Figure 7-13.Control circuit components.
Figure 7-14.Three-phase motor controller diagram.
TWO-WIRE CONTROL.Two-wire control
provides no-voltage release or low-voltage release.
Two-wire control of a starter means that the starter
drops out when there is a voltage failure and picks up as
soon as the voltage returns. In figure 7-15, the pilot
device is unaffected by the loss of voltage. Its contact
remains closed, ready to carry current as soon as line
voltage returns to normal.
difficult to understand. As you already know, most
The phrases no-voltage release and two-wire
electrical circuits are represented as a wiring diagram
control should indicate to you that an automatic pilot
or a line diagram. Work through the examples given
device, such as a limit switch or a float switch, opens
throughout this chapter. This practice will improve
and closes the control circuit through a single contact.
your skills in reading and understanding electrical
diagrams. If the diagrams are too complex, break them
down to a more elementary diagram. These diagrams
are your key to understanding how a machine operates
and how to repair it when it breaks.