434 Electrical Power Systems Technology
than during normal operation. Motor-starting equipment should attempt
to reduce starting currents to a level that can be handled by the electrical
power system where they are being used.
Full-voltage Starting—One method of starting electric motors is
called full-voltage starting. This method is the least expensive and the sim-
plest to install. Since full power-supply voltage is applied to the motor
initially, maximum starting torque and minimum acceleration time result.
However, the power system must be able to handle the starting current
drawn by the motor.
Full-voltage starting is illustrated by the diagram in Figure 16-11. In
this power control circuit, a start-stop pushbutton station is used to con-
trol a three-phase motor. When the NO start pushbutton is pressed, cur-
rent will flow through the relay coil (M), causing the NO contacts to close.
The line contacts allow full voltage to be applied to the motor when they
are closed. When the start pushbutton is released, the relay coil remains
energized, because of the holding contact. This contact provides a current
path from L1 through the NC stop pushbutton, through the holding con-
tact, through the coil (M), through a thermal overload relay, and back to
L2. When the stop pushbutton is pressed, this circuit is opened, causing
the coil to be de-energized.
Primary Resistance Starting—Another motor starting method is
called primary resistance starting. This method uses resistors in series with
the power lines to reduce the motor-starting current. Ordinarily, the re-
sistance connected into the power lines may be reduced in steps until full
voltage is applied to the motor. Thus, starting current is reduced according
to the value of series resistance in the power lines. Since starting torque
is directly proportional to the current flow, starting torque is reduced ac-
cording to the magnitude of current flow.
OPERATIONAL POWER CONTROL SYSTEMS
Figure 16-12 shows the primary resistance starting method used to
control a three-phase motor. When the start pushbutton is pressed, coils
Sand TR are energized. Initially, the start contacts (S) will close, applying
voltage through the primary resistors to the motor. These resistors reduce
the value of starting current. Once the time delay period of timing relay TR
has elapsed, contact TR will close. The run contacts (R) will then close and