Question 1

Q.1. Define heating time constant.

Accepted Answer

The heating time constant is the time required for a machine's temperature rise to reach 63.2% (1-1/e) of its final steady-state value, assuming a constant rate of heat dissipation.

Question 2

Q.2. Why hysteresis loss is less in the case of a soft magnetic material when…

Accepted Answer

Soft magnetic materials have a narrow hysteresis loop (low coercivity), so the area enclosed (representing energy dissipated per cycle) is small, unlike hard magnetic materials which have wide loops and high coercivity.

Question 3

Q.3. What percentage of silicon content is added to electrical sheet steel us…

Accepted Answer

Electrical sheet steel for large transformers typically contains about 3-4.5% silicon content.

Question 4

Q.4. What is CRGO steel?

Accepted Answer

CRGO (Cold Rolled Grain Oriented) steel is silicon steel processed so its magnetic domains are aligned along the rolling direction, giving very low core loss and high permeability specifically along that direction, making it the standard material for transformer cores.

Question 5

Q.5. The heat dissipation by air blast belongs to which of the one category s…

Accepted Answer

Heat dissipation by air blast (forced air cooling) belongs to the convection category, specifically forced convection.

Question 6

Q.6. Write the expression for a volt per turn of winding for transformers.

Accepted Answer

The volt per turn (Et) for a transformer is given by Et = 4.44 f Φm = 4.44 f Bm Ai, relating supply frequency, maximum flux, and net core area.

Question 7

Q.7. Name the different types of windings used in transformers.

Accepted Answer

The main types of transformer windings are cylindrical, helical, cross-over, and disc (continuous/interleaved) windings, chosen based on voltage/current rating and mechanical strength requirements.

Question 8

Q.8. The density of hydrogen as compared to air is (given numeral value).

Accepted Answer

The density of hydrogen compared to air is approximately 0.07 (about 7% of air's density), making hydrogen roughly 14 times lighter than air.

Question 9

Q.9. What is the maximum temperature limit for Class-B insulation?

Accepted Answer

Class-B insulation has a maximum permissible operating temperature of 130°C.

Question 10

Q.10. Write the output equation of the electrical machine giving the relation…

Accepted Answer

The output equation relates machine output Q to the square of bore diameter D, core length L, speed, and specific loadings: Q = Co D²Ln, where Co is the output coefficient depending on specific electric and magnetic loadings.

Question 11

Q.1. Discuss the major considerations and design factors in Electrical Machin…

Accepted Answer

Electrical machine design must balance electromagnetic, thermal, mechanical, and economic considerations — including flux/current densities, insulation and cooling limits, mechanical strength under fault forces, efficiency, and cost of active materials — within relevant standards.

Question 12

Q.2. Discuss the properties that good magnetic materials should possess.

Accepted Answer

Good magnetic materials for machine cores should have high permeability, low hysteresis and eddy current loss, high saturation flux density, high electrical resistivity, and adequate mechanical strength.

Question 13

Q.3. Discuss the types of duties of a machine as per IS:4722-1968.

Accepted Answer

IS:4722-1968 classifies machine duty into continuous (S1), short-time (S2), and intermittent periodic duty (S3-S8 with variations including starting, braking, and varying load), each requiring different thermal design margins.

Question 14

Q.4. A copper bar 12 mm in diameter is insulated with micanite tube which fit…

Accepted Answer

Using the cylindrical-shell heat conduction formula with r1=6mm, r2=7.5mm, thermal resistivity 8 Ω-m, length 0.2m, and ΔT=25°C, the heat loss through the micanite insulation is approximately 17.6 W.

Question 15

Q.5. Calculate the heating time constant of 10 kVA transformer during a heat …

Accepted Answer

Using the two-point exponential heating-curve method with θ(1h)=35°C and θ(2h)=47.5°C, the heating time constant works out to approximately 0.97 hours (about 58.3 minutes).

Question 16

Q.6. Enumerate specifications of a Transformer that are required prior to des…

Accepted Answer

Before designing a transformer, the required specifications include rated kVA, voltage ratio and connection (star/delta), frequency, number of phases, type of cooling, impedance/regulation limits, temperature rise limits, and applicable standards.

Question 17

Q.7. Discuss the short circuit ratio of a synchronous machine and its signifi…

Accepted Answer

Short circuit ratio (SCR) of a synchronous machine is the ratio of the field current needed for rated open-circuit voltage to the field current needed for rated short-circuit armature current; a high SCR indicates better voltage regulation, higher stability limit, but larger machine size.

Question 18

Q.1. Discuss the necessity and type of cooling in turbo-generators. Discuss t…

Accepted Answer

Turbo-generators need forced/direct cooling (air, hydrogen, or water) because of their high losses concentrated in a small rotor volume at high speed; hydrogen cooling offers superior heat transfer, lower windage loss, reduced fire risk, and longer insulation life compared to air.

Question 19

Q.2. What is a Cruciform Core? Design a cruciform core for a 125 kVA, 50 Hz, …

Accepted Answer

A cruciform core is a stepped, cross-shaped core cross-section that better fills a circular coil window than a plain rectangular core; for the given 125 kVA transformer with Bm=1.23T and Et=17V, the required net iron area works out to about 622.6 cm², giving a circumscribing circle diameter of roughly 317 mm using standard cruciform proportions.

Question 20

Q.3. Calculate the core and window area required for a 1000 kVA, 6000/400V, 5…

Accepted Answer

For the 1000 kVA, 6000/400V, 50Hz single-phase core-type transformer with Bm=1.25 Wb/m², voltage/turn=30V, and given current density/window factors, the required net core area works out to about 1081 cm² and the window area to about 833 cm².

Question 21

Q.4. Draw the flow chart for the design of the transformer.

Accepted Answer

The transformer design flow chart proceeds sequentially: input specifications → select Bm, δ, Kw → compute Et and Ai → determine core dimensions → compute turns and conductor sizes → determine window/winding dimensions → check leakage reactance, regulation, losses, temperature rise → iterate/finalize design.

Question 22

Q.5. Draw the flow chart for the design of the induction motor for its air ga…

Accepted Answer

The induction motor design flow chart proceeds: input specifications → main dimensions (D, L) from output equation → determine air gap length → design stator core, slots, and winding → design rotor core, slots, and winding (squirrel-cage or wound) → verify performance (magnetizing current, losses, temperature rise, circle diagram).

Electrical Machine Design