The Repulsion Motor

Author: E.E. Kimberly

The repulsion motor has operating characteristics similar to those of the series direct-current motor. It has a stator with a winding similar to that of a split-phase motor without the starting winding.

Fig. 21-9. Elements of Repulsion Motor

The rotor is similar to the armature of a direct-current motor, but it has no electrical connection to the supply line. The brushes are connected together in a common short-circuit. The principle of operation will be described with the aid of Fig. 21-9.

For simplicity the motor is shown with a salient-pole stator and with a rotor having full-pitch winding in which a conductor followed down in any slot would be found to return in a diametrically opposite slot. Consider the rotor to be stationary and the flux to be rising in the direction shown in Fig. 21-9 (a). No flux threads the loop 1-9, and hence no voltage is generated therein. In all other loops, by Faraday's Law, voltages are generated in the directions shown on the respective conductors. If a set of short-circuited brushes be placed on the horizontal axis connecting the winding at the two points 5 and 13, no current will flow because on either the top half or the bottom half the sum of the voltages is zero.

Fig. 21-10. Repulsion-Motor Starting Characteristics

If the brushes be rotated to the vertical axis, a large current will flow in the brushes and in all conductors in the directions shown in (a). By Lenz's Law, conductors 2, 3, 10, and 11 will produce a torque in a counter-clockwise direction, while conductors 7, 8, 15, and 16 will produce an equal torque in a clockwise direction. Hence, the motor will not tend to rotate.

Fig. 21-11. Repulsion and Induction Characteristics

If however, the brushes be rotated through the angle a to the positions shown in Fig. 21-9 (6), a new pattern of currents will appear, as shown. By Lenz's Law, conductors 16, 1, 2, 8, 9, and 10 will produce a clockwise torque without an opposing torque, and the motor will tend to rotate. If the brushes be displaced counter-clockwise from the vertical axis through a similar angle a, the torque will be of the same magnitude as before, but will be in a counter-clockwise direction. The direction of the torque is the same as the direction of brush displacement. Fig. 21-10 shows a typical curve of torque vs. a. The speed-torque characteristic is shown by curve (a) of Fig. 21-11.

The repulsion motor is well adapted to small-crane service or other applications in which large starting torque is essential and great speed regulation is not objectionable.