Ans. The simple and the compensated types.
Ques. Describe a simple repulsion motor.
Ans. It consists essentially of an armature, commutator and field magnets. The armature is wound exactly like a direct current armature, and the windings are connected to a commutator. The carbon brushes which rest on this commutator are not connected to the outside line, however, but are all connected together through heavy short circuiting connectors. The brushes are placed about 60° or 70° from the neutral axis. The field is wound exactly like that of the usual induction motor.
Ques. What is the action of this type of motor?
Ans. If nothing be done to prevent, the motor will increase in speed at no load until the armature bursts, just as it will in a series direct current motor.
Ques. What provision is made to avoid this danger?
Ans. A governor is usually mounted on the armature which short circuits the windings, after the motor has been started. The motor then runs as a squirrel cage induction motor. As a rule the brushes are lifted off the commutator when the armature is short circuited, so as to prolong their life.
This is a very successful motor, but it is of course more costly than the simple squirrel cage motor used on two and three-phase circuits.
Fig. 1,911.—Diagram of connections of Sprague variable speed single phase compensated repulsion motor and controller. The controller is designed to give speed reduction and speed increase as resistance or reactance is inserted in the energy and compensating circuits. With the exception of the leads brought out from these circuits, the constant speed and variable speed motors are identical. The standard controller gives approximately 2:1 speed variation.