Page 389 - Offshore Electrical Engineering Manual

376 CHAPTER 5 Large Motors

Table 7.5.1 DC Megohmmeter and Polarisation Index Tests for AC Motors
Motor Rated AC Voltage DC Test Motor Rated AC Voltage DC Test
Voltage (V) Voltage (V)

Below 100 500
Above 100 and up to 600 1000
Above 600 and up to 5000 2500
Above 5000 5000

IEEE 43-2000
Insulation Resistance Test Direct
Winding Rated Voltage (V) Voltage (V)
<1000 500
1000–2500 500–1000
2501–5000 1000–2500
5001–12,000 2500–5000
>12,000 5000–10,000
AC, alternating current; DC, direct current; IEEE, Institute of Electrical and Electronics
Engineers.

6. With a Megger or similar instrument perform the following test using the volt-
ages given in Table 7.5.1. With the neutral point disconnected test the phase-to-
phase insulation by applying the test voltage to each phase in turn with all other
phases earthed. This test measures the combined insulation resistance of the
tested phase, both to earth and to the other (earthed) phases.
7. Again using Table 7.5.1, with the neutral point connected, test the winding insu-
lation to earth by applying the test voltage to each phase winding connection in
turn, with all other phase connections insulated from earth. The winding insula-
tion resistance should be corrected to 40°C by using the equation R = K R ,
c
t t
where R is the insulation resistance corrected to 40°C, R is the insulation
c
t
resistance at temperature t in °C and K is the insulation resistance temperature
t
coefficient at temperature t obtained from Table 7.5.2.
POLARISATION INDEX
This test is a continuation of the above-mentioned Megger test and requires the appli-
cation of the appropriate test voltage for 10 min. The ratio of the 10-min to the 1-min
reading is termed the polarisation index (PI) and gives some indication of the dryness
of the windings and their ability to withstand overvoltage tests.
The procedure is as follows. Using a motor-driven Megger or similar instrument
measure the resistance to earth and between the phases of the motor windings. The
test voltage (obtained from Table 7.5.1) should be applied for 10-min readings and
those noted after 1 and 10 min.