Testing And Checking Meters, Switches, Circuits, And Indicating Lights; Check Accuracy Of Voltmeter; Check Accuracy Of Ac Ammete; Check Operation Of Over Voltage Circuit And Indicating Light - Hobart 90CU24 Operation And Maintenance Manual

OM-2084 / Operation and Maintenance Manual
90CU24 / Series 500392 Generator Sets
e. Testing and checking meters, switches, circuits, and indicating lights (See Fig. 1)

(1) Check accuracy of voltmeter

a. Open door of control box and connect a master voltmeter of known accuracy to terminals of
the voltmeter (13).
b. Compare the unit's voltmeter reading with master meter. Error must not exceed 2% of full
scale.
(2) Check accuracy of AC ammeter
a. Connect a master ammeter of known accuracy to the AC ammeter (12).
b. Compare the unit's ammeter reading with master meter under various loads. Error must not
exceed 4% of full scale.
(3) Check operation of meter selector switch (18). A voltage value should be shown in each switch
position.
a. In any LINE-TO-NEUTRAL position, voltmeter reading should be 115 volts.
b. In any LINE-TO-LINE position, voltmeter reading should be 200 volts.
(4) Check accuracy of frequency meter
a. Connect a master frequency meter of known accuracy to the terminals of the frequency meter
(9).
b. Compare meter readings. Error must not exceed 1% of full scale.
NOTE: Make all protective system tests with the unit operating under load.

(5) Check operation of over voltage circuit and indicating light.

a. With the unit running at normal load, adjust the coarse adjustment potentiometer (36, fig. 1)
of the voltage regulator CLOCKWISE to increase voltage until the over voltage sensing
circuit actuates the protective monitor to open the load contactor and turn on the over voltage
indicating light. At 126 volts, the circuit will trip after a 1-second time delay. At higher values
of voltage, time delays for over voltage trips are as follow:
• At 140 volts, the circuit will trip within 160 milliseconds.
• At 180 volts, the circuit will trip within 50 milliseconds.
b. If the load contactor does not open under the conditions described in step (a), refer to the
Troubleshooting Chart in Section 2-4.
c. Return unit to normal operating conditions by adjusting coarse adjustment potentiometer
(turning it counterclockwise) and pressing reset push button switch (19).
(6) This step checks operation of the under voltage sensing circuit, indicating light, and time delay
circuit.
When the voltage is reduced to a predetermined value, the under voltage sensing circuit
activates the under voltage time delay circuit. If the under voltage condition continues for
approximately 7 seconds, the time delay circuitry will activate the protective monitor's under
voltage circuit, which in turn opens the load contactor to stop power delivery and turns on the red
under voltage indicating light (17).
A stopwatch is required for this check.
a. With the unit running at normal load, use the output voltage coarse adjustment potentiometer
(36) on the voltage regulator to reduce the voltage to 104 volts. The load contactor should
NOT open.
b. Reduce voltage in steps of 1 volt, with a time delay of 7 seconds between steps. At a setting
of 100 volts, the load contactor will open and the under voltage light will glow after a
November 27/1997 Chapter 2-3
Page 7