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.
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 troubleshooting controls.
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 troubleshooting.Continue Reading