22 Questions
Why are transformer cores laminated?
To minimize eddy current loss
What type of losses vary as the square of flux density?
Eddy current losses and hysteresis losses
What is the main purpose of using silicon steel in laminations?
To reduce hysteresis loss
What type of loss is reduced by laminating the transformer core?
Eddy current loss
How do eddy current and hysteresis losses vary with frequency?
They vary as the square of frequency
What is the purpose of laminating the transformer core?
To reduce eddy current loss
What is the benefit of using silicon steel in transformer cores?
It reduces hysteresis loss
How do you reduce hysteresis loss in a transformer core?
By using silicon steel laminations
What are hysteresis loss and eddy current loss directly proportional to?
f and f
What happens to hysteresis loss if voltage is doubled, load is doubled, and frequency is halved?
Will remain same
If the supply frequency to the transformer is increased, the iron loss will?
Increase
What is the effect of increasing the frequency on iron loss?
Increases iron loss
What is the effect of increasing the voltage on hysteresis loss?
Increases hysteresis loss
What is the effect of increasing the load on hysteresis loss?
Does not affect hysteresis loss
What type of loss is directly proportional to frequency?
Eddy current loss
What type of loss is directly proportional to frequency squared?
Eddy current loss
What is the load kVA corresponding to maximum efficiency for a 200 kVA transformer with an iron loss of 1 kW and full-load Cu loss of 2 kW?
141.4
If the full-load copper loss of a transformer is 1600 W, what is the copper loss at half-load?
800 W
What is the condition for maximum efficiency in a transformer?
Its Cu loss equals iron loss
A transformer has an iron loss of 100 W and an equivalent resistance referred to the secondary of 0.25 Ω. What is the output current corresponding to maximum efficiency?
20
What is the maximum efficiency of a 100-kVA transformer having an iron loss of 900 kW and full-load copper loss of 1600 W?
At 133.3 kVA
What is the efficiency of a power transformer operating at full load, with a voltage and current of 200 V and 100 A respectively, at a power factor of 0.8? The iron and copper losses are equal to 120 kW and 300 kW respectively.
86.5%
Study Notes
Ideal Transformer
- In an ideal transformer, windings have no resistance, core has no losses, and core has infinite permeability.
Transformer Construction
- Transformer cores are laminated to minimize eddy current loss.
Voltage Regulation
- A transformer has negative voltage regulation when its load power factor is lagging.
Transformer Function
- The function of a transformer is to step down or up AC voltages and currents.
Transformer Impedance
- The impedance of a transformer can be calculated using the turns ratio and the impedance of the primary and secondary windings.
Transformer Windings
- Both resistances and leakage reactances of the transformer windings are series effects.
Equivalent Circuit
- In the approximate equivalent circuit of the transformer, all resistances and inductances are lumped before the magnetizing branch.
Core Material
- Silicon steel is used in laminations because it reduces hysteresis loss and eddy current losses.
Iron Loss
- Iron loss of a transformer varies with the square of flux density.
- Iron loss is directly proportional to frequency and flux density.
Voltage Regulation Formula
- Voltage regulation of a transformer is given by V2-E2/V2.
Practical Transformer
- To convert an ideal transformer into a practical transformer, we add primary winding resistance, leakage reactance, and secondary winding leakage reactance.
Hysteresis Loss
- Hysteresis loss is directly proportional to frequency and flux density.
- If voltage is doubled, load is doubled, and frequency is halved, hysteresis loss will remain the same.
Iron Loss and Frequency
- If the supply frequency to the transformer is increased, the iron loss will increase.
Commutator
- The sole purpose of a commutator in a d.c. generator is to convert the induced a.c. into d.c. and reduce sparking at brushes.
Lap Winding
- Lap winding is suitable for high current, low voltage d.c. generators.
D.C. Motor
- As the load is increased, the speed of a d.c. shunt motor reduces slightly.
Load and Speed
- When load is removed, a series motor will run at the highest speed.
D.C. Generator
- The series field of a short-shunt d.c. generator is excited by armature currents.
- The generated e.m.f. in a d.c. generator depends on the number of poles and the speed of the generator.
Transformer Operation
- When operating a transformer at a frequency lower than its rated frequency, both voltage and kVA rating are reduced in the same proportion as the frequency.
Efficiency
- The maximum efficiency of a transformer occurs when its Cu loss equals iron loss.
- The full-load copper loss of a transformer can be calculated using the output current and equivalent resistance.
- The efficiency of a transformer can be calculated using the input and output power.
Test your knowledge of ideal transformers, transformer cores, and voltage regulation with these questions.
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