AC 3 Phase

What is the hazard to consider when working on parallel resonant a.c. circuits?

• High circulating currents flowing between inductive and capacitive components

• High voltages that may be present across inductive and capacitive components

• High let-through energy associated with inductive and capacitive components

• High power dissipation from inductive and capacitive components

Which device can show harmonic waveforms and indicate the harmonic order?

• A phase sequence indicator

• A power analyser

• A true RMS multimeter

• An RCD tester

How are three phases generated in an alternator?

• Three phase windings electrically displaced by 120° and rotating within a magnetic field

• One phase winding rotating within three magnetic fields electrically displaced by 120°

• Three phase windings mechanically displaced by 120° and rotating within a magnetic field

• One phase winding rotating within three magnetic fields mechanically displaced by 120°

What is the phase relationship between the line and phase voltages of a star connected three-phase system?

• 0°

• 30°

• 90°

• 120°

Calculate the phase voltage for a three-phase star connected load drawing 10A from a 400V supply.

• 400 V

• 231 V

• 10 V

• 100 V

Calculate the line current for a three-phase star connected load drawing 10A from a 400V supply.

• 400 A

• 231 A

• 10 A

• 100 A

Calculate the line voltage for a three-phase delta connected load drawing 10A from a 400V supply.

• 400 V

• 230 V

• 690 V

• 115 V

Calculate the line current for a three-phase delta connected load drawing 10A from a 400V supply.

• 5.8 A

• 10 A

• 15 A

• 20 A

Which three (3) of the following statements are true of ‘open delta’ connected transformers?

• They are a cheaper alternative to a standard three-phase transformer

• They are more common and easier to maintain

• They can be used if one phase of a three-phase transformer goes off-line

• They supply only 57.7% of an equivalent three-phase transformer

• They take up less room

Which of the following conditions must be met for a single-phase wattmeter to measure total power of a three-phase circuit?

• a) The load is balanced

• b) The load is unbalanced

• c) The neutral is disconnected

• d) The voltage coil is connected between phases