Faculty of Engineering, Architecture and Science
Department of Electrical and Computer Engineering
LAB INSTRUCTIONS
EES 612 – ELECTRICAL MACHINES AND ACTUATORS
EXPERIMENT # 2: SINGLE-PHASE TRANSFORMER
2.1 Introduction
A standard transformer consists of two coils wound on a common (laminated) iron core.
The coils are insulated electrically but linked magnetically by the core flux. The coil that
is connected to the source is called the primary, while the coil connected to the load is
called the secondary. For the ideal transformer, the induced voltage in each turn (of
either the primary or secondary) has the same value, and as a result, the voltage ratio is
the same as the turns ratio. In addition, the ideal transformer does not consume any
power, and consequently, the currents ratio is the same as the inverse turn ratio.
Transformers find many applications in electrical distribution. In transporting power over
long distances (from an electric-power generating station to a distant city, for example)
transformers are used at the generating station to step up the voltage for transmission,
and as a result, reduce the power dissipated in the transmission lines. And for safety
and other reasons, step down transformers are then used for distributing power to the
residential building at a low voltage. Transformers are also used in electronic
applications to provide impedance matching and circuit isolation.
Well-designed transformers approximately meet the conditions assumed for the ideal
transformer. Realistically, however, they require excitation current, and consequently,
have core loss (hysteresis and Eddy-current) losses, and copper losses due to the
coil resistances. In addition, not all the magnetic flux stays in the core, some leaks off
into the air.
This experiment examines the performance of a practical transformer. Its excitation
current as well as its core losses are evaluated from the open-circuit testing at the
rated voltage, and the copper losses are evaluated from the short-circuit testing at the
rated current. The equivalent circuit of the practical transformer is then determined.
The transformer’s voltage regulation is also evaluated for different loading conditions.
2.2 Pre-Lab Assignment
1) A 60 VA, 120/60-V, 60 Hz transformer supplies the rated current to a resistive
load.
a- Find the number of the primary turns, and the maximum value of the flux φ,
assuming that the transformer has 50 turns on the secondary winding.
b- Determine the magnitude of the load impedance as seen from the power
source.
2) For the transformer in question (1), determine the efficiency of power
transformation at full-load with a unity power factor, given that the core losses are
3 W and the copper losses are 6 W.
2.3 Procedure
General safety: To prevent injury to persons or damage to equipment, the power
source must be turned OFF prior to the completion (or changes) of any circuit
connections around the transformer.
Equipment
AC power supply module: EMS 8821
Transformer module: EMS 8341. Name plate rating: 60 VA
Resistance module: EMS 8311
Fluke digital multi-meter
Part I: Open-Circuit Test
1- With the AC-power supply turned OFF, and the variac at zero position,
connect the circuit shown in Fig. 2.1. Notice that the higher voltage winding
of the transformer is left open circuited during the test.
23-
Fig.2.1 Open-circuit test circuit layout
2- Connect the clip-on Ammeter loop (of the DMM) around the wire connecting
nodes 6 and 5. Turn the AC power supply ON, and gradually turn the variac’s
knob clockwise until the voltmeter reading becomes equal to 60 V, which is
the low-voltage side rated voltage.
Record the voltage, current, and power readings of the DMM in table (2.1).
Note that these readings have the following relationship: P=V.I.cosθ, where
θ is the phase angle between the voltage and the current phasors (V w.r.t I).
Table 2.1
V (V)
I (mA)
P (W)
θ°
3- Use the data in table (2.1) to find the values of the in-phase (hysteresis) Ic
and the magnetizing Im current components of the excitation current Ie. Then
find the equivalent core resistance RcL, and the equivalent magnetizing
reactance XmL. Record your results in table (2.2). Note that the subscript L in
RcL and XmL is a reminder that these values were obtained on the low-voltage
side of the transformer
Table 2.2
Ic (mA)
Im (mA)
RcL (Ω)
XmL (Ω)
4- Turn OFF the power source. Connect the voltmeter across the high-voltage
side of the transformer (terminals 1 & 2). Turn the power ON, and record the
voltmeter reading in table (2.3). Determine the turns ratio “a” of the
transformer, and evaluate the core resistance RcH and the magnetizing
reactance XmH.
Table 2.3
a = VHV/VLV
RcH (Ω)
XmH (Ω)
Part II: Short-Circuit Test
5- Turn OFF the power source, and turn the variac knob (counter-clockwise)
back to zero position. Modify your circuit connections as shown in Fig.(2.2).
Fig. 2.2 Short-circuit test circuit layout
6- Connect the clip-on Ammeter loop around the wire connecting the nodes 6
and 1. Turn the power ON and very slowly turn the variac’s knob clockwise
until the Ammeter reading becomes equal to 0.5 A, which is the high-voltage
side rated current. Record the voltage, current, and power readings of the
DMM in table (2.4).
Table 2.4
V (V)
I (mA)
P (W)
7- Use the data in table (2.4) to find the values of ReH and XeH and record these
values in table (2.5). Note that ReH and XeH are the equivalent resistance and
equivalent leakage reactance as seen from the high-voltage side of the
transformer.
Table 2.5
ZeH (Ω)
ReH (Ω)
XeH (Ω)
8- What are the values of equivalent resistance and leakage reactance, ReL and
ReL as seen from the low-voltage side of the transformer?
ReL =
Ω
Xe L =
Ω
Part III: Voltage Regulation
9- Turn OFF the power source. Modify the circuit connections as shown in Fig.
(2.3). connect the clip-on Ammeter loop around the wire connecting the
nodes x and y.
Fig. 2.3 Voltage-regulation circuit layout
10- Starting with RL=240 Ω=1200//300, which is the resistive load value that will
draw the rated current at rated voltage on the high-voltage side of the
transformer. Turn the power ON. Gradually turn the variac’s knob clockwise
until the voltmeter reading across RL equal to 120 V. Record the value of the
load voltage VL and current IL in the table (2.6).
11- Switch ON the next value of RL as listed in table (2.6), and measure the
corresponding values of VL and IL. Use Graph (2.1) to plot the voltage
regulation curve (VL vs. IL).
Table 2.6
RL (Ω)
IL (mA)
240
Rated current
VL (V)
300
400
600
1200
∞
No-load
VL(V)
IL(mA)
Graph 2.1
2.4 Conclusions and Remarks
1- Draw the equivalent circuit of the step up transformer as seen from the
power source (the L.V. side is the primary and H.V. side is the
secondary).
2- With the step up transformer operating at the rated load and unity
power factor, find the following performance parameters:
a) The voltage regulation,
b) The efficiency of the power transformation.
Last updated May 13, 2012-S.H.