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Tachometers are the devices employed for measuring the rotational speeds of motors and generators.

The tachometers are classified as follows:

I.            Mechanical tachometers
II.            Electrical tachometers

  1. D.C. tachometer generators
  2. A.C. tachometer generators
  3. Photo-electric tachometers (or Speed-meter)
  4. Toothed motor variable reluctance tachometer etc. Electrical tachometers are preferred (as compared to mechanical tachometers) in view of the advantages they
    offer as electrical transducers.

Give important types of tachometer generators are briefly described below:

1. D.C. Tachometer Generator:

In a D.C. generator the e.m.f generated depends upon the following two factors:

(i) Field excitation

(ii)  Speed

If for the field system permanent magnet pole pieces are used, then the generated voltage depends only on the speed. Hence the speed can be computed by measuring the generated e.m.f.

A D.C. tachometer generator is shown Fig. 32.


  • The shaft whose speed is to be measured is coupled to the armature.
  • A moving coil voltmeter is connected across the brushes to measure the generated voltage. The variable resistance R is incorporated to limit the current through the voltmeter.

Since voltage is proportional to speed, the voltmeter may be calibrated in terms of speed (r.p.m.).

2. A.C. Tachometer Generator:

The inherent demerits associated with D.C. tachometer generator, due to the provision of commutor and brushes, are eliminated in A.C. tachometer generator.

An A.C: tachometer generator is shown in Fig. 33.


  • It consists of, like an alternator, a stationary armature (stator) and a rotating field system (rotor). Owing to the generation of e.m.f in a stationary coil on a stator, commutation problems no longer exist.
  • The alternating e.m.f. induced in the stationary coil is rectified, and the output D.C. voltage is measured with the help of a moving coil voltmeter (V).
  • The ripple content of the rectified voltage is smoothened by the capacitor filter (C).
  • As the speed depends on both the amplitude of the voltage and frequency, anyone of themcan be used as a measure of the speed. In an A.C. tachometer, it is the induced voltage that is considered as the required parameter.

1.           Photo-electric Tachometer:

Fig. 34 shows a photo-electric tachometer.


Fig. 34

  • It consists of a opaque disc mounted on the shaft whose speed is to be measured. The disc has a number of equivalent holes around the periphery. On one side of the disc there is a source of light (L) while on the other side there is a light sensor (may be a photosensitive device or photo-tube) in line with it (light-source).
  • On the rotation of the disc, holes and opaque portions of the disc come alternatory in between the light source and the light sensor. When a hole comes in between the two, light passes through the holes and falls on the light sensor, with the result that an output pulse is generated. But when the opaque portion of the disc comes in between, the light from the source is blocked and hence there is no pulse output. Thus whenever a hole comes in line with the light source and sensor, a pulse is generated. These pulses are counted/measured through an electronic counter.

The number of pulses generated depends upon the foliowing factors:

i.            The number of holes in the disc;
ii.            The shaft speed.

Since the number of holes are fixed, therefore, the number of pulses generated depends on thespeed of the shaft only. The electronic counter may therefore be calibrated in terms of speed (r.p.m.)

Advantage. It is a digital instrument.

Disadvantage. It is required to replace the light source periodically, and if the grating period is small then errors might creep in the output,