A Schrage motor is a polyphase commutator motor with shunt characteristic, in which the rotor carries two windings, of which one receives current from the supply by means of collector rings, while the other is connected to the commutator. The commutator carries two adjustable sets of brushes and supplies each of the separate phases on the stator with adjustable voltages, in order to obtain a variation of speed and of the reactive power taken from the supply.
The Schrage motor is a variable speed motor that was commonly used in applications related to yarn and carpet making. It was developed in Sweden, but is not commonly used today.
Technology
The Schrage motor has a wound rotor. On the rotor, there is a three phase delta winding with the endpoints of the delta brought out on to slip rings. The three phase supply is connected to these slip rings. Also on the rotor, there is a second delta connected winding with tapings brought out to a commutator. On the stator, there are three discrete windings separated by 120 degrees. The ends of these windings are connected to the commutator by two sets of brushes such that one set of brushes connects to the starts of the stator windings and the other set connects to the ends of the stator windings. The two sets of brushes can be moved to connect to the same point on the commutator, or moved apart in either direction. When they are in alignment, the stator windings are shorted and the motor behaves like an induction motor with the rotor and stator swapped. As the brushes are separated, voltage from auxiliary rotor winding is coupled to the stator winding. The degree of separation and the direction of separation varies the voltage and polarity of the voltage. The frequency of the voltage applied to the stator is dependent on the slip.
In effect, the stator has an induced voltage from the slip, plus a driven voltage from the commutator. This causes the speed of the motor to change. Speed variations of up to 10 to one are possible and the motor can provide a high torque at all speeds.
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