What should be the minimum resistance required between 3 phase and grounding (connecting wire to earth)

[Manjunath]

Hi
I am new to electrical engineering. I want to measure the 3 phase voltage near the water pumpset motors.

I have connected phase lines to earth through a diode and six 1 mega ohm resistors (measuring the voltage across resistor). The same earthing wire is connected to ground in my dc circuit.

Now my motor is damaged and the winding is burned.

I am not sure what is the reason? Will this circuit causes damage to the motor windings? Please suggest.

What is the minimum resistance required between 440v to earthing during motor on ( carying current of 20 amps).

P.s: I have taken earthing as i dont have neutral point

Thanks
Manjunath

 

[pc1966]

A schematic might explain what you are doing better than 1000 words.

There is no way that a 1M resistor could pass enough current to damage the motor so they must have been some other action. In most cases a damaged motor is overheating from either a mechanical overload (and not having a proper motor starter set to the right overload limit) or where forced cooling has failed for some reason (vents blocked up, fan disabled, etc)
[automerge]1590064701[/automerge]
Another thing that can damage a 3-phase motor is the loss of one phase (assuming no protection kicks in) as it can limp along rotating but draws a lot more current.

 

[Manjunath]

A schematic might explain what you are doing better than 1000 words.

There is no way that a 1M resistor could pass enough current to damage the motor so they must have been some other action. In most cases a damaged motor is overheating from either a mechanical overload (and not having a proper motor starter set to the right overload limit) or where forced cooling has failed for some reason (vents blocked up, fan disabled, etc)
[automerge]1590064701[/automerge]
Another thing that can damage a 3-phase motor is the loss of one phase (assuming no protection kicks in) as it can limp along rotating but draws a lot more current.

A schematic might explain what you are doing better than 1000 words.

There is no way that a 1M resistor could pass enough current to damage the motor so they must have been some other action. In most cases a damaged motor is overheating from either a mechanical overload (and not having a proper motor starter set to the right overload limit) or where forced cooling has failed for some reason (vents blocked up, fan disabled, etc)
[automerge]1590064701[/automerge]
Another thing that can damage a 3-phase motor is the loss of one phase (assuming no protection kicks in) as it can limp along rotating but draws a lot more current.

Thanks a lot for the reply.
I have attached the schematic. Requesting you to please go through it and suggest if it is wrong.

I have used power resistors and shorted the earthing and ground lines of circuit. The same line voltages are connected to motor also.

The circuit is working good to measure the voltage, but the motor windings are damaged aftersometime. I am not sure about the reason.

Kindly suggest

Thanks
Manjunath

 

[pc1966]

Well the diagram you have shown looks perfectly OK. If it is reading sensible values for each of the 3 phases then it would confirm you have assembled it to match the schematic. It would not explain any damage to the motor.

The cause is likely to be as above (motor overloaded, loss of one phase).

 

[Lucien Nunes]

I think it is an unfortunate coincidence and that the motor would have failed anyway. Or, you have accidentally disturbed a high-resistance or intermittent connection in one line and the overcurrent on the two remaining lines was not detected by a correctly adjusted overload relay, as PC1966 suggests. Before replacing the motor, I would check the overload settings carefully and also the operation of the control circuit to ensure that it does respond when the relay is triggered by overload.

 

[Manjunath]

I think it is an unfortunate coincidence and that the motor would have failed anyway. Or, you have accidentally disturbed a high-resistance or intermittent connection in one line and the overcurrent on the two remaining lines was not detected by a correctly adjusted overload relay, as PC1966 suggests. Before replacing the motor, I would check the overload settings carefully and also the operation of the control circuit to ensure that it does respond when the relay is triggered by overload.

Thank you both for your kind response.
The motor is submersible 3 phase motor of 6hp. I did multiples ON and OFF frequently with 1 or half minutes time difference between ( 10 times) during my testing. I dont think this will affect the motor.
Any comments on this.
Apart from this I didn't do anything. It might have damaged because of some other reasons. Anyway i will try to find out the reasons as per your suggestions.