rotor, maintaining rated speed at an increased torque
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.
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
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
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.