JNTU
JNTU B.Tech II Year ELECTRICAL TECHNOLOGY Previous Question Paper (March - 2021 - R16)
Answer any five questions. All questions carry equal marks.
Main
Sketch the speed-torque characteristic of a shunt motor at fixed field current. Explain the characteristic through relevant fundamental relationships of the machine.
Answer
The speed-torque characteristic of a DC shunt motor is derived from the fundamental equations of the machine. The back EMF is given by Eb = V - IaRa = (PΦNZ)/(60A). Rearranging for speed (N), we get N = (V - IaRa) / (kΦ), where k = (PZ)/(60A) is the machine constant.
- Ideal Characteristic: If the armature resistance drop (IaRa) is neglected, the speed is directly proportional to the terminal voltage and inversely proportional to the flux per pole (Φ). Since V and Φ are constant in a shunt motor, the speed remains theoretically constant regardless of the load torque.
- Practical Characteristic: In reality, as the load torque increases, the armature current (Ia) increases. The term (V - IaRa) decreases slightly due to the voltage drop across the armature resistance. Consequently, the speed drops slightly as the torque increases, resulting in a slightly drooping characteristic.
- Sketch: The graph plots Torque (T) on the x-axis and Speed (N) on the y-axis. It starts at a specific no-load speed and slopes downwards linearly with a small negative gradient as torque increases.
A constant-torque load is being supplied by a 500 V dc shunt motor, having armature and field resistances of 0.8 Ω and 300 Ω respectively. The motor takes a current of 28 A from the mains when running on load at 750 rpm. Find the value of the field resistance, which should be introduced in the field circuit to increase the speed to 1000 rpm. Also find the corresponding armature and line currents drawn by the motor. Assume linear magnetisation.
Answer
Given: V = 500 V, Ra = 0.8 Ω, Rsh = 300 Ω, IL1 = 28 A, N1 = 750 rpm, N2 = 1000 rpm.
- Initial conditions: Ish1 = V / Rsh = 500 / 300 = 1.667 A. Ia1 = IL1 - Ish1 = 28 - 1.667 = 26.333 A. Eb1 = V - Ia1Ra = 500 - (26.333 × 0.8) = 478.93 V.
- Load condition: Since the load torque is constant and the machine is linear, the torque T ∝ ΦIa. For constant torque, Φ1Ia1 = Φ2Ia2. Also, Eb ∝ ΦN. Thus, Eb1 / Eb2 = (Φ1N1) / (Φ2N2).
- Calculations: Since Ia is constant for constant torque load (assuming Φ is constant), Eb2 = V - Ia1Ra = 478.93 V. From N2/N1 = (Φ1/Φ2) × (Eb2/Eb1), we get 1000/750 = (Φ1/Φ2) × 1, so Φ1/Φ2 = 1.333.
- Field Resistance: Since Φ ∝ Ish, Ish1/Ish2 = 1.333. Ish2 = 1.667 / 1.333 = 1.25 A. New total field resistance Rsh_total = 500 / 1.25 = 400 Ω. Added resistance = 400 - 300 = 100 Ω.
- Currents: Armature current Ia2 = 26.333 A (remains constant for constant torque). Line current IL2 = Ia2 + Ish2 = 26.333 + 1.25 = 27.583 A.
Discuss the methods of speed control of a dc series motor.
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A dc shunt motor operating at 300 V has a no-load speed of 1200 rpm. The motor is now reconnected to 600 V mains and draws an armature current of 150 A. Calculate the motor speed. Assume linear magnetisation characteristic if Ra = 0.2 Ω.
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Explain the constructional differences between core and shell-type transformers.
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A 25 kVA, 600/200 V transformer is subjected to an SC test. The voltage applied on one side, with the other shorted, is 5.2% of the rated voltage. The transformer draws rated current and a power of 242 W during the test. Compute equivalent resistance and leakage reactance of the transformer in ohms on either side and in pu.
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Explain the terms coil span, coil pitch, short-pitching and cording of coils.
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A three-phase, 20-pole synchronous generator has a star-connected winding with 180 slots and 8 conductors/slot. The flux/pole is 0.05 Wb (sinusoidally distributed) and the speed is 300 rpm. Find the frequency and the line-to-line induced emf. The total turns/phase may be assumed to be series connected. Also, assume the stator winding to be full-pitched.
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Draw the equivalent circuit of a transformer. Identify the test by which the value of each circuit element can be found.
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The following test results were obtained on a 20 kVA, 2200/220 V transformer: OC test (LV): 220 V, 1.1 A, 125 W; SC test (HV): 52.7 V, 6.4 A, 287 W. The transformer is loaded at unity pf on secondary side with a voltage of 220 V. Determine the maximum efficiency and the load at which it occurs.
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Draw and explain Torque-Slip characteristics of three phase IM?
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Write brief note on different starting methods of three phase IM?
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Explain the construction and working of shaded pole motor.
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List the applications of Stepper motor.
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Explain the working principle of three phase IM.
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A three-phase induction motor runs at a speed of 940 rpm at full load when supplied form a 50 Hz, three-phase mains. i) How many poles does the motor have ii) What is its percent slip at full load iii) What is the corresponding speed of the rotor field with respect to the rotor surface iv) What would be the rotor speed at twice full-load slip.
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