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DRIVES 607
developed in F-3 opposes that in F-2. Additional regulation is secured by the fact that the induction machine is designed for the rated speed of 1200 R. P. M., and imposes a minimum mechanical load on the motor when it is driven at that speed.
Condenser C-l serves to keep the current through F-2 in phase with the current induced in the armature. Without it the motor could not develop normal power. The additional condenser is sometimes connected in parallel to increase the starting torque of this motor. The contact marked "110 V.," which shorts out resistor R-3, is sometimes used for the same purpose.
When the switch D-2 is thrown to "Variable" the induction machine is open-circuited and does not function. Rheostat R-l is then used like the field resistor of any shunt motor to vary the speed as desired.
The Circuit of Figure 149
(13) Figure 149 represents one of four types of motor control cabinets using vacuum tubes. This particular model was chosen as an example of such equipment because it is the one most widely used.
The motor is diagrammed at the extreme left. There are three stationary windings, the stator, the field and the armature. The rotating element consists of two parts, namely, the rotor and the toothed iron wheel drawn just above it. The entire arrangement, taken as a whole, may be regarded not only as a driving motor but as a motor-generator assembly consisting of one motor and two generators. One of the generators is an apparatus electrically independent of the motor, although built into the same casing. This is an inductor generator, consisting of the toothed iron wheel, the field coil and the armature coil. The other generator is the rotating element of the motor itself, the rotor. The stator is the driving field. It induces alternating current in the rotor windings, and by the magnetic reaction between the rotor field and the stator field the motor is compelled to revolve.
Let us next see how the stator winding is powered, and how the field of the inductor generator is excited.