Hobart Brothers 6910, Operation And Maintenance Manual With Illustrated Parts List

Page: 80 / 206

(36) Check engine oil pressure at governed speed (2000 RPM). Gage should indicate at least 38 PSI
when the engine is hot. Check engine coolant temperature. Gage should indicate in the range of 180
degrees to 190 degrees depending upon ambient temperature.
(37) Check operation of fuse interlock
(Ref. 1-1; 2, Fig. 8).
With unit operating normally under load, re-
move protective relay circuit fuse (Ref. 11, Fig. 1). The load contactor should open immediately. This
indicates that the fuse interlock relay is functioning properly. Replace fuse and apply load.
NOTE: Make all protective relay tests with the unit operating under load.
(38) Check operation of overvoltage relay and indicating light.
(a) With the unit running at normal load, adjust the voltage regulator rheostat to increase voltage
(turn clockwise)
until the the over-voltage sensing relay actuates the protective monitor to open
the load contactor and turn on the overvoltage indicating light. The overvoltage sensing relay
should trip when voltage reaches 130 V to 134 V.
(b) Return unit to normal operating conditions by adjusting voltage regulator rheostat and press-
ing reset switch
(7).
(39) Check operation of undervoltage sensing relay, indicating light, and time delay. Before starting
this check, let us understand the sequence of events which should happen when voltage is reduced
to 103 V or lower. At some value between 103 V and 93 V, the undervoltage sensing relay
(Ref. 1-1;
5, Fig. 8)
should function to activate the undervoltage time delay circuit. Five seconds after the time
delay circuit is activated, (if the undervoltage condition continues) it should function to open the pro-
tective monitor relay which, in turn, will open the load contactor to stop power delivery. As a result of
the action, the undervoltage indicating light
(9) should be turned ON. A stopwatch or sweep-second-
hand watch is required for this check. Proceed as follows:
(a) With unit running at normal load, use the voltage regulator rheostat to reduce voltage to 104V.
The load contactor should NOT open after a 5-second delay.
(b) Reduce voltage in steps of 1 V, with a delay of at least 5 seconds between steps. Restart stop-
watch or note position of sweep-secondhand each time voltage is reduced. At some voltage value
between 103 V and 93 V, and 4 to 12 seconds after a new voltage setting is made, the load con-
tactor should be opened and the undervoltage indicating light should be turned on by the step-by-
step action of the undervoltage sensing relay, time delay circuit, and protective monitor relay.
NOTE: 1. The 4 to 12 second time delay is generally set at 5 seconds.
2. It may be necessary to switch to MANUAL control to obtain these low voltages.
(c) If the load contactor is not opened at 103 V to 93 V, it will be necessary to refer to the Trouble
Shooting Chart, Section 3-1, to determine which component of the undervoltage protective circuit
is defective.
(d) If the undervoltage circuit performs satisfactorily, return the unit to normal operation by adjust-
ing the voltage to 115 V, pressing the reset switch, and closing the load contactor.
(40) Check underfrequency sensing relay, protective monitor, and indicating light. At some frequency
value
(Hz, cycles-per-second)
from 385 Hz down to 375 Hz, the underfrequency sensing relay should
function to signal the underfrequency circuit in the protective monitor module to OPEN the load con-
tactor holding circuit, thus OPENING the load contactor. To check the underfrequency protective com-
ponents, proceed as follows:
OM-2028
2-3 June 20/89 Revised
Page 8