Electrical Distribution Systems MOTOR CONTROLLERS2. When a low voltage conditionFigure 13 LVR Controlleroccurs, the M coil will drop out atsome pre-determined value ofvoltage, and the M and Macontacts will open. The STARTbutton must then be pushed torestart the motor.3. Depressing the STOP button de-energizes the M coil, which thenopens the M and Ma contacts.The purpose of the LVR controller is tode-energize the motor in a low voltagecondition and restart the motor whennormal voltage is restored. This type ofcontroller (Figure 13) is used primarily onsmall and/or critical loads (e.g., cooling water pumps required for safety-related equipment).LVR Controller Operation:1. Place the START switch in Run which energizes coil M, closing the M contacts andstarting the motor.2. When a low voltage condition occurs, the M coil drops out, opening the M contacts andde-energizing the motor. When normal voltage is restored, the M coil is again energized,closing the M contacts and restarting the motor.The LVRE controller maintainsFigure 14 LVRE Controllerthe motor across the line at alltimes. This type of controlleris of the manual variety and isfound mostly on small loadsthat must start automaticallyupon restoration of voltage(Figure 14). An LVREcontroller may or may notcontain overloads. Ifoverloads are used, they willbe placed in the lines to theload.The motor controllers that have been discussed are very basic. There are many automatic controlfunctions that can be incorporated into these types of controllers, but they are beyond the scopeof this text.Rev. 0 Page 19 ES-15
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