We have a 230V compound wound DC motor with seperate field supply.
There are also two current limiting resistors totaling 1.6 ohms in
series with the armature. The field is supplied through a rheostat, the
rheostat used as a speed control.

Here is the problem:

We jumped out the two series resistors to apply full voltage to the armature/series windings and dropped the rheostat to minimum. This would give us maximum torque and minimum speed. We started the motor and everthing went as expected - including the motor now running too slowly for the application. Next, we decreased the field voltage by increasing the rheostat - to increase overall speed. We started the motor and it ran BACKWARDS! We scratched our heads for a while - swapped field leads and it still ran BACKWARDS. We swapped the Armature leads - still backwards. We swapped the field leads back to original landings and it was still backwards. We put the series resistors back in the circuit and it started running forwards again.

So basically, we removed the series resistors - the motor ran forward ONE time. Then we decreased the field voltage to increase speed and it ran BACKWARD (no wiring or polarity changes, only field voltage). We put the field voltage back to where it was the first time (when it ran forward) and it still ran backwards. We swapped leads - first field, then armature, then field again - all to no avail. The motor still ran backwards. We put the series resistors back in and it is running forward again.

We are not newbies to this - we work on motor control systems for a living. The senior tech has been working on DC motors for over 40 years and has never seen this.

Any ideas?

Here is the problem:

We jumped out the two series resistors to apply full voltage to the armature/series windings and dropped the rheostat to minimum. This would give us maximum torque and minimum speed. We started the motor and everthing went as expected - including the motor now running too slowly for the application. Next, we decreased the field voltage by increasing the rheostat - to increase overall speed. We started the motor and it ran BACKWARDS! We scratched our heads for a while - swapped field leads and it still ran BACKWARDS. We swapped the Armature leads - still backwards. We swapped the field leads back to original landings and it was still backwards. We put the series resistors back in the circuit and it started running forwards again.

So basically, we removed the series resistors - the motor ran forward ONE time. Then we decreased the field voltage to increase speed and it ran BACKWARD (no wiring or polarity changes, only field voltage). We put the field voltage back to where it was the first time (when it ran forward) and it still ran backwards. We swapped leads - first field, then armature, then field again - all to no avail. The motor still ran backwards. We put the series resistors back in and it is running forward again.

We are not newbies to this - we work on motor control systems for a living. The senior tech has been working on DC motors for over 40 years and has never seen this.

Any ideas?