Faculty of Engineering, Architecture and Science
Department of Electrical and Computer Engineering
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Course Title
EES 612
Electrical Machines and Actuators
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Faculty of Engineering, Architecture and Science
Department of Electrical and Computer Engineering
LAB INSTRUCTIONS
EES 612 – ELECTRICAL MACHINES AND ACTUATORS
EXPERIMENT # 1: SINGLE-PHASE TRANSFORMER
Introduction
Transformers are widely applied in many electrical systems, small or large. A few
of their most tangible areas of application include long-distance, bulk power
transmission systems, medical and consumer electronic devices for level shifting and
galvanic isolation, audio, video, and radio systems for impedance matching, and
measurement devices for level shifting and galvanic isolation.
A typical single-phase transformer consists of two windings wound on a common
(laminated) iron core. The windings are electrically isolated from each other and
from the core, but are magnetically linked through the magnetic field within the
core. The winding that is connected to the power source is commonly referred to as
the primary winding, whereas the one connected to the load is called the
secondary winding. In the ideal transformer, the voltage induced in each turn of a
winding (whether primary or secondary) is the same as the voltage induced in any
other turn of the same or the other winding. Hence, the ratio of the terminal voltages
(or the so-called voltage ratio) is equal to the ratio of the windings number of turns
(or the so-called turns ratio). In addition, the ideal transformer neither consumes
nor generates power. Therefore, the ratio of the terminal currents equals the
reciprocal of the voltage ratio (and turns ratio).
Even though well-designed large real transformers can closely approximate the
ideal transformer, a real transformer often deviates from the ideal transformer, as
follows:
 The field created by a winding does not entirely stay within the core; some of it
leaks off into air and, consequently, does not get linked by the other winding. The
effect is that the voltage ratio in a real transformer is not exactly equal to the turns
ratio.
 The transformer draws a load-invariant current component, the exciting
current, from the source, due to the limited permeability of the core, and also
due to the energy losses of the core. The exciting current is harmonically
distorted, due to the hysteresis and the nonlinear relationship between the
magnetic field intensity andflux density in the core (remember the nonlinear B − H
2
curves). The no-load exciting current produces the flux in the core. This
flux in turn produces the hysteresis loss and the eddy current loss in the
core. These two losses are collectively called the core loss, P c ore .
In this experiment, the core losses of a transformer are evaluated through the opencircuit test at the rated voltage, whereas the copper losses are measured from the
short-circuit test at the rated current. These, in turn, enable one to determine the
parameters of the transformer’s equivalent circuit. Finally, a parameterized equivalent
circuit enables the calculation of transformer’s efficiency and voltage regulation under
different operating conditions.
Pre-Lab Assignment
P1- Calculate the rated currents of the low- and high-voltage sides of a 60 Hz, 120/60-V,
60 VA, single-phase transformer.
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P2- Draw the equivalent circuit of a single-phase transformer and explain how the core
losses are almost invariant to the load (current). How do they depend on the
terminal voltages of the transformer? Also, using the equivalent circuit explain how
the copper losses depend on the load current.
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P3- A single-phase transformer drives a resistive load at some efficiency. Based on your
answers to P2 above, explain how the efficiency will change if the supply voltage is
reduced by 50%?
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P4 – The primary winding of a transformer is supplied by a sinusoidal voltage of V s
(rms) and frequency of fs . The flux density in the magnetic core is also sinusoidal
function of time with a peak value of Bc . The core has a cross section area of Ac ,
and the winding has N turns.
P4a: Write an expression of Vs in terms of Bc, Ac, N and fs . Also provide an alternative
expression in which Bc.Ac has been replaced by the flux in the core, Φc .
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P4b. The expressions you derived in P4a enable one to determine the number of turns
of the windings as the design stage of a transformer. If the core does not
saturate for maximum flux density B m , what should be the minimum
turn N for the supply voltage V s ?
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P4c. Based on your answers to P4a and P4b, what is the risk involved in operating a
transformer at an increased voltage? At a decreased frequency? If a particular
application requires that a 60-Hz transformer be operated off a, for example, 50Hz supply, how should the voltage be derated for proper operation?
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Lab Work
1. General safety note
To prevent injury to persons or damage to equipment, the power source must be
turned OFF prior to wiring up the circuit and making any subsequent changes.
Ask your TA to check.
2. Equipment
AC power supply module EMS 8821 (for energizing the transformer)
Transformer module EMS 8341 (Nameplate rating 120/60 V, 60 VA)
Resistance module EMS 8311 (for loading the transformer)
Digital Multi-Meter (DMM) and its associated clamps
3. Experiments
E1: Open-Circuit Test
E1.1- With the AC power supply turned OFF, and the variac set at zero position (fully
turned counterclockwise), connect the circuit of Figure E1. Note that, as Figure
E1 indicates, in this test the low-voltage (60-V) winding is the one to be
energized, while the high-voltage (120-V) winding is to be left open.
Figure E1: Circuit for the open-circuit test.
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E1.2- Place the ammeter clamp of the DMM around the wire that connects node 5 to
node 6. Then, turn on the power supply and gradually turn the variac’s knob
clockwise, until the voltmeter reads 60 V; this is the transformer’s rated low
voltage. Record in Table E1 the current, power, and voltage readings of the
DMM. Note that we have arbitrarily labeled the low-voltage side as side 1.
Consequently, the high-voltage side will be regarded as side 2. Use the
measured voltages to calculate the turns ratio, 𝒂 = 𝑽𝟏/𝑽𝟐.
Table E1: Meter readings under the open-circuit test.
V1 (V)
I1 (mA)
P1 (W)
V2 (V)
a=V1/V2
60
E1.3- Turn off the power supply and turn the variac’s knob fully counterclockwise.
E2: Short-Circuit Test
E2.1- With the power supply turn off and the variac’s knob parked at zero position (that
is, turned fully counter-clockwise), modify the circuit as shown in Figure E2. Note
that, as Figure E2 indicates, in this test the high-voltage (120-V) winding is the
one to be energized, while the low-voltage (60-V) winding is to be shorted.
Figure E2: Circuit for the short-circuit test.
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E2.2- Place the ammeter clamps of the DMM around the wire connecting the nodes 6
and 1. Turn on the power supply, and very slowly turn the variac’s knob
clockwise until the ammeter reads 0.5 A; this is the rated current of the winding
being energized (i.e., the high-voltage or 120-V winding). Record in Table E2 the
current and power readings of the DMM.
Table E2: Meter readings under the short-circuit test.
I2 (mA)
V2 (V)
P2 (W)
500
E2.3- Turn off the power supply and turn the variac’s knob fully counterclockwise.
E3: Voltage Regulation
E3.1- With the power supply turned off and the variac’s knob parked in zero position
(turned fully counter-clockwise), modify the circuit as shown in Figure E3. Then,
place the ammeter clamp of the DMM around the wire connecting node x to
node y. The voltmeter shall be connected across the high-voltage terminals of
the transformer, that is, to nodes 1 and 2. Note that we are to operate the
transformer as a step-up transformer, since the load voltage will be higher than
the source voltage.
Figure E3: Circuit for transformer loading and measurement of voltage regulation.
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E3.2- Switch on resistances 300Ω and 1200Ω to introduce an effective load resistance
of 240Ω. At this load the transformer will provide the rated current of 0.5A (point
five amperes) at rated voltage of 120V in the high voltage side.
E3.3- Turn on the power supply and gradually turn the variac’s knob clockwise until the
Voltmeter reads V2 =120V. Measure the load current I2 and the supply voltage, V1
(across nodes 5 and 9). Revcord the results in Table E3.3
Table E3.3: Meter readings for the high-voltage rated resistive load.
RL ()
Load Voltage
V2 (V)
240
120
Source Voltage
V1 (V)
E3.4- Change the load resistance according to Table E3.4, and record the results. Note
that the first row of Table E3.4 reports the same result as Table E3.3. More
importantly, for each load resistance, the supply voltage 𝑉1 must be kept at the
same value as the one measured in E3.3 (see Table E3.3). Whenever needed,
re-adjust the variac’s knob to ensure that 𝑉1 remains the same.
Table E3.4: Meter readings under different loads.
RL ()
Source Voltage
V1(V)
240
Same as that in E3.3.
Re-adjust if needed.
300
Same as that in E3.3.
Re-adjust if needed.
400
Same as that in E3.3.
Re-adjust if needed.
600
Same as that in E3.3.
Re-adjust if needed.
1200
Same as that in E3.3.
Re-adjust if needed.
∞
Same as that in E3.3.
Re-adjust if needed.
(all resistors off)
Load Voltage
V2 (V)
Load Current
I2 (mA)
0.0
E3.5- Turn off the power supply and park the variac’s knob at the zero position.
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Conclusions and Remarks:
C1- Using the results of E1 and E2, calculate and draw the equivalent circuit of the
tested transformer, as referred to the high-voltage side (side 2). Also, for the rated
operating conditions, determine 𝑃𝑐𝑜𝑟𝑒 , 𝑃𝑐𝑢 , and the efficiency of the transformer.
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C2- Suppose that the transformer derives at its high voltage side a 240Ω resistive
Load at the rated voltage of 120V. Using the equivalent circuit found in part C1,
calculate the required source voltage and compare it with the value measured in
E3.3. Provide your reasons for discrepancies.
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C3- Suppose that the source voltage is the same as the voltage measured in E3.3.
Using the equivalent circuit found in C1, calculate the load voltage for each of the
load currents measured in E3.4. Complete the Table C3 below. In addition, plot on
Graph C3 both the measured and calculated values of the load voltage, versus the
corresponding values of the measured load current.
Table C3: Load voltage under different loads.
Measured Load
Current
from Table E3.4
I2 (mA)
Measured Source
Voltage from
Table E3.4 V1(V)
Measured Load
Voltage
from Table E3.4.
V2 (V)
Calculated
Load Voltage
V2 (V)
Error (%)
0.0
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V2(V)
I2 (mA) --
Graph C3 : V2 vs I2
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