Transformers and Power Distribution Flashcards

What is the standard secondary voltage for a three-phase commercial building service in Canada?

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.

What is an autotransformer and what is its main advantage?

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.

What voltage would be measured between the high leg and neutral in a 120/240 V four-wire delta system?

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.

What is the efficiency of a transformer with an output of 45 kW and losses of 2.5 kW?

Answer: 94.7%

Efficiency = Output / Input × 100 = Output / (Output + Losses) × 100 = 45 / (45 + 2.5) × 100 = 45/47.5 × 100 = 94.7%.

What type of transformer winding has the most turns?

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.

What is the purpose of a buck-boost transformer?

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.