WER drive, IFT Fault

I have a customer that has a small extruder that is causing problems. This is an older machine that they have only owned for a couple of years. The problem has been happening occasionally since they installed the machine.
The main screw is driven by a DC motor fed from a WER drive, p/n 1310-8090, electrostat 222.

This is a very old drive, and I cannot locate a manual for it.

Every few days the motor will stop, and the drive has a "IFT FAULT" LED lit up. Usually the reset button will reset the drive and all is good for a few days. Yesterday the drive faulted and will not reset. I was only able to spend a few minutes today on site. The only troubleshooting I could do was remove the armature wires from the drive, which will allow the drive to run without a fault. I measured the arm voltage at the drive terminals and read 220 Vdc with the speed pot set at 45%.

I reconnected the arm wires and removed the belt from the motor pully, but the drive would again immediately fault.

The only other thing I noticed during my short visit was the drive is 40hp, but the motor is 60hp. Would this larger motor cause the drive to fault even when there is very minimal load with the belts removed?

Drive data.
Electrostat 222
p/n 1310-8090
1310-1004
Input Voltage, 460
DC Motor hp, 40
Field volts, 300
Arm volts, 500

OMG, an Electrostat still running? I'm amazed.
With that beast, there are a myriad of potential problems causing IFT trips, in order:
Electrolytic Capacitors in the power supplies of the drive.
Failing SCR.
Electrolytic Capacitors in the regulators of the drive.
Oxidation on the drive interconnect wiring.
Loss of spring tension on any plug in or socketed devices/IC's.
Loose/oxidized nuts on the bolt down connections.

In all honesty, while that drive is repairable, being a pure analog 'big component' drive, I think I'd go ahead and just replace it with a newer, properly sized one. I believe you can still get fairly inexpensive, analog no-frills drives from places like Carotron in that HP. The Carotron "Choice" line would be a good try.

Might need to verify field voltage available and field wiring or windings not open.

Good point on the field, but if there is a field problem, it should just slam the drive to current limit, not IFT; especially if the motor is as old as the drive, it will have some residual field at all times.

I suppose, it is possible to have a shorted or grounded armature. Silly question, have you megged the motor?

This is to add to what rdrast said about failing SCRs. You might try adding an additional cooling fan to enhance the drive rating a little. Years ago the very early SCR's would become unstable at higher temps. As soon as it trips you should check the cathode to anode resistance for leakage. If you can find some new SCRs, preferably with a higher amp rating, just go ahead and replace the originals.