Hi
Kelly had once the 120V KDC 400A and 600A SepEx which worked for our Peugeot 106 electric car with Leroy Somer 120V/90V Sep Ex.
Looking at your specs they seem somewhat similar (96V/96V 500A?).
Kelly's KDC does not seem to exist anymore as product.
But you could test your motor by applying a fixed voltage to the field winding (for example 24 V with 2 12V batteries) and a (brushed) 120V 400A Series DC-controller input to the armature.
Be careful to have a light load (lift the wheels at the drivetrain of the car up)
Before that test UNPLUG the electric motor connections and measure the Ohmic resistance of armature and field.
The armature winding should have a resistance of about 0,5 OHM
The field winding should have a resistance of about 6-8 OHM
If confirmed then connect the field to batteries and armature to series-controller
If this is the case the functionality is most likely like this:
At the start (high torque, low speed) field winding has high voltage and armature low.
At the end when car is driving (low torque, high speed) field winding has low voltage and armature high.
(your test with a light load probably can spin up the motor to 3000 RPM).
You could use a (mechanical) switch (with dc-relay) to create 3 stages
1) Field Voltage about 80 V. High torque
Series controller (on armature) spins up motor to max (1000 RPM)
2) Field Voltage about 30V.
Series controller (on armature) spins up motor to max (3000 RPM)
2) Field Voltage about 10V. High speed, low torque
Series controller (on armature)spins up motor to max (6000 RPM)
For the start pedal is only connected to the Series controller