Free Power Electronics MCQs with Answers

• A. They are mechanically strong
• B. They do not require any voltage for filament heating
• C. Both (A) and (B)
• D. None of the above

Explanation: Metal rectifiers are rugged because they have no fragile vacuum envelope, and they require no filament-heating supply.

• A. 10°C
• B. 20°C
• C. 30°C
• D. 45°C

Explanation: Copper oxide rectifiers are strongly temperature-sensitive and are generally designed for operation up to about 45°C.

• A. 5 to 10 per cent
• B. 15 to 20 per cent
• C. 25 to 30 per cent
• D. none of the above

Explanation: Ageing in selenium rectifiers gradually changes the selenium junction characteristics, commonly producing about a 5 to 10 percent change…

• A. 90 to 95%
• B. 85 to 90%
• C. 80 to 85%
• D. 65 to 75%

Explanation: Copper oxide rectifiers have relatively high forward voltage loss and limited current capability, so their efficiency seldom exceeds…

• A. 25°C
• B. 40°C
• C. 60°C
• D. 75°C

Explanation: Selenium rectifiers can withstand considerably higher temperatures than copper oxide rectifiers, with operation possible up to…

• A. 10 V
• B. 30 V
• C. 60 V
• D. 100 V
• E. 200 V

Explanation: Selenium rectifiers were mainly used for low-voltage power supplies, battery charging and similar applications, generally up to about 100…

• A. 25, 35
• B. 40, 50
• C. 60, 70
• D. 75, 85

Explanation: Selenium rectifiers typically achieve efficiencies in the approximate range of 75 to 85 percent.

• A. an induction motor
• B. a synchronous motor
• C. a D.C. series motor
• D. a D.C. shunt motor

Explanation: A commutating rectifier uses a mechanically driven commutator to perform rectification, and a synchronous motor provides the required…

• A. copper
• B. aluminium
• C. silver
• D. graphite
• E. tungsten

Explanation: In a mercury-arc rectifier, the pool of mercury acts as the cathode, while graphite electrodes are commonly used as the anodes.

• A. Voltage drop in arc
• B. Voltage drop at the anode
• C. Voltage drop at the cathode
• D. All of the above

Explanation: A mercury arc rectifier has voltage losses in the arc as well as at the anode and cathode.

• A. Mechanical rectifiers
• B. Copper oxide rectifiers
• C. Selenium rectifiers
• D. Electrolytic rectifiers
• E. Mercury arc rectifiers

Explanation: Electrolytic rectifiers were mainly used for low-voltage, relatively high-current applications such as charging batteries from an AC…

• A. 80%
• B. 70%
• C. 60%
• D. 40%

Explanation: Electrolytic rectifiers have comparatively large electrode and electrolyte losses, giving a typical efficiency of about 60%.

• A. graphite
• B. boron carbide
• C. aluminium
• D. copper

Explanation: The igniting or auxiliary anode is made of boron carbide because it withstands the mercury-arc operating conditions and does not readily…

• A. fuel cell
• B. batteries
• C. thermionic converter
• D. all of the above

Explanation: A thermionic converter directly changes heat into electrical energy through thermionic emission of electrons from a heated emitter.

• A. Identical to line voltage in a VSI
• B. Identical to line current in VSI
• C. Identical to phase voltage in VSI
• D. Identical to phase voltage in CSI

Explanation: Under the usual voltage-current duality, the CSI output line-current waveform corresponds to the VSI output line-voltage waveform when the…

• A. Dependent on the load current
• B. Dependent on the voltage
• C. Dependent on both the load current and load voltage behind the short circuit current
• D. Independent on both the load current and load voltage

Explanation: Commutation overlap is caused by source or leakage inductance, so its duration increases with load current and decreases as the…

• A. silicon has large leakage current than germanium
• B. silicon has small leakage current than germanium
• C. silicon has small leakage voltage than germanium
• D. silicon has large leakage voltage than germanium

Explanation: Silicon has much lower reverse leakage current than germanium, especially at elevated temperatures, so it gives SCRs better blocking…

• A. two SCR's in parallel opposition
• B. two SCR's in series
• C. three SCR's in series
• D. four SCR's in series

Explanation: AC control must conduct during both positive and negative half-cycles, so two SCRs are connected in antiparallel, also called parallel…

• A. Duty ratio only
• B. Frequency only
• C. Duty ratio and frequency
• D. Duty ratio, frequency and time delay

Explanation: A semiconductor switching pattern is specified by how long it conducts relative to a cycle, how often it repeats, and any timing or phase…

• A. Frequencies above 20 kHz
• B. Frequencies above 1000 kHz
• C. Frequencies less than 10 Hz
• D. 50 Hz frequency

Explanation: In conventional power-system applications, power electronics controls and converts AC power at the supply frequency, commonly 50 Hz, using…