Jawaharlal Nehru Technological University Hyderabad
Jawaharlal Nehru Technological University Hyderabad B.Tech IV UTILIZATION OF ELECTRIC POWER Previous Question Paper (July/August - 2022 - R16)
Answer any five questions. All questions carry equal marks.
Main
What are the various points which govern the choice of a motor for a particular industrial drive?
Answer
The selection of an electric motor for an industrial drive is governed by several critical factors to ensure efficiency, reliability, and longevity:
- Nature of Load: The torque-speed characteristics, starting torque requirements, and the duty cycle (continuous, intermittent, or variable) must match the motor's capability.
- Electrical Supply: Availability of AC or DC supply, voltage levels, frequency, and phase requirements.
- Environmental Conditions: The motor must be selected based on ambient temperature, humidity, presence of corrosive fumes, dust, or explosive gases, which dictate the type of enclosure (e.g., drip-proof, totally enclosed, flame-proof).
- Mechanical Considerations: Type of mounting (horizontal/vertical), shaft extension, bearing type, and space constraints.
- Speed Control: Requirements for constant speed, variable speed, or precise speed regulation.
- Efficiency and Power Factor: High efficiency and power factor are essential for reducing operating costs and energy consumption.
- Cost and Maintenance: Initial capital cost, ease of maintenance, and availability of spare parts.
- Overload Capacity: The motor must be able to handle transient overloads without stalling or overheating.
A motor has a heating time constant of 80 minutes and cooling time constant of 100 minutes. When run continuously on full load of 220 KW, the final temperature rise is 50°C. The motor is driving an intermittent load having 15 minutes on load period followed by 20 minutes shut down. What is the maximum value of load it can supply during the on-load period without the maximum temperature rise exceeding 40°C? Assume losses to be proportional to the square of the load.
Answer
Given: Heating time constant τh = 80 min, Cooling time constant τc = 100 min, Pfl = 220 kW, θfl = 50°C, ton = 15 min, toff = 20 min, θmax = 40°C.
The temperature rise during the 'on' period is given by:
The temperature fall during the 'off' period is:
Substituting θmin into the first equation:
Calculating the exponential terms:
Solving for steady-state temperature rise θss:
Since losses are proportional to the square of the load:
Suggest, with reasons the electric drive used for the following applications: i) Lifts and ii) cement mills.
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Explain different methods of speed control used in electric drives with suitable diagrams.
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Explain the principle of working of dielectric heating.
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Distinguish between the direct and indirect type of arc furnaces.
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Discuss the principle of arc welding. Explain the difference between carbon and metallic arc welding and their relative merits and demerits.
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Define the following terms: i) Illumination ii) Luminous Intensity.
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State and explain the laws of illumination.
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A 250 Volt lamp has a total flux of 2500 lumens and takes a current of 0.85 amperes. Calculate i) lumen per watt ii) M.S.C.P per watt.
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Describe the construction and working of high pressure mercury vapour lamp.
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What is flood lighting? Give its applications.
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Sketch neatly two typical speed-time curves (i) for suburban, and (ii) for main line railway service assuming level track between their two stations.
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Explain the advantages of electric braking used in traction.
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Explain the following terms in a locomotive: i) dead weight ii) effective weight and iii) adhesive weight.
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An electric train while going down an incline of 1 in 200 has the following speed-time curve i) Starting from rest a uniform acceleration of 2.5 kmphps for 40 sec ii) Steady speed for 50 sec (with mechanical braking), iii) Coasting for 60 sec and iv) Braking at a rate of 3.5 kmphps. Assume the track resistance as 50 N per tonne, allowance for rotational inertia 8%, overall efficiency 75%, calculate the specific energy consumption.
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