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Measuring Efficiency on a Low Speed, High Frequency,
Variable Reluctance Generator

A 360 pole rotor is used to produce a high number of cycles per revolution. When the rotor pole face moves from alignment with a pole face in the E-stator to maximum misalignment, Lenz's law predicts a generated current surge in the power coil in an attempt to maintain a constant flux in the face of increased reluctance in the magnetic circuit.

The task is to measure efficiency by measuring mechanical input power and dividing it into the electrical output power. As rotor RPM is trivially available along with all the other electrical parameters, the problem reduces to a measurement of mechanical input torque.

Diagram of a variable reluctance generator

While pony brakes are a well established prior art, the system above is a simple alternative. The E-stator is mounted on a parallel leaf spring assembly. This assembly has the advantages of nearly zero force and friction with linear motion near the zero position. Thus, when the tangential magnetic force between the moving rotor and E-stator is overcome by the mechanical input torque, the E-stator is deflected toward the force gage. The force gage and lead screw are used to restore the E-stator to its zero position, as indicated by the dial indicator. The resulting force crossed with the mean radius of the magnetic gap is the mechanical input torque, neglecting the torque required to turn the rotor on its bearings.