Maybe its just my contrarian nature but, whenever someone says it can't be done, my creative juices really start flowing. And I've paid dearly for it too sometimes, I might add!
Anyway, first some basic stuff. Doing the math, I find that this motor develops 7hp at rated speed and torque. That torque calculates to 21ft-lbs.
It is the nature of wound field DC motors to develop full rated torque rather easily right down to zero speed. There is little challenge in controlling speed at very low speeds either as long as the torque isn't dancing around high/low and plus/minus. For the application at hand, I will need to know more about the torque behavior of the machine to get very detailed with solutions.
I hope that the application requires the full speed output of the motor as well as the low speed. Otherwise, a 7hp motor is being asked to do only .09hp of work (21ft-lbs @ 22rpm). Seems a bit of a waste! If high speed isn't needed, a gearbox and much smaller drive/motor is advised.
It wasn't mentioned but the motor field winding style is important. A shunt field is good for motors that need to develop the same torque in both directions. A Stab shunt or comp shunt motor has both series and shunt fields so the motor develops more torque in one direction than the other. It is very important that the two fields are wired properly or the motor will not be stable and will not develop full torque in either direction.
The next issue is the tach which I assume is a 50vdc/1000rpm unit. This is an analog device with 1% accuracy typical. Since we are trying to run at 22rpm, that's only 1.1vdc which can get lost in the noise very easily. Is the tach wiring carefully shielded? At these speeds, an optical encoder would be far better but I don't know if the Fincor drive can deal with an encoder.
Ron Doran mentioned an auxiliary blower for cooling if full torque is needed at these low speeds. It's very important from a heat standpoint but probably is not a factor in getting the motor to settle down.
It is essential that the Current Minor Loop be well-tuned as well as the Speed Major Loop. These have a huge effect on speed stability. Use the drive AutoTune features if it has them.
If the torque isn't changing much and you can stand about 2% speed regulation (that's from no load to full load), you might try operating with Armature Feedback only. That leaves speed loop tuning and tach issues out of the picture. If torque is changing a lot, this will not give good results.
How about coming back with field winding style, load torque characteristics, speed range needed, and confirmation of tach output specs. We can probably help you further with that information.