Electrical Machine Design – 1 — Winter 2018

Derive the output equation DC machine. Also explain the factors affecting the size of rotating machine.

Explain the factors to be considered while selecting the number of poles in the design of DC machine.

Discuss factors to be considered while deciding the length of air gap in the design of a d.c. machine.

What is the importance of temperature as a factor in the life of insulating material? Also classify insulating material as per I.Sspecification.

Determine the main dimensions, number of poles and length of air gap of a 6002 kW, 500 V, 900 r.p.m. generator. Assume average gap density as 0.6 Wb/m and ampere conductors per metre as 35000. The ratio of pole arc to pole pitch is 0.75 and efficiency is 91 percent. The following are the design constraints: peripheral speed: ≤ 40 m/s, frequency of flux reversals: ≤50 Hz, current per brush arm: ≤ 400 Aand armature mmf per pole ≤ 7500 A. The mmf required for air gap is 50 percent of armature mmf and gap contraction factor is 1.15

Explain different types of duties and rating for rotating electrical machines.

Explain guidelines or guiding factors used for the selection of number of armature slots in d.c. machine design.

Describe the methods adopted to reduce the effect of armature reaction in DC machine.

A 4 pole generator supplies a current of 140 A. It has 480 armature conductors

wave connected, (b) lap connected. The brushes are given an actual lead of 10°. Calculate the cross and demagnetizing mmf per pole in each case. The field winding is shunt connected and takes a current of 10 A, find the number of extra shunt field turns to neutralize the demagnetization.

What is design optimization? Derive necessary condition for designing a transformer with minimum cost.

Derive the output equation of a 3-phase core type transformer.

Answer the following with respect to transformer design:

Why cores are stepped? (ii) Why yoke is designed for low flux density? (iii) Why circular coils are preferred in transformer winding?

Calculate approximate overall dimensions for a 200 kVA, 6600/440V, 50 Hz, phase core type transformer. The following data may be assumed: emf per turn=10V; maximum flux density=1.3 Wb/m2, current density=2.5 A/mm2 , window space factor=0.3, overall height=overall width, stacking factor=0.9. Use a 3 stepped core.1 For a 3 stepped core, Width of largest stamping=0.9d and Net iron area= 0.6d2, where d=diameter of circumscribing circle.

Explain the steps involved to calculate no load current of a 3-phase transformer from its design data.

Obtain the expression of leakage reactance of a 3-phase core type distribution transformer

Discuss the steps for designing a shunt field winding of a dc machine.

Discuss the importance of mitred joints in the core assembly of transformers.