CEA Transformers and Distribution Systems 1 — Questions and Answers
Question 1: What is the turns ratio of a transformer with a primary winding of 480 turns and a secondary winding of 120 turns?
- 4:1 (Correct answer)
- 1:4
- 2:1
- 1:2
Correct answer: 4:1
The turns ratio is calculated by dividing primary turns by secondary turns: 480 ÷ 120 = 4:1.
Question 2: A step-down transformer has a 2:1 turns ratio and a primary voltage of 240V. What is the secondary voltage?
- 480V
- 120V (Correct answer)
- 240V
- 60V
Correct answer: 120V
With a 2:1 turns ratio, the secondary voltage is half the primary: 240V ÷ 2 = 120V.
Question 3: What operating principle allows a transformer to transfer electrical energy between two circuits?
- Electrostatic induction
- Mutual electromagnetic induction (Correct answer)
- Direct current amplification
- Resistance heating
Correct answer: Mutual electromagnetic induction
Transformers operate on mutual electromagnetic induction, where a changing magnetic field in the primary winding induces a voltage in the secondary winding.
Question 4: Which transformer type has its secondary winding electrically isolated from the primary winding with no common connection?
- Autotransformer
- Isolation transformer (Correct answer)
- Current transformer
- Potential transformer
Correct answer: Isolation transformer
An isolation transformer has separate primary and secondary windings with no electrical connection between them, providing complete electrical isolation.
Question 5: If an ideal transformer draws 5A at 240V on the primary, what is the secondary current when the secondary voltage is 120V?
- 5A
- 2.5A
- 10A (Correct answer)
- 20A
Correct answer: 10A
In an ideal transformer, primary VA equals secondary VA: 240V × 5A = 120V × I, solving gives I = 10A.
Question 6: What core material is most commonly used in power transformers to reduce eddy current losses?
- Solid copper
- Solid aluminum
- Silicon steel laminations (Correct answer)
- Ferrite powder
Correct answer: Silicon steel laminations
Silicon steel laminations are used in transformer cores because they provide high magnetic permeability and their thin, insulated layers reduce eddy current losses.
Question 7: A transformer nameplate shows a KVA rating of 75 KVA. What does this rating represent?
- The maximum kilowatts of real power the transformer can deliver
- The apparent power capacity in kilovolt-amperes the transformer can handle continuously (Correct answer)
- The transformer's efficiency at full load
- The primary voltage multiplied by secondary current in amperes
Correct answer: The apparent power capacity in kilovolt-amperes the transformer can handle continuously
The KVA rating represents the transformer's apparent power capacity—the maximum load it can handle continuously without exceeding its temperature rating.
CEA Transformers and Distribution Systems 1
What is the turns ratio of a transformer with a primary winding of 480 turns and a secondary winding of 120 turns?