The Shaded-Pole Induction Motor

Author: E.E. Kimberly

Of the many devices used for obtaining starting torque in a single-phase motor, the shaded pole is the simplest and lowest in cost, but it is otherwise the least meritorious. Fig. 21-2 shows schematically the electrical features of a four-pole shaded-pole motor. The name arises from the so-called shading coil of one or more short-circuited turns of conductor surrounding a portion of each pole on the air-gap end. The function of that coil is to delay the time of rise of flux in the shaded portion of the pole, and also to delay the time of fall of the same flux relative to the flux in the unshaded portion. This action simulates very imperfectly the rotating fields of a polyphase motor and produces a torque which tends to rotate the squirrel-cage rotor from the unshaded to the shaded portion of the pole.

Fig. 21-2. Four-Pole Shaded-Pole Motor

Fig. 21-3. Shaded-Pole Motor Characteristics

Fig. 21-3 shows some typical speed-torque curves of shaded-pole motors with low-resistance and high-resistance rotors. These motors are of relatively low efficiency and are unsatisfactory for any use requiring more than a small fraction of 1 hp. The efficiency could be improved by opening the shading-coil circuits after a starting; but, inasmuch as simplicity and low cost are the chief merits of this kind of motor, the added mechanism for opening the coils is not justified. This type of motor is used in small desk fans, domestic-furnace control, and other services in which its inferior characteristics may be tolerated. Fig. 21-4 shows a two-pole, so-called core type of shaded-pole motor commonly used in small fans and phonograph-record players.

Fig. 21-4. Two - Pole Core - Type Shaded-Pole Motor With Two Shading Coils per Pole (More Than One Shading Coil per Pole Improves Performance)