LOW-VOLTAGE CONTROL.Sometimes it
is desirable to operate push buttons or other control
devices at some voltage lower than the motor voltage.
in the control system for such a case. a separate source.
such as an isolating transformer or an independent
voltage supply, provides the power to the control
circuit. This independent voltage is separate from the
main power supply for the motor.
One form of separate control is shown in figure
7-17. When the thermostat calls for cooling and the
high-low pressure control is activated, the com-
pressor motor starter coil M is energized through the
step-down isolating transformer. When coil M is
energized, power contacts in the 240-volt circuit
close to start the refrigeration compressor motor.
Since the control circuit is separated from the power
circuit by the isolating control transformer, there is
no electrical connection between the two circuits.
For this reason, the wire jumper attached to L2 on a
starter should be removed for different voltages;
however, the overload relay control contact must be
included in the separate control wiring.
TROUBLESHOOTING AND TESTING
In this section it is assumed that the motor and fuse
are in good condition. To make certain that the motor is
not at fault, connect a voltmeter at the motor terminals
and determine whether voltage is available when the
contacts of the controller are closed. If there is no
voltage, the trouble probably, lies in the controller.
Figure 7-17.Low-voltage control circuit.
By using a snap-around type of voltmeter-
ohmmeter or individual instruments, you can conduct
many of the tests needed to determine opens, shorts,
grounds. and continuity in just a short time. You can
test malfunctioning circuits for shorted coils; open
coils; grounded coils; open resistances; shorted
resistances; low voltages; high voltages: excessive
amperes; broken, loose, or dirty connections; and
many other problems with comparative ease. This
testing is true of all motors. as well as starters.
You should follow a systematic procedure when
You must exercise extreme caution when test-
ing live components. Always use the one-hand
rule to avoid completing the circuit between the
live component and a metal sur-face. Always
have a second person standing by when work-
ing on energized equipment and ensure the
person is qualified in CPR. When working on
anything that should have the power off, always
shut the power off yourself. Most disconnects
have allowances for a padlock to be used to
keep the power from being turned back on. This
safety precaution is called LOCKOUT. The
NAVOSH Manual, OPNAVINST 5 100.23,
provides guidance on the Lockout/Tag out pro-
gram at shore activi-ties according to OSHA
regulations. It is extremely important to take
this precaution. Controls with voltage over 240
volts should never be energized when you are