castera,
Your crane motor probably had a series field winding cumulatively compounded with the shunt field winding. Cumulative compounding is used where high torque is required (e.g. cranes), since the dc motor torque is proportional to the product of armature amperes and field flux. In cumulative compounding, the series field (called series stability winding) is connected so that series field mmf assists the shunt field mmf (i.e. adds) thus increasing the field flux and torque with increasing armature amperes. In differential compounding, the series field mmf opposes the shunt field mmf. Due to safety reasons (like over speeding due to less flux, reversed rotation at starting due high starting current etc.), differential compounding is seldom used.
Now, there are two ways to reverse the direction of rotation of a cumulative-compound dc motor.
1. You can reverse the shunt field to get a reversed rotation. But the same time, the series field, connected in series with the armature, must also be reversed to get back the cumulative compound connection. If you just leave the series field as connected before, then the motor will become differentially compounded with the above attendant safety problems. So, this was probably why your GEC man asked you to reverse both the field and “armature” connections. But the so called “armature connection” was actually the series field connection.
2. As you can see, the reversal of shunt field to change direction invites its own hassles. So, it will be simpler just to reverse the (armature + interpole) connection alone (not the series field connection) to change direction of rotation. By this method of (armature + interpole) reversal, all the requirement of cumulative compounding and the relationship between the shunt field and interpole (as discussed in our earlier posts)are maintained.
I hope my clarifications helped.