Three Phase Systems In Power Applications MCQ

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• In the transformer the function of a conservator is to
  ক

  Provide fresh air for cooling the transformer

  খ

  Supply cooling oil to transformer in time of need

  গ

  Protect the transformer from damage when oil expends due to heating

  ঘ

  None of the above
• 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?
  ক

  I_θb = 12 ∠150° A, I_θc = 12 ∠-90° A

  খ

  I_θb = 12 ∠120° A, I_θc = 12 ∠30° A

  গ

  I_θb = 12 ∠30° A, I_θc = 12 ∠120° A

  ঘ

  I_θb = 12∠ 90° A, I_θc = 12 ∠90° A
• A simple three-phase generator consists of three conductive loops separated by 120°.
  ক

  True

  খ

  False
• The term 'squirrel-cage' applies to a type of three-phase ac generator.
  ক

  True

  খ

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

  13.3 mA

  খ

  133 mA

  গ

  1.33 A

  ঘ

  6.2 A
• A constant load power means a uniform conversion of
  ক

  Mechanical to electrical energy

  খ

  Electrical to mechanical energy

  গ

  Current to voltage

  ঘ

  Voltage to current
• A balanced load is one in which all the impedances are equal.
  ক

  True

  খ

  False
• In a balanced three-phase load, each phase has
  ক

  An equal amount of power

  খ

  One-third of total power

  গ

  Two-thirds of total power

  ঘ

  A power consumption equal to $$\sqrt {3{{\text{V}}_{\text{L}}}} {I_{\text{L}}}$$
• A simple two-phase generator consists of two conductive loops separated by 90°, rotating in a magnetic field.
  ক

  True

  খ

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

  2,000 V

  খ

  6,000 V

  গ

  666 V

  ঘ

  3,464 V
• Polyphase generators produce simultaneous multiple sinusoidal voltages that are separated by
  ক

  Certain constant phase angles

  খ

  Certain constant frequencies

  গ

  Certain constant voltages

  ঘ

  Certain constant currents
• In a three-phase system, the voltages are separated by
  ক

  45°

  খ

  90°

  গ

  120°

  ঘ

  180°
• In a ∆-connected source feeding a Y-connected load
  ক

  Each phase voltage equals the difference of the corresponding load voltages

  খ

  Each phase voltage equals the corresponding load voltage

  গ

  Each phase voltage is one-third the corresponding load voltage

  ঘ

  Each phase voltage is 60° Ωut of phase with the corresponding load voltage
• Polyphase is characterized by two or more sinusoidal voltages, each having a different phase angle.
  ক

  True

  খ

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

  0 V

  খ

  90 V

  গ

  156 V

  ঘ

  180 V
• 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
  ক

  3 A

  খ

  6 A

  গ

  9 A

  ঘ

  27 A
• In a Y-connected circuit, the magnitude of each line current is
  ক

  One-third the phase current

  খ

  Three times the corresponding phase current

  গ

  Equal to the corresponding phase current

  ঘ

  Zero
• In a ∆-connected generator, all of the phase voltages are
  ক

  Zero

  খ

  Equal in magnitude

  গ

  One-third of total

  ঘ

  One-sixth of total
• The most common type of ac motor is the
  ক

  Single-phase induction motor

  খ

  Two-phase induction motor

  গ

  Three-phase induction motor

  ঘ

  Two-phase squirrel-cage motor
• In a three-phase system, when the loads are perfectly balanced, the neutral current is
  ক

  Zero

  খ

  One-third of maximum

  গ

  Two-thirds of maximum

  ঘ

  At maximum
• In a Y-Y source/load configuration, the
  ক

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

  খ

  Phase current, the line current and the load current are 120° out of phase

  গ

  Phase current and the line current are in phase and both are 120° out of phase with the load current

  ঘ

  Line current and the load current are in phase and both are out of phase with the phase current
• In a ∆-connected source driving a ∆-connected load, the
  ক

  Load voltage and line voltage are one-third the source voltage for a given phase

  খ

  Load voltage and line voltage are two-thirds the source voltage for a given phase

  গ

  Load voltage and line voltage cancel for a given phase

  ঘ

  Load voltage, line voltage, and source phase voltage are all equal for a given phase
• 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?
  ক

  0 A

  খ

  1.33 A

  গ

  3.99 A

  ঘ

  2.66 A
• 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?
  ক

  1.33 A

  খ

  1.88 A

  গ

  2.66 A

  ঘ

  1.77 A
• Power is measured in a three-phase load by using an ammeter.
  ক

  True

  খ

  False
• In a Y-connected generator, there is a 120° difference between each line voltage and the nearest phase voltage.
  ক

  True

  খ

  False
• In a ∆-connected generator, there is a 120° difference between each line current and the nearest phase current.
  ক

  True

  খ

  False
• An alternator is an electromechanical ac generator.
  ক

  True

  খ

  False
• 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.
  ক

  Single-phase 32 A; three-phase 16 A

  খ

  Single-phase 16 A; three-phase 8 A

  গ

  Single-phase 8 A; three-phase 4 A

  ঘ

  Single-phase 16 A; three-phase 0 A
• In a Y-connected circuit, between each line voltage and the nearest phase voltage, there is a phase angle of
  ক

  0°

  খ

  30°

  গ

  60°

  ঘ

  120°