The damaged motor rheostat is what I expect to find in a Reliance MG drive. These are specially wound for the application. Note that the windings consist of one long winding on one side and four short winding sections on the other side. If you check, you'll find that it's resistance is 0 Ohms on the long winding side. From the point where the wiper is at the midpoint, the resistance increases from 0 to about 1400 Ohms (near the break). You need to check and see what the resistance actually is just before the open spot.

The spare rheostat's winding appear to be one continuous winding. I think you'll find that its resistance changes evenly from 0 at one end to 1400 Ohms at the other. That rheostat can't be used in place of the original motor rheostat, despite the fact that they are both about the same resistance. You indicate that another one of these was used to replace the original generator rheostat and that's not going to work correctly either.

A Ward-Leonard DC drive operates in two ranges: base speed and field weakening. In base-speed range, the spindle motor's field is held at 100% (230 VDC in this case) and the armature voltage is increased from near zero at minimum speed, to 100% (230 VDC) at base speed (690 RPM at the motor). With the field and armature at 100% (230 VDC) the motor will run at base speed. To increase above base speed, the field voltage is reduced (the field is weakened), dropping to well under 100 VDC at maximum speed (2070 RPM).

The way these MG drives function, the generator rheostat (connected as a potentiometer) changes its resistance from 0 at minimum speed to maximum resistance at the midpoint of the rheostat and stays at maximum for the second half. This increases the generator's field voltage from 0 to maximum which, in turn, increases the spindle motor's armature voltage from 0 to maximum at the control's midpoint (base speed). From midpoint on, the generator rheostat stays at maximum and the armature voltage stays at 100%.

During the entire time that the generator rheostat's resistance is changing and the generator is ramping up the motor's armature voltage, the motor rheostat stays at 0 Ohms, keeping the spindle motor's field at 100%. After the midpoint on the rheostat wipers, the motor rheostat resistance increases from 0 to maximum (1400 Ohms). The motor rheostat is connected in series with the spindle motor's field and introduces additional resistance in the field circuit, reducing the field voltage. By the time the speed control and the rheostat wipers reach the end of travel, the motor rheostat has introduced 1400 Ohms of resistance into the field circuit, fully weakening the field and causing the motor to run at full speed (2070 RPM). But when you hit the bad spot in the rheostat, the motor loses all field voltage and tries to run away, as previously discussed.

The reason that a rheostat like your spare won't work for the generator rheostat is that it only reaches 50% of the required resistance at the midpoint, where it should be at 100%, and thus the motor doesn't get full armature voltage. The motor will never reach the base speed condition. This is why the control dial is behaving oddly.

The only way to get the controls functioning as designed is to find a replacement generator rheostat and either repair or replace the motor rheostat. The replacement rheostats can be used to control the MG, but you need two knobs, so that you can individually control each rheostat.

By the way, your spindle motor has a 230 VDC armature and 230 VDC field. The newer Reliance 10EE spindle motors and MGs, starting with the piggyback-MG round-dials, have 115 VDC fields, so most 10EE rheostats won't work either.