All Free Electrical Engineering MCQs with Answers

• A. one armature and one field
• B. two armatures and one field
• C. one armature and two fields
• D. none of the above

Explanation: A rotary converter uses one armature carrying both the AC and DC windings, with one field system producing the magnetic flux.

• A. a transformer
• B. an induction motor
• C. an alternator
• D. any D.C. machine

Explanation: Its rotating armature, field system and commutator make a rotary converter structurally similar to a DC machine.

• A. low power factor
• B. high power factor
• C. zero power factor
• D. none of the above

Explanation: A rotary converter normally operates with a high power factor, especially when its excitation is properly adjusted.

• A. d.c. only
• B. a.c. only
• C. partly a.c. and partly d.c.

Explanation: The armature conductors carry alternating currents in the armature winding, while the commutator makes the external DC current…

• A. Synchronous motor
• B. Squirrel cage induction motor
• C. Slip ring induction motor
• D. Any of the above

Explanation: A synchronous motor is commonly used to drive large motor-generator sets because it operates at constant speed and can provide high…

• A. same
• B. less
• C. double
• D. three times

Explanation: A rotary converter performs conversion with fewer armature windings and generally has lower copper resistance than a DC generator of…

• A. Yes
• B. No

Explanation: The force on a current-carrying conductor is proportional to the cross product of field and current, F = BIl.

• A. more wire can be used with less D.C. resistance in coil
• B. the laminations are insulated from each other
• C. the magnetic flux is concentrated in the air gap of the core
• D. the laminations are stacked vertfcally

Explanation: Insulating the thin laminations prevents large circulating eddy-current paths between adjacent sheets, thereby reducing eddy-current loss.

• A. hysteresis losses
• B. eddy current losses
• C. both (A) and (B)
• D. none of the above

Explanation: An air-core coil has no ferromagnetic material, so it has practically no hysteresis loss; its nonmagnetic core also avoids the usual core…

• A. high, high
• B. low, high
• C. high, low
• D. low, low

Explanation: Armature and transformer cores need high permeability so flux is established with a small magnetising force, and low hysteresis loss to…

• A. low hysteresis loss
• B. high permeability
• C. low retentivity
• D. high retentivity

Explanation: Good magnetic memory means the material retains its magnetisation after the magnetising force is removed, so high retentivity is required.

• A. zero
• B. small
• C. large
• D. none of the above

Explanation: The area of a hysteresis loop represents the energy lost per cycle per unit volume of the material.

• A. alnico alloys
• B. aluminium
• C. cast iron
• D. wrought iron

Explanation: Alnico alloys have high retentivity and coercivity, allowing them to maintain strong magnetisation and resist demagnetisation.

• A. lowers their power efficiency
• B. increases their cost of manufacture
• C. leads to their increased weight
• D. produces fringing

Explanation: Leakage flux does not directly dissipate power, although it reduces useful flux and can worsen machine performance.

• A. increase m.m.f.
• B. increase the flux
• C. prevent saturation
• D. none of the above

Explanation: An air gap adds reluctance to the magnetic circuit, which limits excessive flux and helps prevent core saturation.

• A. volume of material
• B. frequency
• C. steinmetz coefficient of material
• D. ambient temperature

Explanation: Hysteresis loss follows the Steinmetz relation, approximately proportional to volume, frequency, and the material coefficient.

• A. low coercivity
• B. low retentivity
• C. low hysteresis loss
• D. high coercivity

Explanation: Silicon steel has a narrow hysteresis loop, resulting in low hysteresis loss when subjected to repeated magnetisation.

• A. ferrites have high resistance
• B. ferrites are magnetic
• C. ferrites have low permeability
• D. ferrites have high hysteresis

Explanation: Ferrites have much higher electrical resistivity than ordinary iron, so circulating eddy currents and the resulting loss are greatly…

• A. copper loss
• B. eddy current loss
• C. hysteresis loss
• D. all of the above

Explanation: Laminating the core breaks up the paths available for circulating eddy currents and increases their effective resistance.

• A. low, high
• B. high, high
• C. high, low
• D. low, low

Explanation: A permanent magnet must retain strong magnetism after the magnetizing field is removed, requiring high retentivity, and must resist…