How to judge the quality of a motor, there are two main parameters, one is to measure the resistance of the motor coil, and the other is to measure the insulation resistance between the coil and the shell. The method of measuring insulation resistance of a three-phase motor is the same as that of a single-phase motor, but the method of judging the resistance of the measuring coil is different.

three phase motor

The three-phase asynchronous motor commonly used in work is composed of three sets of coils, and most of the coil connections are Y-shaped or △-shaped.

Schematic diagram of three-phase motor

Motor junction box

To judge whether the coil of the three-phase motor is good or not, it is necessary to measure whether the resistance value between the three-phase windings is balanced. It is not necessary to know the actual resistance value of each group of coils of the corresponding power motor, and it is not necessary to remove the connecting piece to measure the resistance values of the three groups of coils to judge whether the resistance values of the three-phase coil windings are balanced, which is troublesome.

Whether it is a Y-connected motor, a delta-connected motor, or a two-speed motor. Both can be used to measure the resistance between terminals U1, V1, and W1, and it can also be used to judge whether the three-phase resistance of the motor is balanced.

A multimeter is usually used to measure the resistance of the motor coil. The multimeter is selected in the resistance 200Ω range. Use the red and black test leads to measure the resistance between the motor (U1, V1), (V1, W1), and (U1, W1).

multimeter

When the mutual error between the measured resistance values is 2%, it can be judged that the motor resistance value is normal. When the error between the measured resistance values reaches more than 10%, it can be judged that the motor is broken.

When the error of the measured resistance value is 2%-10%, it is necessary to consider whether the coil of the motor is produced by the original factory, or whether the coil of the motor has been burned out and rewound in the later stage, and the level of maintenance personnel for replacing the coil is a problem, and the resistance of the wound coil There is an error, and when the error is not large, it can be used normally. When you are not sure, you can use the frequency converter to drive the motor to check whether there is an alarm phenomenon to judge whether the motor is good or bad.

When the three-phase resistance of the motor coil is balanced, it cannot be said that the motor is good, and the insulation resistance of the motor must be measured. Generally, when the insulation resistance of the motor is lower than 0.58MΩ, it means that the motor is not well insulated and should not be used. When the insulation resistance of the motor is higher than 0.58MΩ, it means that the motor has good insulation and can be used normally.

When measuring the motor with a multimeter, the gear of the multimeter is in the resistance MΩ range. Put one test lead on any terminal of U1, V1, W1, and the other test lead on the motor casing, pay attention to the place where there is no paint. When the measured resistance value is greater than 0.58MΩ or it is not connected, it can be judged that the motor insulation is normal. . When the measured resistance value is less than 0.58MΩ, the insulation is not good and should not be used. The insulation resistance of the motor measured by the multimeter is not necessarily accurate. When you feel that the insulation resistance value of the motor is different every time you measure it, it will affect your judgment. A megohmmeter can be used. The data measured by the multimeter is only of reference value, while the value measured by the megohmmeter is relatively accurate and can be regarded as real data.

megger

The E and L terminals of the megohmmeter are connected to the measuring wire rods. The measurement method of the two measuring wire rods is the same as that of the multimeter pen to measure the motor. That's it.

single phase motor

Single-phase motors usually have only two sets of coils, the main winding and the auxiliary winding. The auxiliary winding starts and the main winding runs. Inside the motor, the main winding and the auxiliary winding are connected in series, and one wire is drawn out from the connection, and three wires are generally drawn out of a single-phase motor, which can be represented by three wire ends ①②③.

Single-phase motor coil and wiring

Use the 200 ohm range of the multimeter. When the motor power is small, you can increase the ohm range of the multimeter.

Use a multimeter to measure the three wires ①②③ of the single-phase motor. You can choose two-phase measurement, ①②, ②③, and ③① to measure three sets of resistance values. The measured resistance values are normally 1 large and 2 small.

When the sum of the two small resistance values measured is not equal to the maximum resistance value, it proves that the motor is damaged, or when the measured resistance is infinite, it proves that the motor winding is broken and damaged. The main winding and the auxiliary winding of the single-phase motor are intertwined. It is the main winding and the secondary winding that are short-circuited together, and there are very few short-circuits in one group of windings.

When the sum of the two groups of small resistance values is the largest resistance value, it proves that the motor coil winding is normal.

Single-phase motors also need to measure insulation resistance. The measurement method is the same as that of three-phase motors. Please refer to the method for measuring insulation resistance of three-phase motors above. If the insulation resistance is higher than 0.58MΩ, it proves that the insulation resistance of the motor is normal. If the insulation resistance is lower than 0.58MΩ, it proves that the motor is insulated. Not good and should not be used.

Generally, the resistance value with the smallest resistance value is measured as the main winding, the second largest resistance value is the auxiliary winding, and the largest resistance value is the main and auxiliary winding series resistance.

The single-phase motor wiring is to measure the largest resistance wire in parallel with the capacitor, and measure the smallest resistance wire to connect to the single-phase voltage.

When measuring the resistance value of a single-phase motor as 1 large and 2 small, when the two small measured resistance values are the same, it means that the resistance values of the main and auxiliary windings are the same. When the main winding is energized, the motor rotates forward. The motor is reversed. When the resistance values of the primary and secondary windings are the same, it means that the single-phase motor has the function of forward and reverse rotation, and can be used for forward and reverse rotation.