Free Power Electronics MCQs with Answers

• A. anode is heated
• B. tube is evacuated
• C. an auxiliary electrode is used
• D. low mercury vapour pressures are used

Explanation: An auxiliary electrode is used to initiate ionisation and establish the cathode spot in a mercury arc rectifier.

• A. it is light in weight and occupies small floor space
• B. it has high efficiency
• C. it has high overload capacity
• D. it is comparatively noiseless
• E. all of the above

Explanation: Mercury arc rectifiers combine high efficiency, strong overload capability, quiet operation, and relatively compact construction.

• A. voltage only
• B. current only
• C. voltage and current
• D. r.m.s. value of current
• E. none of the above

Explanation: Internal efficiency is mainly determined by the ratio of the useful rectified voltage to the supply voltage, so it depends primarily on…

• A. is directly proportional to load
• B. is inversely proportional to load
• C. varies exponentially with the load current
• D. is almost independent of load current

Explanation: The arc and electrode voltage drops in a mercury pool rectifier remain nearly constant over its normal operating-current range.

• A. the rectifier will not operate
• B. internal losses will be reduced
• C. both ion and electron streams will move in the same direction
• D. the rectifier will operate at reduced efficiency

Explanation: The mercury pool is designed to act as the cathode, while the carbon electrodes act as anodes.

• A. self restoring property of mercury
• B. high ionization potential
• C. energy spent in overcoming the electrostatic field
• D. high temperature inside the rectifier

Explanation: Anode drop occurs because charged particles must expend energy in overcoming the electrostatic field near the anode.

• A. anode drop and cathode drop
• B. anode drop and arc drop
• C. cathode drop and arc drop
• D. anode drop, cathode drop and arc drop

Explanation: The anode-to-cathode voltage includes the voltage drop near the anode, the cathode drop, and the drop through the arc column.

• A. 5.4 V
• B. 8.4 V
• C. 10.4 V
• D. 16.4 V

Explanation: Mercury has an ionization potential of approximately 10.4 V, commonly stated as 10.4 eV when expressed as ionization energy per electron.

• A. 2000 A
• B. 1500 A
• C. 1000 A
• D. 500 A

Explanation: Glass-bulb mercury arc rectifiers are limited by the mechanical and thermal capability of the glass enclosure and are commonly rated up to…

• A. colour of mercury
• B. ionization
• C. high temperature
• D. electron streams

Explanation: The characteristic blue glow is produced by the ionized mercury vapour in the arc.

• A. Voltage drop in transformer reactance
• B. Voltage drop in resistance of transformer and smoothing chokes
• C. Arc voltage drop
• D. All of the above

Explanation: Under load, voltage is lost in transformer reactance, transformer and smoothing-choke resistance, and the rectifier arc itself.

• A. Six-phase
• B. Three-phase
• C. Two-phase
• D. Single-phase

Explanation: A six-phase rectifier produces six voltage pulses per cycle, so the intervals between pulses are shortest and the output ripple is least.

• A. one-third of a cycle
• B. one-fourth of a cycle
• C. one-half a cycle
• D. two-third of a cycle

Explanation: In a three-phase rectifier, the three anodes share one complete cycle equally. Therefore, each anode conducts for one-third of a cycle.

• A. ions
• B. electrons
• C. ions and electrons
• D. any of the above

Explanation: The mercury arc contains positive mercury ions moving from the anode towards the cathode, while electrons move in the opposite direction.

• A. accelerates
• B. decelerates
• C. any of the above
• D. none of the above

Explanation: A grid made positive with respect to the cathode attracts electrons and increases their velocity towards the anode.

• A. The voltage regulation of the rec¬tifier should be low
• B. In the output circuit there should be no harmonics
• C. The power factor of the system should be high
• D. The rectifier supply transformer should be utilized to the best ad-vantage
• E. all of the above

Explanation: The number of phases is selected by considering ripple and harmonics, voltage regulation, power factor, and effective use of the supply…

• A. resistance
• B. capacitance
• C. leakage reactance
• D. any of the above

Explanation: During commutation, current transfer between phases is opposed mainly by the transformer’s leakage reactance.

• A. shunt motor
• B. synchronous motor
• C. induction regulator
• D. synchronous generator

Explanation: An induction regulator can vary the phase of its output voltage relative to the supply voltage, providing the required phase shift between…

• A. maximum grid voltage
• B. critical grid voltage
• C. any of the above
• D. none of the above

Explanation: The minimum grid potential at which the anode-cathode arc can be initiated is called the critical grid voltage.

• A. magnitude
• B. polarity
• C. phase
• D. any of the above
• E. none of the above

Explanation: Phase-shift control changes the firing instant relative to the AC waveform, so the effective phase of the applied grid voltage is varied.