How To Measure The Resistance Of Three-phase Motor And Single-phase Motor To Judge The Quality Of The Motor

How to measure the resistance of three-phase motor and single-phase motor to judge the quality of the motor

 

Before the motor is used, or in the fault, the quality of the motor should be measured before it can be powered on.

 

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 of the coil and the shell. Three-phase motors measure insulation resistance in the same way as single-phase motors, but the method of measuring coil resistance 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 connection methods are Y-type hair extensions or △ connection methods.

 

Schematic diagram of three-phase motor

Motor junction box

 

To judge whether the coil of the three-phase motor is good or bad, it is necessary to measure whether the resistance between the three-phase windings is balanced. There is no need to know what the actual resistance of each group of coils of the corresponding power of the motor is, nor do you need to remove the connecting piece to measure the resistance of the three sets of coils to judge whether the resistance of the three-phase coil winding is balanced, which is more troublesome.

 

Whether it is a Y-type motor, a △ motor, or a two-speed motor. The resistance between the terminals of U1, V1 and W1 can be measured to determine whether the three-phase resistance of the motor is balanced.

 

The multimeter gear is selected in the resistance of the resistance 2002, and the resistance between the motor (U1, V1), (V1W1), and (U1W1) is measured with a red and black pen respectively.

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 bad.

 

When the error of measuring the resistance value is 2%-10%, it is necessary to consider whether the coil of the motor is produced by the original factory, or the coil of the motor has burned out and rewound the coil in the later stage, and the level of replacement coil maintenance personnel, the coil resistance of the winding has an error, and when the error is not large, it can also be used normally. When the grasp is not accurate, you can use the frequency converter to drive the motor to see if there is an alarm phenomenon to judge the quality of the motor.

 

When the motor coil resistance is balanced in three phases, it can not be said that the motor is good, but also measure the insulation resistance of the motor, generally when the motor insulation resistance is lower than 0.58MQ, it means that the motor insulation is not good and should not be used. When the motor insulation resistance is higher than 0.58MQ, it means that the motor insulation is good and can be used normally.

 

When measuring a motor with a multimeter, the multimeter gear is in the resistor MQ file. One meter pen on any terminal of U1, V1, W1, the other table pen on the motor shell, pay attention to the place without paint, the measured resistance value is greater than 0.58MQ or not through, you can judge that the motor insulation is normal. When the measured negative value is 0.58MQ, the insulation is not good and it is not used officially. The multimeter to measure the insulation of the motor is not necessarily accurate, when you feel that the insulation resistance value of the motor is different every time you measure it, it affects your judgment. Megohmmeter can be used, the data measured by the multimeter only has reference value, and the numerical accuracy measured by the megohmmeter is relatively high and can be regarded as real data.

Megohmmeter

 

The E, L terminal posts of the megohmmeter are connected to the measuring rod, and the measurement method of the two measuring rods is the same as the multimeter pen measuring motor, and the megohmmeter has no gear adjustment, as long as the hand stick is shaken at 120 revolutions per minute.

 

Single-phase motor

 

Single-phase motors usually have only two sets of coils, which are the main winding and the secondary winding. The secondary winding starts and the main winding runs. Inside the motor, the main winding and the secondary winding are connected in series, one wire is led out of the connection, and the single-phase motor generally leads out three wires, which can be represented by (1), (2) and (3) three wire heads.

Single-phase motor with wiring

 

Using the 200 ohm gear of the multimeter, when the motor power is small, the multimeter ohm gear can be increased.

 

Use a multimeter to measure the three wire ends of (1), 2, (3) of a single-phase motor, you can choose two and two phase measurements, (1) (2), (2) (3), (3) (1) (1) to measure three sets of resistance, the measured resistance is normally 1 large and 2 small.

 

When the sum of the two small resistance values is not equal to the maximum resistance, it proves that the motor is damaged, or the measurement resistance is infinite, it proves that the motor winding is damaged by a new circuit, and the main winding and the secondary winding of the single-phase motor are intertwined, and when the short circuit is short-circuited, the main winding and the secondary winding are short-circuited together, and the short circuit phenomenon of a group of windings is rare.

 

 

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

 

Single-phase motor also to measure insulation resistance, measurement method is the same as three-phase motor, please refer to the above three-phase motor measurement insulation resistance method, insulation resistance higher than 0.58MQ to prove that the motor insulation resistance is normal, insulation resistance lower than 0.58MQ to prove that the motor insulation is not good and should not be used.

 

Generally, the smallest resistance value measured is the main winding, the second largest resistance value is the secondary winding, and the largest resistance value is the main resistance and the series resistance of the secondary winding.

 

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

 

When the measured single-phase motor resistance is 1 large and 2 small, when the two small measurement resistance values are the same, it means that the main and secondary winding resistance values are the same, when the main winding is energized, the motor is forward, and when the secondary winding is energized, the motor is reversed. When the main and secondary winding resistance values are the same, it means that the single-phase motor has the function of forward and reverse rotation, which can be used for forward and reverse rotation.