Make sure you indicate clearly the questions attempted Show your work to earn marks. • Enjoy Q1. With the help of a well-drawn diagram, explain concept map of an electromechanical system modelling. [20] Q2. Separately-excited generator develops a no load emf of 150V at an armature spend of 20 rev/s and a flux per pole of 0.1 Wb. determine the generated emf when a. The spend increases to 25rev/s and the pole flux remains unchanged. b. The speed remains at 20 rev/s and the pole flux is decreased to 0.08Wb. c. The speed increases to 24 rev/s and the pole flux is decreased to 0.07 Wb Write short notes on the following term a. Copper loss b. Iron loss c. Friction and Windage losses. [20]

An air-gap between two pole pieces is 20 mm in length and the area of the flux path across the gap is 5 cm². If the flux required in the air-gap is 0.75 mWb find the m.m.f. necessary.

(a) Explain briefly what is fringing effect and leakage flux. (b) A ferromagnetic core is shown in Figure 1. The depth of the core is 10 cm. The other dimensions of the core are as shown in the figure. By assuming the magnetic leakage to be negligible and relative permeability of the core is 800. Calculate the total reluctance and find the value of the current that will produce a flux of 0.01 Wb. With this current, calculate the flux density at the top of the core 20 cm 40 cm 10 cm, 30 cm 500 turns 30 cm 30 cm Figure 1

A magnetic circuit has a mean flux path length of 800 mm and a reluctance of 2.5.105 A-t/Wb. A magnetic flux of 5 mWb is produced in this circuit by an energising coil of 800 turns. Determine the: (i) m.m.f. that must be produced by the coil. (ii) magnetic field strength. (iii) voltage induced in the coil circuit if the magnetic field is allowed to collapse to zero in 2 ms. (iv) inductance of the energising coil.

Transformers operate by the principle of   A. plasma fission   B. plasma fusion   C. AC generation   D. EM dispersion   E. EM induction

dc machines

For operation at 60 Hz and a given peak flux density, the core loss of a given core is 1W due to hysteresis and 0.5W due to eddy currents. Estimate the core loss for 400-Hz operation with the same peak flux density.

A 220 V , 60 Hz , supply is connected to 200 – turn armature winding of reluctance 1.7x105 At / Wb . Find : the magnetizing reactance the rms values of the ( magneto – motive force , exciting and armature currents , magnetic fluxflu the inductance Draw the phasor diagram of the circuit

• A DC shunt machine while running as generator develops a voltage of 250 V at 1000 r.p.m. on no-load. It has armature resistance of 0.5 ohm and field resistance of 250 ohm. When the machine runs as motor, input to it at no-load is 4 A at 250 V. Calculate the speed and efficiency of the machine when it runs as a motor taking 40 A at 250 V. Armature reaction weakens the field by 4 %

A flux of 0.0016 Wb flows in a silicon sheet steel coil/core. The arrangement appears as shown below with the following dimensions: Ac = 0.0025 m? Core/gap area: Flux path length: le = 0.350 m lg = 0.0015 m N = 500 Air gap length: Number of turns: %3D A. What is the current, I, through the winding? B. What is the energy stored in the inductance? C. Assume that the current is given by i =9.75 sin (3147) A. What is the RMS voltage that would appear across the winding? D. Assume that the gap is removed from the arrangement with the flux path remaining at 0.350 m. For a flux of 0.0016 Wb, what is the inductance of the arrangement? E. What is the energy stored in the inductance with the gap removed?