DET2033 - THREE PHASE (PART 1)

Choose the advantages of a three phase system over a single phase system:

i. In a balanced three phase system, the conductors need only 75% of the size of conductors for a single phase two wire system of the same power (KVA) rating

ii. The efficiency and power factor of a single phase motors are much better than three phase motor for the same power transferred

iii. Three phase motor have the ability to 'self start', but not in single phase

iv. Three phase transformer are lighter, cheaper and more efficient compared to a single phase transformer of the same size.

• i and ii

• i, ii and iv

• ii, iii and iv

• i, ii, iii and iv

In three phase system, the windings are displaced at __________degree from each other.

• 90

• 120

• 180

• 360

Which of the statements below describe the advantage of three phase system compared to a single phase system?

• The three phase system requires conductor with smaller cross sectional area for the same amount of power transmitted in single phase system.

• The single phase system requires conductor with smaller cross sectional area for the same amount of power transmitted in three phase system.

• The three phase motor is generally big and has self starting properties

• The three phase motor is not too robust

A polyphase system is generated by______?

• Having two or more generator windings separated by equal electrical angle.

• Having generator windings at equal distances

• None of the above

• A and C

Which is correct for single phase supply comparison to three phase supply?

• Low cost of Machinery in single phase

• Low Reliability

• Required less amount of conductor material

• Better power factor and efficiency

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

• 45 degrees

• 90 degrees

• 120 degrees

• 180 degrees

Polyphase generators produce simultaneous multiple sinusoidal voltages that are separated by

• Certain constant phase angles

• Certain constant frequencies

• Certain constant voltages

• Certain constant currents

A simple three-phase generator consists of three conductive loops separated by 120 degrees

• True

• False

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

• Zero

• One-third of maximum

• Two-thirds of maximum

• At maximum

Select the CORRECT explanation for 'three phase supply'.

• Three phase supply is generated when three coils are placed 180 degrees apart and the whole coil is rotated in a uniform magnetic field

• Three phase supply is generated when three coils are placed 60 degrees apart and the whole coil is rotated in a uniform magnetic field

• Three phase supply is generated when two coils are placed 120 degrees apart and the whole coil is rotated in a uniform magnetic field

• Three phase supply is generated when three coils are placed 120 degrees apart and the whole coil is rotated in a uniform electromagnetic field