Third Year Practice Questions

How many conductors from the source to a service are required in a single-phase 120/240 V system?

Three conductors (L1, L2, N)

The number of _______ determines the number of phases

windings

What type of three-phase system does 240 V, 3-phi identify?

A delta connected service

Three-phase circuits are three single-phase circuits combined into one circuit with either three or four conductors

True

What is the minimum number of coils in an ac generator required for producing a three-phase supply?

Three

What is the effect on output voltage of an alternator if the magnetic field strength is increased?

Egen increases

An alternator is constructed so that all three windings have a common connection to one point. This is known as a _______ connection

wye

What happens when a coil cuts the magnetic lines of force?

A voltage is induced in the coil cuts the magnetic lines of forces

As the turns of a coil rotate in an alternator, when is the induced voltage maximum?

The induced voltage is maximum when the turns of the coil are at 90 degrees to the magnetic lines of force

As the turns of a coil rotate in an alternator, when is the induced voltage zero?

The induced voltage is zero when the turns of the coil are moving parallel with the magnetic lines of force

Why is the phase sequence CAB the same as the ABC phase sequence?

The Terminals 1, 2, and 3 still see an ABC sequence and a counterclockwise rotation

The BAC phase sequence is the same as the ABC phase sequence

False

In what two ways can the phase sequence of voltages at a piece of equipment be changed?

The phase sequence can be changed by either reversing the rotation of the alternator supplying the equipment or by interchanging any two line conductors to the equipment

List three methods of observing a phase sequence

1. an oscilloscope

2. a phase sequence indicator

3. a phase rotation meter

List 3 advantages of three-phase systems over single-phase systems

1. constant power

2. smaller equipment for equivalent capacity

3. better efficiency

In which of the following systems would you expect less vibration?

A three-phase system

In which of the following systems would approximately 25% more copper be used for a feeder for the same capacity?

A single-phase system

When does the voltage induced in an alternator coil reach its maximum

When the coil is moving at 90 degrees to the magnetic lines of force

How many mechanical degrees apart are the coils in the most basic three-phase alternator?

120 degrees

How many electrical degrees apart are the voltages produced by three-phase ac generator?

120 degrees

Which of the following is considered a positive phase rotation?

BCA

How should the phase rotation of equipment be confirmed before energizing?

by using a phase rotation meter

What equipment is required to run a three-phase lathe on a single-phase farm service?

phase converter

State the mathematical relationship between phase and line voltage in a wye connection

VL = VP x 1.732 and VL leads VP by 30 degrees

Calculate the line voltage in a wye-connected load when the phase voltage is 347 V

600 V

Calculate the phase voltage when three light bulbs are connected in wye and the supply voltage is three-phase, four-wire, 415 V

239.6 V

What is the angle measurement between phase and line voltage in a wye-connected load?

30 degrees

What is the angle measurement between the phase voltages in a wye-connected load?

120 degrees

What is the relationship between line current and phase current in a wye-connected system?

The phase and line currents in a wye-connected circuit are equal

What is the angle measurement between the line current and line voltage in a wye connection at unity power factor?

At unity power factor the angle between line current and line voltage is 30 degrees

In a wye-connected load, the phase voltage is 200 V and the resistance of each phase is 20 ohms. Calculate the line current.

10 A

If the line voltage of a three-phase wye-connected load is 208 V and the resistance of each phase is 5 ohms. Calculate the phase current.

24 A

What are the two main purposes of a neutral conductor in a three-phase, four-wire wye connected system

1. To maintain phase voltages

2. To carry unbalanced current