JNTU
JNTU B.Tech II ELECTRICAL MACHINES I Previous Question Paper (November/December - 2018 - R16)
Note: This question paper contains two parts A and B. Part A is compulsory which carries 25 marks. Answer all questions in Part A. Part B consists of 5 Units. Answer any one full question from each unit. Each question carries 10 marks and may have a, b, c as sub questions.
Part A
Why equalizer connection is necessary for the armature winding of a dc machine with lap-winding?
Answer
Equalizer connections are necessary in lap-wound DC machines to prevent circulating currents caused by magnetic flux imbalances. Due to slight variations in air gap length or magnetic properties, the induced EMFs in parallel paths may differ, leading to circulating currents that flow through the brushes and cause sparking. Equalizer rings connect points of equal potential in the armature winding, providing a low-resistance path for these currents to circulate within the armature itself, thereby bypassing the brushes and improving commutation.
What are the various methods of improving commutation?
Answer
Commutation is the process of reversing the current in the armature coils as they pass through the magnetic neutral axis. To improve commutation and ensure sparkless operation, the following methods are employed:
- Resistance Commutation: This involves using high-resistance carbon brushes. The higher contact resistance between the brush and the commutator segment forces the current to reverse more rapidly in the short-circuited coil, reducing the sparking effect.
- EMF Commutation (Interpoles): Small auxiliary poles, known as interpoles, are placed between the main poles. These are connected in series with the armature and produce a flux that induces an EMF in the short-circuited coil. This induced EMF opposes the reactance voltage (the cause of sparking), effectively neutralizing it.
- Compensating Windings: These are windings placed in the pole faces of the main poles, connected in series with the armature. They neutralize the cross-magnetizing effect of armature reaction, which otherwise distorts the flux and causes voltage surges that lead to sparking at the brushes.
What is significance of Back EMF?
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Draw the torque-current characteristics of dc compound motors.
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Hopkinson’s test is also called a regenerative test. Justify?
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What are the advantages and disadvantages of Hopkinson’s Test?
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Draw the exact equivalent circuit of a Transformer.
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What is stacking factor? What is its approximate value?
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What are the advantages of auto transformer over two winding transformer?
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Why OC-test conduct on LV-side and SC-test conduct on HV-side?
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Part B
Explain the concept of Armature reaction and how it overcomes.
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A dc shunt generator has the following open circuit magnetization curve running at 800 rpm. Field current (A): 0 0.5 1.0 2.0 3.0 4.0 5.0. EMF (V): 10 50 100 175 220 245 262. Find graphically the critical resistance of shunt field circuit. If the field resistance is changed to 75 ohms, what will be the critical speed for the machine to build up.
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Distinguish between self excited and separetley exited DC Generators. Give the circuit diagrams of Self excited DC Generators and derive the voltage equations.
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Derive e.m.f equation of d.c generator.
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Draw and explain the Electrical characteristics of DC series and shunt motor.
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Derive the expression for the speed of a dc motor interms of back emf and flux per pole.
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Explain with a neat sketch, the function of a 3-point starter.
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A dc shunt motor takes 1.5A on no-load when connected to 250V mains with an armature resistance of 1.0Ω when the field current is 0.5A. Determine the load current corresponding to maximum efficiency.
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Explain the procedure to separate the stray losses in a DC Motor.
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Hopkinson’s test on two similar DC shunt machines gave the following data: Line voltage 230V, line current excluding both the field currents is 40A, motor armature current 350A, field currents 5A and 4.2A. Calculate the efficiency of each machine. Armature resistance of each machine is 0.02Ω.
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With a neat sketch, explain the procedure of Swinburne’s test? Mention its advantages and disadvantages.
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Discuss the working principle of single phase Transformer and also explain the constructional details.
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Explain why hysterisis and eddy current looses occur in a transformer. What are the methods to reduce these losses?
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Draw and explain the phasor diagram of single phase transformer on load considering with winding resistance.
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Derive the equations for the currents supplied by each transformer when two transformers are operating in parallel with equal and unequal voltage ratios.
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What is auto transformer? Explain the working principle of auto transformer.
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What are the advantages of poly phase transformers? Give different configurations.
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