Diode Test; Testing The Transformer-Rectifier - Hobart 60CU20 Operation And Maintenance Manual With Illustrated Parts List

OM-2226 / Operation and Maintenance Manual
60CU20 / Series 500062 / 400 Hz. Generator Set

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 are as follows:
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.
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.
6) Testing the Transformer-Rectifier (for units with the DC option)
The 28.5-VDC transformer-rectifier is an optional add-on to the GPU. The following test procedures may
be used for testing the T-R following repair, or for just checking performance.
a) Preparation for Testing
(1) Connect the T-R to a load bank.
(2) Start the GPU per the operating procedures in Chapter 1, Section 3.
b) Operational Test Procedure
(1) Power Delivery
a Press "DC OUTPUT" push button to close the DC output contactor. The blue "DC OUTPUT"
lamp will glow indicating that DC power is being delivered to the load bank.
b Observe the DC voltmeter on the control panel, under no load for accuracy. It should indicate
approximately 28.5 VDC.
c With the GPU connected to a load bank, place a 600 A load on the GPU. Observe the DC
voltmeter and ammeter for accuracy.
(2) Discontinue Power Delivery
Press "DC OUTPUT" push button to open the DC output contactor. The blue "DC OUTPUT" lamp
will not glow indicating that DC power has been removed from the load bank.
May 2, 2013 Chapter 2-3
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