Answer / biju9cr

Knowing the polarization Index of a motor or generator can
be useful in appraising the fitness of the machine for
service. The index is calculated from measurements of the
winding insulation resistance.

Before measuring the insulation resistance, remove all
external connections to the machine and completely discharge
the windings to the grounded machine frame.

Proceed by applying either 500 or 1000 volts dc between the
winding and ground using a direct-indicating, power-driven
megohmmeter. For machines rated 500 volts and over, the
higher value is used. The voltage is applied for 10 minutes
and kept constant for the duration of the test.

The polarization index is calculated as the ratio of the
10-minute to the 1-minute value of the insulation
resistance, measured consecutively.

Polarization Index =


Resistance after 10 minutes/
Resistance after 1 minute

The recommended minimum value of polarization index for ac
and dc motors and generators is 2.0. Machines having
windings with a lower index are less likely to be suited for
operation.

The polarization index is useful in evaluating windings for:

* Buildup of dirt or moisture.
* Gradual deterioration of the insulation (by comparing
results of tests made earlier on the same machine).
* Fitness for overpotential tests.
* Suitability for operation.

Answer / mohammed sarfraz

POLARIZATION INDEX is ability of the insulation to absorb
the voltage over a period af time, this give an indication
of the overall insulation quality of individual pieces of
insulation in the transformer. this test is usually
performed as ratio of insulation resistance for 10 minutes
to ir value of 1 minutes.

the value should be less then 2.5.

Answer / maheshwar prasad

Polarisation index is the ratio of megger value taken for 10 minutes to the megger value taken for 1 minute. The significance of polarization index test. Let I be the total initial current during polarisation index test or PI test. IC is the capacitive current. IR is resistive or conductive current. IS is surface leakage current. IP is polarization current of the insulator. Value of insulation resistance test or IR value test, i.e. value megger reading just after 1 minute of the test, is- Megger value of 10 minute test, is Therefore, result of polarization index test, is From the above equation it is clear that, if the value of (IR + IS) >> IP, the PI of insulator approaches to 1. And large IR or IS or both indicate unhealthiness of the insulation. Th value of PI becomes high if (IR + IS) is very small compared to IP. This equation indicates that high polarization index of an insulator implies healthiness of insulator. For good insulator resistive leakage current IR is very tiny. It is always desired to have polarisation index of an electrical insulator more than 2. It is hazardous to have polarisation index less than 1.5.

how a single machine can act as both motor and generator? and give explanation interms of connections?

1 Answers  

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