of the same motor control system shown in
the other three diagrams. It is laid out so the
process of operation is easy to trace. The
schematic is sometimes called an elementary
or a single-line diagram. It is also useful for
To understand the electrical operation of
figure 4-8, you must be able to identify all
of the components in the figure. L1, L2,
and L3 are the three phases of power. The
control circuit fuse is placed in the path of
the beginning of the control circuit. Therefore,
when excessive current is drawn through the
control circuit, the fuse will blow and open the
path. The M coil is located in the control circuit,
which, when energized, changes the position of
contacts labeled with an M. OL1 and OL2 are in-
line fuses in the load circuit which protect the
motor. When OL1 or OL3 opens due to excessive
current or too much heat, contacts OL1 or OL3
open, causing the M coil to be de-energized.
Follow the logical sequence in table 4-1. It shows
the electrical operation of this simple, but actual
design, you will see in the field.
Table 4-1Three Phase Motor Controller Circuit
Voltage is present at
L1, L2, and L3.
The path extends to
the top of contacts
M1, M2, and M3.
L1 is providing
voltage through the
control circuit fuse
and the stop button.
The load (motor)
will not operate at
4 - 9