Question:
Our one frequency inverter which drives 0.37 KW 400 V dosing pump motor intermittently (once in a month or once in two months) shows DC link fault and the speed is reduced to zero. This motor used to do changeover weakly. Pump NO: 1 never has such problem, pump NO: 2 only have this problem. We checked the motor found OK, checked the control circuit found ok, replaced with same new inverter still the same problem comes. We thought of incoming power supply problem so we swapped power supply cable from motor 1-2 but still the DC link fault comes in pump NO: 2. Then some of our experts said it is because the inductor is connected in the circuit, once remove the inductor this fault will not come again. But after removal of the inductor also same problem comes. From the previous history of work orders we found that this motor is a rewound motor, before rewinding there was no fault history at all. This motor is running always perfectly without any faults in manual control. Fault comes only in automatic control.
Could you please tell me what is the real problem?
Is it because of rewinding of the motor; winding geometry might have changed that affects the frequency inverter?
If this is the problem then why this fault is not coming whenever it is in service? (It waits for 1month or two months some time the fault comes in a weak also)
Is that the inverter will cause any problem because the inductor is in disconnected condition?
What is exactly the DC link fault and what are the reasons it can come in the inverter?
Why the DC link fault comes in when it is in automatic operation only?

Answer:
Have you compared the good unit to the bad unit?
Could there be any mechanical issues loading the motor?
Check that the current level on the bad motor is the same as the good motor.
It sounds as if the rewind data is not correct and the motor is taking high current. If the rewind data is correct the core loss may be high.

The procedures you have gone through would indicate that the motor is the issue. My advice would be to go to the OEM and purchase a new motor or if it is a standard motor your regular supplier should be able to supply them. It could even be beneficial to purchase two new motors and keep the existing good one as a spare.