II B.Tech I Semester Supplimentary Examinations, February 2008
ELECTRICAL MACHINES – I
(Electrical & Electronic Engineering
1. With the help of neat diagram obtain the expression for the energy stored in a magnetic system for a simple attracted armature type relay. Explain the operation of system.
2. (a) What is an equalizer connection? What is necessity of equalizer connection?
(b) An 8 pole DC generator has per pole flux of 40mWb and winding is connected in lap with 960 conductors. Calculate the generated EMF on open circuit when it runs at 400 rpm. If the armature is wave wound at what speed must the machine be driven to generate the same voltage.
3. (a) Explain about demagnetizing Ampere turn per pole and Cross magnetizing Ampere Turn per pole.
(b) What is the purpose of compensating winding? Explain in detail.
4. (a) Draw the neat graph to show open circuit characteristic of a separately excited DC generator? Why is a field regulator necessary for this machine.
(b) What is critical speed? Explain the significance of critical speed.
5. Six DC generators are running in parallel, each having an armature resistance of 0.15 ohm, running at the same speed and excited to give equal induce e.m.f(s). All generators share load equally at a terminal voltage of 500V. The total load is 360KW. If the field current of one generator is raised by 5% and the speed remains constant, Calculate
(a) New Terminal Voltage
(b) Output of each machine.
6. (a) Define torque. Derive the expression for torque developed by a D.C. motor from fundamentals.
(b) Determine the torque developed when a current of 30A passes through the armature of a motor with the following particulars: lap winding, 310 conductors, 4-pole, pole-shoes 16.2 cm long subtending an angle of 600 at the centre, bore radius 16.2 cm, flux density in air gap 0.7 tesla.
7. (a) Compare the speed control of DC motor by rheostat control method and speed control by semiconductor devices.
(b) A DC shunt motor is required to supply a load requiring a constant torque of 120 Nm, over a speed of 500 to 2000 RPM.
i. Explain the three methods of obtaining the required speed range.
ii. For each method above specify the base speed and power rating.
8. (a) Explain the Testing method to separate rotational losses in DC machine.
(b) Two identical DC machines when tested by Back to back method gave the following test results: Field currents are 2.5 A and 2 A. Line voltage is 220 V. Line current including both field current is 10 A. Motor armature current is 73 A. The armature resistance of each machine is 0.05 . Calculate the efficiency of both machines.