FULL ENJOY Call Girls In Majnu Ka Tilla, Delhi Contact Us 8377877756
Emii
1. Government Engineering College
Kozhikode-5
Electrical Machines Lab Manual
AI09 308(P), ME09 307(P)
(Version 1.1.0 - June 25, 2012)
Prepared by :- Mohammed Sadik.P.K(2010 AEI batch)
Ranjith A.R
(2010 AEI batch)
Nikhil Narayanan
(2010 AEI batch)
Guided by
:- Smt. Sangeetha K
(Associate Professor(EEE))
2. A
Type set in L TEX 2ε , circuit designs and graphs in XCircuit.
Platform : GNU/Linux
Other free softwares used: GNU Emacs, GNU Bash, Gedit, Vim . . . . . .
2011, 2012, Some Rights Reserved.
You may get a copy of this work from www.sadiq.tk.
This work is licensed under :
Creative Commons Attribution-ShareAlike 2.5 India License.
To view a copy of this license, visit http://creativecommons.org/licenses/by-sa/2.5/in/
or send a letter to Creative Commons, 444 Castro Street, Suite 900, Mountain View,
California, 94041, USA.
Thanks to:1.
2.
3.
4.
5.
6.
7.
8.
9.
Smt. Sangeetha K(Associate Professor(EEE))
Sri. Asokan (Assistant Professor(EEE))
Dr. Reena P(Associate Professor(ECE))
Najmudheen P.K (Cherumukku)
Jazeel M (Kolappuram)
Selil C.P(MES Kuttippuram-EC)
Mansoor M (GECK-2008 AEI batch)
Sajith P.P (GECK-2010 AEI batch)
Misthah K.M (AWH Kozhikode-EEE)
ÇÙÖ
Å
ÒÙ
и
× Ò
Ö
Ø
×Ô
ÐÐÝ
Ò
×
ÓÙÖ
ØÓ
Ð
Ú ÖÝÓÒ
××Ñ
Ø ×
¹
Û
Ó
¾¼½¼
Ú
ÐÔ
Á
Ù×
Ø
º
ÓÖ
Ø
Ô Ö
Ø ÓÒ
Ó
Ø
×
4. CONNECTION DIAGRAM OF LOAD TEST
0-10A MI 150V,10A,upf
M
L
P
V
E
S1
V
0-150V
MI
C
P2
NL
0-250V
MI
S2
120/240V 1KVA
TRANSFORMER
Rated current on primary side =
1000
= ......
120
Rated current on secondary side =
1000
= ......
240
REGULATION AND EFFICIENCY CURVES
Efficiency (%)
Regulation
Efficiency
Output (watts)
Experiment 1
S
A
V P1
C
B
Regulation (%)
N
A
10A
230V
1-φ
50 Hz
AC
0-5A MI
4
Load
5. Experiment 1
LOAD TEST ON SINGLE PHASE
TRANSFORMER
AIM
To conduct load test on the given single phase transformer at unity power factor and determine
the efficiency and regulation curve.
APPARATUS REQUIRED
1.
2.
3.
4.
5.
6.
Voltmeter 0-250V MI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Voltmeter 0-150V MI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Ammeter 0-10A MI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Ammeter 0-5A MI. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1
Wattmeter 150V, 10A, upf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Autotransformer(cont. variable) 0-270V, 10A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
no.
no.
no.
no.
no.
no.
THEORY
Regulation of a transformer is defined as the drop in terminal voltage of a transformer expressed
as a percentage of the no-load terminal voltage.
%Regulation =
Vno load − Von load
Vno load
When a purely resistive load is connected across the secondary, the transformer will be
working at unity power factor.
Terminal voltage, V = Induced emf [E2 ] − I2 r2 − I2 x2
Where r2 and x2 are the secondary winding resistance and leakage reactance respectively
and I2 is the secondary load current.
Output
× 100.
The efficiency of transformer is defined as
Input
As the load current increases the power output increases. The iron loss remains constant
from no load to full load. The copper loss increases as the square of the load current. Thus the
efficiency curve starts from zero, increases to a maximum value(When iron loss = Cu loss) and
thereafter starts decreasing.
5
7. PROCEDURE
Connections are made as shown in the circuit diagram. The supply is switched on keeping
the autotransformer in the minimum position and at no load. Adjust the autotransformer to
get the rated voltage of the transformer. The readings of all the meters are noted down. The
secondary voltage at no load is also noted down. This value is VNL . A small load is added
on the secondary side and the meter readings are again noted. The experiment is repeated
for different values of load current till the current on the primary side equals the rated value.
The load is then reduced to zero, the autotransformer is brought back to the zero position and
the supply is switched off. The readings are then tabulated as shown and the regulation and
efficiency curves are plotted.
RESULT
Conducted load test on the given 1-φ transformer and plotted the regulation and efficiency
curves.
VIVA QUESTIONS
1.
2.
3.
4.
5.
What do you understand by regulation of a transformer?
What are the other methods of testing transformers?
What is the disadvantage of testing a transformer using load test?
Is a high or low value of regulation preferred for a transformer? Give reasons.
What are the reasons for the drop in terminal voltage as the secondary current
is increased?
7
9. Experiment 2
LOAD TEST ON 3−φ SQUIRREL CAGE
INDUCTION MOTOR
AIM
To conduct load test on the given 3-φ squirrel cage induction motor and plot the performance
characteristics.
APPARATUS REQUIRED
1.
2.
3.
4.
Voltmeter
Ammeter
Wattmeter
Tachometer
0-600V
MI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
0-10A
MI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
600V,10A, upf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
To measure speed
1 no.
1 no.
2 nos.
THEORY
A squirrel cage induction motor essentially consists of a stator and a rotor. The stator is a
hollow cylindrical structure with slots on the inner periphery and carries a three phase winding.
The winding can be connected in star or delta and is connected across a 3-φ supply.
The rotor is also a cylindrical structure with slots on the outer periphery. The slots carry
thick Al or Cu bars. These bars are short circuited at both ends by means of end rings.
When a 3-φ supply is given to a 3-φ winding displaced by 120◦ in space, a magnetic field
of constant magnitude but rotating at synchronous speed is produced. This flux links with
the stationary rotor, thus inducing an emf in it. As the rotor circuit is closed, a current flows
through it. The direction of the induced current is such as to oppose the cause producing it.
The cause is the relative motion between the stator magnetic field and the rotor. So the rotor
starts rotating in the same direction as the stator magnetic field and tries to catch up with
it. But practically it is never able to do so. Because if it does so, there would be no relative
motion, no emf and hence no torque.
9
11. Thus an induction motor always runs at a speed slightly less than the synchronous speed.
The term slip is of importance in an induction motor and is defined as
%slip =
Where,
Ns - Synchronous speed =
Ns − N
× 100
Ns
120 × f
P
N - rotor speed
f - frequency
P - No. of poles of the machine
An induction motor can never operate at s=0. It always operates between s=0 and s=1(starting).
The performance characteristics are plots of efficiency, torque, speed, slip, pf and line current versus output.
Current and torque increases with increase in output. The induction motor is essentially
a constant speed motor. However speed reduces gradually with increase in output and slip
increases gradually with increase in output. The pf is low at low loads and increases with
increase in output. The efficiency increases with increase in output, reaches a peak value and
then gradually drops with further increase in output.
PROCEDURE
The load on the motor is completely removed by loosening the brake drum. The motor is to
be always started and stopped at no load, The supply is switched on and the motor is started
using a Direct On Line Starter (DOL Starter).
The readings of the voltmeter, ammeter, wattmeters and spring balance are noted down.
The speed is measured using a tachometer. The load is then increased in steps, each time noting
down all the above readings. The experiment is repeated for different values of load currents
till the rated current of the machine is reached.
During the experiment, the machine may get heated up. It is cooled by pouring some water
into the brake drum.
11
13. At low loads,(when pf < 0.5) one of the wattmeters read negative, in such cases, the supply
is switched off and the connections to the M and L terminals of the wattmeter are interchanged.
The meter now reads positive, but it is to be recorded as negative.
The load on the machine is removed completely and the supply is switched off.
The readings are tabulated and the performance characteristics are plotted.
RESULT
Conducted load test on the given 3-φ squirrel cage induction motor and plotted the performance
characteristics.
VIVA QUESTIONS
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.
11.
How are the meter ratings selected for this experiment?
Why does one of the wattmeters read -ve at starting?
What is ‘slip’ in an induction motor?
What are the two types of 3-φ induction motors and what is the difference
between the two?
What is the value of slip at starting?
What are the advantages and disadvantages of squirrel cage induction motor?
What is the condition for maximum torque in an induction motor?
What are the different losses in an induction motor?
Give some applications of 3-φ squirrel cage induction motor?
Explain a typical Torque-slip characteristic.
What is the effect of increased rotor resistance on the performance of an induction machine?
13
14. CONNECTION DIAGRAM
3 POINT STARTER
L F A
+
20A
600Ω
2A
Rh1
220V
DC
S
A1
A1
+
G
M
F1
F1
V
-
A2
A2
20A
0-30V
MC -
-
DETAILS
GENERATOR
3.5 KW
Speed - 1500rpm
volts - 220V
Amps - 16 A
Winding - shunt
Field - 220V,0.46A
OBSERVATION
Sl No.
Field current
Eo at rated speed
Eo at 1000 rpm
To determine O.C.C at 1000 rpm
We have E ∝ N at same flux or field current
E1
N1
=
E2
N2
⇒ E2 =
Where,
N2 is 1000 rpm
N1 → Rated speed = 1500 rpm
Experiment 3
14
N2
N1
E1
+ 0-300V
MC
V
+
F2
MACHINE
MOTOR
3.5 KW
Speed - 1500rpm
volts - 220V
Amps - 18.6 A
Winding - shunt
Field - 220V,0.46A
600Ω
2A
Rh2
A
F2
0-2A
MC
15. Experiment 3
O.C.C OF DC SHUNT GENERATOR
AIM
To conduct no load test on the given d.c shunt generator and determine the following:1. Open circuit characteristics at rated speed.
2. Predetermine the O.C.C at 1000 rpm.
3. The critical field resistance at rated speed.
4. The critical speed of the machine.
APPARATUS REQUIRED
1.
2.
3.
4.
5.
Voltmeter
Voltmeter
Ammeter
Rheostat
Tachometer
MC (0-300V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.
MC (0-30V). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 no.
MC (0-2A) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.
600Ω, 2A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 nos.
THEORY
The O.C.C is a curve showing the relationship between the no load emf generated and the shunt
field current (Eo and If ). Even when the field current is zero there is some residual magnetism
present in the poles. Hence there is a small voltage generated even at zero field current, which
is called the residual voltage. As the field current is increased, Eo also increases and the curve
traced is almost a straight line. As If is further increased the poles start getting saturated, the
straight line relation no longer holds good and the curve bends and becomes almost horizontal.
Critical resistance
It is that value of resistance in the field circuit at which the generator will just excite(or voltage
build up begins). If the resistance is higher, the machine will fail to build up voltage. It is given
by the slope of the tangent drawn to the linear portion of the magnetization curve from the
origin.
15
16. MEASUREMENT OF SHUNT FIELD RESISTANCE
+
0-2A MC
100Ω,2.8A
+
F1
+
220V
DC
A
V
0-250V
MC
F2
-
OBSERVATION
Sl No.
V
I
16
Rf
17. Conditions for voltage build up in a d.c shunt generator
1. There should be some residual magnetism in the poles.
2. For the given direction of rotation, the shunt field coils should be properly
connected. That is, The coils should be so connected that the flux generated
by the field current aids the residual flux.
3. When excited at no load, the shunt field circuit resistance should be less than
the critical resistance.
Critical speed
It is that value of speed at which the given shunt field resistance represents the critical resistance.
It is determined as follows. For the same value of If determine E1 and E2 from the field
resistance lines. Then
E1
N1
E2
N1
=
⇒ Nc =
E2
Nc
E1
Where,
Nc is the Critical speed
PROCEDURE
Connections are made as shown in the diagram. The motor field rheostat (Rh1 ) is kept in
minimum position, the generator field rheostat (Rh2 ) in maximum position and switch ‘S’ is
kept open at starting. Supply is switched on. The starter handle is gradually moved to cut off
the starter resistance. The rheostat Rh1 is varied till the speed equals the rated speed of the
machine. With ‘S’ open, the residual voltage is measured using the smaller range voltmeter.
Switch ‘S’ is then closed. Rheostat Rh2 is then decreased in steps, each time noting down
the voltmeter and ammeter readings. The process is repeated till the voltage equals 120% of
the rated voltage of the machine. [If Eo does not increase, it means that the machine is not
building up voltage. The field terminals F1 and F2 are interchanged and the process is repeated]
Rheostat Rh2 and Rh1 are brought back to the original position and the supply is then switched
off.
17
18. OPEN CIRCUIT CHARACTERISTICS
Eo
(V)
E1
Critical Field
resistance
line
O.C.C at rated
speed
O.C.C at 1000 rpm
Given shunt field
resistance line
E2
residual{
voltage
If1
If (A)
Critical resistance at rated speed,
Rc =
E1
= .........
If 1
Critical speed of the Machine,
Nc =
E2
E1
N1 = . . . . . . . . .
18
19. Measurement of field resistance
Connections are made as shown in the diagram 2. For different values of voltages determine
the current. The ratio gives the field resistance. The O.C.C and field resistance line is drawn
and the critical speed of the machine is determined.
RESULT
No load test was conducted on the given d.c shunt generator and the O.C.C was plotted.
Critical resistance at rated speed = . . . . . . . . .
Critical speed of the machine
= .........
VIVA QUESTIONS
1.
2.
3.
4.
5.
6.
7.
8.
9.
What is the need for starter in a d.c motor?
How does a 3-point starter function?
Why is Rh1 kept in minimum position at starting?
Why is Rh2 kept in maximum position at start up?
What is residual voltage? How is it measured?
What is critical resistance? How can it be determined?
What are the conditions necessary for voltage build up in a d.c shunt generator?
What is critical speed?
Explain the shape of the O.C.C.
19
20. LOAD TEST
+
A
Rh1
600Ω
2A
200V
DC
A1
A1
F1
M
Rh2
+
F1
A2
V
F2
0-2A
MC
F2
20A
600Ω
2A
G
A2
-
0-20A MC
+
-
3 Point Starter
L F A
+
0-300V
MC
20A
-
MACHINE DETAILS
MOTOR
GENERATOR
3.5 KW
3.5 KW
Speed
- 1500rpm
Speed
- 1500rpm
volts
- 220V
volts
- 220V
Amps
- 18.6 A
Amps
- 16 A
Winding - shunt
Winding - shunt
Measurement of Armature Resistance
0-5A MC
+
-
A
+
50Ω
5A
V
0-10V
MC
Sl
No.
Voltage, V
(volts)
Current, I
(Amperes)
A1
G
A2
Resistance, R
(ohms)
Armature Resistance Ra = . . . . . . . . . Ω
Experiment 4
20
Load
A
S1
20V
DC
-
S2
21. Experiment 4
LOAD TEST ON D.C SHUNT
GENERATOR
AIM
To conduct load test on the given D.C shunt generator and plot the external and internal
characteristics.
APPARATUS REQUIRED
1.
2.
3.
4.
5.
6.
Voltmeter
Voltmeter
Ammeter
Ammeter
Rheostat
Tachometer
MC (0-300V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.
MC (0-10V). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1 no.
MC (0-2A) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.
MC (0-5A) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.
600Ω, 2A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 nos.
THEORY
Load characteristics of the machine can be broadly classified into:1) External characteristics
2) Internal Characteristics
External Characteristics(V vs IL )
It is a curve showing the variation in terminal voltage of the generator as the load on the
generator is increased. The characteristics are as shown in the figure.
At no load, the terminal voltage of the generator is at its rated value. As the load current
is increased the terminal voltage drops. The drop in terminal voltage is due to the following
reasons:1. For a generator V = Eg − Ia Ra , as the load current increases, Ia increases, Ia Ra drop
increases, thus decreasing the terminal voltage V.
2. As the load current increases, Ia increases, armature reaction effect also increases. Due
to demagnetizing effect of armature reaction, the induced emf Eg decreases, thereby
decreasing V.
3. Due to reasons (1) and (2), the terminal voltage decreases, which in turn reduces the field
current Ish , thereby decreasing Eg causing further decrease in V.
21
22. OBSERVATION - LOAD TEST
Sl
no.
V
(volts)
IL
(A)
Ish
(A)
Ia
(A)
Eg
(V )
Sample Calculation (set no . . . )
Terminal Voltage (V)
= .........V
Load Current (IL )
= .........A
Shunt Field Current (Ish )
= .........A
Armature Current (IA ) = IL + Ish
= .........A
Generated emf (Eg )
= V + Ia Ra = . . . . . . . . .
Internal and External Characteristics
V/ Eg
(V)
Drop Due
To Armature
Resistance
Drop due to armature
Reaction Effect
Internal Characteristics
Eg vs Ia
External
Characteristics
V vs IL
IL/ Ia (A)
22
23. Internal Characteristics [Eg vs Ia ]
It is a plot of the internally generated emf (Eg ) and armature current (Ia ). It is a curve similar
to the external characteristics and lies above it.
E g = V + I a Ra
& Ia = IL + Ish
PROCEDURE
Connections are made as shown in the diagram. rheostat Rh1 is kept in minimum position and
Rh2 in maximum position. Switch S2 is kept open. Supply is switched on and the motor is
started using a 3-point starter. The motor field rheostat Rh1 is varied till the speed equals the
rated speed of the motor. The generator field rheostat Rh2 is varied till the voltmeter reads the
rated voltage of the machine. Switch S2 is then closed. The load on the generator is increased.
The readings of the voltmeter and ammeters are noted down. The experiment is repeated for
different values of load current till the rated current of the generator is reached. During the
experiment, the speed is to be maintained constant at the rated value.
The load is then switched off completely, the rheostats are brought back to the original
position and the machine is switched off.
Measurement of Ra
Connections are made as shown in the diagram. Keeping the rheostat in the minimum output
voltage position, supply is switched on. The rheostat is then varied in steps and the voltmeter
and ammeter readings are noted. The ratio gives the armature resistance.
The readings are then tabulated as shown. The external and internal characteristics are
then plotted.
RESULT
Conducted load test on the given DC shunt generator and plotted the external and internal
characteristics.
VIVA QUESTIONS
1.
2.
3.
4.
5.
6.
7.
8.
9.
What is the need for starter with a d.c motor?
How does a 3-point starter function?
Why is Rh1 kept in minimum position at starting?
Why is Rh2 kept in maximum position at starting?
Why does the terminal voltage of a generator decrease with increase in load?
How are the meter ratings selected for this experiment?
What are the different losses in a d.c generator?
What is the condition for maximum efficiency in a d.c machine?
What is armature reaction? How does it effect the functioning of the machine?
23
24. CONNECTION DIAGRAM
2 Point Starter
+
-
L A
+
A
20A
F1
0-20A
F2
V
-
0-300V MC
A2
BRAKE
DRUM
20A
Machine Details
HP
5
Volts 230 V
Amp 17 A
speed - 1500 rpm
Radius of brakedrum, R = . . . . . . . . .
OBSERVATIONS
Sl.
No.
V
volts
I
Amp.
S1
Kg
S2
Kg
Speed
rpm
Torque
N −m
Output
watts
Input
watts
Sample Calculation (set no. . . . )
Voltmeter reading (V ) = . . . . . . . . .
Current (I)
= .........
Spring balance readings, S1 = . . . . . . . . .
Speed(N )
Torque(T )
S2 = . . . . . . . . .
= .........
= 9.8 (S1 − S2 ) R = . . . . . . . . .
2πN T
= .........
60
= VI
= .........
Output power
= .........
=
Input power
Output power =
Input power
Efficiency
Experiment 5
S2
A1
+
220V
DC
S1
24
Where R is the radius
of brake drum
Efficiency
(%)
25. Experiment 5
LOAD TEST ON DC SERIES MOTOR
AIM
To conduct load test on the given d.c series motor and plot the performance characteristics.
APPARATUS REQUIRED
1. Voltmeter (0-250)V MC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.
2. Ammeter (0-20)A MC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.
3. Tachometer - to measure speed
THEORY
In a series motor, the field winding is connected in series with the armature winding. Thus the
same current flows through the field and armature windings.
Electrical characteristics(T vs Ia ) :- It shows the variation of torque with the armature
current.
We have
T ∝ φIa where φ is the flux/pole
∝ Ia Ia (as φ ∝ Ia up to the point of magnetic saturation)
Thus
2
T ∝ Ia
However after magnetic saturation φ remains almost constant, Hence T ∝ Ia
Thus the curve is a parabola up to magnetic saturation and shows a linear variation after
the point.
Mechanical Characteristics(N1 vs T ):- It shows the variation of speed with torque.
Eb
1
We have N ∝
∝
as Eb is almost constant
where Eb is back emf
φ
φ
In a series motor φ ∝ Ia
1
∴N ∝
Ia
That is, as Ia increases, Speed decreases.
The same pattern is followed in the N -T characteristics. The curve traced is a rectangular
hyperbola.
A series motor should never be started at no load. At no load, Ia is very small, hence the
speed of the motor becomes dangerously high(as N ∝ I1 ).
a
25
26. Performance Characteristics
T η N
(Nm)
(%)
(rpm)
Torque
Efficiency
Speed
O
Output
(watts)
Electrical Characteristics
Mechanical Characteristics
N
(rpm)
T
(Nm)
Ia (A)
26
T (Nm)
27. Performance characteristics shows the variation of speed, torque and efficiency with change
in output.
PROCEDURE
Connections are made as shown in the connection diagram. A small load is applied to the
motor by tightening the brake drum. The motor should never be started at no load. Supply is
switched on and the motor is started using a 2-point starter. The voltage, current, speed and
spring balance readings are noted down. The experiment is repeated for different loads till the
rated current of the machine is reached.
During the experiment when the machine gets heated up, it is cooled by pouring water into
the brake-drum.
The load is then reduced till the current reaches a small value and the supply is switched
off.
RESULT
Load test was conducted on the given DC series motor and the performance, electrical and
mechanical characteristics are plotted.
VIVA QUESTIONS
1.
2.
3.
4.
5.
6.
7.
8.
What is the precaution to be taken when working with a d.c series motor?
What is the need for starter with a d.c motor?
How does a 2-point starter function?
Explain the shape of the electrical and mechanical characteristics.
What is the condition for maximum efficiency in a d.c motor?
What are the different losses occurring in a d.c machine?
How are the meter ratings selected for this experiment?
Give some applications of d.c series motor.
27
28. CONNECTION DIAGRAM
FLUX AIDING
0-500mA MI
230V
50Hz
1-φ
AC
A
0-250V
MI V
V1
C
N
P1
B
Auto
Transformer
P
230/115V, 3kVA
Transformer
V3
2
0-250V
MI
E
S2
0-150V
MI
1A
S1
P2
NL
V1 ⋍ V2 + V3
FLUX OPPOSING
C
N
P1
B
0-250V
MI V
V1
S1
V3
2
0-250V
MI
S2
P2
E
0-150V
MI
230V
50Hz
1-φ
AC
A
Auto
Transformer
P
230/115V, 3kVA
Transformer
0-500mA MI
1A
NL
V1 ⋍ V2 − V3
MEASUREMENT OF RESISTANCE OF COILS
0-5A MC
+
A
P1
0-20V
50Ω
5A
P2
S2
+
V
-
Experiment 6
S1
0-10V
MC
28
29. Experiment 6
MEASUREMENT OF COUPLING
COEFFICIENT OF TRANSFORMER
COILS
AIM
To determine the self inductance, mutual inductance and coupling coefficient of the given transformer windings.
APPARATUS REQUIRED
1.
2.
3.
4.
5.
6.
7.
Transformer 230/115V, 1KVA . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.
Voltmeter
0-250V
MI. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2 nos.
Voltmeter
0-150V
MI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.
Voltmeter
0-10V
MC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.
Ammeter
0-500mA MI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.
Ammeter
0-5A
MC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.
Autotransformer 0-270V, 10A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.
PRINCIPLE
The property of a coil due to which it opposes any change of current through it is known as self
inductance. The coefficient of self induction(L) is defined as Weber turns per ampere in the coil.
L=
Nφ
N N Iµ0 µr A
N2
=
=
(H)
I
Il
S
Mutual inductance is the ability of one coil to produce an emf in a nearby coil by induction
when the current in the first coil changes. The coefficient of mutual inductance(M) is defined
as the Weber turns in one coil due to ampere current in the other coil.
M=
N2 N1 I 1
N1 N2
N2 φ1
=
=
I1
I1 S
S
29
30. OBSERVATIONS
Measurement of impedance of coils
Condition
Flux aiding
Flux opposing
V1 (v)
V2 (v)
V3 (v)
I(A)
Measurement of resistance of coils
Sl. No.
1
2
3
V (v)
I(A)
Winding Resistance R = r1 + r2 = . . . . . . . . .
30
R(Ω)
Z(Ω)
L(H)
31. Consider two coupled coils A and B
L1 =
2
N1
S
2
N2
S
N1 I1
Flux produced in A due to current I1 is φ1 =
S
L2 =
Let a fraction k1 of this link with the second coil
ie, φ2 = k1 φ1
Then
M=
(k1 φ1 N2 )
(k1 N1 N2 )
=
I1
S
Flux produced in B due to current I2 is φ2 =
........................
(1)
N2 I2
S
Suppose a fraction k2 of this links with A
ie, φ1 = k2 φ2
M=
(k2 N1 N2 )
(k2 φ2 N1 )
=
I2
S
........................
(2)
from (1) and (2)
2 2
N2 N2
(k1 k2 N1 N2 )
= k1 k2 1 2
SS
S S
M
M 2 = k1 k2 L1 L2 or k = √
( L1 L2 )
√
where k = k1 k2
M2 =
The constant k is called the coefficient of coupling and may be defined as the ratio of mutual
inductance actually present between the two coils to the maximum possible value. If the flux
due to one coil completely links the other then k = 1. If the flux of one coil does not link the
other coil at all then k = 0.
31
32. CALCULATION
For coils connected in series with fluxes aiding each other
V1
Total impedance of coils ZA =
= .........
I
2
Reactance of coils XA = ZA − R2 = . . . . . . . . .
Where, R = r1 + r2
is the total resistance
XA
= .........
of both windings
Inductance LA =
2πf
For coils connected in series with fluxes opposing
V1
= .........
Total impedance of coils ZB =
I
2
Reactance of coils XB = ZB − R2 = . . . . . . . . .
XB
Inductance LB =
= .........
2πf
When coils are connected with flux aiding each other
Total inductance, LA = L1 + L2 + 2M
(1)
When coils are connected with flux opposing each other
Total inductance, LB = L1 + L2 − 2M
(2)
subtracting (2) from (1), M =
L1
L2
=
N1
N2
2
=
(LA − LB )
4
2
240
120
=4
L1 = 4L2
Substituting in (1), LA = 5L2 + 2M or L2 =
L1 = 4L2 = . . . . . . . . .
Coupling coefficient k = √
M
= .........
L1 L2
32
LA − 2M
= .........
5
33. PROCEDURE
Measurement of impedance of coils
Connections are made as shown in the first figure. Supply is switched on with autotransformer in
the minimum position. The autotransformer is adjusted to get the rated voltage in voltmeter1.
The corresponding readings in all meters are noted down. In this case the fluxes produced by
both the coils are additive in nature (ie, V1 = V2 + V3 ).
Next the connections of the second coil are reversed. The fluxes produced by the two coils
are now in subtractive polarity (ie, V1 = V2 − V3 ). The autotransformer is adjusted so as to get
the same reading in V2 as with the additive polarity. This is done to maintain the same flux in
both the cases. The readings of all meters are noted down and tabulated as shown.
Measurement of resistance
Connections are made as in the third figure. For different values of voltages the readings of
both meters are noted down and tabulated.
RESULT
The coupling coefficient of the given transformer windings is . . . . . . . . . .
VIVA QUESTIONS
1. What is meant by coupling coefficient of a transformer? What are the limiting values?
2. Why is the voltage V2 maintained constant in the second case?
3. How are the meter ratings selected for this experiment?
33
34. CONNECTION DIAGRAM(OPEN CIRCUIT TEST)
0-2A MI 150V, 2A, lpf
L
M
2A
P
A
B
230V
1-φ
50Hz
AC
C
V
C
NL
N
V
P
0-150V
MI
h.v
l.v
E
120/240V, 1KVA
Transformer
OBSERVATION
Vo
Io
Wo
CONNECTION DIAGRAM(SHORT CIRCUIT TEST)
0-5A MI 75V, 5A, upf
M
L
5A
P
A
B
230V
1-φ
50Hz
AC
C
V
C
NL
N
V
0-50V
MI
h.v
E
l.v
120/240V, 1KVA
Transformer
Rated Current =
Rated KV A
1000
=
= .........
Rated V oltage on h.v side
240
OBSERVATION
VSC
Experiment 7
ISC
34
WSC
35. Experiment 7
O.C AND S.C TESTS ON SINGLE PHASE
TRANSFORMER
AIM
To conduct open circuit and short circuit tests on the given 120/240 V, 1KVA transformer and
predetermine the following:1. Equivalent circuit as referred to l.v side
2. Equivalent circuit a referred to h.v side
3. Efficiency curve at 0.8 pf
4. Regulation curve at 1 /2 Full load
INSTRUMENTS REQUIRED
1.
2.
3.
4.
5.
6.
7.
Voltmeter 0-150V MI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Voltmeter 0-50V MI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Ammeter
0-2.5A MI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Ammeter
0-5A
MI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Wattmeter 150V, 2.5A, lpf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Wattmeter 75V, 5A, upf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Autotransformer 0-270V, 10A
no.
no.
no.
no.
no.
no.
PRINCIPLE
Open Circuit test
This test is usually conducted on the l.v side of the transformer. It is conducted to determine
the core loss(iron loss or no load loss). The low voltage side of the transformer is supplied at
rated voltage with the h.v side left open. The current, voltage and power on the input side is
noted. Since the no-load primary current is small(2-10% of the rated current) the copper losses
in the primary winding can be neglected and the power loss read by the wattmeter is the core
loss of the transformer. Since the flux linking with the core is constant at all loads, the core
loss remains same for all loads. The parameters R0 and X0 (the shunt branch) are determined
using this test.
35
36. From O.C test (l.v side) :V0 = . . . . . . . . . I 0 = . . . . . . . . . W 0 = . . . . . . . . .
W0
W0 = V0 I0 cos φ0 cos φ0 =
= .........
V0 I 0
sin φ0 = . . . . . . . . .
Iw = I0 cos φ0 = . . . . . . . . .
Iµ = I0 sin φ0 = . . . . . . . . .
Core loss component resistance as referred to l.v side R0 =
V0
= .........
Iw
V0
= .........
Iµ
The parameters R0 and X0 as referred to the h.v side are
′
R0 = R0 × k 2 = . . . . . . . . .
N2
E2
240
′
where k =
X0 = X0 × k 2 = . . . . . . . . .
=
=
N1
E1
120
Magnetising reactance as referred to l.v side X0 =
From S.C test (h.v side) :VSC = . . . . . . . . . ISC = . . . . . . . . . WSC = . . . . . . . . .
Total eqvt. wdg. resistance as referred to h.v side R02 =
WSC
= .........
ISC
VSC
= .........
ISC
2
= Z02 − R22 =
0
Total eqvt. impedance as referred to to h.v side Z02 =
Total eqvt. leakage reactance referred to h.v side X02
The parameters R02 , Z02 , and X02 as referred to l.v side are
R01 = R02 |K 2 = . . . . . . . . .
X01 = X02 |K 2 = . . . . . . . . .
Z01 = Z02 |K 2 = . . . . . . . . .
EQUIVALENT CIRCUIT
as referred to l.v side
as referred to h.v side
I1
I'2
I0
120V
=
R0
I1
R0=
1
I0
X01=
Z'L
X0 =
I'2
240V
R'0=
36
R0=
2
X02=
ZL
X'0=
37. Short Circuit test
The short circuit test is conducted to determine the full load copper loss and the equivalent
resistance and leakage reactance referred to the winding in which the test is conducted. The test
is conducted on the h.v side with the l.v side short circuited by a thick conductor. A low voltage
just enough to circulate the rated current of the transformer is supplied to the transformer. The
voltage supplied is usually only 5-10% of the normal supply voltage and so the flux linking with
the core is small. Thus core losses can be neglected and the wattmeter reading gives the full
load Cu loss of the transformer.
PROCEDURE
Open Circuit test
Connections are made as shown in the connection diagram 1. The h.v side is left open. The
supply is switched on with the autotransformer in the minimum position. The autotransformer is
gradually varied till the voltmeter reads the rated voltage of the primary side of the transformer.
The corresponding ammeter and wattmeter readings are noted down and tabulated as shown.
Short Circuit test
Connections are made as shown in the diagram 2. The l.v side is short circuited. Supply is
switched on with the autotransformer in the minimum position. The autotransformer is gradually varied till the ammeter reads the rated current of the transformer on the h.v side.
Rated current =
Rated volt Amperes of transformer
Rated voltage on h.v side
37
38. Calculation of efficiency :Efficiency of the transformer at any load and p.f is given by
η=
Power Output
x × F.L(V A) × cos φ
=
Power Input
x × F.L(V A) × cos φ + Wi + x2 WCu
Where,
Wi - is the core loss
WCu - full load Cu loss
x - is the fraction of full load
x
Output
Wi
x2 WCu
Input
Efficiency
1/
4
1/
2
3/
4
1
Sample Calculation(set no. . . . )
x = .........
F.L(V A) = . . . . . . . . .
p.f cos φ = . . . . . . . . .
Power output = x × F.L(V A) × cos φ = . . . . . . . . .
Power input = Power Output + Wi + x2 WCu = . . . . . . . . .
Power Output
Efficiency
=
= .........
Power Input
η
Efficiency curve
1/4
FL
1/2
FL
38
3/4
FL
O/P
FL
39. The corresponding voltmeter and wattmeter readings are noted down and tabulated as
shown. Using the readings obtained from the two tests, the equivalent circuit as referred to the
l.v side and h.v side are drawn. The efficiency at various fractions of full load are calculated
and tabulated. The efficiency curve is then plotted.
Regulation of the transformer (which gives the variation of the secondary terminal voltage
from no load to full load expressed as a percentage of the secondary terminal voltage with the
primary voltage held constant) is then calculated using the approximate formula at various
power factors and half the full load, Regulation curve is then plotted.
39
40. Calculation of Regulation
Regulation at any load and p.f is given by
% Reg =
I2 R02 cos φ ± I2 X02 sin φ
o
E2
‘+’ for lagging
‘-’ for leading
Where I2 is the current at any load and
= xI2 F L
Where
x → Fraction of full load
I2 F L → full load current on secondary side
0
E2 → rated voltage on secondary side
0
0.2
←− lagging
0.4 0.6 0.8
upf
1
0.8
leading −→
0.6 0.4 0.2
0
Sample calculation . . . . . .
(for one lead and one lag case)
Regulation Curve
%Reg
(upf)
0 0.2 0.4
p.f
(lead)
0.6
0.8
0.8
%Reg
40
0.6 0.4
0.2
0
p.f
(lag)
41. RESULT
O.C and S.C tests were conducted on the given 1-φ transformer and predetermined the regulation and efficiency curves.
VIVA QUESTIONS
1. How are the meter ratings selected for O.C and S.C tests?
2. Why is the O.C test conducted on the l.v side of the transformer and S.C test
on h.v side?
3. What are the losses measured in an O.C test?
4. What are the losses measured in an S.C test?
5. What is the condition for maximum efficiency in a transformer?
6. What is meant by ‘regulation’ of a transformer?
7. Is a high or low value of regulation preferred? Why?
8. How can the parameters on one side of the transformer be transferred to the
other side?
41
42. CONNECTION DIAGRAM
600V, 5A, upf
0-5A MI
R
M
A
5A
L
C
B1
50Ω, 5A
400V 3 - φ 50Hz AC
C1
Y
5A
5A
0-600V
3−φ
Inductive
Load
0-10A
B2
C2
B
V
E1
E2
50Ω, 5A
B3
C
N
NL
C3
M
E3
V
L
600V, 5A, upf
PHASOR DIAGRAM
V BY
IB
V RY
VB
φ 30
30
φ
φ
IY
VY
Experiment 8
42
VR
IR
50Ω, 5A
43. Experiment 8
THREE PHASE POWER
MEASUREMENT BY TWO
WATTMETER METHOD
AIM
To measure the power factor and power consumed by a 3-φ RL load using two wattmeter
method.
APPARATUS REQUIRED
1.
2.
3.
4.
5.
6.
Voltmeter (0-600V) MI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.
Ammeter (0-5A) MI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.
Wattmeter 600V, 5A, upf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 nos.
Rheostat 50Ω, 5A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 nos.
3-φ Inductive load (0-10A) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.
3-φ Autotransformer (0-415V, 10A) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 no.
PRINCIPLE
In two wattmeter method the current coils of two watt meters are connected in two phases and
the potential coils between the corresponding phase and the third phase. It can be proved that
the sum of the wattmeter readings gives the total power.
From the phasor diagram
Reading of Wattmeter 1, W1 = VRY IR cos(30 + φ)
Reading of Wattmeter 2, W2 = VBY IB cos(30 − φ)
W1 + W2 = VRY IR (cos 30 cos φ − sin 30 sin φ) + VBY IB (cos 30 cos φ + sin 30 sin φ)
= VRY IR cos 30 cos φ + VBY IB (cos 30 cos φ)
Assuming balanced load
VRY = VBY = VBR = VL
& IR = IB = IY = IL
Where VL and IL are the line values of voltage and current.
= VL IL cos√ cos φ + VL IL cos 30 cos φ
30
= 2VL IL 23 cos φ
√
= 3VL IL cos φ
43
44. OBSERVATIONS
Sl.
No.
1
2
3
voltage
Current
V(V )
Case
I(A)
Wattmeter
reading
W1
W2
W1 and W2 read +ve
W1 reads +ve, W2 reads
zero
W1 reads +ve, W2 reads
-ve
Sample Calculation (set no . . . )
Voltage V = . . . . . . . . .
Current I = . . . . . . . . .
Wattmeter reading W1 = . . . . . . . . .
Wattmeter reading W2 = . . . . . . . . .
Total power P = W1 + W2 = . . . . . . . . .
Phase angle φ =
tan−1
√
3(W1 − W2 )
W1 + W2
= .........
Power factor = cos φ = . . . . . . . . .
44
Power
P(W )
Phase
angle
φ
Power
factor
cos φ
45. For leading pf φ (When the load is capacitive)
W1 = VL IL cos(30 − φ)
W2 = VL IL cos(30 + φ) = Power in a 3φ circuit
W1 + W2 = VL IL cos(30 − φ) + VL IL cos(30 + φ)
= √ cos 30 cos φ]VL IL
[2
= 3 cos φVL IL
W1 − W2 = VL IL cos(30 − φ) − VL IL cos(30 + φ)
= [−2 sin 30 sin φ]VL IL
= − sin φVL IL
.............................
(1)
.............................
(2)
From (1) and (2)
tan φ
When
When
When
When
=
√ (W1 − W2 )
(W1 − W2 )/VL IL
√
= 3
(W1 + W2 )
(W1 + W2 )/ 3VL IL
pf is unity, φ = 0 and W1 = W2 .
1.0 > pf > 0.5, 0 < φ < 60◦ and both W1 and W2 read positive.
pf = 0.5 , φ = 60◦ and W1 = 0, hence W2 alone reads the total power.
0.5 > pf > 0, 60◦ < φ < 90◦ , W1 reads negative and W2 positive
PROCEDURE
Connections are done as shown in the figure. The resistance is kept in the maximum position
and the inductive load is set to minimum. The supply is switched on with the autotransformer
in the minimum position.
The autotransformer is adjusted to get rated voltage in the voltmeter. The load is purely
resistive, the power factor is nearly unity and both wattmeters read positive. Readings are taken
corresponding to this condition. The inductive load is then increased till one of the wattmeter
becomes zero. This corresponds to a pf of 0.5. Again all readings are noted. On further
increasing the inductive load one of the wattmeters starts deflecting in the negative direction.
This indicates that the power factor of the circuit is less that 0.5. The supply is now switched
off and the pressure coil or current coil (ie, C & V or M & L) connections of the wattmeter
reading negative is interchanged. Supply is switched on and the readings corresponding to
this condition are noted. The reading of the wattmeter whose terminals are interchanged is be
recorded as negative. The power factor and power are calculated using the formula given.
RESULT
Power consumed by a 3-φ RL load is measured using two wattmeter method.
VIVA QUESTIONS
1.
2.
3.
4.
What is the expression for power in a 3-φ circuit?
Derive the expression for power factor in terms of the wattmeter readings.
What are the other methods of measuring 3-φ power.
What does a zero reading in one of the wattmeters signify?
45
46. CONNECTION DIAGRAM
250V, 5A, upf
0-5A MI
L P.C
M
5A
P
A
B
C
230V
1-φ
V
50Hz
AC
NL
P1
E
N2
P2
LAMP
LOAD
ERROR CURVE
Line Current
Experiment 9
N1
0-250V
MI
% Error
N
C
V
C.C
Energy
Meter
K=
240V, 5A
46
47. Experiment 9
CALIBRATION OF SINGLE PHASE
ENERGY METER
AIM
To calibrate the given 1-φ energy meter at unity power factor by direct loading.
APPARATUS REQUIRED
1.
2.
3.
4.
5.
6.
7.
8.
Energy meter 240V, 5A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Wattmeter 250V, 5A, upf . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Ammeter (0-5A)
MI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Voltmeter (0-250V) MI . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1-φ Autotransformer (0-270V, 13A) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Stop watch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Connecting wires
Lamp load
no.
no.
no.
no.
no.
no.
PRINCIPLE
An energy meter is an instrument used to measure electrical energy. It keeps a record of the
total energy consumed in a circuit during a particular period. It is an integrating type of
instrument. It essentially consists of two electromagnets called the shunt magnet and series
magnet. A coil having a large number of turns of fine wire is fitted on the shunt magnet called
the pressure coil and is connected across the supply mains. The series electromagnet is wound
with a few turns of heavy gauge wire called the current coil and is connected in series with the
load. An aluminium disc is mounted on a vertical spindle and is free to rotate between the two
magnets. The reaction between the magnetic fields setup by the two electromagnets and eddy
currents set up a driving torque in the disc and the disc starts rotating. The average torque
thus produced is proportional to the true power of the circuit.
47
48. OBSERVATIONS(direct loading)
Sl. no.
V(v)
I(A)
W(w)
T(s)
T.R(kwh)
I.R(kwh)
% Error
Sample Calculation(set no. . . . )
Energy meter constant k = . . . . . . . . .
Voltmeter reading (V ) = . . . . . . . . .
Ammeter reading (I) = . . . . . . . . .
Time for 5 rev. of energy meter disc (t) = . . . . . . . . .
1
= .........
k
5
Indicated energy for 5 revolution of energy meter disc (IR) = = . . . . . . . . .
k
Wattmeter reading (W ) = . . . . . . . . .
True energy for ‘t’ seconds (T R) = W × t = . . . . . . . . .
I.R − T.R
% Error =
× 100 = . . . . . . . . .
T.R
Indicated energy for 1 revolution of energy meter disc =
48
49. Calibration involves comparing the energy measured by an energy meter with a standard
instrument. The standard chosen here is a wattmeter. Since the wattmeter measures only the
power, it has to be multiplied with time to get the energy reading. The readings are then
compared to find the error in the energy meter.
Calibration can be done either by direct loading or phantom loading. In direct loading both
the current and pressure coils are fed from the same supply at rated voltage. Energy meters of
high rating when tested by direct loading would involve large amount of power. Such meters
are thus tested using phantom loading, wherein the pressure coil is supplied from rated supply
and current coil circuit from a separate low voltage supply.
PROCEDURE
Connections are made as shown in the connection diagram. The supply is switched on, keeping
the autotransformer in the minimum position. The autotransformer is then varied to get the
rated voltage. The lamp load is then switched on and the ammeter adjusted for a small value
of current. The corresponding readings of voltmeter, ammeter and wattmeter are noted down.
The time for five revolutions of the energy meter disc is also noted. The experiment is repeated
in steps adding loads till the rated current of the energy meter is reached. The true energy and
indicated energy is evaluated and the error found out. The error curves are then plotted as
shown.
RESULT
The given energy meter is calibrated by direct loading at upf and the error curve plotted.
VIVA QUESTIONS
1.
2.
3.
4.
5.
What is meant by ‘calibration’ of the energy meter?
What is the standard used for calibration of energy meter?
How does an induction type energy meter work?
What is the disadvantage of direct loading method?
How are the meters selected for this experiment?
49
50. WHEATSTONES BRIDGE
P
Q
I1
G
I2
R
S
PORTABLE FORM OF WHEATSTONES BRIDGE
G
G
A
L
V
O
5
1000
100
M1000
10
6
5
7
4
8
M100 3
1
.01
.001
M10
1
.1
10
x1000
RATIO
5
6
EXT
INT
2
2
B
7
4
9
8
2
9
1
10
x1
MIN
MAX
SENSITIVITY
CONTROL
x10
G
EXT
10
B
X
R1
R2
V
Experiment 10
EXT.
BATT.
50Ω,5Α
8 3
3
G
9
1
7
1
10V
8
SERIES x100 10
ARM 5 6
4
GALV
7
3
9
2
6
4
50
0-30V
INT
51. Experiment 10
RESISTANCE MEASUREMENT USING
WHEATSTONES BRIDGE
AIM
(a) To measure the resistance of given voltmeter (0-30V) using Wheatstones bridge.
(b) To draw the circuit for extending the range of the given voltmeter (0-30V) to read up
to 300V.
APPARATUS REQUIRED
1.
2.
3.
4.
5.
Wheatstones bridge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Voltmeter (0-30V) MC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Galvanometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Rheostat 50Ω, 5A. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1
D.C source (0-30V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
no.
no.
no.
no.
no.
PRINCIPLE
This is the best and most common method of measuring medium resistance (from 1Ω - 0.1
MΩ). The general circuit arrangement is shown in the figure. P and Q are two known and
fixed resistances. S is a known variable resistance and R is the unknown resistance. When the
bridge is balanced, no current flows through the galvanometer, then
I1 × P = I2 × R
I1 × Q = I2 × S, ie,
P
Q
=
R
S
or R =
P
Q
×S
The unknown resistance can then be determined.
In the portable form of the Wheatstones bridge the ratio(P/Q) can be set to values of 1,
10, 100, 1000. The standard resistance S can be adjusted using decade dials of x1, x10, x100
and x1000. R can be found out using the above formula.
51
52. OBSERVATIONS
Sl.
no.
Unknown
resistance
P
Q
S1 ×
1000Ω
S2 ×
100Ω
S3 ×
10Ω
S1 ×
1Ω
S = S1 + S2
+S3 + S4
XΩ
Mean
X
Voltmeter
(0 − 30V )
Resistance of the given voltmeter is . . . . . . . . .
EXTENSION OF INSTRUMENT RANGE
RS
RV
V
0-30V
VV
VT
300V
Let Rv
Rs
Vv
VT
be
be
be
be
the
the
the
the
resistance of the voltmeter.
resistance to be connected in series.
range of the given voltmeter.
range to which the extension is to be made.
The value of the resistance to be connected in series to extend the range is
Rs =
VT
Vv
where ‘m’ =
− 1 Rv = (m − 1)Rv
VT
, is the multiplying factor.
Vv
52
53. PROCEDURE
The given voltmeter(unknown resistance) is connected to the terminal marked R1 and R2 on the
bridge. The toggle switches are adjusted for external battery and galvanometer. An external
battery is connected to terminals BB′ through a rheostat. A galvanometer is connected to the
terminals marked Galvo. on the bridge. The P/Q ratio (range selector) is suitably selected.
The resistance ‘S’ is varied by varying the four decade resistances (one at a time starting from
the highest range) till null deflection is observed in the galvanometer, when the ‘B’ and ‘G’
keys are pressed. Adjustments are made till null deflection is obtained, The reading of the
‘Range selector’ and the four dials of the variable resistance ‘S’ are noted. The readings are
tabulated as shown. The experiment is repeated for different values of range selector(P/Q ratio).
Extension of range of meter:- First the resistance of the given voltmeter Rv is measured
using Wheatstones bridge. To extend the range of given voltmeter a resistance Rs is connected
in series with voltmeter as shown in the figure.
Since they are in series, current is the same through voltmeter and series resistance.
I=
Rs = (VT − Vv )
Vv
(VT − Vv )
=
Rv
Rs
Rv
=
Vv
where ‘m’ is the multiplying factor =
VT
Vv
− 1 Rv = (m − 1)Rv
VT
.
Vv
RESULT
1) Resistance of given volt meter is . . . . . . Ω
2) Resistance to be connected in series to extend its range to 300V is . . . . . . Ω
VIVA QUESTIONS
1. What is the range of resistances that can be measured using a wheatstones
bridge?
2. Why can’t a wheatstones bridge be used for measurement of small value of
resistance?
3. How can a low range voltmeter be used for measurement of high voltages?
53
55. Experiment 11
RESISTANCE MEASUREMENT USING
KELVINS DOUBLE BRIDGE
AIM
a) To measure the resistance of the given ammeter(0-2.5A) using Kelvins double bridge.
b) To draw the circuit for extension of range of the meter to read up to 25A.
APPARATUS REQUIRED
1.
2.
3.
4.
5.
Kelvins Double Bridge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
D.C source (0-30V) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Ammeter (0-2.5A) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Galvanometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Rheostat 45Ω,5A. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1
no.
no.
no.
no.
no.
PRINCIPLE
This method is the best available for precise measurement of low resistances(less than 1Ω). In
the figure ‘R’ is the low resistance to be measured and ‘S’ is a standard variable resistance of the
same order of magnitude, P,Q, p and q are four non-inductive resistances, one pair of which are
variable. These are connected to form two sets of ratio arms, which are used for range selection.
The ratio Q/P is kept same as q/p ratio along with ‘S’ being varied till null deflection of the
galvanometer is obtained.
Then
Q
q
Q
R
=
= or R =
×S
S
P
p
P
55
56. OBSERVATIONS
Sl.
No.
Unknown
resistance
remarks
1
Ammeter
+leads
Leads
alone
S2 × 10−4
Ω
direct
reverse
2
direct
reverse
Resistance
Resistance
Resistance
Resistance
of
of
of
of
Range
multiplier
S1
mΩ
S = S1 + S2
mΩ
X
mΩ
mean
mΩ
ammeter + leads = . . . . . . . . .
leads alone = . . . . . . . . .
ammeter alone = . . . . . . . . .
the given ammeter (0-2.5A) = . . . . . . . . .
Extension of range of ammeter
To extend the range of ammeter a resistance is connected in shunt as shown
IT=25A
2.5A
A
RA
IM
RSH
Let RA
RSH
IM
IT
be the resistance of the ammeter
the resistance to be connected in shunt
the range of the given ammeter
the range to which the extension is to be made
The value of resistance to be connected in shunt to extend the range of the given ammeter
to (0-25A)
= RSH =
(IM × RA )
=
(IT − IM )
where ‘m’ is the multiplying factor =
IT
IM
56
RA
IT
−1
IM
=
RA
m−1
57. PROCEDURE
Connections are made as shown in the figure. Choose a suitable range multiplier . Set the
current switch in forward position. Press the galvanometer initial key first and adjust main
dial and slide wire to get null deflection in the galvanometer. Then press the galvanometer
final key and check whether the galvanometer reads null deflection. If not, adjust the dial
readings to get null deflection. The readings of the main dial and slide wire are noted down.
The current switch is then put to the reverse position. This reverses the direction of current
in circuit. The main dial and slide wire are adjusted to get null deflection and the readings are
noted again. The mean of the two is taken as the correct value. This is done to eliminate errors
due to thermal effect. The ammeter is then disconnected and the resistance of the connecting
leads alone is measured using the same method. The experiment is repeated with different
values of range multiplier. The readings are tabulated as shown.
Resistance of ammeter = (Resistance of ammeter + leads) - (Resistance of leads alone)
Extension of range
To extend the range of ammeter a resistance is connected in shunt as shown. Since both are
in parallel, voltage across both is the same.
IM × RA = ISH × RSH
RSH =
IM
ISH
× RA =
where ‘m’ is the multiplying factor =
(IM × RA )
=
(IT − IM )
RA
IT
−1
IM
=
RA
m−1
IT
IM
RESULT
1) Resistance of given ammeter( 0 − 2.5A) = . . . . . . Ω
2) Resistance to be connected in shunt to extend its range to (0 − 25A) = . . . . . . Ω
VIVA QUESTIONS
1. How does a Kelvins double bridge differ from a wheatstones bridge?
2. What is the range of resistances that can be measured using a Kelvins double
bridge?
3. How can a low range ammeter be used for measurement of larger values of
currents?
57
58. CIRCUIT DIAGRAM FOR LINEAR RESISTANCE
0-1A
2A
+
+
A
-
+
220V
DC
300Ω
1.7A
V
0-250V
1000Ω
1.2A
-
2A
-
CIRCUIT DIAGRAM FOR INCANDESCENT LAMP
0-1A
2A
+
+
A
LAMP
+
220V
DC
300Ω
1.7A
V
0-250V
-
2A
-
OBSERVATION AND CALCULATION
Sl.
no.
Linear Resistance
Voltage Current Resistance
V
I
R=V /I
Incandescent lamp
Voltage Current
V
A
V-I CHARACTERISTICS
V
Linear
Resistance
Incandescent
Lamp
I
Experiment 12
58
59. Experiment 12
V-I CHARACTERISTICS OF
INCANDESCENT LAMP AND LINEAR
RESISTANCE
AIM
To determine the V-I Characteristics of linear resistance and incandescent lamp.
APPARATUS REQUIRED
Rheostat
Voltmeter
Ammeter
Incandescent lamp
Rheostat
-
300Ω, 1.7A
0-300V MC
0-2A MC
240V, 100W
1000Ω, 1.2A
.................................................
.................................................
.................................................
.................................................
.................................................
1
1
1
1
1
no.
no.
no.
no.
no.
PRINCIPLE
The resistance of a material is practically a constant at constant temperature, so for the linear
resistance, according to ohm’s law the current flowing through the circuit is directly proportional
to the voltage applied.
ie, I ∝ V
V = IR
where,
V is voltage applied, I the current and R the resistance.
Here R is a constant therefore we get a linear relationship between voltage and current. But
in the case of incandescent lamp, large amount of heat is produced so there is a considerable
change in the resistance thus as the voltage increases we get a non linear relationship between
voltage and current.
PROCEDURE
The connections are made as per the circuit diagram. The linear resistance is connected in
the circuit first. Keeping the potential divider in the minimum output voltage positions, the
supply is switched on. The rheostat(300Ω, 1.7A) is adjusted to get different voltages till the
rated voltage is reached and corresponding current readings are noted down.
The experiment is repeated by connecting incandescent lamp in place of the rheostat. The
V-I characteristics of linear resistance and incandescent lamp are plotted.
RESULT
V-I characteristic of linear resistance and incandescent lamp are plotted.
59
60. CIRCUIT DIAGRAM - OCC & SCC
+
0-10A MI
L F A
15A
A
Rh1
300Ω
1.7A
220V
DC
A2
-
N
B
S3
R
Y
Y
B
F1
F2
15A
0-300V
MI
GS
M
F1
V
R
A1
F2
S1
+
D.C motor
V -230V
I-17A
H.P -3.5
rpm-1500
Rh2
2A
1000Ω 1.2A
+
-
S2 2A
A
-
alternator
V -415V
I-5A
KV A-3.5
rpm-1500
Conn.-Star
0-2A MC
MEASUREMENT OF ARMATURE RESISTANCE
+
0-5A MC
+
5A
A
50Ω 5A
R
+
20V
DC
-
0-10V
MC
V
N
5A
OBSERVATIONS AND CALCULATIONS
O.C TEST
S.C TEST
If
VOC
Ia
If
Measurement of Ra
V
I
Experiment 13
Ra
60
61. Experiment 13
OPEN CIRCUIT AND SHORT CIRCUIT
TEST ON A THREE PHASE
ALTERNATOR
AIM
To conduct open circuit and short circuit tests on a three phase alternator and predetermine
the regulation curve by emf method at half load and full load.
APPARATUS REQUIRED
Voltmeter
Ammeter
Rheostat
-
0-300V, MI
0-10V, PMMC
0-10A, MI
0-2A, PMMC
0-5A, PMMC
300Ω, 1.7A
1000Ω, 1.2A
50Ω, 5A
......................................................
......................................................
......................................................
......................................................
......................................................
......................................................
......................................................
......................................................
1
1
1
1
1
1
1
1
no.
no.
no.
no.
no.
no.
no.
no.
PRINCIPLE
As the load on the alternator is varied the terminal voltage also varies. This is due to
1. Voltage drop due to armature resistance IR.
2. Voltage drop due to armature reactance IXL .
3. Voltage due to armature reaction effect.
The voltage regulation of a synchronous generator is defined as the rise in voltage at the
terminals when the load is reduced from full load rated value to zero, speed and field current
remaining constant
%Reg =
E−V
× 100
V
Where E - Generated emf
V - Terminal voltage
For small machines the regulation may be found by direct loading. For large machines the
voltage regulation is predetermined by using indirect methods like emf method, mmf method,
Potier and ASA methods All these methods require open circuit characteristics and short circuit
characteristics.
The open circuit characteristics [also called open circuit saturation curve or magnetization
curve] is a plot of no load terminal voltage versus field excitation with the machine running at
rated speed. Under these conditions the induced voltage is directly proportional to the flux.
The shape of curve is therefore a typical B-H curve or magnetization curve. The short circuit
characteristics is a plot between armature current and field excitation with a symmetrical short
circuit applied across the terminals. Under these conditions current in the armature winding
61
62. O.C.C and S.C.C of
3φ alternator
Regulation curve
%Reg
Ia (A)
(half load)
VOC (V)
(full load)
S.C test
O.C test
(upf)
0
0.2
p.f
(lead)
0.4
0.6 0.8
0.8
0.6
0.4
0.2
0
p.f
(lag)
If (A)
%Reg
Line voltage
VL
= .........
VL
= √ = .........
3
= .........
= 1.6 × Ra (dc) = . . . . . . . . .
= VOC /ISC = . . . . . . . . .
2
2
= Z s − Ra = . . . . . . . . .
VP H
Effective value
From graph
Ra (dc)
Ra
Zs
∴ Xs
Sample Calculation
Eo = (V cos φ + IRa )2 + (V sin φ ± IXS )2
‘-ve’ for leading
‘+ve’ for lagging
% regulation =
1/2
Eo − V
× 100
V
0
←− lagging
0.2 0.4 0.6
0.8
Reg.(FL)
Reg.(HL)
62
upf
0.8
leading −→
0.6 0.4 0.2
0
63. wholly depends on the internal impedance consisting of synchronous reactance Xs and the winding resistance Ra .
Now Ra being small compared to Xs the pf under short circuit condition is zero power
factor lagging and therefore the armature reaction mmf is almost wholly demagnetizing.
The short circuit characteristics is a straight line. From O.C.C & S.C.C the synchronous
impedance is evaluated as follows.
For any value of excitation or field current If , if VOC is the open circuit voltage & ISC is the
short circuit current, then synchronous impedance Zs =VOC /ISC . The value of Zs is calculated
for the unsaturated region. For the computation of regulation, it is convenient to take Zs at
such a value of excitation which give rise to Vph [normal voltage per phase]on open circuit. The
armature resistance is measured using ammeter-voltmeter method. Under working conditions
the effective value of Ra is increased due to skin effect and temperature effect. The effective
value of Ra is generally taken as 1.6 times the d.c value.
2
2
Synchronous reactance per phase Xs = Za − Ra Ω per phase.
Eo =
(V cos φ + IRa )2 − (V sin φ ± IXs )2
where +ve sign for lagging power factor and -ve for leading. Now percentage regulation for
each case is computed as
Eo − V
% Regulation =
× 100
V
PROCEDURE
O.C test
Connections are made as shown in the connection diagram. Switches S3 and S2 are kept in the
open position. The motor field rheostat Rh1 is kept in minimum position and the alternator
field rheostat Rh2 in the maximum position. Supply is switched on by closing switch S1 . The
dc motor is started using the 3-point starter. The motor field rheostat Rh1 is varied till the
speed becomes equal to the rated speed. Switch S2 is closed. Rh2 is varied in steps and the field
current and voltmeter reading are noted down. The experiment is repeated for different values
of field current till the voltmeter reading shows 120% of the rated voltage of the alternator.
Rheostat Rh2 is brought back to the maximum resistance position.
S.C test
Switch S3 is closed and rheostat Rh2 is varied till the ammeter reading in the alternator (A2 )
reads the rated current of the machine. The corresponding value of field current is noted down.
Armature resistance is found by voltmeter-ammeter method.
The regulation is then determined at various power factors for half and full loads and the
regulation curve is plotted.
RESULT
The open circuit and short circuit test was conducted on the given 3-φ alternator and the
regulation curves for half load & full load are plotted.
63
64. CONNECTION DIAGRAM - NO LOAD TEST
600V,5A,lpf
0-5A
R
400V 3 - φ 50Hz AC
10A
10A
L
E1
V
R1
R
V
C
B1
C1
Y
M
A
0-500V
B2
B
Y
STATOR
C2
B
10A
R2
R3
ROTOR
E2
B3
C
C3
M
E3
V
L
600V,5A,lpf
BLOCKED ROTOR TEST
250V,10A,upf
0-10A
R
A
400V 3 - φ 50Hz AC
10A
Y
M
10A
E1
V
0-250V
B2
B
Y
STATOR
C2
B
10A
S1
R1
R
V
C
B1
C1
L
R3
ROTOR
R2
E2
B3
C
C3
E3
V
M
L
Machine Details
Voltage - 415V
Current - 7.5A
speed - 1440rpm
Phase 3-φ
H.P - 5.0
250V,10A,upf
Experiment 14
64
BLOCKED
ROTOR
S2
65. Experiment 14
NO LOAD AND BLOCKED ROTOR
TESTS ON 3 PHASE SLIP RING
INDUCTION MOTOR
AIM
To perform no load and blocked rotor test on a three phase slip ring induction motor and
determine the equivalent circuit.
APPARATUS REQUIRED
Voltmeter
Ammeter
Wattmeter
Rheostat
Autotransformer
-
(0-500V) MI
(0-250V) MI
(0-30V) PMMC
(0-5A) MI
(0-10A) MI
(0-10A)PMMC
500V, 5A, lpf
250V, 10A, upf
9Ω, 8.5A
...............................................
...............................................
...............................................
...............................................
...............................................
...............................................
...............................................
...............................................
...............................................
1
1
1
1
1
1
2
2
1
no.
no.
no.
no.
no.
no.
nos.
nos.
no.
PRINCIPLE
Slip ring motors are always started with full line voltage applied across the stator terminals. The
value of starting current is adjusted by introducing a variable resistance in the rotor circuit.The
controlling resistance is in the form of resistances connected in star. The resistance is gradually
cut out of the rotor circuit as the motor gathers speed.
65
66. OBSERVATIONS AND CALCULATIONS
No load test:
V0 (V )
I0 (A)
W1
W2
W0 =W1 + W2
Blocked rotor test:
VSC (V )
ISC (A)
W1 (w)
W2 (w)
WSC =W1 + W2
MEASUREMENT OF STATOR RESISTANCE
0-5A
5A
+
+
A
-
R
50Ω 5A
+
220V
DC
0-20V
5A
-
V
Y
B
For finding stator resistance, Rs :
No.
V (V )
I(A)
Rs
R
R(meas)
R
R
R × 2R
2
= R
3R
3
2
Rs /ph(dc) = R(meas)
3
2
Rs /ph(ac) = 1.6 × × R(meas) = . . . . . . . . .
3
R(meas)
66
=
67. By introducing the rotor resistance, the rotor current is reduced at starting and the starting
torque is increased the latter due to improvement in power factor.
No load test:If the motor is run at rated voltage and frequency without any mechanical load, it will draw
power necessary to supply the no load losses. The no load current will have two components.
The active component and the magnetizing component, the former being very small as the no
load losses are small. The power factor at no load is therefore very low. The no load power
factor is always less than 0.5 and hence at no load one of the wattmeter at input side reads
negative.
The no load input W0 to the stator consists of
1. Small stator copper loss
2. Core losses
3. The loss due to friction and windage.
The rotor copper loss can be neglected, since slip is small at no load.
Blocked rotor test :The stator is supplied with a low voltage of rated frequency just sufficient to circulate rated
current through the stator with the rotor blocked and short circuited. The power input, current
and the voltage applied are noted down.
The power input during the blocked rotor test is wholly consumed in the stator and rotor
copper losses. The core loss is low because the applied voltage is only a small percentage of the
normal voltage. Again since the rotor is at stand still the mechanical losses are absent. Hence
the blocked rotor input can be taken as approximately equal to the copper losses.
67
68. From no load test:V0 = . . . . . . . . . I 0 = . . . . . . . . . W 0 = . . . . . . . . .
V0 /ph = V0 = . . . . . . . . .
Line current(IL ) =
I0
= .........
IL
phase current(I0 /ph) = √
= .........
3
Power consumed =
W0 = . . . . . . . . .
W0
3V0 I0
cos φ0
= √
∴ φ0
sin φ0
= .........
= .........
V0 /ph
=
I0 /ph cos φ0
R0 /ph
X0 /ph
=
V0 /ph
I0 /ph sin φ0
= .........
= .........
= .........
From blocked rotor test:VSC = . . . . . . . . . ISC = . . . . . . . . . WSC = . . . . . . . . .
VSC /ph = VSC
= .........
WSC
= .........
WSC /ph =
3
ISC
ISC /ph
= √
= .........
3
WSC /ph
(Total winding resistance as R01 = 2
ISC /ph
referred to the stator side)
VSC /ph
Z01 =
ISC /ph
= .........
= .........
2
2
Z01 − R01
= .........
(Total leakage reactance as referred to the stator side)
(Rotor resistance as referred
to the stater side)
X01
=
′
R2
= R01 − RS(ef f )
(Electrical equivalent of the
mechanical load)
RL
′
= R2
1−s
s
R0=
1
V/ph=
= .........
X01=
RL
R0=
X0=
68
69. PROCEDURE
No load test:Connections are made as shown in the diagram for no load test. Brake drum is made free to
rotate by loosening the belt. The autotransformer is placed in zero position. Then the supply
is switched on and the auto transformer is adjusted to supply the rated voltage to the machine.
The handle of the starter is rotated to cut out the rotor resistance. Readings of the wattmeters,
voltmeter and ammeter are noted and tabulated.
Blocked rotor test:Connections are made as shown. The rotor is blocked by tightening the belt on the brake
drum. The auto transformer is set to the zero voltage position. Then the three phase supply
is switched on. By adjusting the autotransformer, the ammeter reading is made equal to rated
current of the machine. Readings of the two wattmeters, voltmeter and the ammeter are noted
and tabulated.
Measurement of stator resistance:Connections are done for the stator resistance measurements. It is measured using the voltmeter2
ammeter method. The measured value is Rph as the machine is ∆ connected. Thus Rph =
3
1.5Rmeas . Rs(ef f ) is taken as 1.6 times Rph to account for skin effect and heating effect.
RESULT
No load and blocked rotor tests were conducted on the given three phase slip ring induction
motor and the equivalent circuit parameters were determined.
69
