Addis Ababa Institute of Technology (AAiT)

Department of Electrical and Computer Engineering

Electrical Machines (Lab IV) Laboratory Report
Course number: Eceg:-3207 - Electrical Machines Laboratory

Experiment Number: 02

Title: Three Phase Transformer

Figure A: A Three Phase Transformer (Delta – Star connection) Layout Diagram

By: Besufekad Mekuria

Lab Group: 4,
ID: TCR/0590/01
Date of Lab. session: November 30, 2010
Date of submission: December 04, 2010

AAIT, DEPARTMENT OF ELECTRICAL ENGINEERING, ELECTRICAL MACHINES LABORATORY REPORT, THREE PHASE TRANSFORMERS 0
 TABLE OF CONTENTS:

Section Sub-Title Location

NO
1.1 Introduction Page Two

1.2 Objective Page Two

1.3 Preparation Page Two

1.4 Used Equipment Page Two

1.5 Components used Page Two

1.6 Theory Page Three

1.7 Procedure Page Five

1.8 Results Page Five

1.9 Conclusion Page Seven

AAIT, DEPARTMENT OF ELECTRICAL ENGINEERING, ELECTRICAL MACHINES LABORATORY REPORT, THREE PHASE TRANSFORMERS 1
 Three Phase Transformers
1.1 INTRODUCTION
Instead of three - single-phase transformers, a three-phase bank may consist of one three-phase
transformer having all six windings on a common multi-legged core and contained in a single tank.
Advantages of three-phase transformers over connections of three single-phase transformers are
that they cost less, weigh less, require less floor space, and have somewhat higher efficiency.

Therefore, the main purpose of the lab session was to introduce three phase transformers and their
uses over single phase transformers.

1.2 OBJECTIVE
1. To determine the transformation ratio for different kinds of connections.
2. To perform no-load and short circuit tests.
3. To perform load tests for different kinds of load.
4. To determine the transformer efficiency.

1.3 PRE-LAB PREPARATION

Some preparatory analysis were performed before the laboratory session as a preparation given to
us by the Laboratory instructors. This manual also included details on what we were expected to
perform in the Lab.

1.4 USED EQUIPMENT

No Description Code/Lab Reference Quantity
1 Three Phase Transformer - 1
2 DC Source - 1
3 Ammeter - 2
4 Voltmeter - 2
5 Wattmeter - 1
6 Variable Resistor (Load) - 2
7 Switch - 2
Table 1: Electrical Equipment that were used to set up the Network for the laboratory procedure

1.5 USED COMPONENTS

No Description Type Quantity
1 Switches Normally open 2
2 Conducting Wires Designed for circuit board 8
Table 2: Electrical Components that were used to set up the circuit for the laboratory procedure

AAIT, DEPARTMENT OF ELECTRICAL ENGINEERING, ELECTRICAL MACHINES LABORATORY REPORT, THREE PHASE TRANSFORMERS 2
 1.6 THEORY
Three single phase transformers can be connected together to work as a three phase
transformer. There are different ways which the primary and the secondary windings of a group of
three single phase transformers or of a single three phase transformer may be connected together
to transfer energy from one three phase circuit to another.

Star – Star Connection

The fig below shows three single – phase transformer connected to work as a start – start
three – phase transformer. Note that X, Y and Z are connected together and let us call the common
point o. Similarly x, y and z are connected together and let us cal it o’.

Fig -1 star – star connection

If the primary and secondary line voltages are V1 and V2, the primary and secondary voltages of the
individual transformers must be V1/ 30.5 and V2 / 30.5 respectively.

1. Delta – Delta Connection

Fig. below shows three single phase transformers of the windings of a single three phase transformer
connected in delta (Δ) on both primary and secondary sides. Note that in this type of connection the
phase and line voltages are equal. The main advantage of this connection lies in the fact that the
system can still operate on 58% of it’s rated capacity even in case of failure of one of the
transformers.

Figure - 2 Delta – delta connection

AAIT, DEPARTMENT OF ELECTRICAL ENGINEERING, ELECTRICAL MACHINES LABORATORY REPORT, THREE PHASE TRANSFORMERS 3
 2. Star – Delta Connection
The Fig. below shows transformer windings where the primary side is connected in star and the
secondary is connected in delta.

Fig -3 Star – delta connection

For this type of connection, on the primary side the primary side the line voltage is equal to 30.5 times
the phase voltage. But on secondary side the phase and the line voltage are equal. This connection is
commonly used at the receiving end of high-voltage transmission lines.

3. Delta – Star Connection

In this case the primary side is connected in delta and the secondary side is connected in start.

Fig - 4 Delta – star connection

Here the phase and line voltages are equal on the primary side. On the secondary side the
line voltage = 30.5 phase voltage.

This connection gives a higher secondary voltage for transmission purposes than the
connections with Δ secondaries without increasing the strain on the insulation of the transformer. It
is the connection commonly used at the generating end of transmission lines. The neutral of the Y –
connected secondary is generally grounded.

AAIT, DEPARTMENT OF ELECTRICAL ENGINEERING, ELECTRICAL MACHINES LABORATORY REPORT, THREE PHASE TRANSFORMERS 4
 1.7 PROCEDURE
1. Identify the name-plate on the transformer.
2. Measure the winding resistance of the transformer by the ammeter voltmeter method.
3. Connect the transformer in Y-Y. Apply rated voltage to the primary. Measure the Secondary
voltage. Determine the transformation ratio. Repeat the procedure for Δ- Δ, Y- Δ and Δ-Y
connections.
4. NO LOAD TEST
Assemble the circuit below. Vary the input V1 from 0.5 rated to 1.2v rated and at each step
measure the open circuit power poc, the phase voltages and the currents IA and IC.

Figure 5: connection for open circuit test

5. SHORT CIRCUIT TEST

Connect the above circuit with the secondary sire short circuited. Increase gradually the
short circuit primary voltage so that the current on the secondary side is about 0.5I rated,
0.75I rated and I rated. At each step record the short circuit voltage, current and power.

6. LOAD TEST
Apply rated voltage to the primary and increase gradually the load current on the secondary side
by varying the resistance of the load resistor. At each step measure the current and the
secondary voltage.

1.8 Results
Firstly, the Name plate of the transformer:

Siemens Power = 2.5kVA/2.17kVA

frequency = 50 - 60Hz

Primary Resistance: 0.8 Ohm

Secondary Resistance: 0.5 Ohm

AAIT, DEPARTMENT OF ELECTRICAL ENGINEERING, ELECTRICAL MACHINES LABORATORY REPORT, THREE PHASE TRANSFORMERS 5
 Ratio Test:

Vp 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380

V 58 76 88 98 108 118 128 140 150 164 172 184 192 204 216

Pin 10 11 20 20 20 22 22 40 40 40 40 48 48 60 76

Ip 70 75 80 85 90 95 100 105 110 125 130 145 150 165 180

V2 53.6 62.3 75 84.5 93.5 103.5 111 121.8 131 144.5 153 162 170 180 191

YY VP Vs Y* ** *Y

UV 100 50 UV 29 UV 50 VW 85

UW 100 50 UW 29 UW 50 UV 85

VW 100 50 VW 29 VW 50 UW 85

Short Circuit

0.5 of rated (1.9)

P=10W, Ip = 0.95A, Is = 1.9A

0.75 of rated

Ip = 1.4, Is=2.85A, P = 25W

Irated

Ip = 1.75, Is = 3.8, P=35W

Finally changing the resistance of the variac

Pin Ip Vl Is
60 0.4 4V 0.4
100 0.65 4V 0.9
560 0.9 4V 1.4
680 1.1 4V 1.9
880 1.45 4V 2.4
1040 1.7 4V 2.9
1200 1.95 4V 3.4
1240 1.9 4V 3.7

AAIT, DEPARTMENT OF ELECTRICAL ENGINEERING, ELECTRICAL MACHINES LABORATORY REPORT, THREE PHASE TRANSFORMERS 6
 1.10 Conclusion
Upon completion of the laboratory session, many observations were made; these observations have
been dealt with here under their own subheading.

Observations made about three phase transformers

 This lab has tried to come up with different result that may be crucial in determining the behavior of a three
phase transformer.

 Firstly the winding resistance of the transformer is calculated by ammeter/voltmeter method followed by
the three tests: no load, short – circuit and Load test. These test verify the transformer ratio, resistance
efficiency and other important results.

 Even though the results are within good precision range, some may deviate from the theory and may lead
to false conclusions. But this all consideration are taken into account. A better result may be obtained by
using more accurate equipments and a healthy transformer.

Observations made about the Operation of Transformers.

 The two coils of the transformer can be used interchangeably two different outputs, these are;
One: As a step up Transformer where the Primary turn is greater than the Secondary turn. And
Two: As a step down Transformer where the Secondary turn is greater than the Primary turn.
 In the design of the transformer, the magnetic component (the iron core) is composed of many strips of
lamination. The reason that we don’t use only a single bar of iron is to avoid power loss due to a
phenomenon called “Eddy Current Loss”.

Measurement of the open Characteristics of the Transformer (Nameplate)

 When measuring the input and output impedances of the Transistor the input Voltage was measured
using a voltmeter while the input current was simultaneously measured using the ammeter. These
two values were recorded for a set of different inputs, then the voltages were divided with their
corresponding currents to obtain different values of input (primary coil) input impedance
(Resistance, since the impedance has no imaginary part because of DC source).

 The two coils of the wattmeter are used to measure the power, one for voltage and the other for current.

Observations made about practical errors

It is a well known fact that there does not exist a circuit element that functions with a hundred percent
efficiency. Therefore slight deviations of the measured values from the calculated ones are to be expected
under any circumstances. However the possible causes for these irregularities include:

 The Transformers are not ideal in nature, this is due to there are power losses due to many reasons:

1. The windings on the coil contribute to a considerable amount of power loss due to the fact that their
resistance due to a very great length. 2 Magnetic power loss exists due to eddy current generation as
stated above. And 3. Hysteresis loss

 The DC voltmeter does not measure a potential difference with a magnitude that it is programmed and
expected to do so. This is generally due to manufacturing defects, defects from improper usage and
gradual wearing off. Also since its internal resistance is great yet not equal to infinity.

 Similarly, the Ammeter does not reveal a current with a magnitude precision that it is expected to do so.
This is generally due to manufacturing defects, defects from improper usage and gradual wearing off. Also
since its internal resistance is very small, yet not equal to Zero.

 Also the AC Ammeter used was an analog device; therefore readings are perceptible to be misconstrued.
And also continuous mistreatment of the device leads to incorrect readings in the future.

End of Manual

AAIT, DEPARTMENT OF ELECTRICAL ENGINEERING, ELECTRICAL MACHINES LABORATORY REPORT, THREE PHASE TRANSFORMERS 7