1. Choose the appropriate megger: If the rated operating voltage of the motor being tested is 380 volts, then we can select a 500V megger.
2. Place the megger flat, perform a short circuit test by shorting the two probes, and if the pointer approaches 0 while cranking the handle, it indicates good condition.
3. Then separate the two probes, crank the handle, and if the pointer approaches infinity, it indicates good condition.
4. When measuring, it is best to remove the connection plate of the three-phase motor, ground the casing, and label the three winding terminals from left to right as U, V, W.
5. Step one: Measure the insulation resistance between the three-phase output terminals and the casing. Connect E to the motor casing and L to the three terminals U, V, W respectively. Crank the handle at about 120 RPM, and when the pointer stabilizes near infinity, it indicates good insulation.
6. Step two: Measure the insulation between the three points U, V, W, measuring each pair once. If all three sets of data show the pointer at infinity, it indicates good insulation.
7. If you do not remove the connection plate, you can still measure. This is the difference between star and delta connections. In a star connection, you can measure the resistance between the three points U, V, W and the neutral point. If the three resistance values are close, it indicates good condition. In a delta connection, measure the resistance between each pair of U, V, W, and if the values are close, it indicates good condition. Using a multimeter for resistance measurement is more accurate, and also measure the resistance to ground.
How to quickly and simply determine the condition of a motor?
1. How to test the condition of an AC three-phase motor?
1. Use a megger, a 500V megger is sufficient, to measure the insulation resistance between the three terminals and the motor casing. It should be above 0.5M ohms to indicate no short circuit to ground.
2. Use a multimeter to measure the resistance between phases A/B/C. They should be approximately equal. If the difference is too large, the motor can still run, but it won’t last long. Remember, the larger the motor, the smaller the resistance! However, all three phases should not be 0 ohms unless it is a very large motor, such as one over 50KW! Remember, if it is a variable speed motor, the resistance values of the six terminals can be different!
3. Check the bearings and fan; generally, if the motor is wound, it is best to replace everything! Sometimes a seized bearing can also burn out the motor!
4. The no-load current of the motor is generally 10% to 50% of the rated current, and sometimes the no-load current can even be zero!
5. The rated current of the motor during operation is at full load, and the output power is basically 100%. If the running current is low, it indicates that the motor’s output power is reduced, indicating light load operation.
2. How to test the condition of an AC single-phase motor?
Use a 500V megger to measure the insulation resistance between the motor windings and the casing, which should not be less than 0.5 megohms; use a multimeter to check for continuity in the winding leads. If all the above requirements are met, the motor is in good condition. To test the capacitor’s condition, a pointer multimeter is more convenient (there are also digital multimeters with a capacitor range that can measure directly). Set the multimeter to the 1K or 10K resistance range, measure the two leads of the capacitor, and if the needle quickly deflects to the right and then slowly returns to the left, the capacitor is good; if it stays deflected to the right, the capacitor is shorted; if the needle does not move, the capacitor has an internal break or no capacitance. This method can only determine the condition of the capacitor.
3. How to determine the condition of a DC motor?
First, check for any broken wires and measure the resistance to see if it is normal. If it is a brushed DC motor, you can rotate the rotor and use a multimeter to measure if the output DC voltage is normal. If it is a brushless DC motor and has three-phase leads, you can rotate the rotor and use a multimeter to measure if the output AC voltage is normal. The output voltage is proportional to the speed.
4. How to determine the condition of an offline motor?
1. Use a multimeter to measure the DC resistance of the three-phase windings. The closer the results of the three measurements are, the better. If the difference is more than 10%, the phase with the lower resistance may have an inter-turn short circuit.
2. Use a 500V insulation megger to measure the insulation resistance between the three-phase windings and between each phase and ground (to the casing). For motors that have been stored for a long time, the insulation resistance should be above 0.5 megohms to be considered good and usable. If it is above 0.1 megohms but below 0.5 megohms, it indicates that the motor has absorbed moisture and should be dried before use. Very low insulation resistance indicates that the insulation has been damaged.
3. Rotate the motor rotor; it should turn smoothly and without noise or friction, indicating that the bearings and air gap are normal. It can also turn when connected to 220V.
Source: This article is reprinted from the internet, and the copyright belongs to the original author. If there are any copyright issues, please contact us for deletion. Thank you!
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