Electrician And Wireman : Practice Questions and Answers

The correct answer is B: Voltages.

Phase sequence in a three-phase system refers to the order in which the voltages of the three phases attain their maximum positive value.

Working Principle:

• In a three-phase system, the three phases are designated as R, Y, and B (or A, B, C).
• The voltages are sinusoidal and mutually displaced by 120 degrees.
• In a positive (standard) sequence, voltages peak in the order R → Y → B.
• Reversing the sequence (e.g., R → B → Y) changes the direction of motor rotation.

Importance of Phase Sequence:

• Motor Rotation: Phase sequence directly determines the direction of rotation of three-phase motors. An incorrect sequence causes reverse rotation, potentially damaging equipment.
• Power Distribution: Correct phase sequence is essential for proper operation of all equipment connected to a three-phase supply.
• Synchronization: Critical when synchronizing multiple generators or motors in a system.

The correct answer is D: Induction.

An energy meter operates on the principle of electromagnetic induction. It measures the amount of electrical energy consumed over a specific period (in kWh) and is essential for utility billing.

How it works:

• Voltage Coil: Connected across the load; produces a magnetic field proportional to the applied voltage.
• Current Coil: Connected in series with the load; generates a magnetic field proportional to the current through the load.
• Interaction of Magnetic Fields: The interaction between these two magnetic fields induces eddy currents in an aluminum disc, creating a torque that causes the disc to rotate.
• Disc Rotation: The rotational speed of the aluminum disc is directly proportional to power consumption — higher power means faster rotation.
• Counting Mechanism: The disc rotation drives a gear mechanism that translates rotation into a numerical display of energy consumed in kilowatt-hours (kWh).

The correct answer is C: High voltage.

• A good distribution system requires a proper voltage level at the consumer terminal.
• Voltage variations at the consumer terminal must be kept as low as possible.
• Changes in voltage are generally caused by variation of load on the system.
• High voltage causes lamps to burn out permanently and may cause failure of other appliances.
• The permissible voltage variation is ±5% of the rated voltage.