power factors.