Baker D30R User Manual - page 27
Principles of High Voltage DC Testing
12/3/2009 | 71-022 V6
D30R
27
4)
Sometimes users don’t have or want to take the time to do the full 10 minute PI test.
The DA test is basically a shortened version of the PI test. Instead of forming the ratio of
insulation resistances at 10 minutes and 1 minute, the DA test, as Baker Instrument
Company, an SKF Group Company has it implemented, is the IR ratio at 3 minutes and 30
seconds:
There are no accepted minimum or maximum values of the DA test. However, the DA value is
considered to be trendable. Any change in the DA value indicates that something is changing
in the ground wall insulation system. The stator may be contaminated or wet. The stator may
also be running hot and burning insulation. Usually, changes in the DA will be accompanied by
a change in one of the other “recognized” tests such as the Megohm test, PI test or the DC
Over Voltage test.
The
HiPot
test demonstrates that the ground wall insulation system can withstand a “high”
applied voltage without exhibiting an extraordinarily high leakage current or actually breaking
down. The test consists of applying a DC voltage to the windings of the machine, same as a
Megohm test, but at a higher voltage – usually more than twice the voltage of the machine’s
operating voltage. Therefore, the HiPot test is often called a “Proof” test. The insulation
resistance value at the high applied voltage is not of interest with the HiPot test. However, the
value of the leakage current is and, more specifically, whether or not the observed leakage
current is within acceptable limits.
he choice of test voltage depends on whether a new motor (or coil) is being tested for
acceptance or whether an existing motor is being tested for continued service. Consult your
organizations’ policies regarding the HiPot test voltage to be used. The simple formula of
“2V+1000” generally results in a good test voltage for the HiPot test for motors already in
service. Other recommended HiPot test voltages can be found in IEEE 95, ANSI C50.10-
1977, IEC 34.1 and NEMA MG-1 (see chapter 2).
The HiPot test usually lasts one minute and the leakage current recorded at the end of the
minute. The leakage current at the end of this minute is to be recorded for future
comparisons. Between the time when the voltage is applied to the motor and the time when
the leakage current measurement is taken, the operator should carefully observe the leakage
current and watch for sporadically varying leakage current that might indicate weak
insulation. Such variations should be considered a failure of the insulation.
sec)
30
(
min)
3
(
IR
IR
DA
=