Free Electrical Machines MCQs with Answers

• 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…

• A. 1
• B. 1 H/m
• C. 1/4JI
• D. 4n x 10-' H/m

Explanation: Relative permeability is defined as μr = μ/μ0, so vacuum has μ = μ0 and therefore μr = 1.

• A. Mica capacitor
• B. Paper capacitor
• C. Electrolytic capacitor
• D. Ceramic capacitor

Explanation: Single-phase motors commonly use an electrolytic capacitor for the starting circuit because it provides a large capacitance in a compact…

• A. dilute sulphuric acid
• B. concentrated sulphuric acid
• C. NaOH
• D. KOH

Explanation: Alkaline cells use an alkaline hydroxide electrolyte, most commonly potassium hydroxide, KOH.

• A. minimum efficiency
• B. minimum current capacity
• C. low internal resistance
• D. high internal resistance

Explanation: A constant-voltage generator must maintain nearly constant terminal voltage as load current changes.

• A. Temperature of surroundings
• B. Specific gravity of electrolyte
• C. Rate of discharge
• D. All of the above

Explanation: Battery capacity depends on temperature, electrolyte specific gravity, and discharge rate.

• A. charged ions
• B. chemical energy
• C. electrostatic energy
• D. electromagnetic energy

Explanation: A lead-acid battery converts electrical energy into chemical energy during charging and releases it as electrical energy during discharge.