Licensed Electrical Contractor Transformers and Transformer Calculations 1 — Questions and Answers
Question 1: A transformer has 480 primary turns and 120 secondary turns. What is the turns ratio?
- 1:4
- 4:1 (Correct answer)
- 2:1
- 1:2
Correct answer: 4:1
The turns ratio is primary turns divided by secondary turns: 480 / 120 = 4:1.
Question 2: A transformer has a primary voltage of 480V and a turns ratio of 4:1. What is the secondary voltage?
- 120V (Correct answer)
- 240V
- 960V
- 1920V
Correct answer: 120V
Secondary voltage = Primary voltage / turns ratio = 480V / 4 = 120V.
Question 3: Which type of transformer provides electrical separation between its primary and secondary windings with no direct electrical connection?
- Autotransformer
- Buck-boost transformer
- Isolation transformer (Correct answer)
- Step-up transformer
Correct answer: Isolation transformer
Isolation transformers separate primary and secondary windings to prevent direct electrical connection, protecting equipment from power line disturbances.
Question 4: What does the kVA rating on a transformer nameplate represent?
- The transformer's voltage output
- The transformer's apparent power capacity (Correct answer)
- The transformer's efficiency percentage
- The transformer's impedance value
Correct answer: The transformer's apparent power capacity
The kVA rating indicates the apparent power (kilovolt-amperes) the transformer can continuously deliver without exceeding its temperature rating.
Question 5: In an ideal transformer, when voltage is stepped up from primary to secondary, what happens to the current?
- Current increases proportionally
- Current remains unchanged
- Current decreases proportionally (Correct answer)
- Current doubles regardless of ratio
Correct answer: Current decreases proportionally
In an ideal transformer, apparent power in equals apparent power out; as voltage increases, current must decrease proportionally to maintain constant power.
Question 6: What is the primary purpose of dielectric oil in a liquid-filled transformer?
- To lubricate the core laminations
- To insulate windings and dissipate heat (Correct answer)
- To prevent corrosion on the tank
- To regulate output voltage
Correct answer: To insulate windings and dissipate heat
Transformer oil serves dual functions: it acts as an electrical insulator between windings and as a cooling medium to transfer heat away from the core and coils.
Question 7: Which NEC article governs the installation requirements for transformers, including guarding, clearances, and overcurrent protection?
- Article 240
- Article 430
- Article 450 (Correct answer)
- Article 480
Correct answer: Article 450
NEC Article 450 covers all transformer installation requirements including protection, location, grounding, and construction standards.
Licensed Electrical Contractor Transformers and Transformer Calculations 1
A transformer has 480 primary turns and 120 secondary turns.
What is the turns ratio?