Electrical Machine Design – 1 — Summer 2019

Answer the following with respect to transformer design :

Why cores are stepped? (b) Why yoke is designed for low flux density?

Calculate the diameter and length of armature for a 7.5 kW, 4 pole, 1000 rpm, 220 V Shunt motor. Full load efficiency=0.83, maximum gap flux density=0.9 Wb/m2, specific electric loading =30,000 ampere conductors per meter, field form factor=0.7, assume maximum efficiency occurs at full load and field current is 2.5% of rated current. The pole face is square.

Explain how the choice of number of poles in a d.c. machine affects : (1) Losses in the machine (2) Weight of machine

A 500 kW, 375 rpm, 8 pole D.C. generator has a flux per pole of 0.0885 wb. Determine the armature demagnetizing and cross-magnetizing mmf per pole if the brushes are given a lead of 5% of pole pitch. Assume power developed by armature to be equal to rating of machine.

A 350 kW, 500 V 450 rpm, 6 pole dc generator is built with an armature diameter of 0.87 m and core length of 0.32 m. The lap wound armature has 660 conductors. Calculate the specific electric and magnetic loadings.

Explain different methods used to improve armature reaction effect in DC machine.

Write a short note on a square and stepped cores in transformer design.

Discuss the behavior of current transformer under system short circuit condition.

Derive the expression for no load current of Transformer.

Explain various factors affecting choice of Average flux density and Ampere conductors per meter for D.C. machine.

Explain : (a) Significance of mitered joints in transformer.

Design difference between power & distribution transformer.

Write a Short Note on : Duty Cycle

Derive output equation of 3 –Φ Transformer. Write the significance of constant ‘K’.

How will the output and losses in transformer change with the linear dimensions?

The ratio of flux to full load mmf in a 400 kVA, 50 Hz single phase core type power transformer is 2.4 x 10-6. Calculate the net iron area and the window area of transformer. Maximum flux density in the core is 1.3 Wb/m2, current density 2.7 A/mm2 and window space factor 0.26. Also calculate the full load mmf.1

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

A 40 Hz transformer is to be used on a 50 Hz system. Assuming the Steinmetz’s coefficient as 1.6 and losses at lower frequency 1.2%, 0.7% and 0.5% for I2R, hysteresis and eddy current respectively. Find losses on 50 Hz for the same supply voltage and current.