Question 1

If in a Y-connected ac generator, each phase voltage has a magnitude of 90 VRMS, what is the magnitude of each line voltage?

Accepted Answer

156 V

Question 2

A three-phase △-connected generator is driving a balanced load such that each phase current is 12 A in magnitude. When I θa = 12 ∠30° A, what are the polar expressions for the other phase currents?

Accepted Answer

I θb = 12 ∠150° A, I θc = 12∠–90° A

Question 3

In a △-connected generator, all of the phase voltages are

Accepted Answer

equal in magnitude

Question 4

In a certain three-wire Y-connected generator, the phase voltages are 2 kV. The magnitudes of the line voltages are

Accepted Answer

3,464 V

Question 5

The most common type of ac motor is the

Accepted Answer

three-phase induction motor

Question 6

In a three-phase system, when the loads are perfectly balanced, the neutral current is

Accepted Answer

zero

Question 7

In a △-connected source feeding a Y-connected load,

Accepted Answer

each phase voltage equals the difference of the corresponding load voltages

Question 8

In a balanced three-phase load, each phase has

Accepted Answer

an equal amount of power

Question 9

In a Y-connected circuit, the magnitude of each line current is

Accepted Answer

equal to the corresponding phase current

Question 10

Polyphase generators produce simultaneous multiple sinusoidal voltages that are separated by

Accepted Answer

certain constant phase angles

Question 11

In a △-connected source driving a △-connected load, the

Accepted Answer

load voltage, line voltage, and source phase voltage are all equal for a given phase

Question 12

A constant load power means a uniform conversion of

Accepted Answer

electrical to mechanical energy

Question 13

A single-phase sinusoidal voltage of 120 V is connected to a 90 Ω load. Current in the circuit is

Accepted Answer

1.33 A

Question 14

Compare the total copper cross sections in terms of current-carrying capacity for a single-phase and a three-phase 120 V system with effective load resistance of 15 Ω.

Accepted Answer

single-phase 16 A; three-phase 8 A

Question 15

In a Y-Y source/load configuration, the

Accepted Answer

phase current, the line current, and the load current are all equal in each phase

Question 16

A three-phase generator is connected to three 90 Ω load resistors. Each coil generates 120 V ac. A common neutral line exists. How much current flows through the common neutral line?

Accepted Answer

0 A

Question 17

A two-phase generator is connected to two 90 Ω load resistors. Each coil generates 120 V ac. A common neutral line exists. How much current flows through the common neutral line?

Accepted Answer

1.77 A

Question 18

In a certain Y-Y system, the source phase currents each have a magnitude of 9 A. The magnitude of each load current for a balanced load condition is

Accepted Answer

9 A

Question 19

In a three-phase system, the voltages are separated by

Accepted Answer

120°

Question 20

In a Y-connected circuit, between each line voltage and the nearest phase voltage, there is a phase angle of

Accepted Answer

30°

Three-Phase Systems in Power Applications Questions