309A Transformers and Power Distribution 2

Question 1

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

Accepted Answer

347/600 V

Answer

120/208 V

Answer

120/240 V

Answer

277/480 V

Question 2

What is an autotransformer and what is its main advantage?

Accepted Answer

A single-winding transformer with a shared primary/secondary; smaller size and cost

Answer

A transformer with automatic voltage regulation; highest efficiency

Answer

A transformer that automatically adjusts for power factor; best for motors

Answer

A self-cooling transformer; no oil required

Question 3

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

Accepted Answer

208 V

Answer

120 V

Answer

240 V

Answer

277 V

Question 4

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

Accepted Answer

94.7%

Answer

90.0%

Answer

95.0%

Answer

97.5%

Question 5

What type of transformer winding has the most turns?

Accepted Answer

The high-voltage winding always has more turns

Answer

The low-voltage winding always has more turns

Answer

Both windings always have equal turns

Answer

It depends on the core material

Question 6

What is the purpose of a buck-boost transformer?

Accepted Answer

To make small adjustments (typically ±10-20%) to an existing voltage

Answer

To provide complete isolation between circuits

Answer

To convert single-phase to three-phase power

Answer

To provide emergency backup power