Insulation Resistance of an Induction Motor

The heating may have removed moisture, but if the insulation has become cracked from drying out over a long time (or some other embrittlement process), you may still encounter low phase-ground or phase-phase resistances.

Insulation Resistance:

Insulation resistance of the motor shall be measured between the windings of the machine and its frame by means of a meggar. Often the motors are kept in a store for some time or they are transported under very damp conditions and in such cases, the insulation resistance generally becomes low and it is dangerous for the motor to be connected up before the condition has been rectified.

Insulation resistance measurements are affected by the factors given below :

A) Surface condition,
B) Moisture,
C) Temperature,
D) Magnitude of test voltage,
E) Duration of application of test voltage,
F) Residual charge in the winding,
G) Ageing of insulation, and
H) Mechanical stresses.

Drying Out:
If the measured insulation resistance of the motor is less than 1 MΩ/KV with a minimum of 1 MΩ when the machine is cold, it should first be dried out before full voltage is applied to the terminals of the motors.

Principles of drying out:

Whatever method is employed for drying out a motor, the general principle is to apply heat continuously for a considerable time so as to drive out any moisture which may have become entrapped in the windings. Severe damage can result from improper heating of winding. Avoid too rapid heating. Provide some ventilation to carry off moisture and to ensure circulation of heated air. During drying out period, it is recommended to take measurement of the insulation resistance periodically. It will generally be found that at the beginning of the drying out, the insulation resistance decreases. This is due to the fact that as the heating starts the moisture is redistributed in the windings. After some time, the insulation resistance value reaches a minimum and then stabilizes at this for some time, after which it begins to increase until it reaches its maximum value. When the maximum value of the insulation resistance has been reached, it is safe to put the motor into service.

The convenient method of drying out the motor is to place heaters or lamps around it and inside it also. It is recommended to employ suitable guarding and covering arrangements so as to conserve the heat. Heating by infrared lamps may also be used for drying the windings.

An alternative method of drying out is to block the motor so that it cannot rotate and then apply such a low voltage to the starter terminals that full-load current flows in the starter. The temperature of the windings should not be allowed to exceed 90ºC.

Also, hot air may be blown into the motor but the air should be clean and dry and at a temperature of not more than 90ºC.

Close supervision is necessary during the process of drying out by the methods given above. The heat generated in the windings is not easily dissipated and one part of the winding may be exceedingly hot before another part has had time to expel the moisture. This may be obviated to some extent by taking every precaution to exclude draughts from the exposed parts of the windings.

The method of heating employed for drying out shall be continuous and the process shall be carefully watched to ensure that the winding does not attain a temperature sufficiently high to damage the insulation. The maximum safe temperature of the windings measured by thermometer is 90ºC. At the same time, the temperature should not he allowed to fall too low as otherwise re-absorption of moisture would take place.

The drying out should be continued as long as the insulation resistance rises, or until a sufficiently high value, that is, not less than 1 MΩ/KV with a minimum of 1 MΩ at 75ºC, has been reached.

During the drying out period, readings of temperature and insulation resistance shall be taken at least once an hour in order to see how the drying out is progressing. The temperature of the motor should he kept as constant as possible, otherwise the insulation resistance reading may be misleading.

If it is found that the insulation resistance of the motor does not rise even after drying out or is extremely low and persistently remains so, damage in the windings should be suspected. In some cases, this damage can be located by visual inspection but in case this damage has taken place either in the slots or in the bottom layer of the windings, cause should be investigated and defect remedied as it is very dangerous to put the motor into service if the insulation resistance is very low. If the winding insulation is very dirty and wet, insulation resistance may not increase by drying with heat. Then it has to be flushed with water jet and dried by beating. In case, insulation resistance does not improve, manufacturer may be consulted.

The recommended minimum insulation resistance (R_m) for ac and dc machine armature windings and for field windings of ac and dc machines may be determined by the following empirical relationship:

R_m = kV+1

R_m = recommended minimum insulation resistance in mega ohms at
40°C of the entire machine winding, and
kV = rated machine voltage, in kilovolts.

So, R_m at 40°C is in effect one mega ohm per 1000 volts plus one mega ohm.

_{(Reference "IS-900-1992 Code Of practice For Installation and Maintenance Of Induction Motors" and "IS-7816-1975 Guide For Testing Insulation Resistance Of Rotating Machines")}

Vinu_Answers Sure_Answers

GA from me.. BUT.. does he understand what he's copied??

Could you give the curve showing insulation resistance Vs temperature during the drying process?.

The typical drying-out curve for a DC motor armature shows how insulation resistance changes. During the first part of the run, the resistance actually decreases because of the higher temperature. Then it rises at aconstant temperature as drying proceeds. Finally, it rises to a high value, as room temperature (20°C) is reached. Note that if you conduct insulation resistance tests during drying, and you have readings of previous tests on the dry equipment, you'll know when you've reached the safe value for the unit. You may prefer to use a time resistance test, taken periodically (say, once a shift), using the dielectric absorption ratio or polarization index to follow dry-out progress (no need to correct for temperature).

EFFECT OF TEMPERATURE ON INSULATION RESISTANCE:
The resistance of insulating materials decreases markedly with an increase in
temperature. If you want to make reliable comparisons between readings, you should
correct the readings to a base temperature, such as 20°C, or take all your readings at approximately the same temperature (usually not difficult to do). We will cover some general guides to temperature correction.

One thumb rule is:

For every 10°C increase in temperature,
halve the resistance;
or, for every 10°C decrease,
double the resistance.

For example, a two-mega ohm resistance at 20°C reduces to 1/2 mega ohm at 40°C.

Each type of insulating material will have a different degree of resistance change with temperature. Factors have been developed, however, to simplify the correction of resistance values. Table below gives such factors for rotating equipment, transformers and cable. You multiply the readings you get by the factor corresponding to the temperature (which you need to measure).

For example, assume you have a motor with Class A insulation and you get a reading of 2.0 megohms at a temperature (in the windings) of 104°F (40°C).
From Table below you read across at 104°F to the next column (for Class A) and obtain the factor 4.80. Your corrected value of resistance is then:

2.0 MΩ (reading at 104ºF) X 4.80 (corrected value for class A at 104ºF) = 9.6 MΩ (Reading for 68ºF or 20ºC)

For more information refer link attached below:

http://www.slideshare.net/PowerpointEngineering/guide-to-insulation-testing

May this will help you.

VInu_answers Sure_Answers

I have to say I agree with the guy who wishes to remain unknown... you do seem to copy a lot of info, without the detailed understanding of what you are saying.

sharing is fine and dandy and I'm sure you mean it in a "I want to help" kinda way, but you MUST know the subject matter BEFORE you copy and paste.

You my friend do not.... and looking at your past offerings, this is not the first time you've been "pulled" on your blatant plagiarism.

There's an old saying my father taught me.. if you don't know what you are talking about, don't talk!

Can anyone suggest whether to inject D.C. through the stator coil in series to improve IR values?

I have heard of applying low voltage to the windings to heat them and try to bake out moisture. It can help if that is the problem. However, it will not help deteriorated insulation or other problems.

If an oven is not available,one can fabricate a box lined with thermal insulation material and fix some carbon filament bulbs. In 1960s when no oven was available we used it even to bake rewound motors after applying shellac varnish.

not had to rewind a motor for some time now, but I think motors are still cooked to set the varnish, like the idea of the DIY oven!!

you can pass current in the winding connecting three phases in series across 415 volts which dissipates moisture and improves insulation reisistance.