Figure 1-27.-Block diagram of current flow.

rotor, maintaining rated speed at an increased torque

demand.

Since the synchronous speed of the stator flux is

directly proportional to the input frequency of the supply

to the motor, a change is necessary in the rotor torque to

maintain constant speed for this variation also. The

frequency regulator supplies the proper triggering pulse

to the rotor SCRs controlling the current flow in the

rotor, hence controlling the speed/torque of the motor.

Figure 1-26.-Schematic symbol silicon controlled rectifier.

CONTROL EQUIPMENT

The motor control consists of an ac magnetic starter

containing overload protection, start and stop switches,

type of SCR. Recise firing is attained by a short gate

and a frequency-regulating system. The first two com-

pulse with an amplitude of at least 3 volts and is capable

ponents are standard; however, the frequency-

of delivering the maximum firing requirements of the

regulating system is further divided into a detector, a

SCR.

preamp and trigger, a starter, a motor rotor control unit,

and a resistor unit (fig. 1-27).

Short or delayed SCR firing time allows a small

rotor current to flow, thus limiting the torque developed

FREQUENCY REGULATOR

by the rotor required to maintain rated speed (necessary

for 400 Hz) at no-load or light loads. As generator load

The detector in the frequency-regulating system

is primarily a frequency-sensing transformer with a

is increased, a greater current is allowed to flow in the

Figure 1-27.-Block diagram of current flow.

1-23