1) A laminated tooth of the armature steel in an electrical machine is 30mm long and has a taper such that maxi. width is 1.4 times the minimum. Determine the E.M.F needed for the mean flux density of 1.9 wb/m2 in this tooth. Use the simpson’s rule. The B. at curve for the material of tooth is
B wb/m2 1. 6 1.8 1.9 2.0 2.1 2.2 2.3
at A/m 3700 10,000 17,000 27,000 41,000 70,000 1,08,000
2) Describe the choice of specific electric loading in the electrical machines.
3) Calculate the major dimensions, number of poles and the length of the airgap of a 600kw, 500v, 900 rpm generator. Suppose the average gap density as 0.6 wb/m2 35,000. The ratio of pole arc to pole pitch is 0.75 and the efficiency is 91%. The following are the design constraints: peripheral speed 40 m/s, frequency of flux reversals 50 HZ, current per brush arm 400 A > and armature mmf per pole > 7500A. The e.m.f needed for the air gap is 50% of armature e.m.f. per pole and gap contraction factor is 1.15.
4) The commutator of the 50 rpm machine is 0.3m in diameter. The brush friction loss is 100 w. If at full load the commutator loss is twice the brush friction loss, determine the length of commutator that will give the final temperature rise of 40°C. Suppose that the commutator of this diameter and 75mm in length running at 700 rpm provides the temperature length running at 700 rpm provides a temperature rise of 40°C with the commutator loss of 300W.
5) Find the dimensions for core and yoke for a 5KVA, single phase core type transformer. A rectangular core is used with long side twice as long side twice as long as short side. The window height is 3 times the width. Voltage per turn is 1.8 V. Space factor 0.2, current density 1.8 A/mm2