Question 1

Q.1. What are the factors that affect the size of rotating machines?

Accepted Answer

The size of rotating machines is affected by output rating, speed, specific magnetic and electric loadings, permissible temperature rise/cooling method, and the choice of materials.

Question 2

Q.2. What do you understand by cogging?

Accepted Answer

Cogging is a phenomenon where an induction motor refuses to start (or starts with great difficulty) due to magnetic locking between stator and rotor teeth when the number of stator and rotor slots are equal or in certain unfavorable ratios.

Question 3

Q.3. How iron loss can be reduced in transformers?

Accepted Answer

Iron loss in transformers is reduced by using high-resistivity CRGO silicon steel, thin laminations (to limit eddy currents), and by operating at a suitably chosen (not excessive) flux density to limit hysteresis loss.

Question 4

Q.4. What are the various coding methods used for transformers?

Accepted Answer

Vector group coding (e.g., Dyn11, Yy0, Dyn1) is the standard method used to specify transformer winding connections and phase-displacement relationships between primary and secondary.

Question 5

Q.5. What are the various factors considered in designing of electrical machi…

Accepted Answer

Factors in electrical machine design include electromagnetic loadings, thermal/cooling limits, mechanical strength, insulation class, economic material cost, and compliance with relevant standards.

Question 6

Q.6. Why damper windings are used in synchronous machines?

Accepted Answer

Damper windings in synchronous machines suppress hunting/oscillations, provide starting torque (for synchronous motors), and reduce the effect of unbalanced loading and negative-sequence currents.

Question 7

Q.7. Define specific electric loading.

Accepted Answer

Specific electric loading (ac) is the total ampere-conductors around the armature periphery divided by the periphery length, expressed in ampere-conductors per meter.

Question 8

Q.8. Mention the advantages and disadvantages of using open slots in machines.

Accepted Answer

Open slots simplify winding insertion/repair and reduce leakage reactance but increase air-gap flux pulsation/ripple and hence increase iron loss and noise compared to semi-closed or closed slots.

Question 9

Q.9. What are the advantages of using synthesis design method in CAD?

Accepted Answer

The synthesis method in CAD directly generates a complete machine design from specifications using pre-programmed formulas/logic, offering fast, automated design generation without requiring an initial designer-supplied trial design.

Question 10

Q.10. Mention various duty cycles of motor.

Accepted Answer

Standard motor duty cycles (per IS:4722/IEC 60034-1) include S1 (continuous), S2 (short-time), S3-S5 (intermittent periodic, with variants including starting/braking), S6 (continuous with intermittent loading), S7 (continuous with starting/braking), and S8 (continuous with periodic speed changes).

Question 11

Q.1. Explain the synthesis method of computer aided design with a neat flowch…

Accepted Answer

The synthesis method directly generates a design from specifications through a fixed sequence of formula-based calculation steps (flowchart), offering fast automated design but limited flexibility for highly customized or novel machine designs compared to the analysis method.

Question 12

Q.2. Derive the expression for output equation of induction motor.

Accepted Answer

The induction motor output equation Q = 11 Kw Bav ac D²L n ×10⁻³ (kW) is derived from the machine's EMF equation, torque expression, and the definitions of specific magnetic and electric loading.

Question 13

Q.3. A 3-phase, 4-pole, 50 Hz induction motor has 24 stator slots and 28 roto…

Accepted Answer

Since rotor slots S2=28 and poles P=4 give S2/P=7, the resulting 7th-order tooth-ripple harmonic field is stationary relative to the rotor at Ns/7 = 1500/7 = 214.3 rpm, causing the motor to lock into synchronous crawling at this sub-synchronous speed if not avoided by proper slot combination selection.

Question 14

Q.4. What do you mean by Short Circuit Ratio (SCR) with reference to synchron…

Accepted Answer

Short Circuit Ratio (SCR) is the ratio of field current for rated open-circuit voltage to field current for rated short-circuit current; it affects voltage regulation, stability limit, machine size/cost, and short-circuit current magnitude.

Question 15

Q.5. Explain the construction of synchronous machine with neat diagrams.

Accepted Answer

A synchronous machine consists of a stationary armature (stator) carrying the AC winding, a rotor carrying the DC-excited field winding (either salient-pole or cylindrical/non-salient construction), slip rings and brushes (or a brushless exciter) for field supply, and a frame/bearing structure supporting the rotating assembly.

Question 16

Q.6. Explain the necessary steps for the design of wound rotor of an inductio…

Accepted Answer

Designing a wound rotor involves selecting rotor slot number (avoiding cogging/crawling combinations), determining rotor turns from the desired slip-ring voltage, sizing rotor conductors from the rotor current, choosing coil span, and designing slip rings/brush gear.

Question 17

Q.7. State and explain the specific magnetic loading and the choice of magnet…

Accepted Answer

Specific magnetic loading (Bav) is the average flux density over the air-gap surface; its choice balances core/iron loss and magnetizing current against the size of the machine, with higher Bav giving a smaller machine but greater iron loss and magnetizing current.

Question 18

Q.1. Mention the various steps of determination of main dimensions for core, …

Accepted Answer

Transformer core/yoke/window dimensioning proceeds from the required net iron area (from Et) through core-shape-specific proportions to gross dimensions; LV winding is typically built as cylindrical layers close to the core for lower insulation stress, while HV winding uses disc/cross-over construction further out with heavier insulation.

Question 19

Q.2. What do you understand by CAD analysis? Explain different approaches use…

Accepted Answer

CAD analysis evaluates the performance of an already-specified trial design (as opposed to synthesis, which generates a design from specifications); the two main approaches are the analysis method (iterative evaluation/refinement of a trial design) and the optimization method (systematic search over a design-parameter space to minimize/maximize an objective function subject to constraints).

Question 20

Q.3. Estimate the main dimensions including winding conductor area of a 3-pha…

Accepted Answer

For the 300kVA, 6600/440V, 3-phase delta-star transformer with the given 3-stepped core (0.25m circumscribing circle, 0.4m leg spacing), the net iron area works out to about 375 cm² from the emf/turn, and the LV/HV winding conductor areas are determined from the phase currents and specified current density.

Question 21

Q.4. Derive the expressions for design of rotor and end rings of squirrel cag…

Accepted Answer

Squirrel-cage rotor bar cross-section is derived from the expected rotor bar current (via the MMF-balance relation with the stator), end-ring cross-section is derived from the sum of bar currents distributed around the ring circumference; limitations include fixed rotor resistance (poor starting torque/current trade-off) and inability to add external resistance for speed/torque control.

Question 22

Q.5. Write short notes on:

Accepted Answer

FEM-based machine design uses numerical finite element analysis to solve for the detailed magnetic field distribution in complex machine geometries, offering far greater accuracy than classical analytical formulas; induction motor operating characteristics (torque-slip, current-slip, efficiency-load curves) describe how key performance quantities vary with load/slip.

Electrical Machine Design