DESIGN OF ELECTRICAL MACHINES (DEM)–April / May 2011 Question Paper

Anna University


Sixth Semester

Electrical and Electronics Engineering


(Regulation 2008)

Time : Three hours

Maximum : 100 marks

Answer ALL questions

PART A — (10 × 2 = 20 marks)

1. What is specific Electric Loading?

2. How materials are classified according to their degree of magnetism?

3. Name any two methods to reduce armature reaction.

4. What is slot loading?

5. Give the relationship between emf per turn and kVA rating in a transformer.

6. What are the factors affecting the choice of flux density of core in a transformer?

7. How crawling can be prevented by design in an Induction motor?

8. Define dispersion coefficient of an Induction Motor.

9. What is run away speed of Synchronous Machine?

10. Give the need for damper winding in Synchronous Machine.

PART B — (5 × 16 = 80 marks)

11. (a) Discuss about various duties and ratings of Rotating Machines and give their respective temperature — time curves. (16)


(b) A field coil has a heat dissipating surface of 0.15 m2 and length of mean turn 1 m. It dissipates loss of 150 W, the emissivity being 34 W/m2_°C. Estimate the final steady temperature rise of the coil and its time constant if the cross section of the coil is 100*50 mm2. Specific heat of copper is 390 J/kg° C. The space factor is 0.56. Copper weighs 8900 kg/m3. (16)

12. (a) (i) Explain the effects of choice of number of poles in a DC Machine on (1) Frequency of flux reversal (2) Weight of iron (3) Weight of copper and (4) Length of commutator. (8)

(ii) A 5 kw, 250 V, 4 pole, 1500 rpm DC Shunt Generator is designed to have a square pole face. The specific magnetic loading and specific electric loadings are 0.42 Wb/m2 and 15000 AC/m respectively. Find the main dimensions of the machine. Assume full load efficiency = 0.87 and pole arc to pole pitch ratio is 0.66. (8) Or

(b) (i) Discuss various methods to determine mmf required for teeth of an Electric Machine. (8)

(ii) Determine the apparent flux density in teeth of a DC machine if the real flux density in teeth is 2.15 Wb/m2, slot pitch is 28 mm, slot width is 10 mm, gross core length is 0.35 m, no. of ventilating ducts is 4 each 10 mm wide. Magnetizing force corresponding to flux density of 2.15 Wb/m2 is 55000 AT/m and iron stacking factor is 0.9. (8)

13. (a) (i) Derive the output equation of a three phase transformer. (8)

(ii) The ratio of flux to full load mmf in a 400 kVA, 50 Hz single phase core type transformer is 2.4*10–6. Calculate the net iron area and the window area of the transformer if the maximum flux density in the core is 1.3 Wb/m2, Current density is 2.7 A/mm2 and window space factor is 0.26. Also calculate the full load mmf. (8)


(b) A 250 kVA, 6600/400V three phase core type transformer has a total loss of 4800 W at full load. The transformer tank is 1.25 m in height and 1 m × 0.5 m in plan. Design a suitable scheme for tubes if the average temperature rise is to be limited to 35° C. The diameter of each tube is 50 mm and are spaced 75 mm from each other. The average height of tubes is 1.05 mm. Specific heat dissipation due to radiation and convection is respectively 6 and 6.5 W/bm2_°C. Assume that convection is improved by 35% due to the provision of tubes. (16)

14. (a) Determine the main dimensions, number of radial ventilating ducts, number of stator slots and turns per phase of a 3.7 kW, three phase, 400 V, 4 Pole, 50 Hz squirrel cage Induction Motor to be started by a Star-Delta starter. Given that the average flux density in the air gap = 0.45 Wb/m2; Ampere Conductor per meter of armature periphery = 23000, full load efficiency = 0.85, full load power factor = 0.84 and kw = 0.955. Take L/ τ = 1.5. (16)


(b) (i) Discuss the factors to be considered in estimating the length of air gap of an Induction Motor. (8)

(ii) Discuss the step by step procedure to design the rotor of a squirrel cage Induction Motor. (8)

15. (a) Define Short Circuit Ratio. Explain how it is determined for an alternator. Also discuss its effects on the performance of alternator. (16)


(b) (i) Derive the output equation of an AC machine. (8)

(ii) Determine the main dimensions of a 100 kVA, 50 Hz, three phase 375 rpm alternator. The average air gap flux density is 0.55 Wb/m2 and ampere conductors per metre is 28000. Given that τ / L must be between 1 to 5. The maximum permissible peripheral speed is 50 m/sec. The runaway speed is 1.8 times synchronous speed. (8)