An axial airgap brushless DC tachogenerator

Tachogenerators are used in servosystems to provide a voltage signal which is proportional to rotational speed. Traditionally, these have been purpose-designed DC generators with alnico permanent magnets, skewed armatures, multi-segment commutators and silver loaded brushes. However, the widespread application of brushless motors in servosystems has led to a demand for brushless tachogenerators. One way of making a brushless DC tachogenerator is to construct an AC generator which has: a voltage waveform that is essentially trapezoidal rather an sinusoidal; a number of phases sufficient to ensure that the flat tops of successive phase voltages overlap; and an electronic commutation system comprising elements which can switch successive phases to the output terminals at appropriate instants and which can conduct current in both directions; the switching elements should also obey Ohm´s Law. The author describes a machine with these features