Paul,   I remember the big motor starters and had some which also did from
one to five speed control of the three phase motor. They require maintenance
on the solid silver contacts and resister elements and fail due to heat
cycle fatigue and arcing as the points close.  We usually changed to a Wye
Delta starter contactor rather than fixing them.
         My prefferred method of transforming power is with a hydraulic
piston pump. A variable displacement pump that will fit in a 10" cube can
easily need 120 HP to turn it at 3500 RPM and full flow at 3000 PSI. One
common frame size goes up to 60 GPM.  If you know the pressure and RPM of
the pump you can determine HP to a fraction of a horsepower. There are also
mechanical lever controls available on these pumps ( think Bobcat skid
loader) that can set the displacement for different loads.
        The pumped oil would be sent thru an adjustable valve or an orfice
to return to the tank and the HP would go off as heat. This makes a lot of
heat and in fact I used to have a relief valve that I could drop the
pressure to just 600 PSI to warm up the oil in the winter if the hydraylics
were outside the building.This was done to reduce the viscosity of cold oil
to prevent pump damage.
       If you set pump volume at 1000 Rpm down to where the turbocompound
can turn it then read the pressure rise  as the HP rises and record the
pressures you can read the HP of the pump curve charts.

george grimes