How do you find out which phase is bad in a three-phase motor?

Here are the methods to find out which phase of a three-phase motor has a fault:

I. Observation method

Check the motor appearance

First, observe the appearance of the motor to see if there are obvious signs of damage, such as burned windings or broken casings. If the winding of a certain phase is charred, it is very likely that this phase has a fault. For example, when the motor is overloaded or short-circuited, the winding of the faulty phase may be charred black due to overheating.

At the same time, check the junction box of the motor to see if there are loose, fallen off or charred terminal blocks. If the terminal block of a certain phase is loose or charred, it may also indicate that there is a problem with this phase.

Observe the motor running state

When the motor is running, observe the vibration, noise and temperature of the motor. If a certain phase has a fault, the motor may have abnormal vibration, increased noise or increased temperature. For example, when a phase winding is open-circuited, the motor may have severe vibration and noise; when a phase winding is short-circuited, the temperature of the motor may rise rapidly.

You can touch the motor casing with your hand to feel the temperature difference of each phase. If the temperature of a certain phase is significantly higher than that of the other two phases, then this phase may have a fault. However, be careful when touching the motor casing to avoid burns.

II. Measurement method

Use a multimeter to measure resistance

Disconnect the power supply of the motor, open the junction box of the motor, and use the resistance range of the multimeter to measure the resistance values of the three-phase windings respectively. Under normal circumstances, the resistance values of the three-phase windings should be equal or close. If the resistance value of a certain phase is significantly different from that of the other two phases, then this phase may have an open circuit, short circuit or ground fault.

For example, when measuring the winding resistance of a three-phase motor, suppose the resistance of phase A is 10 ohms, the resistance of phase B is 10.2 ohms, and the resistance of phase C is 2 ohms. The resistance value of phase C is significantly different from that of phase A and phase B, indicating that phase C may have a fault.

When measuring resistance, pay attention to choosing the appropriate resistance range and ensure that the test leads of the multimeter have good contact with the windings.

Use a megohmmeter to measure insulation resistance

Use a megohmmeter to measure the ground insulation resistance and interphase insulation resistance of the three-phase windings. Under normal circumstances, the insulation resistance should be within a certain range. If the insulation resistance value of a certain phase is too low, it means that this phase may have a ground fault or interphase short circuit fault.

For example, when measuring the insulation resistance of a three-phase motor, suppose the ground insulation resistance requirement is not less than 0.5 megohm. If the ground insulation resistance of phase A and phase B is 1 megohm, and the ground insulation resistance of phase C is 0.2 megohm, then phase C may have a ground fault.

When measuring insulation resistance, disconnect the motor winding from the power supply and ensure that the motor casing is well grounded.

Use a clamp ammeter to measure current

When the motor is running, use a clamp ammeter to measure the three-phase currents respectively. Under normal circumstances, the three-phase currents should be balanced or close to balanced. If the current of a certain phase is significantly higher or lower than that of the other two phases, then this phase may have a fault.

For example, when a three-phase motor is running normally, the current of each phase should be around 10 amperes. If it is found that the current of phase A is 10 amperes, the current of phase B is 10.5 amperes, and the current of phase C is 15 amperes. The current of phase C is significantly higher than that of the other two phases, indicating that phase C may have overload, short circuit or other faults.

When measuring current, pay attention to choosing the appropriate current range and ensure that the clamp of the clamp ammeter has good contact with the wire.

III. Other methods

Motor fault detector

Use a professional motor fault detector to quickly and accurately detect the faulty phase of the motor. Motor fault detectors usually can measure parameters such as winding resistance, insulation resistance, current, voltage, etc. of the motor, and judge the fault type and location of the motor by analyzing these parameters.

For example, some high-end motor fault detectors can detect early faults of the motor, such as local short circuit of windings and insulation aging, through technologies such as spectrum analysis.

Replacement method

If a certain phase is suspected to have a fault, you can try to replace the winding of that phase with the winding of a normal phase. If the fault of the motor disappears after replacement, then it can be determined that the original phase has a fault.

For example, when a three-phase motor has a fault and phase C winding is suspected to have a problem. You can replace the phase C winding with the phase A or phase B winding. If the motor operates normally after replacement, then it can be determined that the phase C winding has a fault.

In short, through the comprehensive application of observation methods, measurement methods and other methods, the faulty phase in a three-phase motor can be found relatively accurately. When detecting faults, pay attention to safety, ensure that the motor is disconnected from the power supply, and follow the correct detection methods and steps.