428 Electrical Power Systems Technology
Single-phase AC Motors
DC Motors
Motor Starting Protection
Dynamic Braking
Speed Control Circuits
Frequency Conversion Systems
Programmable Logic Controller
BASIC CONTROL SYSTEMS
Electrical power control systems are used with many types of loads.
The most common electrical loads are motors, so our discussion will deal
mainly with electric motor control. However, many of the basic control sys-
tems are also used to control lighting and heating loads. Generally, the
controls for lighting and heating loads are less complex.
Several power control circuits are summarized in Figures 16-1 through
16-9. Figure 16-1 is a start-stop pushbutton control circuit with overload pro-
tection (OL). Notice that the “start” pushbutton is normally open (NO),
and the “stop” pushbutton is normally closed (NC). Single-phase lines L1
and L2 are connected across the control circuit. When the start pushbut-
ton is pushed, a momentary contact is made between points 2 and 3. This
causes the NO contact (M) to close. A complete circuit between L1 and L2
results, which causes the electromagnetic coil M to be energized. When the
NC stop pushbutton is pressed, the circuit between L1 and L2 will open.
This causes contact M to open and turn the circuit off.
The circuit of Figure 16-2 is the same type of control as the circuit
given in Figure 16-1. In the circuit of Figure 16-2, the start-stop control of
a load can be accomplished from three separate locations. Notice that the
start pushbuttons are connected in parallel, and the stop pushbuttons are
connected in series. The control of one load, from as many locations as is
desired, can be accomplished with this type of control circuit.
The next circuit (Figure 16-3) is the same as the circuit in Figure 16-
1, except that a “safe-run” switch is provided. The “safe” position assures
that the start pushbutton will not activate the load. A “start-safe” switch
circuit often contains a key, which the machine operator uses to turn the
control circuit on or off.
Figure 16-4 is also like the circuit of Figure 16-1, but with a “jog-run”
switch added in series with the NO contact (M). In the “run” position, the