UNIT-II
Measurement of speed
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Tachometer, What’s That?
Tachometer is used for measuring rotationalspeed
Can be used to measure speed of a rotating shaft
Can also be used to measure flow of liquid by
attaching a wheel with inclinedvanes
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What Are the Different Types of
Tachometers?
Classification of tachometers:
• Mechanical Tachometers
• Revolution counter
• Hand speed indicator
• Tachoscope
• Centrifugal tachometer
• Resonance (vibrating read) tachometer
• Electrical Tachometers
• Eddy current or drag cup tachometer
• Tachogenerator (DC and AC)
• Contactless electrical Tachometers
• Magnetic pickup tachometer
• Photo-electric tachometer
• Stroboscope
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Tachometers can also be classified on the basis of data
acquisition – contact or non contact types
They can also be classified on the basis of the
measurement technique – time based orfrequency
based technique of measurement
They can also be classified as analog or digital type
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Mechanical
Tachometers
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Revolution counter
Revolution counter is used to measure an average of rational
speed instead of instantaneous rotational speed.
It consists of a worm gear that is usually attached to a
spindle. It has two dials, an inner one and an outer one.
The inner dials represent one revolution of the outer dials and
the outer dials represent on revolution of the spindle.
The tachometer has a stopwatch attached to the revolution
counter and is used to indicated time.
These are limited to low speed engines and measure
satisfactory upto 2000-3000r.p.m.
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1. Revolution counter
•Speed measure upto 2000-3000 rpm.
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2. Hand speed indicator
Hand Speed Indicator has an integral stopwatchand
counter with automaticdisconnect.
The spindle operates when brought in contact with
shaft.
Counter does not function until start and wind button is
pressrd to start watch and engage the clutch.
The instrument indicates average speed over short
interval in r.p.m .
Accuracy about 1% of the full scale.
Measure speed within range 20,000 to 30,000 rpm
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Hand speed indicator
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3. Tachoscope
Tachoscope consists of revolution counter for timing
device.
The two components are integrally mounted and start
simultaneously when contact point is pressed against
rotating shaft.
The rotational speed is computed from readingof
counter and timer.
Tachometer can be used to measure speeds up to
5000r.p.m.
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Tachoscope
Tachoscope
4. Centrifugal force tachometer
Centrifugal Tachometer operates on principle that
centrifugal force is proportional to speed ofrotation.
It consists two balls arranged about spindle. Centrifugal
force developed by these ballscompress spring as
function of speed positionspointer.
They are suitable for40,000r.p.m. with an accuracy of
about ±1 %.
Centrifugal force tachometer
w = angular speed, 1 = shaft,
2 and 3 = masses, 4 = displacement-sensitive element.
5. Resonance (vibrating reed)
tachometer
In Vibrating Read Tachometers a series of consecutively
timed steel rods are used to determine speed on basis of
vibrations created bymachine.
One end of rod is fixed to a base which is kept in
contact with any non-moving part of machine and
other is attached to calibratedscale.
These can be used in speed range of 600-10000 rpm with
an accuracy of ±0.5 %.
Resonance (vibrating reed)
tachometer
Electrical
Tachometers
1. Eddy current or drag cup
tachometer
An eddy-current tachometer uses the interaction of the
magnetic fields generated by a permanent magnet and a
rotor, whose speed of rotation is proportional to the eddy
currentsgenerated.
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.
Used for measuring rotational speeds upto 12,000rpm
with an accuracy of ± 3%.
Eddy current or drag cup tachometer
Tachogenerators
•Employ small magnet type dc or ac generators which
translate the rotational speeds into dc or ac voltage signal.
•Relative perpendicular motion between a magnetic field
and conductor results in voltage generation in the
conductor.
• Magnitude of this voltage is a direct function of the
strength f the magnetic field and the speed with which the
conductor moves perpendicular to it.
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D.C. Tachogenerator
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 generatede.m.f.
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.).
D.C. Tachogenerator
A.C. Tachogenerator
The inherent demerits associated with D.C. tachometer
generator, due to the provision of commuter and brushes, are
eliminated in A.C. tachometergenerator.
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 longerexist.
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).
A.C. Tachogenerator
As the speed depends on both the amplitude of the voltage
and frequency, anyone of them can 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.
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Contactless
electrical
Tachometers
1.Inductive type pickup tachometer
A coil wounded on permanent magnet not on iron
core, this configuration enable us to measure
rotational speed of thesystems.
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.
Let, T-> No. of teeth on rotor
N-> Revolutions per second
P -> Number of pulses per second
Speed,N = pulses per second / Number of teeth
N = P/T
= P/T * 60 rpm
•If rotor has 60 teeth, and if the counter counts the pulses in one
second, then the counter will directly display the speed in
revolutions per minute.
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2. Capacitive type Pickup tachometer
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 digitalcounter.
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.
Photo-electric tachometer
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.
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Photo-electric tachometer
The number of pulses generated depends upon the foliowing factors:
i. The number of holes in thedisc;
ii. The shaftspeed.
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.)
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Photo-electric tachometer
Computer mouse with a ball
Stroboscope
The instrument operates on the principle that if a repeating
event is only viewed when at one particular point in it’s cycle it
appears to be stationary. A mark is made on rotating
shaft, and a flashing light is subjected on the shaft. The
frequency of the flashing is one very short flash per revolution.
To determine the shaft speed we increases the frequency of
flashing gradually from small value until the rotating shaft
appears to be stationary, then note the frequency. The frequency
then doubled, if there is still one apparent stationary image, the
frequency is again doubled. This continued until two images
appear 180 degrees apart. When first appear for these two
images the flash frequency is twice the speed of rotation.
i) Single mark on the shaft
fr= ff
fr= nff
Where, n=1,2,3,4 …etc.
(a) fr= ff /2 (b) fr= ff /3 (c) fr= ff /4
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2) Multiple marks on the shaft
• For getting stationary pattern when,
fr= ff /4, ff /5 ……etc.
• For certain values of fr which is smaller than ff namely,
fr= ff /N, 2ff /N, 3ff /N……….
Where N represents the number of distinguishing made
on shaft.
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Shaft speed = (disk speed) x ( no. of opening in the
disk)/ no. of images
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Stroboscope
Stroboscopes are used to measure angular speed
between 600 to 20,000 rpm .
It’s advantage is that it doesn’t need to make contact with
the rotating shaft.
• For exact speed measurement, the flashing rate is
adjusted and synchronism is attained (appearance
of a single line stationary image) for the highest
rate of flashing.
• If synchronism occurs at n different flashing rates
f1,f2,….fn, then the actual shaft speed is
calculated from the relation;
f1 f n n 1
fr
f1 f n
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