400v and 690 current rating of contactors

[Ali-rashid]

I was choosing contactors for DOL starting a 690v motor. I was reviewing the specs of Schneider LC1F400M7. It is written:
200kW on 400vAC. This means the rated current is about 400Amps on 400vAC.
280kW on 690vAC. This means the rated current is about 280Amps on 690vAC.

Why the contactors is not capable of conducting 400Amps in 690vAC? in other words, why when voltage goes up, the current carrying capability of this contactor is decreased?

 

[waross]

They are both probably capable of carrying much more current.
I saw a lot of damage done to a 400 HP motor and driven fan.
An exploded coupling and two bent shafts.
An engineer determined by testing that a 250 HP rated contactor could safely carry the current of a 400 HP motor.
Unfortunately the 250 HP contactor could not safely interrupt the current of a loaded 400 HP motor.
When the stop button was pushed, the ensuing arcs joined up phase to phase and put a short on the motor terminals.
The motor became a shorted induction generator and stopped so fast that the coupling exploded and both shafts were bent.
Don't argue with the manufacturer's ratings.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[LionelHutz]

I think you'll find it's mostly related to the power in the arc as the contactor breaks the current.

 

[waross]

I agree Lionel.
As a matter of interest, we had a set of contactors that would blow the arcs right through the de-ion plates and out of the front of the contactor. In damp weather the arcs had a tendency to join up, phase to phase, and blow the fuses.
The owner eventually had us change out all the contactors for a type which did not have exposed de-ion plates.
No more problems.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[crshears]

Could I infer that one would have to be much more cautious and conservative about contactor choices for switching fully loaded induction motors than if block switching purely resistive loads? Reading between the lines, it seems like inductive loads have a much greater arc persistence, if that isn't insufficiently technically accurate.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[ScottyUK]

crshears,

That would be a reasonable inference.

Contactors for inching and plugging duty - AC-4 in my part of the world - have to switch locked rotor current at a highly lagging power factor, which is why they are so much larger than their AC-3 equivalents which only have to break running current at a more reasonable PF of about 0.8-ish.

 

[jraef]

It seems to be something specific to that contactor. I looked at the A-B IEC contactor that size, and it is rated AC-3 for 315kW at 400V, 355kW at 690V. It does drop to 280kW at 1000V, so that points to there being an issue with voltage increase. So I too think it will have to do with arc suppression and the added heating that takes place if the arc lasts longer.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[waross]

In the event of a motor stall the contactor must break locked rotor current.
That doesn't happen with resistors.

Bill
--------------------
"Why not the best?"
Jimmy Carter