Figure 7-18.Three-phase starter controlled by a float
push button, the float, the pressure, or the limit
switches, are connected between line 1 and the holding
coil. The normally closed overload contacts are always
located between the holding coil and line 2. A wiring
diagram usually can be found in the cover of the
controller. Now it has been established that the motor
and line voltage are in working order. This checking
has narrowed the problem to the control circuit and the
chance that some components are open.
You can locate opens in the control circuit with a
voltmeter. Connect one lead of the voltmeter to line 1,
and touch the other lead to first one terminal or the
holding coil and then the other terminal. There should
be the same voltage reading that is read between line 1
and line 2. If the control circuit voltage is supplied with
a transformer. the voltage read should be that of the
transformer output. If there is no voltage on either side
of the holding coil, the overload contacts are open.
Pushing the RESET button should close the overload
contacts. If they do not close after they have had time to
cool. they may be defective. In this case. they should be
If there is a voltage on one terminal of the holding
coil but not the other, the coil is open. You must then
replace the coil. If there is a voltage on both terminals
of the holding coil, the coil and the overload contacts
can be assumed to be in working order. To double-
check these components, short out line 1 and the
terminal marked 3 with a piece of wire. This action will
bypass the external control, and then the holding coil
should close the contacts. You can use a current-
limiting resistor in place of a w-ire. If the control
functions, the problem is in the external controlling
Solid-state controllers have very complicated
circuitry; thus, troubleshooting these units requires a
good background in electronics and electric motors.
These controllers have repair instructions with them as
well as a list of parts that should be stocked for repair
purposes. Repairs consist of replacing boards or
modules that plug into the circuity.
A combination starter consists of a magnetic
starter and disconnect switch mounted in the same
enclosure. These starters are supplied with either a
fused disconnect switch or a circuit breaker. The fuses
(or circuit breaker) provide short-circuit protection by
disconnecting the line. A combination starter and
circuit breaker will prevent single phasing by
simultaneously opening all lines when a fault occurs in
any one phase. This type of starter can be quickly reset
after the fault has been cleared. Figure 7-19 shows a
fused combination starter. Figure 7-20 shows a
combination starter and a thermal-magnetic circuit
We will now show you a number of control circuits
with various combinations of push-button stations. A!!
of these diagrams use one type of magnetic switch, but
others can be used. Figure 7-21 shows a magnetic
switch that is operated from any of three stations.
Figure 7-22 shows a straight-line diagram of the
control circuit of three start-stop stations. Figure 7-23
shows the control circuit of two start-stop stations. In
these diagrams, the START buttons are connected in
parallel, and the STOP buttons are connected in series.
These button connections must be done regardless of
the number of stations. Note that the maintaining
contact is always connected across the START button.
All STOP buttons are connected in series with one
another and in series with the holding coil, so the motor
can be stopped from any position in case of emergency.