AC TACHOMETER

AC TACHOMETER

 An a.c. the tachometer is used in the feedback control system to feedback an a.c. a voltage proportional to the speed of the shaft. This is basically a two-phase induction motor. One of the stator windings is used as the reference winding and the other is the control winding. The reference winding is fed a suitable a.c. the voltage of constant frequency and magnitude. Therefore, a voltage of the same frequency is induced in the control winding. This output voltage is fed to the high input impedance circuit of an amplifier so that the control winding can be considered as open-circuited. It is essential that the voltage induced in the control winding is directly proportional to the shaft speed and phase of this voltage be fixed with respect to the voltage supplied to the reference winding. The principle of operation of an a.c. the tachometer can be explained using double-revolving field theory. With reference to reference winding the tachometer can be considered equivalent to a single-phase induction motor. At standstill, the forward and backward fields are equal and hence voltage induced in the control winding is zero.

When the rotor is revolving, the rotor current due to the forward rotating field decreases since its effective impedance increases whereas for the backward rotating field the impedance decreases, the difference between them is a function of speed. Therefore, the voltage developed across control winding is a function of speed. Reversal of direction of rotation reverses the phase of output voltage.

 For a constant phase angle of output voltage and linear relationship between output volt[1]age and speed, a suitable value of the ratio of rotor reactance to rotor resistance should be chosen. If it is low, the sensitivity i.e. volts per revolution per minute is sacrificed but the linear speed range is wide. However, if it is high the speed range is limited to a fairly small fraction of synchronous speed to meet the condition of linearity of voltage and consistency of phase angle. An a.c. tachometer should have low inertia when rapid speed variations are encountered as in an automatic control system.