Transformer Step Up and Step Down
Chapter: Alternating Current | Topic: Transformer Step Up and Step Down
TRANSFORMER STEP UP AND STEP DOWN
Topic: Transformer Step Up and Step Down
Multiple Choice Questions
Q1. A transformer works on the principle of:
- Ohm's law
- Electromagnetic induction
- Heating effect of current
- Photoelectric effect
Q2. In a step-up transformer:
- N_s < N_p and V_s < V_p
- N_s > N_p and V_s > V_p
- N_s = N_p and V_s = V_p
- N_s > N_p and V_s < V_p
Q3. For an ideal transformer, the ratio of secondary current to primary current is:
- Equal to the turns ratio
- Equal to the inverse of the turns ratio
- Equal to the square of the turns ratio
- Zero
Q4. The core of a transformer is laminated to:
- Increase the magnetic flux
- Reduce eddy current losses
- Increase the weight
- Make it more durable
Q5. In a transformer, electrical energy is transferred from primary to secondary coil through:
- Direct electrical connection
- Mutual induction
- Conduction
- Radiation
Q6. A step-down transformer converts:
- High voltage AC to low voltage AC
- Low voltage AC to high voltage AC
- AC to DC
- DC to AC
Short Answer Questions
Q7. State the principle on which a transformer works. Can a transformer work with DC? Explain.
Q8. Distinguish between a step-up transformer and a step-down transformer in terms of turns ratio and voltage ratio.
Q9. Why is the core of a transformer made of soft iron and laminated? Explain.
Q10. For an ideal transformer, show that V_s/V_p = N_s/N_p = I_p/I_s.
Q11. Why is electrical power transmitted at high voltage? How do transformers help in this process?
Q12. What would happen if the primary coil of a transformer were connected to a DC source of the same voltage rating?
Long Answer QuestionsME
Q13. Explain the construction and working of a transformer with a neat diagram. Derive the relation between primary and secondary voltages and currents for an ideal transformer.
Q14. Explain why transformers cannot be used with DC. What would happen if DC were applied to the primary of a transformer?
Q15. Describe the energy losses in a transformer and explain how each loss can be minimized.
Numerical & Application-Based Problems
Q16. A transformer has 500 turns in the primary coil and 50 turns in the secondary coil. The primary is connected to a 220 V, 50 Hz AC supply.
(a) Calculate the voltage across the secondary coil.
(b) If a load resistor of 10 ฮฉ is connected across the secondary, calculate the current in the secondary and primary coils (assume ideal transformer).
(c) Calculate the power output of the transformer.
Q17. A step-up transformer has a turns ratio of 1:25. The primary is connected to a 220 V AC supply and draws a current of 10 A.
(a) Calculate the secondary voltage.
(b) Calculate the secondary current (assuming 90% efficiency).
(c) Calculate the power output.
(d) Explain why the secondary current is less than what would be expected for an ideal transformer.
Q18. In a rural area of India, electricity is supplied at 11 kV and needs to be stepped down to 220 V for household use. A transformer is installed for this purpose.
(a) Calculate the turns ratio of the transformer.
(b) If the transformer supplies power to 100 houses, each drawing an average current of 5 A at 220 V, calculate the current in the primary coil (assume 95% efficiency).
(c) The local electricity board uses step-up transformers at the generating station to raise the voltage to 440 kV for transmission. Explain why this is done and calculate the current in the transmission line if the power transmitted is 100 MW.
(d) A student argues that since transformers are very efficient, there is no energy loss in power transmission. Point out the error in this reasoning and explain where energy losses still occur.