any advantage to switch armature wires over field wires?

Thread Starter

strantor

Joined Oct 3, 2010
5,177
I have a 50hp Louis Allis shunt wound DC motor circa 1970's/ early 80's, controlled by a newer bardac DC drive. Originally was controlled by a louis allis drive. What puzzles me is in the drive cabinet, they have 2 giant contactors (each about the size of a cinder block) for switching the armature wire for direction control. I am puzzled as to why they would not just use a couple of multipurpose relays and switch the field wires.

In a stranger note; one of the contactors quit working & I had to run the machine backwards before it was mechanically safe to turn off the machine to do electrical work, so I swapped the field wires and tried to run it. All that happened was I blew a couple of 100A fuses (likely due to a seperate issue - water in a junction box between the drive cabinet & motor).

I thought that sawpping either the field OR armature wires would change direction; am I wrong?
 

t_n_k

Joined Mar 6, 2009
5,447
Perhaps switching the more (highly?) inductive field winding off would give rise to substantial arcing in the switching device. DC arcs are difficult to clear. You might need just as big a switch with a chamber big enough to cool/quench the arc.

I guess if you turned off the armature supply and slowly turned the field current down then reversed / re-energized the field supply connection, it should work in principle.
 

Thread Starter

strantor

Joined Oct 3, 2010
5,177
Perhaps switching the more (highly?) inductive field winding off would give rise to substantial arcing in the switching device. DC arcs are difficult to clear. You might need just as big a switch with a chamber big enough to cool/quench the arc.

I guess if you turned off the armature supply and slowly turned the field current down then reversed / re-energized the field supply connection, it should work in principle.
That makes sense
 
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