c.
Checking Magnetic Pick-up Signal
Connect a high impedance voltmeter to magnetic pick-up input terminals (terminals 10 and
11, Fig. 5) on controller terminal board. The voltage value at no load, governed speed should
be 4-V AC MINIMUM.
NOTE:
A signal as low as 2.5-V is sufficient to operate the control unit satisfactorily. If a no-voltage
condition is indicated, the magnetic pick-up is too far away from flywheel teeth, or it is defective.
4.
Generator and Exciter Test
The generator fields and exciter stators may be tested with a Kelvin bridge. This is a double-bridge type
instrument required for the very low resistance’s encountered in this test. It is understood that 0
resistance indicates a SHORT CIRCUITED condition. An infinite resistance reading indicates an OPEN
CIRCUITED condition. See Fig. 6 for resistance values.
a.
Disconnect generator stator leads at the output module panel.
b.
Disconnect the two black exciter field leads from terminal block mounted on output module panel.
c.
Check resistance and compare to values given in Figure 6.
Test Connection
Resistance (Ohms)
Generator Stator Phase A to N (G1)*
0.00235
Generator Stator Phase B to N (G1)*
0.00235
Generator Stator Phase C to N (G1)*
0.00235
Exciter Stator Field (L2)
26.5
A - B, B - C, C - A Exciter Armature (G2)
0.1
Generator Revolving Field (L1)
2.1
*NOTE:
The two leads of a phase must be connected when test is made. Take readings when unit is
cold and in an ambient temperature of 70º F (21º C).
Generator and Exciter Stator Tests
Figure 6
5.
Diode Test
Test values for diodes are not given here because they could be misleading. Test values may vary
even between diodes of the same part number, rating, and manufacturer. General instructions for
testing diodes follow:
a.
Disconnect exciter windings from diode lead(s).
b.
Use a good quality ohmmeter. An instrument which indicates 50 ohms at the center of the scale is
preferable.
NOTE:
Make certain the battery is in good condition and the pointer is adjusted to zero when the test
lead points are shorted together. Set the scale selector to RX1.
c.
Hold one ohmmeter lead point on the threaded end of the diode. Hold the other lead point on the
wire terminal end. Observe and note the indicated resistance. Now reverse the lead connection on
the diode. Again observe and note the ohmmeter indicated resistance. Generally speaking, if an
infinite or very high resistance was indicated with the leads connected one way and a low, readable
resistance was indicated with the leads connected the opposite way, the diode may be considered
good.
OM-2084 / Operation and Maintenance Manual
90CU24 / Series 500392 Generator Sets
November 27/1997
Chapter 2-3
Page 17
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