What is the most effective way to perform a megger test on an electrical motor?

1 Prepare the motor

Before you start the megger test, you need to disconnect the motor from the power supply and isolate it from any other equipment. You also need to discharge any stored energy in the motor, such as capacitors, by grounding it for at least five minutes. Then, you need to clean the motor terminals and remove any dust or grease that might affect the test results. You should also check the temperature and humidity of the motor and the surrounding environment, as these factors can influence the insulation resistance.

2 Connect the megger

A megger is a device that applies a high voltage to the motor windings and measures the current that flows through them. The ratio of the voltage to the current is the insulation resistance, which indicates the quality of the insulation. To connect the megger, you need to attach one lead to the motor frame and the other lead to one of the motor terminals. You should test each terminal separately and record the results. You can also test all the terminals together by connecting them with a jumper wire.

3 Perform the test

To perform the test, you need to turn on the megger and select the desired test voltage. The test voltage should be appropriate for the motor rating and type. For example, a 480-volt motor can be tested with a 1,000-volt megger, while a 2,300-volt motor can be tested with a 2,500-volt megger. You should avoid using a higher voltage than necessary, as this might damage the insulation. Then, you need to press the test button and hold it for at least one minute. The megger will display the insulation resistance value on the screen. You should compare this value with the manufacturer's specifications or industry standards to determine if the motor is in good condition or not.

4 Interpret the results

The results of the megger test can vary depending on several factors, such as the age, size, type, and usage of the motor, as well as the temperature, humidity, and cleanliness of the environment. However, as a general rule, a good insulation resistance value should be above 1 megohm per 1,000 volts of applied voltage. For example, if you test a 480-volt motor with a 1,000-volt megger, you should get a reading of at least 500 megohms. If the reading is lower than that, it means that there is some problem with the insulation, such as deterioration, contamination, or damage.