Baker D30R, User Manual

Page: 36 / 82

High Voltage DC Testing
12/3/2009 | 71-022 V6
D30R 36
−
~900uA in the 100uA/div range,
−
~90uA in the 10uA/div range,
−
~9uA in the 1uA/div range or
−
~0.9uA in the 0.1uA/div range
The over-current trip will remove the high voltage from the test leads, stop the test and
illuminate the red HIPOT TRIP lamp on the front panel. Releasing the TEST button resets the
trip circuitry, extinguishes the red HIPOT TRIP lamp, and readies the tester for another test.
Effects of temperature
Temperature has a very strong effect on Megohm readings because insulation resistance
varies inversely with temperature on an exponential basis. (IEEE 43 has a very good
description of this effect.)
Simply put, the insulation resistance drops in half for every 10
o
C rise in temperature.
Therefore, before any judgments are made regarding the health of a motor’s insulation based
on a trend of past Megohm measurements, all the measurements used in the trend should be
compensated or corrected for temperature. The temperature compensation of the insulation
resistance means to convert all the IR measurements used in the analysis to the same
temperature. The recommended temperature to use is 40
o
C. The following formula should be
used to make the calculation.
For example: An insulation resistance/Megohm value is 5000Mohms at 30
o
C, the
compensated IR value at 40
o
C is 2500Mohms.
Step Voltage test
Another test that can be performed using the Digital tester is the Step-Voltage test. This test
is used to indicate the condition of winding insulation by observing the linearity of leakage
current as voltage is increased in steps. The best results can be achieved if historical records
are maintained of multiple Step-Voltage tests, beginning with measurements made when the
winding is new.
Use the same voltage increments and time intervals for all Step-Voltage tests of a particular
winding. Baker Instrument Company, an SKF Group Company, recommends that results are
plotted on graph paper in order to make comparisons of different tests.
Note: It is important for the insulation to be free of moisture and dirt when this test is made.
Step Voltage test procedure
Determine the number of steps to perform, and the voltages and time increments that best
suits testing needs.
For example, if the maximum voltage should be 30,000 volts, it may be best to use six steps
of 5000 volts. The time interval will depend on the capacitance of the test piece and the type
of insulation it uses. Choose an interval that lets a noticeable change in resistance readings
occur at each step. One minute step intervals are fairly standard or many windings (See IEEE
95 for more information).
For example, with a 12 kV test instrument:
( ) (
)
R R
T c
T
∗
⎥
⎦
⎤
⎢
⎣
⎡
⎭
⎬
⎫
⎩
⎨
⎧
−
∧ =
10
40
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