309A Transformers and Power Distribution 2 — Questions and Answers
Question 1: What is the standard secondary voltage for a three-phase commercial building service in Canada?
- 120/208 V
- 120/240 V
- 277/480 V
- 347/600 V (Correct answer)
Correct answer: 347/600 V
The standard three-phase commercial/industrial voltage in Canada is 347/600 V (wye-connected), where 347 V is the phase-to-neutral voltage used for lighting and 600 V is the line-to-line voltage for motors.
Question 2: What is an autotransformer and what is its main advantage?
- A transformer with automatic voltage regulation; highest efficiency
- A single-winding transformer with a shared primary/secondary; smaller size and cost (Correct answer)
- A transformer that automatically adjusts for power factor; best for motors
- A self-cooling transformer; no oil required
Correct answer: A single-winding transformer with a shared primary/secondary; smaller size and cost
An autotransformer uses a single winding with a tap, sharing conductors between primary and secondary. This makes it smaller, lighter, and less expensive than a two-winding transformer of the same rating.
Question 3: What voltage would be measured between the high leg and neutral in a 120/240 V four-wire delta system?
- 120 V
- 208 V (Correct answer)
- 240 V
- 277 V
Correct answer: 208 V
In a 4-wire delta system, the high leg (wild leg) to neutral voltage is 240 × (√3/2) = 208 V. This is higher than the normal 120 V phase-to-neutral voltage of the other two phases.
Question 4: What is the efficiency of a transformer with an output of 45 kW and losses of 2.5 kW?
- 90.0%
- 94.7% (Correct answer)
- 95.0%
- 97.5%
Correct answer: 94.7%
Efficiency = Output / Input × 100 = Output / (Output + Losses) × 100 = 45 / (45 + 2.5) × 100 = 45/47.5 × 100 = 94.7%.
Question 5: What type of transformer winding has the most turns?
- The high-voltage winding always has more turns (Correct answer)
- The low-voltage winding always has more turns
- Both windings always have equal turns
- It depends on the core material
Correct answer: The high-voltage winding always has more turns
The high-voltage winding always has more turns than the low-voltage winding. The voltage ratio is directly proportional to the turns ratio: VP/VS = NP/NS.
Question 6: What is the purpose of a buck-boost transformer?
- To provide complete isolation between circuits
- To make small adjustments (typically ±10-20%) to an existing voltage (Correct answer)
- To convert single-phase to three-phase power
- To provide emergency backup power
Correct answer: To make small adjustments (typically ±10-20%) to an existing voltage
Buck-boost transformers are small autotransformers used to increase (boost) or decrease (buck) supply voltage by small amounts, typically 5-20%, to correct for voltage variations.
309A Transformers and Power Distribution 2
What is the standard secondary voltage for a three-phase commercial building service in Canada?