ADDIS ABABA UNIVERSITY

COLLEGE OF TECHNOLOGY AND BUILT ENVIRONMENT

School of Electrical and Computer Engineering
Electrical Machines (Lab IV)

Course number: Eceg:-3152 - Electrical Machines Laboratory

Experiment Number: 04 and 05

Title: Lab report for tests on three-phase induction motor

and starting methods of induction motor
Date of experiment: April may 7, 2025

Group members ID
1. EYOB FIKIRADDIS………………….UGR/8200/15
2. EYUEL ENGIDA……………………..UGR/6642/15
3. FETHULMUBIN KEDIR……………..UGR/3294/15
4. FULEA THEGAYE……………………UGR/6125/15
5. HAILEMICHAEL AWOKE……………UGR/1045/15
6. HERAN FIKADU……………………...UGR/9979/15
7. IBRAHIM MUSA………………………UGR/8614/15
8. KALEAB SOLOMON…………………UGR/5092/15

Date of submission: May 15, 2025

Introduction​
In experiment 4, tests were conducted on a three-phase induction motor to identify
winding terminals, measure resistances, and determine winding connections.
Experiment 5 focused on motor starting methods: direct online, Y-Δ, auto-transformer,
and rotor resistance to study their effects on starting current and torque. These
experiments aimed to enhance understanding of motor operation and starting
techniques.

Objective​
Experiment 4:​
​ - To determine the terminals of the stator and rotor windings.​
​ - To measure the winding resistance.​
​ - To determine the transformation ratio between the stator and rotor windings for
a wound rotor induction machine.​
Experiment 5:​
​ - To start an induction motor using different methods of starting.

Used Equipment and Components

Components Used

No. Description Quantity

1 Three-Phase Induction Motor 1

Equipments Used

No. Description Quantity

1 VARIAC 1

2 Voltmeter 2

3 Ammeter 2

4 Switches 2

5 Conducting Wires -

6 Variable Resistor 1

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Theory of Three-Phase Induction Motors
An induction motor is an electromechanical energy conversion device that transforms
three-phase electrical input power into mechanical output power. It operates based on
the principle of electromagnetic induction and the Lorentz force. The fundamental
components of a three-phase induction motor are the stator and the rotor.

1. Stator:

The stator is the stationary part of the motor. It consists of a core made of laminated
steel to reduce eddy current losses, and it houses a three-phase winding. When this
winding is supplied with a balanced three-phase AC voltage, it produces a Rotating
Magnetic Flux (RMF).

The three-phase currents flowing through the stator windings create individual magnetic
fields. Due to the phase difference of 120 electrical degrees between the three phases,
these magnetic fields combine to produce a resultant magnetic field that rotates in
space at a constant angular speed. This speed is known as the Synchronous Speed ,
and it is determined by the frequency of the applied AC power and the number of poles
of the stator winding, according to the following equation:

The rotating magnetic flux can be mathematically represented by sinusoidal functions

with a spatial displacement of 120 degrees:

Where is the maximum flux and is the angular frequency.

Fig 1.

2. Rotor:

The rotor is the rotating part of the motor, separated from the stator by an air gap. There
are two main types of rotors used in three-phase induction motors:
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 ●​ Squirrel Cage Rotor: This type consists of copper or aluminum bars embedded
in slots in the rotor core. These bars are short-circuited at both ends by
conducting end rings, forming a permanently short-circuited winding. This is the
more common and robust type.
●​ Wound Rotor: This type has a three-phase winding similar to the stator winding,
but the terminals are connected to slip rings mounted on the rotor shaft. External
resistances can be connected to these slip rings to control the motor's starting
torque and speed.

Fig 2. Squirrel cage type rotor Fig 3. Squirrel cage type rotor

3. Working Principle:

The operation of the three-phase induction motor relies on the principle of

electromagnetic induction:

1.​ Induction of Voltage: As the rotating magnetic flux (RMF) produced by the
stator windings sweeps across the rotor conductors, it cuts these conductors.
According to Faraday's law of electromagnetic induction , this induces a voltage
in the rotor conductors.
2.​ Production of Rotor Current: Since the rotor winding (in the case of a squirrel
cage rotor, the bars are short-circuited) forms a closed circuit, the induced
voltage drives a current through the rotor conductors.
3.​ Development of Force: When the current-carrying rotor conductors are placed
in the magnetic field produced by the stator, they experience a mechanical force.
This force is described by the Lorentz force law:

or for a current-carrying wire,

The direction of this force is such that it tends to rotate the rotor in the same direction as
the rotating magnetic flux.

4.​ Rotor Speed and Slip: The rotor starts rotating due to the developed torque.
However, the rotor speed will always be less than the synchronous speed . If the
rotor were to reach synchronous speed, the relative speed between the rotating

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 magnetic flux and the rotor conductors would become zero, and no voltage or
current would be induced in the rotor, resulting in zero torque. The difference
between the synchronous speed and the mechanical speed of the rotor is
expressed in terms of slip (s):

The slip is often expressed as a percentage:

4. Nameplate Information and Insulation:

"Name plate identification" which is crucial for understanding the motor's specifications.
This typically includes:

●​ Number of phases: Usually three for the motors discussed here.

●​ Type of power supply: Specifies whether it's designed for a specific AC voltage
and frequency.
●​ Power rating: The mechanical output power the motor can deliver (e.g., in
kilowatts or horsepower).
●​ Voltage and current ratings: The rated voltage and current for proper operation.
●​ Speed: The rated full-load speed of the motor.
●​ Frequency: The frequency of the AC power supply for which the motor is
designed.
●​ Insulation class: Indicates the thermal endurance of the winding insulation,
specifying the maximum allowable operating temperature for a specified period of
time. This "ability to survive at specified temperature for a period of time" is
crucial for the motor's lifespan and reliability.

5. Determining Winding Resistances and Turns Ratio:

●​ DC Resistance Test: This method involves supplying a DC voltage to the

windings and measuring the current to determine the resistance . We use DC
voltage instead of AC to avoid the skin effect. The formulas provided:

The subsequent calculations are methods to determine the individual phase resistances
of the stator or rotor windings by measuring the resistances between the terminals.

●​ Determining Transformer Turns Ratio (for Slip-Ring Induction Motors): This

test is specifically for slip-ring induction motors with the rotor short-circuited. By
increasing the input voltage to the rated value and measuring the line-to-line
voltage on the stator side and the induced voltage on the rotor side, the turns
ratio can be determined. The formulas provided:

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show how to calculate the turns ratio between the stator and rotor windings for each
phase, and then find the average value.

Theory on the Starting Methods of Three-Phase Induction Motors

Three-phase induction motors are widely used in industrial applications due to their
rugged construction, simplicity, and reliability. These motors are self-starting, as the
three-phase supply applied to the stator winding generates a rotating magnetic field that
induces current in the rotor, producing torque.

However, direct connection of an induction motor to the supply can result in a very high
starting current, mostly 10 times the full load current. This large current can cause
damage on the motor windings. Therefore, starting methods are employed to limit the
high current and provide safe, controlled motor acceleration. The most commonly used
methods are:

1. Direct-On-Line (DOL) Starter

The DOL starter is the simplest and most direct method of starting a three-phase
induction motor. In this method, the motor is connected directly to the full supply
voltage. It is also called full voltage starting method

It has its own advantages and disadvantage:

●​ Advantages: Simple design, low cost, and high starting torque.

●​ Disadvantages: Very high starting current, which can be damaging to both the
motor and the power system.
●​ Applications: Suitable for small motors where the high current will not
significantly affect our motors’ winding and the power supply.

2. Star-Delta Starter

The Star-Delta starter reduces the starting current by connecting the motor windings in
a star (Y) configuration firstly, and then switching to a delta (Δ) configuration using
star-delta switch once the motor reaches a wanted speed.

●​ Working Principle: When the windings are connected in star, the phase voltage
is reduced to 1/√3 (about 58%) of the line voltage, reducing the starting current to
approximately one-third of what it would be in DOL.
●​ Advantages: Simple and cost-effective method to reduce starting current.
●​ Disadvantages: Reduced starting torque 1/3 full voltage starting torque
●​ Not suitable for loads requiring high starting torque.

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 ●​ Applications: Commonly used for medium-size motors where reduced starting
current is necessary, but high starting torque is not required

3. Auto-Transformer Starter
The Auto-Transformer starter uses an auto-transformer to apply a reduced voltage to
the motor during starting. After the motor accelerates to a preset speed, the full line
voltage is applied.

●​ Working Principle: At starting, the auto-transformer reduces the voltage applied

to the motor, this limits both starting current and torque proportionally.
●​ After the motor reaches a certain speed, the auto-transformer is bypassed and
full line voltage is applied.
●​ Advantages: Offers better control over the starting voltage and provides higher
starting torque than star-delta starters.
●​ Disadvantages: More complex.
●​ Applications: Suitable for large motors or applications where high starting
torque is needed with limited starting current.​

Procedure: For Experiment 4

Activity 1: Identifying the Nameplate

1. The induction motor was positioned on the bench, and its nameplate was
carefully examined.​
2. Key details from the nameplate were recorded, including:​
Rated voltage​
Rated current​
Rated frequency​
Rated power​
Connection type (Star or Delta)​
Number of poles​
3. The nameplate information was later compared with the values measured
during the experiment to verify accuracy and performance.

Activity 2: Identifying the Terminals for rotor and stator winding

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 1. A multimeter and voltmeter setup were used to identify the terminal pairs of
each winding in the induction motor.​
2. Continuity between terminals was checked to determine the correct winding
pairs (e.g., U1–U2, V1–V2, W1–W2).​
3. For each identified pair, a voltmeter was connected according to the provided
wiring diagram to verify terminal relationships and confirm correctness.​
4. Once the winding pairs were confirmed, the terminals were properly labeled.​
5. To determine the correct direction of each winding terminal, a reference
method shown in the figure was followed.

Activity 3: Measuring Winding Resistance

1. A standard multimeter was used to measure the resistance of the stator or

motor windings.​
2. The resistance between each pair of winding terminals was carefully
measured.​
3. All readings were recorded and compared to assess balance and detect any
irregularities.​
4. Significant variations in resistance values indicated the possibility of faulty or
damaged windings.

Activity 4: Determining the Transformation Ratio

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 1. A known voltage was applied to the stator winding to determine the
transformation ratio of the motor.​
2. A voltmeter was connected across the rotor terminals to measure the induced
voltage.​
3. Both the supply voltage (applied to the stator) and the resulting rotor voltage
were recorded accurately.​
4. The transformation ratio was calculated using the formula:

k=Supply Voltage/Rotor Voltage

​ . The polarity and direction of voltage were carefully observed and verified with
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reference to the provided wiring diagram.

Procedure: FOR EXPERIMENT 5

A. DIRECT ON-LINE STARTING METHOD

1.​ The main power supply was switched off before beginning the connection
process.
2.​ The line supply was connected to the input terminals of the Direct-On-Line (DOL)
starter.
3.​ The output terminals of the DOL starter were connected to the stator terminals of
the three-phase induction motor.
4.​ A contactor, overload relay, and start/stop push buttons were identified and
connected as per the provided diagram.
5.​ After verifying the correctness of all the connections, the power supply was
switched on.
6.​ The start button was pressed to initiate the motor, and the stop button was used
to turn off the motor.
7.​ The motor’s operation was observed, and the starting current was measured
using an ammeter connected in series.

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B. Reduced voltage starting​
i) Procedure (Star-Delta Starter):

1. A star-delta starter equipped with three contactors and a timer was used
for the motor starting process.​
2. Initially, the motor was started using the star connection to limit the
starting current.​
3. After a brief preset delay, the connection automatically switched from
star to delta.​
4. The starting current during both the star and delta stages was observed
and recorded.

II) Procedure (Auto-Transformer Starter):

1. An auto-transformer was connected between the power supply and the

motor to provide reduced voltage during startup.​
2. A connector or switching mechanism was used to transition from
reduced voltage to full line voltage.​
3. The motor was initially started with the reduced voltage supplied through
the auto-transformer.​
4. After the initial startup phase, the connection was switched to the full
voltage supply.​
5. The motor's current was monitored throughout the starting process using
an appropriate measuring instrument.

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III) Procedure (Rotor Resistance Starter):

1. External resistors were connected to the rotor windings through slip rings
to control starting conditions.​
2. The stator was directly connected to the three-phase power supply.​
3. The motor was started with a high resistance in the rotor circuit to limit
starting current and improve starting torque.​
4. As the motor picked up speed, the rotor resistance was gradually
reduced in steps.
5. The motor’s current and torque performance were observed and
monitored during the starting process.

Calculations

Resistance measurement

𝑅𝑥𝑢 + 𝑅𝑦𝑣 + 𝑅𝑧𝑤 4.6+4.6+4.6​

𝑅𝑠𝑡𝑎𝑡𝑜𝑟 = 3
= 3
= 4. 6Ω

𝑅𝑘+𝑅𝑙+𝑅𝑚 6.6+1.2+3.1​​
𝑅𝑟𝑜𝑡𝑜𝑟 = 3
= 3
= 3. 633Ω

On the transformation ratio

Individual ratios:

𝑉1 150​
𝐾1 = 𝑉𝑤𝑢
= 32.2
= 4. 66

𝑉 150​
𝐾2 = 𝑉𝑥𝑤
= 32.4
= 4. 62

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 𝑉 150​
𝐾3 = 𝑉𝑢𝑤
= 32.2
= 4. 66

Average ratio

𝑘1 + 𝑘2 + 𝑘3 4.66 + 4.62 + 4.66

𝐾𝑎𝑣 = 3
= 3
= 4. 65

Average rotor voltage

32.2 + 32.4 + 32.2

𝑉𝑎𝑣 = 3
= 32. 27𝑉

RESULT

Experiment 4​
Activity 1: Name plate reading of the induction motor

Name plate Name plate Star(Y) Delta (Δ)

Number of phases 3 phase Rated voltage 380 V 220 V

Service duty 3hr

Power factor 0.69 Rated current 18.7 A 15 A

Degree protection IP 00/44

Insulation class B/F

Motor rated power 3kW

Full load speed 1390 rpm

Activity 2: Determination of the terminals​

· By assuming the X is the start terminal, U is the end terminal (X –U)​
· By assuming the Y is the start terminal, V is the end terminal (Y –V)​
· By assuming the Z is the start terminal, W is the end terminal (Z–W)

Activity 3: Resistance measurement

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 For stator winding RA =4.6Ω

RB = 4.6 Ω Rstator = 4.6 Ω

RC = 4.6 Ω

For rotor winding rKL = 7.8 Ω

rLM =4.3 Ω Rrotor = 3.63 Ω

rKM = 9.7 Ω

Activity 4: Transformation Ratio​

The input voltage V1 =150V,

Line voltage of the rotor Ratio (K)

VKL =32.2V K1 =4.66 Kav =4.65

VLM = 32.4V K2 = 4.62

VKM =32.2V K3 = 4.66

Experiment 5

1. Direct online Starting method

Voltage Starting current Steady current

100V 9A 1.6A

150V 15A 1.6A

2. Using the Y-Δ method

Starting current Steady current

Starting by Star( Y ) 10A 1.6A
Then change to Delta(Δ) 6A 2.9A

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3. Autotransformer method​
Starting voltage V = 70V ​
Starting current 5A

Steady current 1.6A

4. Rotor Resistance starting method

Transient current Steady current

Current at maximum 2.2A 1.6A
external resistance
Current at a minimum 11A 1.6A
external resistance

Conclusion

This experiment on the three-phase induction motor provided a comprehensive

understanding of its operational characteristics, starting methods, and key parameters.
These practical investigations deepened our insight into the motor's behavior under
different conditions.
The measured results were compared with theoretical expectations. While the values
generally aligned, minor deviations were observed. These discrepancies can be
attributed to:
1.​ Instrument Limitations: Small inaccuracies in the measuring devices (voltmeters,
ammeters) could have influenced the results.
2.​ Connection Resistances: Unavoidable contact resistances in the test setup may
have introduced errors.
3.​ Non-Ideal Conditions: Theoretical models assume perfect symmetry and uniform
flux, whereas real-world motors exhibit slight imbalances.
The exploration of starting methods revealed distinct advantages and limitations for
each technique:
●​ Direct-On-Line Starting: Simple and cost-effective but draws high inrush current,
which may strain the power supply.
●​ Reduced Voltage Starting: Mitigates inrush current but reduces starting torque,
making it suitable for light-load applications.

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 ●​ Rotor Resistance Starting: Effective for slip-ring motors, providing high starting
torque with controlled current, though it requires additional rotor circuitry.
●​ Autotransformer Starting: Balances current and torque well but involves higher
complexity and cost.
These observations highlighted the trade-offs between starting torque, current, and
system complexity, emphasizing the importance of selecting the appropriate method
based on application requirements.
In conclusion, the experiment successfully bridged theoretical principles with practical
motor operation. It reinforced the significance of accurate parameter measurement and
the critical role of starting methods in motor performance. The hands-on experience
gained will be invaluable for future work in electrical machine analysis, design, and
troubleshooting.

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