EXPERIMENT 8: SINGLE-PHASE TRANSFORMER B-H LOOP

MOTIVATION:

To reduce the transmission losses, it is desirable that when transmitting power over long
distances it should be done at the highest possible voltage. The order of which, in modern
practice, is several hundred kilovolts (KV). Insulation considerations, however, limit the voltage
of generation to a few kilovolts and considerations of economy and safety dictate that in large
power systems, generation, transmission and distribution should be done at different voltages.
This requires free conversion from one voltage to another. In a typical system, generated voltage
has first to be stepped up for transmission and then stepped down - most of the time in stages for
distribution. In addition transformation of voltage is frequently used in electronic circuits like
radio and T.V. receivers, battery chargers and low voltage high current devices such as projector
lamps etc.

The transformation of voltage can be done more easily and efficiently in case of AC systems due
to availability of alternating current transformers; this in fact is the main reason of the popularity
of ac systems over DC systems.

OBJECTIVES:

1. To study the constructional details of a single-phase transformer.

2. To display the B-H curve for the core material used on an oscilloscope.
3. To determine the core losses and exciting current by conducting open circuit test on the
given transformer.
4. Waveforms of exciting current and induced voltage at different flux levels.

THEORETICAL BACKGROUND:

A single-phase transformer essentially consists of two magnetically coupled windings capable of

transforming the voltage and current level of the alternating supply to different values. The two
windings, one called the input or the primary winding and the other called the output or the
secondary winding are placed on a core made of silicon steel stampings. The core provides a low
reluctance path for the changing magnetic flux, which links both the windings. The voltage and
current transformation take place due to the mutual inductance caused by the magnetic coupling.
Transfer of power from the primary winding to the secondary winding at an altered voltage and
current level, therefore takes place through the medium of changing magnetic field.
Principles of Electrical Engineering (GEL-104), Lab Manual

The magnetic core, which has been introduced for the flux path, introduces hysteresis and eddy
current losses. Using silicon steel minimizes hysteresis and the eddy current losses are reduced
by laminated construction. Those core losses and the copper losses in the winding appear as heat
energy thus necessitating efficient means of cooling. Smaller transformers are cooled by the free
flow of air, whereas the larger ones are immersed in a tank of oil for cooling as well as
insulation.

The hysteresis loss per cycle is equal to the area of the hysteresis loop of the magnetic material
used. Hysteresis loop or the B-H curve for a transformer can be easily displayed on an
oscilloscope. When an ac voltage is applied to the primary winding with the secondary left open
circuited the primary current is proportional to the field 'H' and the induced emf in the secondary
winding is proportional to the rate of change of flux(or flux density B). If now a signal
proportional to the primary current is applied to the horizontal or 'X' plate of the oscilloscope and
a signal proportional to (∫edt) where e is the induced secondary voltage is applied to the vertical
or 'Y' plate, we see a B-H curve on the oscilloscope; integration of e is effected by putting a
suitable RC circuit at the secondary output.

With the secondary open circuited the primary winding of the transformer behaves like an iron-
cored inductance. If a dc supply is switched on suddenly a flux is setup in the iron core so as to
oppose the flow of current. The dc current has therefore an exponential build up like that in an
RL circuit and only the resistance of the winding restricts the steady state current. If the
resistance is very small as generally is the case for a transformer, the steady state dc current is
likely to be quite large. If, however, an ac supply is applied to the primary winding a pulsating
flux establishes itself in the magnetic core and according to Lenz's law this flux is such that it
induces an alternating voltage in the primary winding which opposes the applied voltage. The
primary current is now restricted to a small value.

If we assume the pulsating flux Φ to have a sinusoidal waveform ΦmCosωt, where ω is the
supply frequency in radians per second, the induced primary voltage,
E1=-N1dǾm/dt
= N1dǾmSinώt
where N1 is the number of primary turns. Thus the primary RMS induced voltage E1 is
proportional to the primary turns N1 and lags behind the flux by 900. Since the flux Φm also
links with the secondary winding another voltage E2 is induced in the secondary winding, which
is proportional to the number of secondary turns and is in phase with E1.

E2/E1 = N2/N1

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Dept. of Electrical Engineering, IIT Ropar
Principles of Electrical Engineering (GEL-104), Lab Manual

Now E1 opposes the applied voltage say V1 and is nearly equal to V1 if the primary resistance and
leakage reactance is very small. Figures 8.1, 8.2 and 8.3 respectively show the schematic
diagram, equivalent circuit and phase diagram of a single-phase transformer with primary and
secondary resistances and leakage inductances neglected.

V1 EE1 EE22 V2
1

Fig 8.1

In Fig. 8.1, the primary and secondary windings are shown on two limbs of the core but in
practice the two windings are provided along with each other on both the core limbs. This is
done to reduce the leakage inductance of the winding. The parallel branch shows an inductance
causing the magnetizing current Im which sustains the magnetic flux Φ and should be in phase
with Im and 900 lagging the applied voltage. Resistance in the parallel branch represents the core
losses, which manifests them as heat energy. Thus the phase diagram shows magnetizing current
Im and loss current IC and the resultant no load current IO.

Now with the primary resistance and leakage inductance neglected, E 1 = V1 and E2 = V2. Since
E2/E1 = N2/N1, we can say that V2/V1 = N2/N1. Thus approximately the output voltage bears the
same ratio to the input voltage, which secondary turns bear to the primary turns.

I1 I2

I0
Im
E1 Xm Rc E2 V
V
1 2

Fig 8.2

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Dept. of Electrical Engineering, IIT Ropar
Principles of Electrical Engineering (GEL-104), Lab Manual

I1
V1=-E1 -I2

I0
Ic

Ǿm
Im

I2
V2=-E2
E1

Fig 8.3
So far the secondary has been considered open circuited. If it is, however, loaded a current I 2 will
flow through the load and the secondary winding. The effect of this current is to reduce the flux
øm because it will induce a flux opposing the flux ø m. This will mean a decrease in the induced
emf E1. The current now rushes from the primary supply to cancel the effect of the secondary
current and to establish equilibrium flux øm. Thus the power has been transferred from the
primary to the secondary winding through the mutual flux. Now ignoring the losses one can say
that output power is equal to input power. Thus

I2E2 = I1E1

I2/I1 = E1/E2 = N1/N2

Therefore the current ratio is equal to the inverse turn’s ratio. This situation is shown in the phase
diagram where I2 is shown to balance the effect of I2 and total input current on load becomes I1.
The phase diagram can now be modified to include the effect of resistance and leakage
reactances of the windings.

EQUIPMENTS REQUIRED:

1. Transformer under test

2. Single phase two winding transformers with tapings at say 25%, 50% and 75% to be used
as an isolated supply for the experiment

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Dept. of Electrical Engineering, IIT Ropar
Principles of Electrical Engineering (GEL-104), Lab Manual

3. Oscilloscope
4. A rheostat, with a current rating more than no load current of the transformer under test
5. Two voltmeters, one with voltage range equivalent to the full voltage rating and the other
with 10% of the voltage rating: or one multi range voltmeter.
6. Two ammeters, one with current range equivalent to full load current rating and the other
with 10% rating: or one multi range ammeter
7. One multi range wattmeter
8. Load to be used on the secondary
9. A 0.5 or 1 watt resistance of the order of 1 mega ohm and a mica or paper capacitor of the
order of 1micro F for making an integrating circuit
10. Single phase auto transformer with full load current rating

CRO
X-Plate
Isolating Transformer
Transformer Under Test

Single-
CRO
Phase,
Y-Plate
50Hz,230
V,
AC Fig. 8.4
Supply
PROCEDURE:

For doing the experiment in the laboratory a 1:1 turn ratio transformer may be used as the
transformer under test without loss of generality since a 1:n turn ratio transformer can always be
represented by a 1:1 ratio transformer.

DISPLAY OF B-H CURVE:

Connect as shown in Fig 8.4. An isolating transformer is to be provided because one plate each
of the X and Y set of plates of CRO is grounded, otherwise the neutral of the single phase supply
with interfere either X and Y plate connections. R of the integrating circuit should be sufficiently
large to restrict the current and to provide a large time constant. B-H curve is now seen on the
CRO. B-H for various no load input voltages (thus various flux densities) can be shown by
changing the taps.

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Dept. of Electrical Engineering, IIT Ropar
 Principles of Electrical Engineering (GEL-104), Lab Manual

EXPERIMENT 9: SINGLE PHASE TRANSFORMER OC & SC TEST

OBJECTIVE: To determine parameters and losses by conducting open circuit and short circuit
tests on a single phase transformer.

EQUIPMENTS:
1. Rheostat
2. Voltmeter(0-125/250)
3. Ammeter(0-10-20)
4. One multi range wattmeter

PROCEDURE:

M cc
Test Transformer
A C V
A
pc
Single-phase
Wattmeter V V
230V,50Hz Supply

V
V

A
Auto
Transformer
Fig 9.1

OPEN CIRCUIT TEST:

1. Make the connections as shown in Fig 9.1.

2. The secondary of transformer under test is kept open and the full voltage is applied to the
primary. Low current range ammeter and high voltage voltmeter are used.
3. Read the no load current, power and applied voltage.
4. Take the readings by varying the applied voltage.
5. Plot the Open Circuit Characteristic (OCC).

SHORT CIRCUIT TEST:

1. Now use the high current rating of ammeter and low voltage rating of voltmeter.
2. Short the secondary and apply the reduced voltage to the primary so that the full load
current passes through the windings.
3. Take the voltage, current and power readings.

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Dept. of Electrical Engineering, IIT Ropar
Principles of Electrical Engineering (GEL-104), Lab Manual

4. Take readings at 50% and 25% of rated current.

5. Plot Short Circuit Characteristic (SCC).

OBSERVATIONS:

Short circuit test

S.No Primary voltage Primary Current Secondary Voltage Secondary Current

Open circuit test

S.No Primary voltage Primary Current Secondary Voltage Secondary Current

To be done:
1. Compute the equivalent resistance and equivalent reactance.
2. Determine the efficiency and regulation of the transformer.
Efficiency=power output/ (power output+ core losses+ copper losses)
Where, core loss is determined from open circuit test and copper loss is determined from
short circuit test.

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Dept. of Electrical Engineering, IIT Ropar