The SSD (Shackleton System Drives) 5401 Field Regulator has been obsolete for some years.
The modern SSD 590P Drives all have built-in Field Regulators and it will be better to replace/modernise the main drive and take advantage of the integrated Field Regulator.
The 5401 has a well designed IxR compensation circuit that is fundamental to the performance, stability and torque accuracy of some test rig drives. This is important for motors of medium to large powers combined with low base speeds. Froude Consine and many other rolling road Dynamometers fall into this catagory. Contact us if you have this requirement.