i) Photoelectric Tachometer:
This method of measuring speed consists of mounting an opaque disc on the rotating shaft. The disc has a number of equidistant holes on its periphery. At one side of the disc a light source is fixed. On other side of the disc, and on the line of the light source, a light sensor like phototube or some photosensitive semi-conducting device is placed. When the opaque portion of the disc is between the light source and the light sensor, the light sensor is not illuminated and it does not produce any output. When a hole appears between two, the light falling upon the sensor produces an output pulse. The frequency at which the pulses are produced depends on the number of holes in the disc and its speed of rotation. As the number of holes is fixed, the pulse rate is a function of speed of rotation. The pulse rate is measured by an electronic counter which is directly calibrated in terms of speed.
ii) Toothed rotor variable reluctance Tachometer (Magnetic Pick up) :
This tachometer consists of a metallic toothed rotor mounted on the shaft whose speed is to be measured. The magnetic pickup consists of a housing containing a small permanent magnet with a coil wound round it. When the rotor rotates, the reluctance of the air gap between pickup and the toothed rotor changes giving rise to the induced e.m.f in the pickup coil. This output is in the form of pulses. The frequency of the pulses of induced voltage depends upon the number of teeth of the rotor and its speed of rotation. As the number of teeth of the rotor is known, the speed of rotation can be determined by measuring the frequency of pulses with an electronic counter. If the rotor has T teeth, the speed of rotation is n rps and number of pulses per second is P
Number of pulses per revolution = T
Speed n = (pulses per second /number of teeth)
= (P/T) rps = (P/T) *60 rpm.