Current Imbalance

[rainsong]

What could be the possible cause of the following situation?

Soft Start load (300hp MOTOR) reading as follows:

A phase/Parallel 250MCM #1 - 134A
A phase/Parallel 250MCM #2 - 134A
*B phase/Parallel 250MCM #1 - 124A*
B phase/Parallel 250MCM #2 - 134A
C phase/Parallel 250MCM #1 - 134A
C phase/Parallel 250MCM #2 - 134A

When B phase leads were reversed in the lug configuration on the load side of the Soft Start, the OTHER B phase lead also read 124A.

That rules out that the wire with the first reading was a different length.

Very strange. Could smeone maybe shed som light?

 

[sreid]

If I understand what you are saying, it would appear to me that one of the lugs in the soft starter has a higher resistance than the other lugs.

 

[jraef]

That's the way I would interpret that too. Are they separate lugs, or separate holes in a common lug? If separate holes, I'd then say it is some seemingly minor corrosion built-up inside the hole, the set screw isn't compressing well under the same torque values, or some other issue with the way the lug is making contact with the cable strands.

For instance, sometimes if the threads of the steel set screw are binding or galling into the aluminum lug body, you are tightening to the specified torque value but not all of that force is translating into compression strength in the cable strands. Try using a little conductive anti-seize compound on the threads such as Loctite Graphite 50 or CRC Nickel Anti Seize compound.

If separate lugs, look also at the connection of the lug to the heat sink. If a little non-conductive grease got on there during assembly it could be affecting the metal-to-metal contact.

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[davidbeach]

A higher resistance on one lug would make more sense if the other B phase conductor was at 144A. Why is the total current on B lower than on A or C?

 

[jraef]

Because if it were balanced out between those 2 conductors it would be 129A on that phase? That would not be unheard of or even enough to be concerned with from a phase-to-phase standpoint (4%). The weird part is that it didn't follow the cable.

 

[electricpete]

I have never seen a softstarter, so don't have much experience in that.

What do you mean about swapping the leads at the softstarter... does the softstarter have two output terminals per phase or one terminal per phase with two lugs landed? If two output terminals, can you see where they're joined or is that internal (if joined all)?

How long is the run of parallel cable?

Small conductors have primarily resistive impedance but large conductors like this have primarily reactive impedance. For a high resistance loose connection to signficantly effect the initially-reactive impedance of a long run of large-diameter cable, you have to have a pretty womping high ressitance.... to the point I would expect you would see discoloration at terminations by simple visual inspections. Far lesser hotspots will be easily visible with thermography, assuming the terminations are accessible for inspection while under load.

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