II B.Tech I Semester Supplimentary Examinations, February 2008
ELECTRICAL MACHINES – I
(Electrical & Electronic Engineering
1. (a) Explain the mechanical energy and work done in singly excited system when actual displacement occurs?
(b) An electromechanical device has following self and mutual inductances. L11= L22= 1.6 + cos2θ andL12= 1 + 0.4cosθ. Find the expression for torque as a function of θ. Find Tav forθ = 30o. when coils are connected to a voltage source of 160 sin 314t, find Tav for θ= 30o.
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) With relevant connection diagrams, explain the different types of DC generators and give their application.
(b) Explain voltage build-up process of self excited DC generator?
5. (a) Distinguish between internal and external characteristic of a DC generator.
How can the internal characteristic be derived from the external characteristic of a separately excited generator.
(b) A separately excited generator with constant excitation is connected to a constant load. When the speed is 1500 rpm, it delivers 120A at 500V. At what speed will the current be reduced to 60A ? Armature resistance is 0.1 ohm, Contact drop/brush is 1V. Armature reaction may be ignored.
6. (a) Distinguish between motor and generator action. Derive the equation for the back e.m.f induced in a DC motor.
(b) A 6-pole DC motor has a wave connected armature with 87 slots, each slot containing 6 conductors. The flux per pole is 20 m.wb and the armature has a resistance of 0.13 ohm when the motor is connected to 240V supply and the armature draws a current of 80A driving a load of 16KW. Calculate
ii. Armature Torque and
iii. shaft Torque.
7. (a) What are the different speed control methods of DC Shunt motor? Explain each method and enumerate advantages and disadvantages.
(b) A DC series motor is driving a fan load, whose torque varies a cube of speed. The total armature and series field resistance is 1
. It takes 10 Amp from 200 V mains and runs at 1000 RPM. Find the resistance to be connected in series with the motor to make it run at 800 RPM.
8. (a) With neat diagram explain the Swinburne’s test conducted on DC shunt motors. What are the advantages and limitations of this test? Enumerate all calculations.
(b) The Hopkinson’s test on two machines gave the following results for full load:Line voltage 250 V, line current (excluding field current) = 50 A. Motor armature current = 380 Amp. Field currents are 5 A and 4.2 A. Calculate the efficiency of each machine. Armature resistance of each machine = 0.02Ω