Free Electrical Machines MCQs with Answers

• A. Where lubrication is difficult or im-possible
• B. Where corrosive chemical action exists
• C. Where high temperature exists
• D. All of the above

Explanation: Carbon bearings are suitable where oil or grease lubrication is impractical, and they withstand corrosive chemicals and relatively high…

• A. Speed of the machine
• B. Contact pressure
• C. Surface conditions of brush and commutator
• D. Excessive sparking
• E. All of the above

Explanation: Brush wear depends on speed, contact pressure, brush and commutator surface condition, and sparking.

• A. copper
• B. kanthal
• C. manganin
• D. nichrome

Explanation: Starter coils require relatively high resistance to limit current, so manganin wire is used because it has high resistivity and a low…

• A. To avoid initial rush of current
• B. To avoid change in kW rating with temperature
• C. Both (A) and (B)
• D. Either (A) or (B)

Explanation: A low temperature coefficient keeps the element resistance nearly constant as it heats, reducing the cold-start current surge and…

• A. zero
• B. 0.707 lagging
• C. unity
• D. 0.707 leading

Explanation: Arc furnaces behave largely as inductive loads and commonly operate at a lagging power factor near 0.7.

• A. it is filled below core level
• B. it is filled above core level
• C. it is fully empty
• D. none of the above

Explanation: The Ajax-Wyatt furnace is a core-type induction furnace, so the molten or conducting charge must surround the core sufficiently to form…

• A. Variation of resistance
• B. Variation of voltage
• C. Periodical switching on and off of the supply
• D. All of the above methods

Explanation: Furnace temperature can be regulated by changing resistance, varying the applied voltage, or switching the supply periodically.

• A. to stabilize the arc
• B. to imprpve, power factor
• C. to reduce severity of the surge
• D. none of the above

Explanation: The choke adds inductive reactance and limits rapid current changes, helping maintain a more stable arc.

• A. induction heating of steel
• B. dielectric heating
• C. induction heating of brass
• D. resistance heating

Explanation: Induction heating of steel produces heat through both eddy-current loss and hysteresis loss because steel is electrically conducting and…

• A. bad, good
• B. bad, bad
• C. good, bad
• D. good, good

Explanation: Mica transfers heat from the heating element to the press surface while electrically insulating the user and metal parts.

• A. induced current flow through the charge
• B. hysteresis loss taking place below curie temperature
• C. due to hysteresis loss as well as eddy current loss takingplace in the charge
• D. none of the above factors

Explanation: In ferromagnetic materials below the Curie temperature, the alternating magnetic field causes both induced eddy currents and repeated…

• A. Unity
• B. Low, lagging
• C. Low, leading
• D. Any of the above

Explanation: The furnace transformer and arc circuit have considerable inductive reactance, so the current lags the voltage and the power factor is low…

• A. The amount of heat generated can be controlled accurately
• B. Heat at very high rate can be gene-rated
• C. The area of the surface over which heat is produced can be accurately controlled
• D. All of the above

Explanation: Eddy-current heating permits rapid heating, precise control of generated heat, and concentration of heat over a selected surface region.

• A. Annealing of metals
• B. Melting of ferrous metals
• C. Heating of liquids in electric kettle
• D. Drying of paints and varnishes

Explanation: Radiant heating transfers heat mainly through infrared radiation, making it especially suitable for surface processes such as drying…

• A. copper alloy
• B. carbon
• C. tungsten alloy
• D. stainless steel alloy

Explanation: Tungsten-alloy elements can withstand temperatures around 2250°C because tungsten has a very high melting point and retains strength at…

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

Explanation: Graphite is used for direct-arc furnace electrodes because it conducts current and withstands the extremely high arc temperature.

• A. Current
• B. Voltage
• C. Power factor
• D. All of the above

Explanation: A direct-arc furnace operates with very high current and comparatively low voltage to produce the arc and intense heat.

• A. To achieve better heating
• B. To increase the life of roof refractory
• C. To have better stirring action
• D. To reduce problem of oxidation
• E. All of the above

Explanation: A short arc concentrates heat near the charge, reduces radiation toward the roof, limits refractory damage and oxidation, and supports…

• A. as an insulator
• B. as a device for power factor improvement
• C. for dielectric heating
• D. for induction heating

Explanation: Mica has high dielectric strength and good heat resistance, so it is used as an electrical insulator in presses and heating equipment.

• A. Coreless transformers
• B. Current transformers
• C. Motor-generator set
• D. Multi-phase transformer

Explanation: Older induction furnaces obtain the required high-frequency supply from a motor-generator set, whose generator is designed for the…