The power load is designed to calibrate the drag on a truck wheel. The equipment consists of a relatively small generator coupled to the wheel and the switched load. The rotational speed, and therefore the output voltage, is relatively low. This means that the current is proportionally high to get 1kW of load. The heat from the three phase load has to be dissipated and removed from the area near the wheel and control electronics. The load can be set to one third, two thirds or full power.

The picture below shows the load when it was calibrated for torque at a range of speeds and powers on a dynamometer rig. The large heatsink keeps the temperature of the unit at a safe level.

The picture below, courtesy of TRL, shows the fully instrumented carriage at the TRL Pavement Test Facility. This work was part of a project for the UK Highways Agency. The calibration equipment consists of a specially wound generator shown coupled to the wheel and the 1kW three-phase switched load mounted on the side of the carriage.

A high speed datalogger captures real-time signals from the instrumented suspension and the variable steps in known torque from the load allowed the rig to be calibrated for the drag force.