Star Conexion Terminals Burnt

[yulinios]

Hi there!

After one year of new motor pumps operation, I have found 3 of 6 motors (with the same submergible pump) with 2 phases of 3 with terminal burnt in the conection of star, I have read phase insulation and it´s good and similar comparing with good ones. Some data is 44 kW, 80 A (plate), 70 A operation.

Thanks in advance

 

[rbulsara]

Please clarify what you mean by
" wiht 2 phases of 3 with terminal burnt in the conection of star"??

Do you have all three phase wires connected to the motor?

Did two of the three terminals and the star terminal burn out?

 

[aolalde]

yulinios.
It apears to be due to a high resistance connection, please explain with more detail your findings.

 

[yulinios]

rbulsara:
Motor is connected in star, the terminals wich are adjusted by plates and nuts were good fixed, I have found only 2 cables and terminals with signs of high temperarture, cable insulation were burned in 2 inches, in service I took 57 C of temperatura in these terminals. Motor temperature is 50 C, well we will change the motor to check inside.
Aolalde: I hope that this clarify the landscape.

 

[powerjunx]

aoladle, is right.

Burnt terminal wires usually due to high resistance connection, that is; obviously a poor or loose connection.

yulinios,
" ..in service I took 57 C of temperatura in these terminals. Motor temperature is 50 C,.."

I wonder how you took this temperature while your submersible pumps were inaccessible - submerged. How you do it?

Actually when we test run this machine after such repair, we soak it down to the pit as though it is completely submerged and evidently the temperature is unaccountable unless an emmbedded temperature sensor is installed.

 

[Connection]

It is unusual for star connections to burn as they are effectively neutral with no potential difference. To have 3 out of 6 fail is even more unusual. I have seen smaller sub-m pumps that have a star-point thermal overload. Sometimes the overload fails creating a high resistance joint which would then burn the cables.
In a fixed bridge on a block with good connections I could only imagine that you possibly have a reverse phase or similar. IE the labouring 2 phases would burn out under higher load trying to compensate for the reverse phase. This could only be a connection problem or a design problem.

Cheers

 

[Laplacian]

If this motor has a VPI'd stator, check for epoxy in the lead wires. I've seen several motor lead failures due to VPI insulation in the cable strands on both new and rewound motors.

Connection - even though the star is neutral, full winding current still flows in and out of it along the leads connected to it - only the node sums to zero, not the physical connections.

 

[rbulsara]

If two of the 3 wires got heated up and burnt, one of the possibilities may be single phasing. Do these motors have single phasing protecion?