Tachometers are the most commonly used devices for measurement of speed. Tachometer is defined as, “an instrument used to measure the angular velocity of shaft and express it in number of revolutions made per minute (in r.p.m.).” It may continuously indicate the value of angular speed. Also, it may indicate average angular speed measured within short intervals of time.
CLASSIFICATION OF TACHOMETERS
Mechanical Type
- Revolution counter and timer
- Slipping clutch tachometer
- Hand speed indicator
- Centrifugal force type tachometer
Electrical Type
- Eddy current or Drag type tachometer (Contact type)
- Contactless type
(a) Inductive type tachometer
(b) Capacitive type tachometer
(c) Photoelectric tachometer
(d) Stroboscope
Disadvantages of Mechanical Tachometers
Mechanical tachometers impose load on the shafts, to which, they are connected. On account of inertia of mechanical parts, the mechanical tachometers (except centrifugal tachometer) are not able to detect the variations in the speed. They indicate only the average speed over a specific period of time. Mechanical tachometer normally uses an arrangement, where a counter is used for counting the number of revolutions and a stop-watch is used for measurement of time. The speed is calculated on the basis of these measurements. This involves large error, especially, when stopwatch is operated manually.
What is Speed Measurement?
Speed can be linear or angular according to movement of object.
Linear movement
Linear movement means displacement per unit time, which is measured as Linear speed (V) in m/sec.
Rotational movement
It is measured in,
- Number of revolutions per minute (N) in r.p.m.
- Angular speed in rad/sec.
What is Electrical Tachometer?
Electrical tachometer converts the rotational speed into electrical signal, Which can be measured and calibrated to give rotational speed directly. In short, it is a transducer, which produces an electrical signal in proportion to the rotational speed. Depending upon the type of transduction, electrical tachometers are classified into various types.