FACULTY OF ENGINEERING & THE BUILT ENVIRONMENT
Department of Electrical Power Engineering
Practical Number 2: 2021 Semester A
ELECTRICAL MACHINES II (EMCH 201)
Practical Title: No-load and Blocked-rotor test of a three-phase
induction motor.
DUE DATE: 28th JUNE 2021
STUDENT
Surname, initials
INFORMATION
Mhlanga, S
LECTURER
Dr. M. Kabeya
LAB COORDINATOR
Mr. S. P. Lafleni
Student number
21435873
1
Table of Contents
List of figures .................................................................................................................................. 4
List of tables ................................................................................................................................ 4
Abstract ....................................................................................................................................... 5
Laboratory, Health and Safety Rules and Regulations .............................................................. 6
Introduction .................................................................................................................................... 7
No-load test ................................................................................................................................. 7
Blocked-Rotor test...................................................................................................................... 7
Aim .............................................................................................................................................. 7
Objective ..................................................................................................................................... 8
Apparatus ................................................................................................................................... 8
Specifications of the apparatus which was used .................................................................. 8
Methodology ................................................................................................................................. 10
Three-phase induction motor no-load test ............................................................................. 10
The procedure for performing a no-load test on a three-phase induction motor is
explained below. ............................................................................................................. 11
Guidelines for the No-load test............................................................................................ 11
Blocked-rotor test on induction motor ................................................................................... 12
Blocked-Rotor test procedures for three-phase induction motors. ................................. 12
Preventive measures for the blocked-rotor test. ................................................................ 13
Results ........................................................................................................................................... 14
Measured values of three-phase induction motor No-Load test. ..................................... 14
Calculations .......................................................................................................................... 14
Calculated values for No-Load test on three-phase induction motor .............................. 14
Separating the induction motor's mechanical losses......................................................... 15
The results of blocked-rotor testing.................................................................................... 16
Calculations .......................................................................................................................... 16
Assume the motor is of NEMA design C to obtain X1 and X2. ....................................... 16
Stator Resistance Test .............................................................................................................. 17
Calculations .......................................................................................................................... 17
Practical 3 ..................................................................................................................................... 21
Introduction .............................................................................................................................. 21
Methodology ............................................................................................................................. 22
Procedure .................................................................................................................................. 22
Discussion of the characteristic curve .................................................................................... 25
References ................................................................................................................................. 26
2
List of figures
Figure 1: An induction motor's three-phase equivalent circuit ...................................................... 10
Figure 2: At no load, the three-phase equivalent circuit of an induction motor. ........................... 10
Figure 3: The wiring diagram for the no-load test. ........................................................................ 11
Figure 4: Blocked-Rotor equivalent circuit.................................................................................... 12
Figure 5: The wiring schematic for the Blocked-rotor test. ........................................................... 12
Figure 6: The graph of Poc vs Voc ................................................................................................ 15
Figure 7: The equivalent circuit of the induction motor. ............................................................... 18
Figure 8: The connection of induction motor. ............................................................................... 22
Figure 9: The graph of Torque versus Speed of a three-phase induction motor. ........................... 24
List of tables
Table 1: No-load test measured values. ......................................................................................... 14
Table 2: No-load test calculated values. ........................................................................................ 14
Table 3: Separation of the mechanical losses of the induction motor............................................ 15
Table 4: The measured values of the blocked-rotor test. ............................................................... 16
Table 5: Calculated values for the blocked-rotor test. ................................................................... 17
Table 6: Measured values for the blocked-rotor test. ..................................................................... 17
Table 7: The interpretation table .................................................................................................... 23
3
Abstract
This practical report focuses on the following tests: no-load and blocked rotor test of a threephase induction motor. These tests were conducted on the 21st of May 2021 in the Electrical
Machines laboratory at Durban University of Technology.
The experiment tests were monitored by laboratory coordinator Sipho Lafleni from Department
of Electrical Power Engineering. He was responsible in making sure that what was taught by
Dr. Musasa Kabeya we can be able to apply in practical manner. We connected the No-load
and blocked-rotor tests using the schematic schematics provided by Sipho Pelican Lafleni, a
lab technician from the Department of Electrical Power Engineering. The Laboratory
coordinator guided and monitored us through the whole rotor test connections, and he ensured
that Dr. Kabeya's concept was applied through technical and numerical study. The lab
technician described all the practical apparatus to us and showed us their ratings.
The aim was to complete the task given to us by the lab technician, which was to establish the
characteristics of an induction motor's approximate equivalents circuit. Another aim of this
experiment was to collect data for calculating the efficiency of the machine and evaluating its
performance.
4
Laboratory, Health and Safety Rules and Regulations
Laboratory rules
1. Smoking, drinking, and eating are prohibited.
2. Bags and bookcases should be put against the wall in front of the lab.
3. No talking and fooling around by students during lectures will be tolerated.
4. No unauthorized articles are allowed into the lab without the permission of the
lecturer.
5. Any faulty equipment should be reported immediately to the lecturer.
6. The laboratory and workstations should be always kept neat.
7. If any of the above rules are not adhered to, access to the laboratory may be denied.
Laboratory health and safety rules and regulations
1. Safety clothing suitable for laboratory environment should be worn all the time.
2. No person can switch on any power source without the authority of the lecturer or the
student assistant(s).
3. Any faulty equipment should be reported immediately to the lecturer.
4. Strong action will be taken against persons who wilfully damage equipment’s
(vandalism).
5. Cell phones should be kept off during lectures and practical session.
5
Introduction
An induction motor is a type of common alternating current electric motor (sometimes known
as a synchronous motor). In an induction motor, the electric current in the rotor required to
produce torque is generated by electromagnetic induction from the rotating magnetic field of
the stator winding. The rotor of an induction motor can be either a squirrel cage rotor or a
wound type of rotor. The experiment was carried out on a three-phase induction motor to
perform a no-load and blocked-rotor test.
No-load test
The magnetizing branch parameters (shunt parameters) in the induction machine equivalent
circuit are determined using the open circuit on a transformer and the no load test. During this
test, the motor is allowed to run at the rated voltage and frequency with no load or as an open
circuit between its terminals.
Blocked-Rotor test
The transformer short circuit test is quite like the blocked rotor test. It is performed to determine
the series characteristics of the induction machine, such as leakage impedances. The objective
of blocking the rotor is to prevent it from spinning and to supply balanced voltages to the stator
terminals at 25% of the rated frequency and at a voltage that achieves the specified current.
Because of the decreased voltage conditions and rated current, an equivalent circuit is
constructed, and the core loss and magnetizing component of the current are very tiny
percentages of the total current.
Aim
The purpose of this experiment is to determine the parameters of an approximated equivalent
circuit of an induction motor. The data obtained will be used to calculate the efficiency and
evaluate the machine's performance. The other aim is to find how no-load power, current,
blocked rotor power, and current vary when the applied voltage to the stator changes.
6
Objective

To perform blocked rotor testing on a three-phase induction motor.

To perform a no-load test on the three-phase induction motor.

To determine the motor circuit characteristics, utilize the readings from the lock rotor
and no-load tests.
Apparatus
 Voltage regulator/ Variac

Conducting Wires

Three-phase induction motor trainer

Power analyser

AC power supply

Three-phase induction motor
Specifications of the apparatus which was used.
Name of the apparatus
Specifications of apparatus
Rated power = 6 kVA
Input [A-B-C-O] 380 V
Phase: 3, 50 Hz/60 Hz
Rated output current = 8 A
Weight = 26 kg
Output [a-b-c-o]-430 V
Voltage regulator/ Variac
Digital power analysis system
7
Hp = 1.0
Hz = 50
Amps = 1.95
N= 1420 rpm
Connection = Star
Three-phase induction motor.
Three-phase induction motor trainer
AC power supply
8
Methodology
Three-phase induction motor no-load test
The No-load test simplifies the equivalent circuit diagram of the induction machine shown in
Figure 1 to the circuit illustrated in Figure 2 by making the rotor equal to an open circuit. As
shown in figure 2, measurements will be taken, and the results will be used as data from this
no-load test. The no-load parameters will be calculated using the voltmeter, ammeter, and
wattmeter data obtained when the machine is set to No-load, as shown below.
Figure 1: An induction motor's three-phase equivalent circuit.
Figure 2: At no load, the three-phase equivalent circuit of an induction motor.
The no-load test is important because it helps calculate the parameters of the excitation branch
of a three-phase induction motor's per-phase equivalent circuit. The connection below shows
the no-load test diagram. During the experiment, the instructor's safety instructions were
followed.
9
Figure 3: The wiring diagram for the no-load test.
The procedure for performing a no-load test on a three-phase induction motor is
explained below.

The three-phase source wires are connected to the voltage regulation inputs.

The wires from the voltage regulator's output terminals are connected to the power
analyser's input terminals.

The wires from the power analyser's output terminals are connected to the equivalent
circuit of the three-phase induction motor.

The equivalent circuit of a three-phase induction motor is a voltmeter connected across
the supply lines and an ammeter connected in series with the supply line.

Using the two-wattmeter method, the provided power P1 & P2 to the induction motor
trainer is then measured.
Guidelines for the No-load test

At the output, the voltage regulation must be adjusted to zero.

The power supply must first be turned on, whether the per phase equivalent circuit's
switch is switched on or the switch is closed.

The digital power analyser's switch must be turned on.

The applied voltage must be gradually raised until the rated voltage is reached.

The induction motor trainer's generated power P1 and P2 are then measured using the
two-wattmeter method.
10
Blocked-rotor test on induction motor.
The Blocked-rotor test and its equivalent circuit are formed because of the decreased voltage
conditions and rated current, core loss, and magnetizing component of the current are very tiny
percentages of the overall current.
Figure 4: Blocked-Rotor equivalent circuit.
The blocked-rotor test is useful in determining the series equivalent characteristics of the threephase induction motor equivalent circuit. The lecturer's safety advice was followed during this
experiment. The figure below illustrates the blocked-rotor wiring diagram.
Figure 5: The wiring schematic for the Blocked-rotor test.
Blocked-Rotor test procedures for three-phase induction motors.

The three-phase source wires are connected to the voltage regulation inputs.

The wires from the voltage regulator's output terminals are connected to the power
analyser's input terminals.

The wires from the power analyser's output terminals are connected to the equivalent
circuit of the three-phase induction motor.
11

The equivalent circuit for a three-phase induction motor is one in which the voltmeter
is connected across the supply lines and the ammeter is connected in series with the
supply line.
Preventive measures for the blocked-rotor test.

At the output, the voltage regulation must be adjusted to zero.

First, switch on the power supply.

The per phase equivalent circuit's switch must be closed.

The digital power analyser must be switched on.

The vice or belt must be used to either keep or stop the rotor from spinning.

The machine's voltage must be gradually increased until the required rated current
flows.

The final step is to read the measured current, voltage, and power delivered by the
induction motor (P1 &P2).
12
Results
Measured values of three-phase induction motor No-Load test.
The findings of the no-load test achieved during the practical experiment are termed "Measured
values." The aim was to determine the shunt or no-load branch characteristics of the induction
motor's equivalent circuit, as well as the iron losses and no-load current values from the test
results.
Paramet V1
Ioc1
Ioc2
Ioc3
P1
P2
Voc
Ioc
cos∅oc Poc
A
A
A
W
W
V
A
-
W
1.434
240
33
218
1.360
0.368
190
er
Units
V
Measure 380
1.275 1.363
d values
Table 1: No-load test measured values.
Calculations
Voc,ph = 𝑉𝑙𝑖𝑛𝑒 =
√3
218
√3
= 125.862 V
Cos∅oc = 0.368
∅oc = 68.409
𝐼𝑐 = 𝐼𝑜𝑐. Cos∅oc = 0.50045 𝐴
𝐼𝑚 = 𝐼𝑜𝑐. 𝑆𝑖𝑛∅oc = 1.265 𝐴
Rc = 𝑉𝑜𝑐,𝑝ℎ = 251.498 Ω
𝐼𝑐
Xm =𝑉𝑜𝑐,𝑝ℎ = 99.496 Ω
𝐼𝑚
Calculated values for No-Load test on three-phase induction motor.
Parameters
∅oc
Ic
Im
Units
-
A
A
Calculated values
68.409 0.50045 1.265
Rc.LV Xm.LV Re.LV Xe.LV
Ω
Ω
252.498 99.496
Ω
-
Ω
-
Table 2: No-load test calculated values.
13
Separating the induction motor's mechanical losses
The supply voltage must be increased in four steps until it approaches the rated voltage, and
the motor's corresponding power input must be measured using the two wattmeters, as shown
in figure 3. The table below shows the total mechanical losses of the motor as measured by the
No-load test.
Parameters V
Voc1=0 V
Voc2=90 V
Voc3=180V Voc4=270 V
Voc5=360 V
P1
W
0
0
20
90
240
P2
W
0
0
5
30
170
Poc
W
0
0
15
60
70
Table 3: Separation of the mechanical losses of the induction motor.
The graph showing the relationship between the Poc and Voc is shown in the figure below.
Figure 6: The graph of Poc vs Voc.
14
The results of blocked-rotor testing
The measured values of the blocked-rotor findings achieved during the actual experiment. The
blocked-rotor test is used to establish the equivalent circuit's series branch characteristics, such
as equivalent impendence, as well as the copper losses at any desired load and total voltage
drop of the induction motor.
Parameters V1
Isc1 Isc2
Isc3
P1
P2
Vsc
Isc
cos∅sc Psc
Units
V
A
A
W
W
V
A
-
Measured
377.5 1.933 2.13
200
377.6 1.820 0.382
A
1.864 610
kW
0.834
Values
Table 4: The measured values of the blocked-rotor test.
Calculations
Vsc, ph = 𝑉𝑙𝑖𝑛𝑒 = 218.007 V
√3
Cos∅sc = 0.382
∅𝑠𝑐 = 67.54
P = 𝑃𝑠𝑐 = 278 W
3
Re =
𝑃𝑠𝑐
3𝐼𝑠𝑐2
= 83.92 Ω
Ze = 𝑉𝑠𝑐 = 119.78 Ω
𝐼𝑠𝑐
Xe = √𝑍𝑒2 − 𝑅𝑒2 = 85.46 Ω
Xe = XBR50 = 85.46 Ω
Assume the motor is of NEMA design C to obtain X1 and X2.
X1 = 0.3 XBR50 = 25.638 Ω
X2 = 0.7 XBR50 = 59.822 Ω
15
Parameters
∅sc
P∅
Units
deg.
W
Ω
Ω
Ω
Calculated
67.54
278
119.78
83.92
85.46
Ze
Re
Xe
X1
Ω
25..638
X2
Ω
59.822
Values
Table 5: Calculated values for the blocked-rotor test.
Stator Resistance Test
Calculations
𝑅𝑎𝑣𝑒𝑟𝑎𝑔𝑒 = 𝑹𝑨𝑩+𝑹𝑩𝑪+𝑹𝑪𝑨 = 14.83 Ω
3
𝑅1 =
𝑅𝑎𝑣𝑒𝑟𝑎𝑔𝑒
= 7.415 Ω
2
Following the blocked rotor test, three resistor readings were taken, as shown in the table
below.
𝑹𝑨𝑩
Parameters
Units
Measured
𝑹𝑩𝑪
𝑹𝑪𝑨
𝑹𝒂𝒗𝒆𝒓𝒂𝒈𝒆
𝑹𝟏
Ω
Ω
Ω
Ω
Ω
14.9
14.8
14.8
14.83
7.415
Values
Table 6: Measured values for the blocked-rotor test.
16
Questions
1.Vrated = 380V, Prated = 746W and Irated = 1.95A
2.Calculations obtained:
Re = 83.92Ω
R2 = RBR50-R1
R2 =76.11Ω
Given Nr = 1420 rpm
Then Ns =120×𝐹 = 3000 rpm
𝑃
Slip = 𝑁𝑠−𝑁𝑟 = 0.527
𝑁𝑠
𝑅2
= 144,421Ω
2
The below figure shows the equivalent circuit of the induction motor.
Where: Rc = 251.498 Ω, Xm = 99.496 Ω, X1= 25.638 Ω, R1= 7.415 Ω, V1 = 380 V
Figure 7: The equivalent circuit of the induction motor.
3. Calculations Done:
Given: Rc = 251.498 Ω, Xm = 99.496 Ω, X1 = 25.638 Ω, R1 =7.415 Ω, VL = 380V,
Vph = 380 =219.393V, R2 = 76.11 Ω, X2 = 59.822 Ω
√3
(a) Stator Current
Z2 = 𝑅2 + 𝑗𝑋
𝑆
2
= 156,32 < 22.5° Ω
Z1 = 𝑅1 + 𝐽𝑋1 = 26.689 < 73.869° Ω
17
Z0 =
Zp=
(𝑅𝑐)(𝑗𝑋𝑚)
𝑅𝐶+𝑗𝑋𝑚
𝑍2∗𝑍0
𝑍2+𝑍0
= 92.497
< 68.42° Ω
= 13.66 < 28.59° Ω
Zin = Zp + Z1 = 37.576 < 58.899° Ω
Istator =
𝑉
𝑍𝑖𝑛
= 16.06 < -28.59° A
P.F = cos(-28.59) = 0.88
(b) Stator Copper Loss
Stator copper loss = (𝐼12)(R1) = 1901.705 W
(c)Stator core losses
E2 = (Istator) (Zp) = 219.379 V
(d) Air Gap Power
2
Stator core losses = 𝐸2 = 191.361 W
𝑅𝑐
Stator input = √3 (𝑉𝐿)(𝐼𝐿)𝑃. 𝐹 = √3 (380)( 16.06 < −28.59°)(0.88) = 8167.701 W
Pgap = Stator input – Stator copper loss – Stator core loss = 6074.641 W
(e) Rotor Current
I2 = E2/Z2 = 1.40 < -22.5° A
(f) Rotor copper losses
= (𝐼2)2 (𝑅2) = 149.176 W
(g) Developed Power (Pmech) and Torque
Pmech = Pgap (1 – S) = 2873.304 W
T=
7.04𝑃𝑚𝑒𝑐ℎ
= 14.245 16-ft
𝑁𝑟
(h) Output power and Torque and efficiency
Pout = Pin – Plosses = Stator input – (Pcore + Prcl + Pscl)
= 8167.701 – (219.379 + 149.176 + 1901.705)
= 5897. 447 W
Tout =
7.04𝑃𝑜𝑢𝑡
= 29.23 16-ft
𝑁𝑟
Efficiency = Pout/Pin *100 =
𝑆𝑡𝑎𝑡𝑜𝑟 𝑖𝑛𝑝𝑢𝑡
𝑃𝑜𝑢𝑡
× 100 = 72.2%
18
Discussion and Conclusion of Results
The goal of the experiment was to determine the parameters of the induction motor's equivalent
circuit using no-load and blocked rotor tests. During the experiment, the values were measured
and used as data to assist determine the estimated values. The stated goal of this experiment
was to calculate the equivalent circuit parameters using the measured values, but in this
experiment, with the help of NEMA design, which was taught during the lecture by Dr. M.
Kabeya, the calculations were used to find the parameters of the induction motor's equivalent
circuit, so the goal was met.
The exciting branch parameters are acquired in the blocked rotor test, as previously mentioned.
The parameters obtained by theoretical calculations for the equivalent circuit of the induction
motor are known as Re, Xe, Rc, Xm, and so on. The conclusion is that the experiment's
objectives were met using measurements and theory calculations.
19
Practical 3
Introduction
The goal of this experiment is to obtain the torque-speed characteristic curve using tachometer
readings. The induction motor torque speed indicates the relationship between induced
mechanical torque and mechanical speed. This curve is also referred to as the torque-slip
characteristic, with slip defined as speed relative to synchronous speed given as a fraction.
The curve itself contains information about how the induction motor works. At synchronous
speed, the induced torque is zero. The maximum potential torque that the machine can generate
is known as the pull-out torque or breakdown torque, and it restricts the motor's short-term
overload capacity. It is generally two or three times the motor's rated full-load torque.
Since the beginning motor's torque is somewhat more than its full-load torque, this motor will
begin carrying any load that it is capable of supplying at full power.
The goal of this experiment is to use a star/delta technique to connect and start an induction
motor.
20
Methodology
The connections are illustrated in the figure below.
Figure 8: The connection of induction motor.
Procedure
1. First and foremost, ensure that your laboratory is properly grounded and that it is connected
to the connection supplied on the rear side of the panel.
2. Before beginning the experiment, bear in mind that the motor's belt must be slack so that the
motor may operate freely.
3. Ensure that the three phase mains and the panel's MCB are both turned off.
4. Connect terminal R of the Three Phase Supply block to the first terminal (on the left side) of
the ac voltmeter V1 and the ac ammeter A1.
5. Connect terminal Y of the three-phase supply block to terminal Y of the motor block directly.
6. Connect Three Phase Terminal B Connect the supply block to the second terminal (right
side) of the AC voltmeter V1, as well as the terminal B of the motor block.
7. Connect the second terminal (right side) of AC ammeter A1 to motor block terminal R.
8. Connect the terminals R, Y, and B of the motor block to the terminals R, Y, and B of the
real motor. These terminals are located on the plate that is attached to the top of the motor.
9. Compare your connections to the connections shown in the Three Phase Induction Motor
figure, and if all of them are right, turn on the Three Phase Mains as well as the panel's MCB.
21
10. Using a tachometer, measure the motor's no-load speed (RPM) and enter your findings in
the observation table.
11. Slowly tighten the load such that the motor's speed decreases.
12. Measure the speed once more and record it in the table.
13. Calculate on the belt, but first verify the reading on the springs.
14. Now, if the first spring balance reading is F1 and the second one is F2, the total force on
the pulley will be F = F1-F2 and the torque will be calculated into the observation table at the
relevant speed using the following formula:
T = Fr = 9.8 Nm, F = force on the pulley, and r – pulley radius.
15. Increase the load to the specified current (see reference table provided before tests for
different machine ratings) and record the number of N and T readings. r = 15.2cm/2 = 7.6cm.
The interpretation table for the corresponding speed is shown in the table below.
0
Speed ‘N’
(rpm)
1499
Torque ‘T’
(Nm)
0
1.4
0.7
1494
0.52136
1
2.4
1.4
1486
1.04272
1.5
3
1.5
1478
1.1172
2
3.9
1.9
1471
1.41512
2.1
5
2.9
1466
2.15992
2.4
6
3.6
1461
2.68128
3
7.5
4.5
1447
3.3516
3.2
8.4
5.2
1434
3.87296
3.6
9.5
5.9
1429
4.39432
Interpretation F2 (Kg)
Table: S/N
0.6
F1 (Kg)
F (Kg)
0.6
0.7
Table 7: The interpretation table.
22
The graph below shows the relationship between speed and torque characteristics of a threephase induction motor.
Figure 9: The graph of Torque versus Speed of a three-phase induction motor.
23
Discussion of the characteristic curve
The maximum torque is determined by rotor resistance. However, the exact position of the
maximum torque is dependent on it. The higher the value of R2, the higher the value of the slip
at which maximum torque occurs. The motor's pullout speed reduces as the rotor resistance
increases. The maximum torque remains constant in this condition.
The speed of the induction motor depends on the following factors:

The frequency of the supply source.

The load coupled to the motor shaft.
From equations for pull out torque and pullout slip, it can be seen that:

The slip at which maximum torque occurs is proportional to rotor resistance.

The magnitude of the maximum torque is independent of rotor resistance.
If all other parameters remain constant, increasing the rotor resistance will:

Reduce the speed at which maximum torque occurs.

Increase the starting torque (until spo=1).

Increase slip for a given torque.

Reduce the speed for a given torque.

Increase the rotor losses at a given torque.

The last point above can be shown by considering that the torque equation,
is rotor copper loss divided by slip speed. If slip increases, losses must increase to
maintain the torque.
24
References
[1] E. Hughes, “Electrical Technology”, 5th Edition, Longman publishers, 1977.
[2] B.L. Theraja and A.K. Theraja, “Textbook of Electrical Technology”, S. Chand, 2006.
[3] S. J. Chapman, “Torque versus Speed of a three-phase induction motor”, Electric
Machinery Fundamentals, 5th Edition, D. M. Schueller, Ed. New York: McGraw Hill
Company, 2012, pp. 116-126.
25