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How to Use Megger to Test Brushless Generator

Sep. 28, 2019

Generator test is a preventive test, an important link in the operation and maintenance of power equipment, and one of the effective means to ensure the safe operation of power system. Today Dingbo manufacturer shares the way to test brushless generator by using megger.

What is Megger?

Megger is a kind of measuring instrument, which commonly used in electricians. It is mainly used to check the insulation resistance of electrical equipment, household appliances or electrical lines, so that ensure these equipment, electrical appliances and lines to work in a normal state and avoid accidents such as electric shock casualties and equipment damage.

Test on insulation resistance and absorption ratio of generator stator and rotor windings


Measuring insulation resistance is one of the contents of preventive test. It is the simplest and most convenient test method. Usually, megger is used to measure insulation resistance level. According to the measured size of insulation pad within one minute, it can detect whether there are defects in insulation and damp phenomena.

Yuchai diesel generator

When test, we should pay attention:

1.The generator should be disconnected from the power supply, and the generator winding should be fully discharged for no less than 15 minutes.

2.According to the rated voltage of the generator to select correctly the voltage grade of megger. We usually choose 2500V megger for high-voltage generator, and 500-1000V megger for the low-voltage generator generally. The 500V megger is usually used for the rotor. But when choose megger, should according to actual situation.

3.For long-term placement meggers, before measure, we should check whether it is good. Method: In open circuit test, the indication of the meter head should be “∞” and in short circuit test, the handle of the megohm meter should be shaken lightly and the indication of the meter head should be “0”, which indicating the meter is good.

4.The measuring lead should have enough insulation level. The insulating surface of winding B and Y should be shielded by wires, so as to eliminate the influence of edge leakage on the measured value.


When measuring, the ground wire and generator housing should be in good contact with the starting megohm meter. When the meter head indicates “∞”, then connect the fire wire to the winding under test, at the same time, record the time, read insulation value at 15s and 60s. During the whole continuous measurement process, the megger should maintain a steady rated speed about 120 revolutions per minute. Too fast speed will make the measured value low.


After finished measurement, the fire wire is disconnected at the rated speed of the megger to prevent the megohmmeter from being damaged by reverse charging.


Absorption ratio is the resistance ratio of insulation resistance in 60 seconds and in 15 seconds. The damp and oil pollution of the insulation of the generator's electronic winding will not only decrease the insulation, but also shorten the attenuation time of the absorption ratio characteristics. Because the absorption ratio is particularly sensitive to damp insulation, generally, it is used as one of the main indexes to judge whether insulation is dry or not.

Judging whether the stator is qualified:

1.Insulation resistance not less than 1MΩ.

2.At the same temperature, the same voltage level megohmeter, the same withstand voltage time conditions, comparing the measured insulation resistance with the previous one, it should not be less than 1/3 of the previous one.

3.For generators insulated by mica with asphalt impregnated rubber, absorption ratio is not less than 1.3 or polarization index is not less than 1.5. For the generator with epoxy mica insulation, the absorption ratio should not be less than 1.6 or the polarization index should not be less than 2.0.

If it does not meet the requirements, it means that the generator windings have been damped and should be dried.


Judging whether the rotor is qualified: 

As long as the insulation resistance of generator rotor winding is not less than 0.5MΩ, it can be considered qualified.


Hope above information can help you to test brushless generators

Dingbo manufacturer can supply 20kw-3000kw diesel generator sets with different brands, such as Cummins, Perkins, Volvo, Yuchai, Deutz, Shangchai etc. We focus on high quality products, if you are interested, welcome to contact us.

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