DC motor tripping on Over & under excitation simultaneously

What's the rated working voltage of the motor?

1000 volts

Need to give more information pls.

Independant shunt motor? Rating of the shunt (field), ie volts and current. Base speed 185rpm, whats the max?(460rpm), min speed? Power rating of the motor at the various speed?

Yes, it's independent shunt motor. Field rating is 125 Volts, 128A ( full field). It reduces after base speed of 185 rpm and goes to almost 32A at max speed of 460 rpm. Motor power is 4478KW, armature current is 4700A & armature voltage of 1000V.

What's the supply to the field doing (volts), when going into field weakening? The current seems to drop very drastically from the current drawn at 185rpm (from 128A to 32A). I believe your problem lies in the supply to the shunt and hence the overspeed/field shunt trips.

The shunt current (for max speed, 32Amps) is only 25% of the original value at base speed (128Amps), which is to severe. According to my calculations the shunt current (for a max speed of 460rpm) should be approximiately 50amps.

Also from a table comparing (Hughes electrical technology), shunt current with the flux densities, a 25% flux density will result in half the current drawn on the shunt.

Check the supply to the shunt during fieldm weakening..

See, excitation current of 128A or 32A are design parameters. But our problem does n't happen during field weakening regime. Problem happens much before field weakening starts. Excitation current at that time remains at full 132A. And armature voltage is around 500 volts at the time of problem and field weakening starts at 1000 volts.

What is happening to the actual shunt voltage when increasing the speed above base speed (185rpm) ? I assume it must be dropping off from the 125v at full field? Do you apply an actual 125v to the field at 185rpm? What is the voltage applied to the armature at this speed?

In healthy condition, excitation current above base speed of 185 rpm starts falling upto 32A and speed gets to 465 rpm. Armature voltage remains constant at 1000 V during field weakening period.

Now this problem is happening much below base speed and armature voltage is 500 V at that time. Field current remains constant to full field till that period.

I forgot to post the final result. Later on after detailed checking, found out root of problem was repeatedly low insulation between motor armature's rocker rings.