Operational Power Control Systems 441
part of the parallel-connected motor windings, thus reducing starting cur-
rent. Once the motor has started, the line voltage is placed across all of the
motor windings. This method is undesirable for many heavy-load appli-
cations, because of the reduction of starting torque.
In Figure 16-15, when the start pushbutton is pressed, current will
flow through coil M1 of the time-delay relay. This will cause the NO con-
tacts of M1 to close, and a three-phase voltage will be applied to windings
T1, T2, and T3. After the time-delay period has elapsed, the NO contact
located below coil M1 in Figure 16-15 will close. This action energizes coil
M2 and causes its NO contacts to close. The M2 contacts then connect
the T7, T8, and T9 windings in parallel with the T1, T2, and T3 windings.
When the stop pushbutton is pressed, coils M1 and M2 will be de-ener-
gized.
Direct Current Starting Systems—Since DC motors have no coun-
terelectromotive force (cemf) when they are not rotating (see Chapter
14), they have tremendously high starting currents. Therefore, they must
use some type of control system to reduce the initial starting current.
Ordinarily, a series resistance is used. This resistance can be manually or
automatically reduced until a full voltage is applied. The four types of
control systems commonly used with DC motors are (1) current limit, (2)
definite time, (3) counter-emf, and (4) variable voltage. The current-lim-
it method allows the starting current to be reduced to a specified level,
and then advanced to the next resistance step. The definite-time method
causes the motor to increase speed in timed intervals, with no regard to
the amount of armature current or to the speed of the motor. The counter-
emf method samples the amount of cemf generated by the armature of the
motor to reduce the series resistance accordingly. This method can be used
effectively, since cemf is proportional to both the speed and the armature
current of a DC motor. The variable-voltage method employs a variable DC
power source to apply a reduced voltage to the motor initially, and then to
gradually increase the voltage. No series resistances are needed when the
latter method is used.SPECIALIZED CONTROL SYSTEMS
Electrical power control is usually desired for some specific applica-
tion. In this section, we will discuss some common types of specialized,
electrical power control systems that are used today.