Free Electrical Machines MCQs with Answers

• A. Insulating materials
• B. Conducting materials which are magnetic
• C. Conducting materials which are non-magnetic
• D. Conducting materials which may or may not be magnetic

Explanation: Induction heating requires electrical currents induced in a conductor, so insulating materials cannot be heated directly by this method.

• A. By disconnecting some of the heating elements
• B. By varying the operating voltage
• C. By varying the current through heating elements
• D. By any of the above method

Explanation: An arc furnace’s heat input depends on its electrical power, approximately given by VI.

• A. phase angle
• B. frequency
• C. current
• D. voltage

Explanation: Induction heating uses a frequency much higher than ordinary power frequency to induce strong eddy currents and concentrate heating near…

• A. oxidizing
• B. deoxidising
• C. reducing
• D. neutral

Explanation: A conventional resistance furnace operates with air around its heating elements, giving it an oxidizing atmosphere.

• A. Water
• B. Steam
• C. Sold ice
• D. Melting ice

Explanation: Solid ice has a thermal conductivity of roughly 2.2 W/m·K, higher than water and much higher than steam.

• A. bodies are kept in vacuum
• B. bodies are immersed in water
• C. bodies are exposed to thermal radiations
• D. temperatures of the two bodies are identical

Explanation: Conduction is driven by a temperature gradient, so no net heat is conducted between bodies at identical temperatures.

• A. inside boiler furnaces
• B. during melting of ice
• C. through the surface of the insulted pipe carrying steam
• D. from refrigerator coils to freezer of a refrigerator

Explanation: Inside a boiler furnace, heat reaches the surroundings by conduction through solids, convection through moving hot gases, and radiation…

• A. temperature of the two bodies must be different
• B. both bodies must be solids
• C. both bodies must be in contact
• D. at least one of the bodies must have some source of heating

Explanation: Net heat transfer requires a temperature difference, and the bodies need not be solids or in direct contact because radiation and…

• A. transmits all incident radiations
• B. absorbs all incident radiations
• C. reflects all incident radiations
• D. absorbs, reflects and transmits all incident radiations

Explanation: A perfect black body absorbs 100% of the radiation incident on it, so its absorptivity is unity.

• A. ferromagnetic material
• B. diamagnetic material
• C. paramagnetic material
• D. conducting material

Explanation: Diamagnetic materials develop an induced magnetic moment opposite to the applied field, so they are slightly repelled.

• A. direction of magnetic field due to current carrying conductor
• B. direction of flux in a solenoid
• C. direction of force on a current carrying conductor in a magnetic field
• D. polarity of a magnetic pole

Explanation: Fleming's left-hand rule gives the direction of force or motion on a current-carrying conductor placed in a magnetic field.

• A. it corrodes easily
• B. it has high permeability
• C. it has high specific gravity
• D. it has low permeability

Explanation: Steel has comparatively low permeability, so it requires a strong magnetising force to establish flux in it.

• A. the magnetic lines of force will bend away from their usual paths in order to go away from the piece
• B. the magnetic lines of force will bend away from their usual paths in order to pass through the piece
• C. the magnetic field will not be affected
• D. the iron piece will break

Explanation: Iron has high permeability, offering an easier path for magnetic flux than the surrounding air.

• A. flux density
• B. susceptibility
• C. relative permeability
• D. none of the above

Explanation: Magnetic susceptibility is defined as the ratio of intensity of magnetisation M to magnetising field intensity H, written as χ = M/H.

• A. large area oiB-H loop
• B. high permeability and low hysteresis loss
• C. high co-ercivity and high retentivity
• D. high co-ercivity and low density

Explanation: Rapidly reversing magnetic materials should magnetise easily and waste little energy, requiring high permeability and a narrow B-H loop…

• A. Soft iron
• B. Stainless steel
• C. Hardened steel
• D. None of the above

Explanation: Soft iron has low retentivity, so its magnetism disappears nearly completely when the magnetising field is removed.

• A. radius of conductors
• B. current in one conductor
• C. product of current in two conductors
• D. distance between the conductors

Explanation: For two long parallel conductors, force per unit length is F/L = μI₁I₂/(2πd), so it varies inversely with their separation d.

• A. current, induced e.m.f. and direction of force on a conductor
• B. magnetic field, electric field and direction of force on a conductor
• C. self induction, mutual induction and direction of force on a conductor
• D. current, magnetic field and direction of force on a conductor

Explanation: Fleming’s left-hand rule relates the magnetic field, conventional current, and the force or motion of a current-carrying conductor.

• A. zero
• B. BLI
• C. B2LI
• D. BLI2

Explanation: The force on a conductor is F = BIL sin θ, where θ is the angle between the conductor current and the magnetic field.

• A. magnetoes
• B. energy meters
• C. transformers
• D. loud-speakers

Explanation: A transformer operates by electromagnetic induction and therefore does not require a permanent magnet.