Electromagnetic Tachometer Generator

Overview

In an electromagnetic tachometer generator, a DC or AC voltage at the output is proportional to the angular speed of its shaft. This shaft is driven by a rotating member whose speed is measured. Three types of tachometers are in use; all of them contain an electrical-machine-like stator and rotor.

a) The stator of a DC-output tachometer contains several permanent-magnet poles or DC-excited electromagnetic poles. A wound rotor is provided with a commutator and brushes. The output voltage is picked up from the brushes. The frequency of this voltage is also proportional to the speed.

b) The stator of an AC-induction tachometer contains two windings around its poles. One of them is excited by an AC signal and induces a voltage in the other one as in the secondary of a regular transformer. The rotor has a short-circuited path for the current similar to that of an induction motor. When the rotor rotates, the flux distribution in the stator is changed, providing the change in the coupling coefficient between the coils. The output voltage across the secondary follows this change indicating the speed.

c) The rotor of a permanent-magnet rotor tachometer carries a rotating magnet whose field induces, in the stationary stator coils, AC voltage having an amplitude and frequency proportional to the speed.



Electromagnetic tachometer generators, a = DC-output tachometer, b = AC-induction tachometer, c = permanent-magnet rotor tachometer; w = angular speed, U_ex = excitation voltage, U_O= output voltage, 1 = driving shaft, 2 and 3 = permanent-magnet poles, 4 = wound rotor, 5 = commutator, 6 and 7 = brushes, 8 and 9 = poles, 10 = excitation coil, 11 = output coil.

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