Why is the three-phase resistance balanced when measured by a multimeter?
Three phase water pumps may experience some leakage when measured with a megohmmeter. To be precise, it should have been caused by shaking with a megohmmeter during a power outage, indicating poor insulation. The cable or motor may have poor insulation between certain parts and the casing. But there is no problem with the coil of the motor itself. So it's good for you to measure the three-phase resistance with a multimeter. This is not contradictory. Find and handle the areas with poor insulation. That's enough.
I will only talk about the handling methods in practical work. Generally, a 500 volt megohmmeter is used to measure the insulation resistance of the coil and the shell. There are three results: one is that the resistance value is zero, as we say, if the shell is touched, it is grounded and the motor is broken. The second is that the resistance value is infinitely large, indicating that the motor is good. The third is that there is a resistance value, indicating that there is an insulation problem. Based on my experience, if the insulation resistance value is greater than 0.5 megohms, it can still be used, but if it is lower than 0.5 megohms, it cannot be used. The second issue is that it is not possible to use a multimeter to measure the balance of three-phase resistance values. The reason is that the short circuit between the turns of the motor coil cannot be measured. In my experience, even good motors do not have exactly the same resistance value. Some high-power motors have thick coils, and the resistance value you measure is zero ohms. This does not necessarily indicate a problem. To check the balance of the three phases, the motor can only be started and operated. Use a multimeter to check the three-phase current, and you will know if it is normal or not.
If you use a megohmmeter to measure the resistance between three-phase coils without disconnecting them, it means that you have no knowledge of instrumentation and should not play with it. If a megohmmeter is used to measure the resistance between three-phase coils and the casing, and a multimeter is used to measure the resistance between coils, it indicates that the water pump is heating up and causing insulation aging, and the coil winding needs to be replaced! I think you should be in the latter situation.
