Schrage motor is an inverted 3-phase induction motor with primary winding on the rotor and the secondary winding on the stator. The schematic diagram of a 2-pole Schrage motor is shown in the figure.
In a Schrage motor, a 3-phase supply is fed to the rotor circuit. The resultant air-gap flux runs at synchronous speed Ns with respect to the rotor and the rotor runs at a speed Nr in the opposite direction with the short-circuited rotor windings. As a result, the magnetic field runs at slip speed with respect to the stator inducing current in it which is having frequency equal to the slip frequency. Hence, the torque is produced in the motor.
The rotor also has a DC winding in the same slots as the primary winding. The DC winding of the Schrage motor is also known as tertiary winding or regulating winding. The DC winding is connected to a commutator on which three sets of brush pairs are placed for a 3-phase EMF injection into the stator winding or secondary winding to control the speed and power factor of the motor.
When the injected EMF adds to the secondary EMF, the speed of the motor increases and when it opposes, the speed decreases. The placement of brushes on the same commutator segment cancels the effect of secondary winding and hence the machine works as an inverted induction motor. The frequency of the brush EMF is always the slip frequency because the rotating field moves at slip speed with respect to the brushes.
The advantages of a Schrage motor over an induction motor are given as follows −
Schrage motor provides a constant torque over a wide range of speeds.
The power developed in the Schrage motor is proportion to the speed of the motor.
No external resistances are required for speed control of the motor, hence, the overall efficiency of the Schrage motor is improved.
The speed of the Schrage motor is independent of the load.
The speed of the Schrage motor can be easily increased or decreased over a wide range of 0.5 Ns to 1.5 Ns.
The Schrage motor has the following disadvantages −
A Schrage motor is more expensive than an induction motor of the same rating.
The maintenance cost of a Schrage motor is higher than that of an induction motor.
The moment of inertia of rotor of a Schrage motor is greater than that of an induction motor of the same size.
Due to the third winding (or DC winding), the losses are increased in a Schrage motor.
The placement of primary winding on the rotor limits the supply voltage, in turn limiting the output of the motor.