Page 392 - Offshore Electrical Engineering Manual
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Motor Commissioning Tests 379
Table 7.5.4 Recommended Minimum Values of PI
(a) Minimum Insulation Resistance for the Entire Winding at 40°C
Motor rated voltage (V) 11,000 6600 3300 440
Minimum insulation resistance (MΩ) 12 7.6 4.5 1.5
(b) Minimum PI Values
Motor insulation class A B F
PI 1.5 2.0 2.0
PI, polarisation index.
For details of guard circuits see Thorn EMI’s: A Simple Guide to Insulation and Continuity
Testing, 1984.
FIGURE 7.5.1
Polarisation index curve for motor windings.
Therefore, to some extent, acceptance will depend on the individual circum-
stances, such as motor size, number of poles, motor operating environment and fre-
quency of operation.
Typical resistance/time characteristics are shown in Figs 7.5.1 and 7.5.2, which
illustrate the behaviour of insulation under different conditions and show the sig-
nificance of PIs. It must be stressed that the PI depends on a number of contributory
factors, such as the winding condition, the insulation class and the type of machine.
