Device for indicating the angular (rotary) speed of
a rotating shaft. Either by registering the total no.
of revolution during the period of contact or by
indicating the number of revolution per min.
Tachometers are classified as follows.
What Are the Different Types of Tachometers?
Tachometers can be classified
1.On the basis of data acquisition
Non contact types
2. Classified as data type
3. On the basis of power .
1) The tachometer has to be in
physical contact with the
2) Preferred where the
tachometer is generally fixed
to the machine.
3) Generally, optical encoder /
magnetic sensor is attached to
shaft of tachometer.
Non Contact Type
1) The tachometer does not need
to be in physical contact with
the rotating shaft.
2) Preferred where the tachometer
needs to be mobile.
3) Generally, laser is used or an
optical disk id attached to
rotating shaft and read by a IR
beam or laser.
How To Choose a Tachometer?
4) Acquisition Time
5) Contact type / Non Contact type
6) Portable / Fixed
7) Digital / Analog
Mechanical tachometers utilize the fact that the
centrifugal force on a rotating mass depends on the
speed of rotation and can be used to stretch or
compress a mechanical spring.
These employ only mechanical parts and
2.Centrifugal Force Tachometer.
1) Mechanical tachometers imposes load on rotating
shaft which they are connected therefore they
absorb the power. Not applicable for shafts
involving small power .
2) More maintenance require.
3) Faulty parts doesn’t give accurate reading.
4) It is normally use an arrangement where in a
counter is used for counting the numbers of
revolution and a stop what is used for
measurement of time.
Revolution counter is used to measure an average of
rational speed instead of instantaneous rotational
It consists of a worm gear that is usually attached to
a spindle. It has two dials, an inner one and an outer
The inner dials represent one revolution of the outer
dials and the outer dials represent on revolution of
The tachometer has a stopwatch attached to the
revolution counter and is used to indicated time.
1. Simple to operate.
2. No attachment require to shaft.
1.Limited to low speed.
2.Chances of operational error.
2.Centrifugal Force Tachometer
A sliding collar is mounted
on a shaft. The coupling has
hinged arms carrying mass
that spread apart when the
shaft rotates, moving the
sliding coupling along the
shaft against a
The position of the coupling on the shaft depends
on the speed of rotation and is transmitted by an
arm mechanism to an indicator pointer; the
indicator dial is calibrated in revolutions per
minute. The tachometer shaft may be driven
directly, by the controlled mechanism, or
indirectly, by a flexible shaft.
Electrical tachometers are of several types.
The eddy-current or drag type is widely used in
automobile speedometers; a magnet rotated with the
shaft being measured produces eddy currents that
are proportional to angular speed.
Electric-generator tachometers work by generating
either an alternating or a direct current.
The stroboscope, an instrument that illuminates
rotating objects so that they appear to have stopped
moving, can be used as a tachometer.
a) Eddy current or drag type tachometer.
b) Electric generator tachometer.
c) Contactless type tachometer.
d) Frequency type tachometer.
e) Ignition type tachometer.
f) Stroboscopic tachometer.
Eddy Current tachometer
(1) permanent magnet, (2) rotor, (3) shaft with pointer, (4) spring
It consist of a permanent magnet as stator and a low
resistance solid metallic cylinder as rotor, which drives
the pointer and scale arrangement. the rotor is also
connected to shaft which has a provision to get contact
with the rotating machines, whose rpm has to be
measured. when it get contact to a rotating machine the
shaft rotates which eventually rotates the metallic
cylinder in strong magnetic field , its known that when a
low resistant metal rotated in a strong magnetic field
eddy current gets induced in the low resistance metal
opposing the cause of action which held the pointer in
equilibrium position showing some reading which will be
calibrated for the corresponding speeds hence it shows
the correct rpm in which the machine rotates
Tachogenerators provide a voltage value which is
proportional to the speed and may be a.c. or d.c.
instruments. The d.c. tachogenerator is a small d.c,
generator with a permanent field. The output voltage
is proportional to speed and may be measured on a
voltmeter calibrated in units of speed. The a.c.
tachogenerator is a small brushless alternator with a
rotating multi-pole permanent magnet. The output
voltage is again measured by a voltmeter although
the varying frequency will affect the accuracy of this
An eddy-current tachometer (Figure 2) uses the
interaction of the magnetic fields generated by a
permanent magnet and a rotor, whose speed of
rotation is proportional to the eddy currents
generated. The currents tend to deflect a disk, which
is mounted on the shaft and restrained by a spring,
through a certain angle. The deflection of the disk,
which is rigidly connected to a pointer, is indicated
on a dial.
Electric tachometers may be of the generator or impulse
type. In tachometer generators the electromotive force of
a DC or AC generator is proportional to the angular
velocity, from which the shaft speed can be determined;
the readings are transmitted to a remote measuring
instrument. The operation of impulse tachometers is
based on conversion of pulses generated in the primary
circuit of an ignition system by the opening of interrupter
contacts into a current that is fed to a permanent-magnet
indicator. The frequency of pulses in the primary circuit
is proportional to the speed of rotation of the engine
Various pick-up devices can be used in conjunction
with a digital counter to give a direct reading of
speed. An inductive pick-up tachometer is shown in
Figure (a). As the individual teeth pass the coil they
induce an e.m.f. pulse which is appropriately
modified and then fed to a digital counter. A
capacitive pick-up tachometer is shown in Figure (b).
As the rotating vane passes between the plates a
capacitance change occurs in the form of a pulse.
This is modified and then fed to the digital counter.
In case of variable reluctance sensors, we have coil
wounded on permanent magnet not on iron core,
this configuration enable us to measure rotational
speed of the systems. - See more at:
In the construction of variable reluctance sensor, we
use ferromagnetic gearwheel. As the gearwheel
rotates, change in magnetic flux take place in the
pickup coil which further induces voltage. This
change in magnitude is proportional to the voltage
induced in the sensor. - See more at: