Free Power Electronics MCQs with Answers

• A. Thermal ion release principle
• B. Nucleate heating principle
• C. Resistance heating principle
• D. Electro-magnetic induction princi¬ple

Explanation: An alternating magnetic field induces eddy currents in the workpiece, and the resulting electrical losses produce heat.

• A. nickel copper
• B. nichrome
• C. silicon carbide
• D. silver

Explanation: Silicon carbide heating elements can operate at much higher temperatures than metallic elements such as nichrome or nickel-copper.

• A. reduce the length of heating element
• B. increase the life of the heating element
• C. reduce the effect of oxidation
• D. produce large amount of heat

Explanation: For a required resistance, R = ρL/A, so higher resistivity allows the element to achieve that resistance with a shorter length or larger…

• A. radiation
• B. convection
• C. conduction
• D. any of the above

Explanation: In indirect resistance heating, the element heats the charge without direct electrical contact, and radiation is usually the dominant…

• A. white body
• B. grey body
• C. black body
• D. transparent body

Explanation: A white body reflects essentially all incident radiation, whereas a black body absorbs it and a transparent body transmits it.

• A. Eddy current heating
• B. Arc heating
• C. Induction heating
• D. Dielectric heating

Explanation: Dielectric heating uses a high-frequency alternating electric field to produce molecular agitation and heat inside insulating materials.

• A. Arc heating
• B. Dielectric heating
• C. Induction heating
• D. Resistance heating

Explanation: Resistance heating has a purely resistive load, so its power factor is unity, the maximum possible value.

• A. Quicker operation
• B. Higher efficiency
• C. Absence of flue gases
• D. All of the above

Explanation: Electric heating provides rapid and efficient heat transfer and produces no combustion flue gases at the point of use.

• A. Resistance heating
• B. Dielectric heating
• C. Arc heating
• D. Induction heating

Explanation: Dielectric heating uses an insulating load that behaves predominantly capacitively, so its current leads the applied voltage.

• A. Induction heating
• B. Dielectric heating
• C. Electric resistance heating
• D. All of the above

Explanation: Resistance heating depends mainly on the value of resistance and applied voltage, not on supply frequency.

• A. Rectifiers
• B. Motor converters
• C. Motor-generator sets
• D. Rotary converters
• E. All of the above

Explanation: AC can be changed to DC using static rectifiers or electromechanical systems such as motor converters, motor-generator sets, and rotary…

• A. Load
• B. Aluminium
• C. Copper
• D. Zinc
• E. None of the above

Explanation: Aluminium is commonly obtained industrially by electrolytic reduction of alumina in molten cryolite, since it cannot be extracted…

• A. Motor-generator set
• B. Motor converter
• C. Rotary converter
• D. Mercury arc rectifier

Explanation: Historically, mercury-arc rectifiers were used for large-scale AC-to-DC conversion because they handled high voltage and current…

• A. Illumination
• B. Electrolysis
• C. Variable speed operation
• D. Traction

Explanation: Electrolysis requires a unidirectional flow of current so that ions consistently move toward their respective electrodes and chemical…

• A. anode
• B. cathode
• C. shell bottom
• D. mercury pool

Explanation: Positive ions are attracted to the negatively charged cathode because opposite charges attract.

• A. Telephones
• B. Relays
• C. Time switches
• D. All of the above

Explanation: Telephones use DC for their line supply, while relays and many time switches require DC to energise their coils or control mechanisms.

• A. 0.05 V to 0.2 V per cm length of the arc
• B. 0.5 V to 1.5 V per cm length of the arc
• C. 2 V to 3.5 V per cm length of the arc
• D. none of the above

Explanation: The arc voltage gradient in a mercury arc rectifier is comparatively high, typically about 2 to 3.5 V per centimetre of arc length.

• A. its ionization potential is relatively low
• B. its atomic weight is quite high
• C. its boiling point and specific heat are low
• D. it remains in liquid state at ordi¬nary temperature
• E. all of the above

Explanation: Mercury is suitable as a cathode because it is liquid at room temperature, has relatively low ionisation potential, and its heavy atoms…

• A. 100 A at 100 V
• B. 200 A at 200 V
• C. 300 A at 300 V
• D. 400 A at 400 V
• E. 500 A at 500 V

Explanation: Glass mercury-arc rectifiers were commonly manufactured for high-current, high-voltage service, with standard maximum continuous ratings…

• A. expenditure of energy in ionization
• B. surface resistance
• C. expenditure of energy in overcoming the electrostatic field
• D. expenditure of energy in liberating electrons from the mercury

Explanation: The cathode fall is the energy needed to liberate electrons from the mercury surface and establish emission at the cathode spot.