Jawaharlal Nehru Engineering College
Laboratory Manual
ELECTRICAL MACHINES
For
Second Year Students
 Author JNEC INSTRU DEPT., Aurangabad
SUBJECT INDEX
1. Study of PMMC
2. Study of Wheatstone bridge.
3. Study of Kelvin’s Bridge.
4. To determine power factor of RLC series circuit.
5. Measurement of Q-Factor.
6. Study of Cathode Ray Oscilloscope (CRO).
7. Study of Strip Chart Recorder.
8. Study of Multimeter.
9. Study the 7-Segment Display.
EXPERIMENT NO. 1
Study of PMMC (Permanent moving magnet coil)
OBJECTIVE: To study the construction and working principle of PMMC, to study how it is used as
Ammeter and Voltmeter, to study how different torque acts, and how damping is provided.
EXPERIMENTAL SETUP: PMMC Instrument such as voltmeter, ammeter etc.
THEORY:
WORKING PRINCIPLE:
The working principle of PMMC instrument is same as that of `D`-Arsonval type of
galvanometer. In this instrument, we have a coil suspended in the magnetic field of a permanent magnet
in the shape of a horseshoe. The coil is suspended so that it can rotate freely in the magnetic field.
When current flows in the coil, the developed (electromagnetic) torque causes the coil to rotate. The
electromagnetic (EM) Torque is counterbalanced by a mechanical torque of control springs attached to
the movable coil. The balance of torques, and therefore the angular position of the movable coil is
indicated by a pointer against a fixed reference called a scale. The equation for the developed torque,
derived from the basic law for electromagnetic torque is
T = B ×A ×I ×N
Where, T= torque, Newton-meter
B= flux density in the air gap, Wb/m 2
A= effective coil area (m2)
N= number of turns of wire of the coil
I= current in the movable coil (amperes)
The equation shows that the developed torque is proportional to the flux density of the flux
density of the field in which the coil rotates, the current coil constants (area and number of turns).
Since both flux density and coil constants are fixed for a given instrument, the developed torque is a
direct indication of the current in the coil. The pointer deflection can therefore be used to measure
current.
Therefore, T= G ×I
Where, G= a constant= N ×B ×A
Hence, Torque T ∝ I
CONSTRUCTION:
The general construction details are as follows:
Moving coil: The moving coil is wound with many turns of enameled or silk covered copper wire. The coil
is mounted on rectangular aluminium former, which is pivoted on jwelled bearings. The coil moves freely
in the field of permanent.
Magnet System: The magnet of materials like Alcomax and Alnico have a high co-ercive force, so it is
possible to use smaller magnets and high field intensities. The flux density used in PMMC vary from 0.1
Wb|m 2 to 1 Wb|m 2. Thus in small instrument it is possible to use a small coil having small number of turns
and hence the size of the instrument achieved is reduced.
Control: when the coil is supported between two jewel bearings two phosphor bronze hairsprings provide
the control torque. These springs also serve to lead current in and out of the coil. The control torque is
provided by the ribbon suspension. This method is comparatively new and is claimed to be advantageous
as it eliminates bearing friction.
Damping: Damping means dissipation of energy of rotation. This dissipation of energy is due partly to
mechanical effects, and usually to a greater extent, due to electromagnetic effects from the coil
circuit. Damping torque is produced by movement of the aluminium former moving in the magnetic field
of the permanent magnet.
Pointer and Scale: The pointer is carried by the spindle and moves over a graduated scale. The pointer
is of lightweight construction and, apart from those used in some inexpensive instruments has the
section over the scale twisted to form a fine blade. This helps to reduce parallax errors in the reading
of the scale. In many instruments such errors may be reduced further by careful alignment of the
pointer blade and its reflection in the mirror adjacent to scale. The weight of the instrument is normally
counter balanced by weights situated diametrically opposite and rigidly connected to it.
Fig: - D’Arsonval Galvanometer
DYNAMIC BEHAVIOUR OF GALVANOMETER:
When we pass current through a galvanometer it does not reach its steady state deflection
immediately but there is a time interval or period of transition during which the moving system of the
galvanometer deflects from it initial position to the final steady state position. The dynamic behaviour
to the galvanometer during this period is examined by the equation of motion.
Constants of the galvanometer “Intrinsic Constant” –
i.
Displacement constant: The deflecting torque is given by T d=Gi Where G is the displacement
constant of the galvanometer and is equal to NBld. The units of G are Nm/A.
ii.
Inertia constant: A retarding torque is produced owing to inertia of moving system. This
torque is dependent upon the moment of inertia of moving system and the angular
acceleration.
Inertia Torque
Tj = J (d2θ/dt2)
Where,
J=moment of inertia of moving system about the axis of rotation: Kg-m2
also called “Inertia Constant”
θ=deflection at any time t ; rad.
d2θ/dt2= angular acceleration.
iii.
Damping Constant: Damping is provided by the friction due to motion of the coil in air and
also by induced electrical effects if a closed circuit is provided.
Damping Torque
TD= D (dθ/dt)
Where D is damping constant: Nm/rad s-1
dθ/dt= angular velocity.
iv.
Control Constant: A controlling torque is produced due to elasticity of the system which
tries to restore the moving system back to its original position.
Controlling torque TC=Kθ, Where K is control constant; N-m/rad
Fig: - Torque acting on the moving system of a galvanometer.
Equation of Motion: There are four torques acting on the moving system. Deflecting torque Td , Tries to accelarate
the system while inertia torque Tj , damping torque TD and control torque TC try to retard the system.
Therefore, for any deflection θ at any instant t,
Tj + TD + TC = Td or
J (d2θ/dt2) + D (dθ/dt) + Kθ = Gi
--------(1)
We have now a linear, second order differential equation whose solution is the sum of a “ complementary
function” representing a transient condition and a “particular integral” representing the steady state
condition.
Complementary Function: The auxiliary equation is Jm2 + Dm + K = 0.
The roots of this equation are:
and
m2 = -D - √(D2-4KJ)
m1 = -D + √(D2-4KJ)
2J
2J
θ= A exp (m1t) + B exp (m2t)
Thus
Where A and B are constants.
Particular Integral: We pass a steady current i through the galvanometer.
Under steady state conditions d2θ/ dt2 = 0, dθ / dt = 0, and θ = θF.
Putting the above conditions in eqn.1, the final steady state deflection is,
θF = Gi/K
Thus the complete solution of differential equation is ,
θ= A exp (m1t) + B exp (m2t) + θF
Now θF is the final steady deflection and the term A exp (m1t) + B exp (m2t) represents a motion which
may or may not be oscillatory. We can tell the type of behavior from the form of the roots m1 and m2.
There are three possible cases.
Case- I
D2 < 4KJ
For this case, the two roots m1 and m2 are imaginary. Thus under these conditions the motion
is oscillatory. The galvanometer oscillates about its final steady state position with decreasing amplitude
before finally settling at its final steady position. The galvanometer is under damped in this case.
Case-II
D2 = 4KJ
For this case, the two roots m1 and m2 are real and equal. The motion in this case is nonoscillatory and the final steady state position is reached in shortest time without any overshoot. The
galvanometer under this condition is critically damped.
Case-III
D2 > 4KJ
For this case, the two roots m1 and m2 are real. Thus under these conditions the motion is nonoscillatory but the galvanometer reaches the final steady state position in sluggish manner. The
galvanometer is over damped in this case.
Fig: - Setting Curves for different types of damping
METHODS FOR PRODUCING DAMPING TORQUE ARE:
I. Air friction damping.
Fig: - Air friction damping
II. Fluid friction damping.
Fig: - Fluid friction damping
III. Eddy current damping.
IV. Electromagnetic damping.
PMMC AS AN AMMETER: -
Let
Rm = internal resistance of the movement.
I = full scale current of the ammeter + shunt (i.e. total current)
Rsh = shunt resistance in ohms.
Im = full-scale deflection current of instrument in ampere.
Ish = (I- Im) = shunt current in ampere.
Since the shunt resistance is in parallel with the meter movement, the voltage drop across the
shunt and movement must be the same.
Therefore,
Vsh = Vm
∴
IshRsh = ImRm,
Rsh = (ImRm)/Ish
But Ish = I – Im
Hence
Rsh = (ImRm) / (I-Im)
For each required value of full-scale meter current, we can determine the value of shunt
resistance.
PMMC AS A VOLTMETER-
Let
Rm = internal resistance of the movement.
Rs = multiplier resistance in ohms.
Im = full-scale deflection current of instrument in ampere.
V = full range voltage of the instrument.
From the circuit,
V = Im (Rs+Rm)
Rs = (V-ImRm)/Im = (V/Im) - Rm
Therefore
Rs = (V/Im) - Rm
The multiplier limits the current through the movement, so as to not exceed the value of
the full-scale deflection.
CONCLUSION: -
REVIEW QUESTIONS AND ANSWERS:
1. List other instrument types generally used for A.C. and D.C. Voltages and Current measurement.
2. Is it possible to use PMMC for A.C. measurement, give reasons for your answer?
3. What are the other methods used for providing controlling Torque other than the spring
balance.
EXPERIMENT NO. 2
Study of Wheatstone bridge.
OBJECTIVE: To measure the resistance value using Wheatstone’s Bridge.
EXPERIMENTAL SETUP: Wheat stone bridge, galvanometer, D.C. power supply, unknown resistance,
connecting leads etc.
THEORY:
INTRODUCTION:
Bridge circuits are extensively used for measuring components values such as R, L and C. Since the
bridge circuit merely compares the value of tan unknown component with that of an accurately
known component (a standard), its measurement accuracy can be very high.
The Wheatstone bridge is used for accurate measurement of resistance.
BRIDGE CIRCUIT DETAILS:
The source of emf and switch is connected to points A and B, while a sensitive current indicating
meter, the galvanometer, is connected to points C and D. The galvanometer is a sensitive microammeter,
with a zero center scale. When there is no current through the meter, the galvanometer pointer resets
at 0, i.e. mid scale. Current in one direction causes the pointer to deflect on one side and current in the
opposite direction to the other side.
When SW1 is closed, current flows and divides into the two arms at point A, i.e. I1 and I2. The
bridge is balanced when there is no current through the galvanometer, or when the potential difference
at points C and D is equal, i.e. the potential across the galvanometer is zero.
To obtain the bridge balance equation, we have from the fig.
I1R1 = I2R2
------------------------ (1)
For the galvanometer current to be zero, the following condition should be satisfied.
I1 = I3 =
E
R1 + R3
I2 = I4 =
-----------------(2)
E
--------------------(3)
R2 + R4
Substituting in eq. (1)
R4 =
R2 R3
R1
This is the equation for the bridge to be balanced.
Balance Equation:
R4 = R2 R3
R1
SENSITIVITY OF WHETSTONE BRIDGE:
Sensitivity is deflection per unit current.
Sensitivity =
deflection
Unit current.
Where, S=linear or angular per micro-A
S=mm/micro-A
S=radians/micro-A
Therefore, total deflection is given by
D=S*I
Where I=current in amperes (micro-A)
THEVENINS EQUIVALENT FOR UNBALANCED WHEATSTONE’S BRIDGE:
Thevenin’s equivalent voltage is found by disconnecting the galvanometer from the bridge circuit,
as shown in the above figure, and determining the open circuit voltage between terminals a and b.
Applying the voltage divider equation, the voltage at point ‘ a’, can be determined as follows
Ea =
E × R3
R1 + R3
Eb = E × R4
R2 + R4
and at point ‘ b’,
Therefore, the voltage between a and b is the difference between Ea and Eb , Which represents
Thevenin’s equivalent voltage.
E × R3
R1 + R3
Eth = Eab = Ea – Eb =
Therefore Eab = E
R3
R1 + R3
-
-
E × R4
R2 + R4
R4
R2 + R4
Thevenin’s equivalent resistance can be determined by replacing the voltage source E with its internal
impedance or otherwise short-circuit and calculating the resistance looking into terminals a and b. Since
the internal resistance looking into terminals a and b. Since the internal resistance is assumed to be very
low, we treat it as 0Ω. Thevenin’s equivalent resistance circuit is shown below.
The equivalent resistance of the circuit is R1R3 in series with R2R4
i.e. R1R3 + R2R4 .
Therefore, Thevenin’s equivalent circuit is given in figure below. If the galvanometer is connected
across the terminals a and b of fig 2 or its Thevenin’s equivalent fig 4 it will experience the same
deflection at the output of the bridge.
The magnitude of current is limited by both Thevenin’s equivalent resistance and any resistance
connected between point a and b. The resistance between a and b consists only of the galvanometer
resistance Rg. The deflection current in the galvanometer is therefore given by
Ig =
Eth
Rth + Rg
LIMITATIONS:
For low resistance measurement, the resistance of the leads and contacts becomes significant
and introduces an error. This can be eliminated by Kelvin’s Double Bridge.
Another difficulty in Wheatstone’s bridge is the change in resistance of the bridge arms due to
the heating effect of current through the resistance. The rise in temperature causes a change in the
resistance, and excessive current may cause a permanent change in value.
APPLICATIONS:
The Wheatstone’s bridge can be used to measure the dc resistance of various types of wire,
either for the purpose of quality control of the wire itself, or of some assembly in which it is used. For
example, the resistance of motor windings, transformers, solenoids, and relay coils can be measured.
Wheatstone’s bridge is also used extensively by telephone companies and others to locate cable
faults. The fault may be two lines shorted together, or a single line shorted to ground.
CONCLUSION:
REVIEW QUESTIONS AND ANSWERS:
1. Compare the measuring accuracy of a Wheatstone’s bridge with the accuracy of an ordinary
ohmmeter?
2. Define the term null as it applies to bridge measurement.
3. What are different types of null detector used in bridge measurement?
4. A wheatstone’s bridge cannot be used for precision measurements. Give reasons.
EXPERIMENT NO.3
To Study Kelvin’s Bridge.
OBJECTIVE: To measure the resistance value using Kelvin’s Bridge.
EXPERIMENTAL SETUP: Kelvin’s bridge, galvanometer, D.C. power supply, unknown resistance,
connecting leads etc.
THEORY:
When the resistance to be measured is of the order of magnitude of bridge contact and lead
resistance, a modified form of Wheatstone's bridge, the Kelvin bridge is employed.
Kelvin's bridge is a modification of Wheat- stone's bridge and is used to measure values of
resistance below
1Ω. In low resistance measurement, the resistance of the leads connecting the unknown resistance to
the terminals of the bridge circuit may affect the measurement.
Consider the circuit in Fig.1, where Ry represents the resistance of the connecting leads from
R3 to Rx (unknown resistance). The galvanometer can be connected either to point c or to point a. When
it is connected to point a, the resistance Ry, of the connecting lead is added to the unknown resistance R
x' resulting in too an high indication for Rx. When the connection is made to point c, Ry is added to the
bridge arm R3 and resulting measurement of Rx is lower than the actual value, because now the actual
value of R3 is higher than its nominal value by the resistance Ry. If the galvanometer is connected to
point b , in between points c and a, in such a way that the ratio of the resistance from c to b and that
from a to b equals the ratio of resistances R 1 and R2,
Equation (3) is the usual Wheatstone's balance equation and it indicates that the effect of the
resistance of the connecting leads from point a to point c has been eliminated by connecting the
galvanometer to an intermediate position, b.
The above principle forms the basis of the construction of Kelvin's Double Bridge, popularly
known as Kelvin's Bridge. It is a Double bridge because it incorporates a second set of ratio arms. Figure
2 shows a schematic diagram of Kelvin's double bridge.
The second set of arms, a and b , connect the galvanometer to a point c at the appropriate
potential between m and n connection, i.e. Ry. The ratio of the resistances of the arms a and b is the
same as the ratio of R1 and R2. The galvanometer indication is zero when the potentials at k and c are
equal.
This is the usual equation for Kelvin's bridge. It indicates that the resistance of the connecting
lead Ry, has no effect on the measurement, provided that the ratios of the two sets of arms are equal.
In a typical Kelvin's bridge the 'range of a resistance covered is 1-0.0001Ω (10 µohm) with an accuracy
of ± 0.05% to ±0.2%.
PRACTICAL KELVIN’S DOUBLE BRIDGE:
Figure No.3 shows a commercial Kelvin’s bridge capable of measuring resistance from 10 0.00001Ω.
Contact potential drops in the circuit may cause large errors. This effect is reduced by varying a
standard resistance consisting of nine steps of 0.001Ω each, plus a calibrated managing bar of
0.0011Ωwith a sliding contact. When both contacts are switched to select the suitable value of standard
resistance, the voltage drop between the ratio arm connection points is changed, but the total
resistance around the battery circuit is unchanged.
This arrangement places any contact resistance in series with the relatively high resistance
value of the ratio arms, rendering the contact resistance effect negligible. The ratio R 1 / R2 is selected
(as given in Fig.No.3) such that a relatively large part of the standard resistance is used and hence R X is
determined to the largest possible number of significant figures. Therefore, measurement a.c. current
improves.
CONCLUSION:
REVIEW QUESTIONS AND ANSWERS:
1. Compare the measuring accuracy of a Kelvin’s bridge with the accuracy of an ordinary
ohmmeter?
2. What are the advantages of Kelvin’s bridge over Wheatstone’s bridge?
EXPERIMENT NO.4
RLC SERIES A.C. CIRCUIT.
OBJECTIVE: To determine power factor of RLC series circuit.
EXPERIMENTAL SETUP:
i. Ammeter A.C.(0-1A), 1 No.
ii. Voltmeter A.C.(0-500V), 1 No.
iii. Wattmeter- (0-1200W)(5A,440V), 1 No.
iv. Resistance (300Ω/1.7Amp)
v. Capacitor, Choke
PROCEDURE:
1.
2.
3.
4.
5.
Make the connections as per circuit diagram.
Give the supply to circuit.
Take the reading of Ammeter, Wattmeter & Voltmeter.
Plot voltage triangle by using VR, VL & VC.
Calculate power factor for ckt.
OBSERVATION:
1. Ammeter reading -------------------------------- = I = ________ Amp.
2. Wattmeter reading ----------------------------- = W= P= _______Watts
3. Supply Voltage ----------------------------------= V = ________ Volts.
4. Voltage across resistance-----------------------=VR= ________ Volts.
5. Voltage across capacitor------------------------=VC=________ Volts.
6. Voltage across inductor (choke)---------------=VL=_________Volts.
CALCULATIONS:
Active power = Wattmeter reading
P = W = VI cosΦ.
Cos Φ = Wattmeter reading (W) / Supply voltage (V) X Ammeter reading.
VOLTAGE TRIANGLE:
From voltage triangle,
Power factor (cos Φ ) = ________ Leading . If Vc > VL
VL
VR
(VC-VL)
VC
I
CIRCUIT DIAGRAM:
CONCLUSION:
Power factor of the RLC circuit is obtained as = _______ Lead
(From voltage triangle)
=________ Lead.
(From calculations)
from this we conclude that ckt. is capacitive in structure because
power factor is leading.
EXPERIMENT NO.5
Measurement of Q-Factor.
OBJECTIVE: To measure Q-Factor of a coil
EXPERIMENTAL SETUP:
i. Ammeter
ii. Ammeter
iii. Voltmeter
iv. Inductor coil
v. Rheostat
(0-5 A)
(0-1 A)
(0-300 V)
(300 Ω, 1.7A)
THEORY:
In series resonating circuit, Q-factor is defined as the ratio of voltage across inductor
or capacitor to the applied voltage.
It is also the voltage magnification in the circuit at resonance
Q = VL / V = VC / V
Where, VL is the voltage across the inductor and
VC is the voltage across the capacitor.
V is the applied voltage.
Q = VL / V = IO XL / R IO = XL / R = WO L / R (For coil)
Q= w L / R
PROCEDURE:
1. Make the connection as per the circuit diagram.
2. Apply D.C. supply and measure the voltage applied, current drawn by the coil.
3. Apply A.C. supply and measure the voltage applied, current drawn by the coil.
4. Calculate Q-factor from given formula.
CIRCUIT DIAGRAM:
CONCLUSION:
Q-factor or quality factor of a coil is given by Q= w L / R and according to the observations it
comes out to be Q = _____.
EXPERIMENT NO.6
Study of Cathode Ray Oscilloscope (CRO).
OBJECTIVE: To study construction, front Panel of CRO. To measure the Voltage, Current, Frequency,
Time period of the Input waveform. To study lissajous pattern.
FIG: FUNCTIONAL BLOCK OF A SIMPLE CRO
TYPICAL SPECIFICATIONS:
VERTICAL DEFLECTION:
Bandwidth (-3dB): d.c. to 20MHz ( 2Hz to 20KHz on a.c.)
Sensitivity: 2mV/cm to 10V/cm
Accuracy: ± 3%
Input Impedance: 1MΩ/28pf approx.
Input Coupling: DC-GND-AC
Input Protection: 400V d.c. or pk a.c.
HORIZONTAL DEFLECTION:
Timebase: 0.5µs/cm to 0.2µs/cm, 18 ranges
Accuracy: ± 3%
ADDITIONAL FACILITIES:
Calibrator: 1V, 2% squarewave at approx. 1KHz.
Ramp Output: Approx. ± 3.5V ramp from 5KΩ.
SUPPLY:
220/240V± 10%
45 TO 65 Hz approx. 40VA.
Fig No. 2: - Simple CRO
APPLICATIONS OF OSCILLOSCOPE:
I.
Measurement of Voltage:
The most direct voltage measurement made with the help of an oscilloscope is the peak to
peak (p-p) value. The rms value of the voltage can then be easily calculated from the p-p value.
To measure the voltage from the CRT display, one must observe the setting of the vertical
attenuator expressed in V/div and the peak to peak deflection of beam, i.e. the number of divisions.
The peak value of voltage is then computed as follows.
Vp-p = (volts/div) × (no. Of div)
Fig No. 3: - Sine Waveform
II.
Period and Frequency Measurement:
The period and frequency of periodic signals are easily measured with an oscilloscope. The
waveform must be displayed such that a complete cycle is displayed on the CRT screen.
Accuracy is generally improved if a signal cycle displayed fills as much of the horizontal
distance across the screen as possible.
The period is calculated as follows.
T = (time/div) × (No. of div/cycle)
The frequency is then calculated as f = 1/T
III.
Measurement of Frequency by Lissajous Method:
This particular pattern results when sine waves are applied simultaneously to both pairs
of the deflection plates. If one frequency is an integral multiple (harmonic) of the other, the
pattern will be stationary, and is called a lissajous figure.
In this method of measurement a standard frequency is applied to one set of deflection
plates of the CRT tube while the unknown frequency (of approximately the same amplitude)
is simultaneously applied to the other set of plates. However, the unknown frequency is
presented to the vertical plates and the known frequency (standard) to the horizontal
plates. The resulting patterns depend on the integral and phase relationship between the two
frequencies. (The horizontal signal is designated as fh and the vertical signal as fv.)
Unknow
n
Frequen
Fig No. 4: - Basic circuit for frequency Measurements with
Lissajous Pattern for Integral Frequencies
Measurement Procedure:
Set up the oscilloscope and switch off the internal sweep (change to Ext). Switch off sync
control. Connect the signal source as given in Fig. 4. Set the horizontal and vertical gain control for
the desired width and height of the pattern. Keep frequency fv constant and vary frequency fh ,
noting that the pattern spins in alternate directions and changes shape. The pattern stands still
whenever fv and fh are in an integral ratio ( either even or odd). The fv = fh pattern stands still and
is a single circle or ellipse. When fv = 2fh , a two loop horizontal pattern is obtained as shown in Fig.
5.
To determine the frequency from any Lissajous figure, count the number of horizontal loops in
the pattern, divide it by the number of vertical loops and multiply this quantity by fh , (known or
standard frequency).
In Fig.5 (g), there is one horizontal loop and 3 vertical loops, giving a fraction of 1/3. The
unknown frequency fv is therefore 1/3 fh. An accurately calibrated, variable frequency oscillator will
supply the horizontal search frequency for frequency measurement. For the case where the two
frequencies are equal and in phase, the pattern appears as a straight line at an angle of 45° with the
horizontal. As the phase between the two alternating signals changes, the pattern changes cyclically,
i.e. an ellipse (at 45° with the horizontal) when the phase difference is π/4, a circle when the phase
difference is π/2 and an ellipse (at 135° with horizontal) when the phase difference is 3π/4, and a
straight line pattern (at 135° with the horizontal) when the phase difference is π radians.
Fig No. 5: - Lissajous Pattern for Integral Frequencies
As the phase angle between the two signals changes from π to 2π radians, the pattern changes
correspondingly through the ellipse-circle-ellipse cycle to a straight line. Hence the two frequencies,
as well as the phase displacement can be compared using Lissajous figures techniques.
When the two frequencies being compared are not equal, but are fractionally related, a more
complex stationary pattern results, whose form is dependent on the frequency ratio and the relative
phase between the two signals as in fig 6.
Fig No. 6 : - Lissajous Pattern for Non-Integral Frequencies
The fractional relationship between the two frequencies is determined by counting the number
of cycles in the vertical and horizontal.
fv = (fraction) × fh
or
fv = number of horizontal tangencies
fh
number of vertical tangencies
CONCLUSION:
REVIEW QUESTIONS AND ANSWERS:
1. What precautions are taken before the CRO is plugged in for operation?
2. Differentiate between Dual Trace and Dual Beam Oscilloscope?
3. Is there any control for beam rotation in CRO? Explain.
4. How the oscilloscope is calibrated to ensure correct sensitivity?
5. How capacitors measurement is performed using CRO?
EXPERIMENT NO. 7
Study of Strip Chart Recorder.
OBJECTIVE: To study Principle, Construction and Working of Strip Chart Recorders.
EXPERIMENTAL SETUP: Strip chart recorder, function generator.
THEORY:
It is often necessary to have a permanent recorder. In many of industrial & research processes it
is necessary to monitor continuously the condition, state or value of the process variables such as flow,
force, pressure, temprature, current voltage, electrical power etc. A recorder thus records electric &
non-electric quantities as a function of time.
This record may be written or printed & later on, can be examined & analyses to obtain a better
understanding & control of processes. Currents & voltages can be recorded directly while the nonelectrical quantities are recorded directly by first converting them to equivalent currents or voltages
with the help of sensors or transducers.
The every increasing emphasis on automation, continuously recording instruments are finding many
applications in industry.
There are two types of recording devices.
1. Analog recorders.
2. Digital recorders.
ANALOG RECORDRES:
There are numerous types of analog recorders. They are broadly classified into:
a. Graphic recorders
b. Oscillographic recorders
c. Magnetic Tape recorders
GRAPHIC RECORDERS:
i. Stripchart recorder: A strip chart records one or more dependent variable with respect to
time. It is X-t recorder.
ii. X-Y recorders: An X-Y recorder records one or more dependent variable With respect to an
independent
Variable.
STRIP CHART RECORDERS:
CONSTRUCTION:
A strip chart recorder consists of:
I. A long roll of graph paper moving vertically.
II. A system for driving the paper at some selected speed. A speed selector switch is genera1ly
provided. Chart speeds of 1 -100 mm/s are usually used.
III. A stylus for making marks on the moving graph paper. The stylus moves horizontally in
proportional to the quantity being recorded.
IV. A stylus driving system, which moves the stylus in a nearly exact replica or analog of the
quantity being recorded.
Fig No.1: - Strip Chart Recorder
A range selector switch is used so that input to the recorder drive system is within the
acceptable level.
Most recorder use a pointer attached to the stylus. This pointer moves over a calibrated scale
thus showing the instantaneous value of the quantity being recorded. An external control circuit for the
stylus may be used.
A. Paper Drive Systems. The paper drive system should move the paper at a uniform speed. A
spring wound mechanism may be used but in most of the recorders a synchronous motor is used
for driving the paper.
B. Marking Mechanisms. There are many types of mechanisms used for making marks on the paper.
The most commonly used ones are :
1.
Marking with Ink filled Stylus. The stylus is filled with ink by gravity or capillary
actions. This requires that the pointer shall support an ink reservoir and a pen, or
contain a capillary connection between the pen and a pen reservoir as shown in Fig.
No.2. In general red ink is used but other colours are available and in instrumentation
display a colour code can be adopted. The stylus, moving over the paper with
preprinted scales, traces the variations of the input signal. This method is most
commonly employed as ordinary paper can be used and therefore the cost is low. Other
advantages are that with this system, operation over a very wide range of recording
speeds is possible and also there is little friction between the stylus tip and the paper.
These disadvantages of this method are that ink splatters at high speeds, batches at
low speeds and clogs when the stylus is at rest. The frequency limit of recorders
incorporating this method of writing is only a few Hz.
2.
Marking with Heated Stylus. Some recorders use a heated stylus, which writes on a
special paper. This method overcomes the difficulties encountered in ink writing
systems.
The heated stylus melts a thin, white wax like coating on a
black paper base. Since the paper required is a special one, the cost is high. This
method cannot be used for recording certain processes which produce heat which
indirectly effect the recordings. But this method is quite reliable and offers high
3.
4.
5.
6.
contrast traces. Sophisticated recorders using papers with waxed surfaces and special
pens, have a frequency response upto 40 Hz are available
Chopper Bar. If a chart made from a pressure sensitive paper is used a simple
recording process is possible. A V -shaped pointer is passed under a chopper bar which
presses the pen into the paper once per second (or any other selected interval) thus
making a series of marks on the special paper. In fact this system is not purely
continuous and hence is suitable for recording some slowly varying quantities, for
example those which have a variation of 1 cycle per hour. This type of marking has the
advantage of a straight line horizontal scale without the use of complex linkage
arrangement.
Electric Stylus Marking. This method employs a paper with a special coating which is
sensitive to current. When current is conducted from the stylus to the paper, a trace
appears on the paper. It is clear that the electric stylus marking method has a wide
range of marking speeds, has low stylus friction and a long stylus life. The
disadvantage is that the cost of paper is very high.
Electrostatic Stylus. This method uses a stylus, which produces a high voltage
discharge thereby producing a permanent trace on an electrosensitive paper. This
arrangement has been incorporated in a recorder having a 50 mm wide chart nine
voltage ranges from 10 mV/mm to 5 V/mm; eight chart speeds from 300 mm/s to 10
mm/min and a frequency response of 60 Hz at maximum amplitude of I db.
Optical Marking Method. This method uses a beam of light to write on a
photosensitive paper. Thus this method allows higher frequencies to be recorded and
permits a relatively large chart speed with good resolution. The disadvantages are that
the paper cost is very high. Secondly the writing process is a photographic one, the
paper must be developed before a record is available and hence this method is not
suitable for processes where instantaneous monitoring is to be done.
C. Tracing Systems. There are two types of tracing systems used for producing graphic
representations.
1. Curvilinear System. In the curvilinear system, the stylus is mounted on a central pivot and
moves through an
arc, which allows a full-width chart marking. If the stylus makes a full
range recording, the line drawn across the chart will be curved and the time intervals will he
along this curved segments. This type of system is used on many records, with PMMC
galvanometers actuating the stylus filled with ink as shown in Fig.no.3. The disadvantage of
this method of tracing is the charts are difficult to analyze because of curved time base lines.
2. Rectilinear System. It is noticed that a line of constant time is perpendicular to the time axis
and therefore this system produces a straight line across the width of the chart. Here the
stylus is actuated by a drive cord over pulleys to produce the forward and reverse motion as
determined by the drive mechanism. The stylus may be actuated by a self-balancing
potentiometer system, a photoelectric deflection system, a photoelectric potentiometer
system, or a bridge-balancing system. This system is usually used with thermal or electric
writing.
Types of Strip Chart Recorders.
The different types of strip chart recorders are:
1.
Galvanometer Type. This type of strip chart recorder operates on the deflection
principle. The deflection is produced by a galvanometer, which produces a torque on
2.
account of a current passing through its coil. This current is proportional to the
quantity being measured.
Null Type. This type of recorder operates on comparison basis.
Galvanometer Type Recorders.
These recorders use a d' Arsonval galvanometer. The pointer is equipped with a
recording pen mechanism (stylus). A cut-away view of the moving coil element is shown in Fig. 2.
As the current flows through the coil, it deflects. The greater the amplitude of the incoming
signal (which is proportional to the quantity being measured), the greater is the deflection.
When the pointer comes to rest on account of controlling torque exerted by springs, the stylus
also comes to rest. Thus, the value of the quantity is recorded. In recorders, the movement of
the instrument requires an appreciable torque. To obtain this torque the d' Arsonval movement
consists of a large moving coil situated in a strong magnetic field.
It should be understood that the instrument must be critically damped or nearly
critically damped so that there is no significant overshoot. But this results in slow response, the
response time being 0.75 to 1.5 s.
Thus this type of recorder
is not useful for recording fast variations in either current or voltage or power. This records
only the average values and hence it should be designed for these. A galvanometer type recorder
is shown in Fig. 2. It is a modified version of the PMMC instrument. The modification is done in
order that the chart may be driven at a constant speed by a clockwork mechanism or an electric
motor. The type of chart used depends upon the form of movement. The recorder shown in Fig. 2
uses a chart having a curvilinear system of tracing. This system is used because it allows the
direct use of simple moving coil movement. . However, some instruments employ additional linkage
system in the system, which allow the use of rectilinear system of tracing as shown in Fig. 3.
The recorders can work on ranges from a few mA to several mA or from a few mV to
several mV. This moving galvanometer type recorder is comparatively inexpensive instrument
having a narrow bandwidth of 0 to 10 Hz. It has a sensitivity of 0.4 mV/mm or from a chart of
100 mm width a full scale deflection of 40 mV is obtained. For measurement of smaller voltages
liner amplifiers are used.
Fig No.2: - Cut-away view of moving coil element
Fig No. 3: - Components of a galvanometer type recorder
Null Type Recorders:
Many recorders operate on the principle whereby a change in its input, produced by the signal from the
sensor or transducer (which is used to convert a non-electrical quantity to an equivalent electric signal),
upsets the balance of the measuring circuit of the recorder. As a result of this unbalance an error
Signal is produced that operates some device which restores balance or brings the system to Null
conditions. The amount of movement of this balance restoring device, then, is an indication of the
magnitude of the error signal, and the direction of the movement is an indication of the direction of the
quantity being measured has deviated from normal.
The signal from the transducer may take any of the several forms. It may be a voltage (a.c. or
d.c.), a current (a.c. or d.c.) or it may be a value of resistance, inductance or capacitance. The recorder,
therefore, must be of a type able to accept the form of the input signal.
There are a number of null type recorders. They are: (i) Potentiometric recorders (ii). Bridge
recorders and (iii) LVDT recorders.
Fig:- Block Diagram of Self-Balancing Potentiometer Recorder
TYPICAL SPECIFICATIONS:
Specifications of YokoGawa Strip Chart Recorder.
Model: - SR1000 3 pen
1. Inputs:
DCV: Direct Current Voltage input, Measuring range 20mV to 20V.
TC: Thermocouple, Measuring range -200°C to 1760°C.
RTD: Resistance Temperature Detector, Measuring range of Pt100 -200°C to 600°C.
DI: Digital Input, Voltage i/p Less than 2.4V: OFF
More than 2.4V: ON (TTL)
1. Effective recording width: 100mm
2. Chart: Plain-Paper Z-fold chart (16m)
3. Chart Speed: Pen Model range 10mm/h to 1500mm/h.
4. Recording Colors: Pen 1=red Pen 2=green Pen 3=blue Plotter pen= Purple
5. Rated Power Voltage: 100 to 240VAC, Automatically selected depending on the power supply
voltage
6. Usable Power Voltage ranges: 90 to 132, 180 to 250VAC
7. Rated Power Frequency: 50/60 Hz, Automatically Selected.
CONCLUSION:
REVIEW QUESTIONS
1.
2.
3.
4.
5.
6.
AND ANSWERS:
What are the requirements of chart drive mechanism?
What are the limitations of such recorders?
What is the major advantage of strip chart recorder?
What are the specifications of the paper chart used for writing?
What are the sources of error in the strip chart recorder?
Give the selection criteria of a recorder for given application.
EXPERIMENT NO.8
Study of Multimeter.
OBJECTIVE: To study the construction, modes and working of Multimeter.
EXPERIMENTAL SETUP: Handheld Multimeter, Bench type Multimeter.
THEORY:
ANALOG MULTIMETER:
A Multimeter is basically a PMMC meter. To measure dc current the meter acts as an ammeter
with a low series resistance.
Range Changing is accomplished by shunts in such a way that the current passing through the
meter does not exceed the maximum rated value.
A multimeter consists of an ammeter, voltmeter and ohmmeter combined with a function switch
to connect the appropriate circuit to the D’Arsonval movement.
Figure shows a meter consisting of a dc milliammeter, a dc voltmeter, an ac voltmeter, a
microammeter, and an ohmmeter.
Analog meters require no Power Supply, they give a better visual indication of changes and
suffer less from electric noise and isolation problems. These meters are simple and inexpensive.
DIGITAL MULTIMETER:
Digital meters offer high accuracy, have high input impedance and are smaller in size. They give an
unambiguous reading at greater viewing distances. The output available is electrical (for interfacing with
external equipment), in addition to a visual readout.
To enable digital systems to to recognize information, inputs which are analog in nature must be
converted to digital form. Hence any digital instrument would invariably consist of an analog to digital
converter in its input stage. The basic buiolding block of digital instrument is shown in fig.1
The three major classes of digital meters are panel meters, bench type meters and system
meters.
All digital meters employ some kind of analog to digital (A/D) converters (often dual slope
integrating type) and have a visible readout display at the converter output.
Panel meters are usually placed at one location ( and perhaps even a fixed range ), while bench
meters and system meters are often multimeters, i.e. they can read ac and dc voltage currents and
resistances over several ranges.
The basic circuit shown in fig 2 is always a dc voltmeter. Current is converted to voltage by
passing it through a precision low shunt resistance while alternating current is converted into dc by
employing rectifiers and filters. For resistance measurement, the meters includes a precision low
current source that is applied across the unknown resistance; again this gives a dc voltage which is
digitized and readouts as ohms.
Bench meters are intended mainly for stand alone operation and visual operation reading, while
system meters provide at least an electrical binary coded decimal output ( in parallel with the usual
display), and perhaps sophisticated interconnection and control capabilities, or even microprocessor
based computing power.
A basic digital multimeters (DMM) is made up of several A/D converter, circuitry for counting
and an attenuation circuit. A basic block diagram of a DMM is shown in fig 3. The current to voltage
converter shown in the block diagram of Fig3 can be implemented with the circuit shown in Fig 4.
The current to be measured is applied to the summing junction (∑ I ) at the input of the op-amp.
Since the current at the amplifier is close to zero because of the very high input impedance of the
amplifier, the current IR is very nearly equal to Ii, the current IR causee a voltage drop which is
proportional to the current, to be developed across the resistors. This voltage drop is the input to the
current, to be developed across the resistors. This voltage drop is the input to the A/D converter,
thereby providing a reading that is proportional to the unknown current.
Resistance is measured by passing a known current, from a constant current source, through an
unknown resistance. The voltage drop across the resistor is applied to the A/D converter, thereby
producing an indication of thye value of the unknown resistance.
CONCLUSION:
REVIEW QUESTIONS AND ANSWERS:
1. What are the various parameters which can be measured using multimeter.
2. What is the type of mechanism for pointer deflection in multimeter?
3. Which one of the two multimeters type viz. analog and digital is more accurate?
4. What is auto ranging multimeter?
EXPERIMENT NO.9
Study of 7-Segment display.
OBJECTIVE: To Study the operation of LED and 7-Segment display.
EXPERIMENTAL SETUP: 7-Segment, Power Supply (0-5V), Bread Board, LED, Resistance.
THEORY:
Light Emitting Diode (LED). A relatively new family of display devices utilizes "light emitting
diodes”. The LED is perhaps the most important of the display devices available today for use in
Instrumentation systems. The LED is a PN junction device, which emits light when a current passes
through it in the forward direction.
Charge carrier recombination occurs at a PN junction as electrons cross from N side and
recombines with holes on the P side. When recombination takes place, the charge carriers give up energy
in the form of heat and light. If the Semiconducting material is translucent the light is emitted, and the
junction is source of light. This is the light emitting diode i.e. LED. Fig. 1 shows a cross-sectional view of
a typical LED charge carrier recombinations takes place in the P type material. Therefore, the P region
becomes the surface of the devices. For maximum light emission, a metal film anode is deposited around
the edge of the p type material. The cathode connection for the device is usually a gold film at the
bottom of the N type region. This helps in reflecting the light to the surface. Semiconductor materials
used for manufacture of LED are gallium arsenide phosphide (GaAsP) which emits red or yellow light of
gallium arsenide (GaAs) which gives green or red Ijght emission. LEDs are used extensively in segmental
and dot matrix displays of numeric and alphanumeric characters. Several LEDs are used in series to
form one Segment while a single LED may be used to form a decimal point. LEDs are available in many
colours like green, yellow, amber and red.
Fig. 1. Cross-Section of LED
A common supply voltage drives the anodes of the LEDs and when a switch closes, the corresponding
LED is forward biased and emits light.
Fig 2 LED controlled by Transistor Switch
A simple transistor can be used for OFF/ON of an LED as shown in Fig. 2. When the transistor is
driven into saturation by base current IB , it conducts heavily (switch is closed and the LED emits light).
The LED current is limited by resistance Rc.
A common supply voltage drives the anodes of the LEDs and when a switch closes, the
corresponding LED is forward biased and emits light.
Fig 3: 7-Segemnt readout and circuit.
PROCEDURE:
1. Make the circuit as shown in the figure.
2. Give a power supply of 5V to IC7447 and the common Anode & 7 -segment display.
3. Change the BCD input as 5V for logic 1 and 0V for logic 0, to IC7447 and see the display
output.
CIRCUIT DIAGRAM:
CONCLUSION:
REVIEW QUESTIONS AND ANSWERS:
1. What are the advantages of LED over other Displays?
2. Convert BCD to decimal from 0000 to 1111.
3. What is the maximum supply on which LED can operate?