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31. Single-Phase Repulsion-Induction Motor.- A repulsion-induction motor is used as a projector drive motor on PG-10 equipment. One of these motors is illustrated in Figure 19. The rotor of this machine contains a commutator and is similar to the armature of a DC motor. The stator is wound with a four-pole single-phase winding. This winding is somewhat similar to the field winding of a four-pole DC motor. The starting device consists of two brushes

mounted so as to make contact with the commutator, a resistor to prevent the motor from coming up to speed too rapidly, and a centrifugal mechanism for shorting out the entire commutator after the motor has attained sufficient speed to run as a single-phase induction motor. As stated in section 30 the action of a single-phase induction motor, when its rotor is stationary, has the tendency to turn the rotor in both directions at once. This action, of course, results in no rotation at all, but, if by some means this balance can be disturbed, rotation will result. The brushes short-circuit a part of the commutator so as to unbalance the torque set up by the rotor current. The current in the rotor is due to the transformer action between the stator and the rotor. The unbalancing action caused by the short-circuited portion of the rotor gives a predominance of torque in one direction so that the motor starts to rotate. The direction of rotation depends upon the position in which the brushes are mounted. The running winding will keep the motor running in either direction once it is up to speed. If the brushes were short-circuited through a very low resistance, such as a copper wire, the starting torque would be too great and an excessive strain would be put on the projector mechanism. A resistor of 9 ohms resistance is used to connect the two brushes together through the frame of the motor itself. This reduces the starting torque sufficiently to prevent undue strain on the projector.

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Chapter Three Pages
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
[16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27]

Chapter
[1] [2] [3] [4] [5] [6] [7] [8] [9]
[10] [11] [12] [13] [14] [15]

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