Question 1

Q.1. Write the factors for consideration in electrical machine design.

Accepted Answer

Electrical machine design must consider specific magnetic and electric loadings, permissible temperature rise, mechanical strength, efficiency, and material cost.

Question 2

Q.2. What are the various cooling methods used for oil type transformers?

Accepted Answer

Oil-type transformers use cooling methods including ONAN, ONAF, OFAF, and OFWF, representing increasing levels of forced circulation of oil and/or air/water for progressively higher cooling capacity.

Question 3

Q.3. Define short circuit current of induction motor.

Accepted Answer

Short circuit current of an induction motor is the very high transient current drawn from the supply at the instant of a sudden short circuit (or equivalently, at starting/locked-rotor condition), limited primarily by the motor's leakage reactance.

Question 4

Q.4. Explain the use of damper winding in synchronous machine.

Accepted Answer

Damper winding in a synchronous machine damps hunting oscillations, provides starting torque for synchronous motors, and reduces negative-sequence effects from unbalanced loading.

Question 5

Q.5. What are the various types of commonly used conducting materials for ele…

Accepted Answer

The commonly used conducting materials for electrical machines are copper (most common, for windings) and aluminum (lighter, cheaper alternative, used especially for cage rotor bars and some windings).

Question 6

Q.6. Define the short circuit ratio of synchronous machine and no-load curren…

Accepted Answer

SCR of a synchronous machine is the ratio of field current for rated open-circuit voltage to field current for rated short-circuit current; no-load current of a transformer is the small current drawn on open circuit, needed to establish the core flux and supply core loss.

Question 7

Q.7. What is the difference between brushless DC motor and switched reluctanc…

Accepted Answer

A brushless DC motor uses permanent magnets on the rotor with electronically commutated stator windings (no brushes/commutator), while a switched reluctance motor has no rotor magnets/windings at all, relying purely on reluctance torque from a salient, ferromagnetic rotor.

Question 8

Q.8. What are the advantages and disadvantages of synthesis method of compute…

Accepted Answer

The synthesis CAD method's advantages are speed and directness (no trial design needed); its disadvantages are limited flexibility for novel designs and no built-in optimization for cost/performance beyond meeting basic specifications.

Question 9

Q.9. Define voltage regulation of a transformer and explain in brief temperat…

Accepted Answer

Voltage regulation is the percentage change in terminal voltage from no-load to full-load (at constant primary voltage); temperature rise in transformers refers to the increase in winding/oil temperature above ambient due to load losses, which must stay within insulation class limits.

Question 10

Q.10. Which type of rotor is generally used for high speed alternators? Defin…

Accepted Answer

Cylindrical (non-salient, round) rotor construction is used for high-speed alternators; heat flow is the transfer of thermal energy from a region of higher temperature to one of lower temperature, driven by a temperature gradient.

Question 11

Q.1. Determine the dimensions of the core and yoke for a 100 kVA, 50 Hz, 1-φ,…

Accepted Answer

For the 100 kVA, 50Hz, single-phase, square-core transformer with Et=14V, Bm=1.1Wb/m², the net iron area works out to about 573 cm², giving a square core side of about 252mm and an inter-limb spacing of about 404mm.

Question 12

Q.2. Derive the kVA output equation of a three-phase transformer. Also define…

Accepted Answer

The 3-phase transformer kVA output equation Q = 3.33 f Bm δ Kw Ai Aw ×10³ is derived from the EMF equation and current-density-based conductor sizing for each of the three phases; window space factor Kw is the ratio of net copper conductor area to total window area.

Question 13

Q.3. Explain the synthesis method of computer aided design with neat flow-cha…

Accepted Answer

The synthesis CAD method generates a design via a fixed sequence of formula-based steps directly from specifications (Input → design constants → main dimensions → magnetic circuit → windings → performance → output), offering speed but limited flexibility compared to iterative/optimization approaches.

Question 14

Q.4. Write short notes on:

Accepted Answer

Permanent magnet synchronous machines use rotor-mounted permanent magnets instead of a DC field winding, eliminating field copper loss and slip rings; the analysis method of CAD evaluates and iteratively refines an existing trial design's performance against specifications, offering flexibility and design insight at the cost of requiring more iterations than synthesis.

Question 15

Q.5. A 3-φ induction motor has 54 slots with 8 conductors per slot and 72 rot…

Accepted Answer

Using the standard turns-per-phase and voltage-ratio relationships, this star-star connected 3-phase induction motor has 72 stator turns/phase and 48 rotor turns/phase, giving a rotor open-circuit slip-ring (line) voltage of approximately 267V for the given 400V stator supply.

Question 16

Q.6. Define space factor and rating of a machines. Explain the choice of spec…

Accepted Answer

Space factor (Kw) is the ratio of copper to window area; machine rating is its specified continuous/duty-cycle output capacity; choice of specific magnetic loading balances machine size against iron loss/magnetizing current, influenced by material, cooling, machine type, and frequency.

Question 17

Q.7. Explain the steps for the design of wound rotor of an induction motor.

Accepted Answer

Wound rotor design involves selecting the rotor slot number (avoiding cogging/crawling combinations with the stator), determining rotor turns from the target slip-ring voltage, sizing conductors from rotor current and current density, and designing the slip rings/brush gear.

Question 18

Q.1. A 90 kW, 500V, 50 Hz, three phase, 8-pole induction motor has a star-con…

Accepted Answer

For the 90kW, 500V, 8-pole, 63-slot stator (6 cond/slot) motor targeting 400V open-circuit rotor voltage, the design gives stator turns/phase=63, a turns ratio of about 1.25, requiring roughly 50 rotor turns/phase and about 48-54 rotor slots (chosen to avoid cogging/crawling), with an approximate full-load rotor phase current of about 168A.

Question 19

Q.2. Explain the procedure to draw circle diagram of an induction motor. What…

Accepted Answer

The circle diagram is constructed from no-load and blocked-rotor test data as a graphical locus of the stator current phasor at constant voltage and varying slip; it directly yields output power, torque, power factor, efficiency, and maximum torque/power for any operating point without further calculation.

Question 20

Q.3. Explain about the shape of pole face of a synchronous machine. Also expl…

Accepted Answer

Synchronous machine pole faces are shaped (chamfered/non-uniform air gap) to produce a near-sinusoidal air-gap flux distribution despite the concentrated pole structure; turbo-alternator design uses a cylindrical rotor with distributed field slots rather than salient poles, and damper windings are embedded as short-circuited bars in the pole face/rotor surface slots.

Question 21

Q.4. What are the various steps of determination of main dimensions for core,…

Accepted Answer

Main dimension determination for core/yoke/window and the LV/HV winding design procedure for a transformer follow the same standard sequence detailed for the equivalent question in the 2023 paper: net iron area from EMF/turn, core-shape-specific gross dimensions, yoke sizing relative to limb flux density, and window sizing from conductor areas and space factor, with LV wound as inner cylindrical layers and HV as outer disc/cross-over windings.

Question 22

Q.5. Determine the main dimensions of a 12MVA, 13.8 kV, 50Hz, 1500 rpm, three…

Accepted Answer

For the 12MVA, 13.8kV, 1500rpm, star-connected alternator with Bav=0.6T, ac=42000 A/m, peripheral speed=80m/s, the design gives a stator bore diameter of about 1.02m, a core length of about 1.75m (using a standard output coefficient), with corresponding maximum flux, stator slot count, and turns per phase determined from the resulting dimensions.

Electrical Machine Design