40 Questions
Where is Dr. Philip Yaw Okyere currently teaching?
Kwame Nkrumah University of Science and Technology, Kumasi
What is the title of the course Dr. Philip Yaw Okyere is teaching?
Electromechanical Energy Conversion and Transformers
Where did Dr. Philip Yaw Okyere pursue his postgraduate studies?
L’Institut National Polytechnique de Grenoble (INPG), France
What is Dr. Philip Yaw Okyere's current position?
Senior Lecturer
What is the degree Dr. Philip Yaw Okyere graduated with in 1979?
B.Sc.(First Class Honours)
Where can Dr. Philip Yaw Okyere be contacted?
Department of Electrical & Electronic Engineering, KNUST
What is the credit value of the course EE 162?
3
What is Dr. Philip Yaw Okyere's field of specialization?
Electrical Engineering
What is the equivalent of the secondary terminal voltage on no load?
V2r
What is the relationship between the current in the primary and the current in the secondary?
I1 = aI2
What is the voltage to be applied to the primary to obtain the rated current in the secondary when the secondary terminal voltage is 240 V?
2462 V
What is the load power factor angle?
36.9°
What is the voltage across the equivalent leakage impedance?
35 + j57 V
What is the input power factor?
0.79 lagging
What is the primary current when the secondary terminal voltage is 240 V?
4.237 A
What is the phase angle between the primary voltage and current?
37.9°
What is the relationship between V1,2 and V1,1 when the number of turns and cross section remain the same?
V1,2 = V1,1 * (Bmax2 / Bmax1)
What is the unit of hysteresis loss WH?
J/m3
How does the hysteresis loss change with frequency?
It increases linearly with frequency
What is the relationship between the eddy-current loss and frequency?
It increases quadratically with frequency
What is the purpose of the phasor diagram on no load?
To furnish the power loss in the core
What is the total iron loss in the transformer core at 60 Hz?
2693 W
What is the hysteresis loss at 25 Hz in Example 6?
300 W
What is the eddy-current loss at 50 Hz in Example 6?
800 W
What is the primary application of a delta-star connection?
To step up alternator voltage to transmission line voltage
What is the main use of a star-delta connection?
As a step-down transformer at the load end of transmission line
What is the problem that can occur in a star-star connection if the secondary load is unbalanced?
Floating neutral
What is the purpose of connecting the neutral of the primary and the neutral of the source together in a star-star connection?
To eliminate the floating neutral problem
Which of the following connections is economical for high-voltage transformers?
Star-star connection
What is the characteristic of a delta-star connection that allows it to provide a four-wire supply?
The secondary star point is earthed
What is the type of connection that has no secondary neutral and cannot provide a four-wire supply?
Star-delta connection
What is the common application of a delta-star connection in distribution service?
As a step-down transformer, the windings are not the most economical
What is the primary reason for using high-grade grain-oriented steel in transformer core?
To reduce the hysteresis loss
What is the main purpose of laminating the transformer core?
To reduce the eddy current loss
What is the advantage of core-type construction in transformer?
Reducing the amount of copper used
Why are the transformer windings carefully insulated from each other and from the core?
To prevent electrical shock
What is the purpose of adding 3 to 4% silicon to the transformer core?
To increase the resistivity of the steel
What type of transformer construction has the primary and secondary windings distributed over both core legs?
Core-type construction
What is the purpose of polarity tests on a transformer?
To determine the additive or subtractive polarity
What is the main advantage of using iron as the core material in transformer?
High permeability
Study Notes
Transformer Parameters and Calculations
- A transformer is rated 10 kVA, 2400/240 V, 60 Hz, with parameters Rm = 80 kΩ, Xm = 35 kΩ, Re1 = 8.4 Ω, and Xe1 = 13.7 Ω for the approximate equivalent circuit.
- To obtain the rated current in the secondary, a voltage needs to be applied to the primary, which can be calculated using the given parameters.
Phasor Diagrams and Power Factor
- The input power factor can be calculated using the phase angle between V1 and I1.
- The load power factor angle can be calculated using the cosine inverse of the load power factor.
Transformer Construction and Design
- Power transformers are designed to approach the characteristics of an ideal transformer.
- The core is made of iron and forms a closed magnetic circuit to attain high permeability and a small magnetizing current.
- High-grade grain-oriented steel with a narrow hysteresis loop is used to reduce hysteresis loss.
- The core is laminated with 3-4% silicon added to increase the resistivity of the steel and reduce eddy current loss.
Magnetic Circuits and Windings
- Two main forms of magnetic circuits are used: core-type and shell-type arrangements.
- The primary and secondary windings are distributed over both core legs to reduce the amount of copper and minimize copper losses.
- The transformer windings are carefully insulated from each other and from the core.
Polarity Tests and Connections
- The four terminals of a single-phase transformer can have either additive or subtractive polarity.
- Common connections include delta-delta, delta-star, star-delta, and star-star connections, each with its own application and characteristics.
Iron Losses and Eddy Currents
- Hysteresis loss and eddy current loss can be calculated using the area of the hysteresis loop and the frequency of operation.
- The total iron loss in a transformer core can be calculated by summing the hysteresis loss and eddy current loss.
Calculate the voltage and current in a transformer's equivalent circuit using given parameters. This quiz is for electrical engineering students.
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