Tacho-generators are used for measurement of angular velocity and give output voltage proportional to rotational speed. There are mainly two types of tacho-generators based on output voltage—d.c. tacho-generator and a.c. tacho-generator.

D.C. Tacho-generator

Schematic of d.c. tacho-generator is shown below—

In this tacho-generator, magnitude of voltage produced by armature is proportional to the shaft speed. Direction of rotation is indicated by the polarity of output voltage.

A.C. Tacho-generator

This is a sensing device which gives an output voltage of constant frequency, output voltage being proportional to angular velocity of the shaft. It consists of a small two-phase induction motor, one phase of which, called the reference winding, is excited from a single phase constant voltage, constant frequency supply. A schematic of tach-generator is shown below—

When the rotor rorates at an angular velocity ωr, the flux ϕd produces a dynamic voltage in the pseudo-stationary rotor coil along d-axis. The magnitude of this voltage is proportional to the speed of the motor because flux ϕd is constant (as input voltage is constant).

This induced voltage in the d-axis coil produces a proportional current in the d-axis coil, producing a pulsating flux along q-axis, whose frequency is same as the frequency of the induced dymanic voltage and therefore the frequency of the supply across the reference winding. This q-axis pulsating flux therefore produces by transformer action a voltage across the control winding proportional to speed, which is desired.

The time-lag between the speed variation and the corresponding variation in the output voltage should be small and this is achieved by using a drag-cup type of rotor instead of normal squirrel cage rotor.


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