Hawkins Electrical Guide v. 06 (of 10) / Questions, Answers, & Illustrations, A progressive course of study for engineers, electricians, students and those desiring to acquire a working knowledge of electricity and its applications

by N. Hawkins

Ans. Starting coils.

Ques. What are "shading" coils?

Ans. Auxiliary coils as placed on fan motors in the manner shown in fig. 1,863.

Ques. How can single phase motors be started without the use of external phase splitting devices?

Ans. Such apparatus may be avoided by having the auxiliary winding of larger self-inductance than the main winding.

Ques. What is the character of the starting torque produced by splitting the phase?

Ans. It does not give strong starting torque.

Fig. 1,863.—Single phase fan motor with shading coils for starting. In addition to the main field coils, one tip of each pole piece is surrounded by a short circuited coil of wire or frame of copper, as indicated in the figure. This coil, or copper frame, is called a shading coil and it causes a phase difference between the pulsating flux that emanates from the main portion of each polar projection and the pulsating flux which emanates from the pole tip, thus introducing a two phase action on the armature which is sufficiently pronounced to start the motor.