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138 RICHARDSON'S BLUEBOOK OF PROJECTION
(23) Some theatres are located where alternating current is not available, therefore cannot use synchronous motors driven by the supply line, but must rely on d.c. equipment. Projectors in such theatres may be driven by d.c. motors fitted with special speed controls. A shunt field d.c. motor is used, with a speed control in series with the field winding.
(24) A rheostat introduced in series with the field controls the motor speed by controlling the field current. As the rheostat control is adjusted to change the amount of resistance in series with the field, the current flowing through the field changes, hence the motor speed is altered.
(25 ) It is important to note that increasing the current through the field decreases the speed of the motor. The reason is as follows: When the armature of a motor revolves, the magnetic force of the field coils, which causes the armature to revolve, also acts to. generate a voltage in the armature by induction. This induced voltage is opposite in direction to the driving voltage admitted by the brushes and the commutator, and is called a counter-voltage or counter-e.m.f . The counter-e.m.f . has a slowing-down effect. Under conditions of normal operation an increase in field current is more effective in generating counter-e.m.f. than in increasing speed of rotation, consequently increasing the field current by means of the field rheostat actually has the effect of slowing the motor, whereas decreasing the field current drastically cuts the counter-e.m.f. in the armature, thus causing the motor to speed up.
(26) Automatic speed control for d.c. projector motors may be effected by a centrifugal electric contact, which closes when the motor runs too fast, and in so doing short-circuits the field resistance. The field then receives more current, the motor slows down and the centrifugal contact reopens. The field current is thereby reduced, the motor speeds up until the contact closes again and the cycle of events repeats itself. The motor can never run appreciably faster or slower than the critical speed for which the centrifugal contact is set.