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

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?

Accepted Answer

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

Answer

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?

Accepted Answer

208 V

Answer

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?

Accepted Answer

94.7%

Answer

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?

Accepted Answer

The high-voltage winding always has more turns

Answer

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.

Red Seal Electrician 309A Practice Test

Red Seal Electrician 309A Practice Test

309A Transformers and Power Distribution 2