Electrical machines


1) Design a suitable double –layer lap winding for a 6-pole dc armature with 18 slots and two coil sides per slot. Give values of front-pitch, brack–pitch and commutator pitch. Draw the developed diagram and show the positions of brushes.

2) A dc shunt motor is being operated from300Vmains.Its no-load speed is 1200rpm.When fully loaded it delivers a torque of 400 N m and its speed drops to 1100rpm.Find its speed and power output when delivering the same torque; if operated with an armature voltage of 600 V. Excitation is assumed unchanged, i.e. motor fields still excited at 300VState any assumption you are required to make.

3) Calculate the efficiency of a 500V shunt motor when taking 700A from the following data taken when the motor was hot: motor stationary, voltage drop in the armature winding 15V.when the armature current was 510Afield current is 9 A at a normal voltage. Motor running at normal speed unloaded the armature current was 22.5Awhen the applied voltage was 550V; allow 2 V for brush contact drop and 1%of the rated output of 400kw for stray-load losses . 4) Two star-connected generators are connected in a parallel and supply a balanced load of 1500kva at 11kV line voltage and 0.8lagging power factor. The Synchronous reactnces of the two machines respectively are: 35 ohms and 40 ohms. The prime mover governors of the two machines are adjusted so as to equally share the power load The phase current in one machine is 43A at a lagging power factor Calculate: a) The phase current in the second machine b) The induced emf of each machine c) The power factor at which each machine operates.

5) Calculate the ratio of transformation of an autotransformer starter for a 25 Kw, 400V, 3-phase induction motor if the starting torque is to be 75% of full-loaded torque. Assume the slip at full-load to be 3.5% and short circuit current to be six times full- load current. Ignore magnetizing current of the transformer and of the motor.

6) For a 2 phase servomotor with a high resistance rotor find approximate expression for forward and backward torques in terms of phase voltages (differing 90 degree in phase) and motor speed. Assume the stator impedance and rotor reactance to be negligible.

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