JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech IV Year ADVANCED CONTROL OF ELECTRIC DRIVES Previous Question Paper (July/August - 2022 - R18)
Answer any five questions. All questions carry equal marks.
Main
What are the advantages of PWM inverters? Derive the expression for rms output voltage of multiple PWM inverters.
Answer
Pulse Width Modulation (PWM) inverters offer several advantages: they allow for independent control of both output voltage magnitude and frequency, eliminate lower-order harmonics by shifting them to higher frequencies (which are easier to filter), and provide improved power factor and efficiency in drive applications. In Multiple PWM, where 'p' pulses are present per half-cycle, the output voltage waveform consists of pulses of width '2d'. The rms output voltage is derived by integrating the square of the output voltage over one period:
For a square wave with 'p' pulses of width '2d' per half-cycle, the output voltage is Vs during the pulse and 0 otherwise. The integral becomes:
The single phase full bridge inverter is operated at 1 kHz and uses a uniform PWM with four pulses per half-cycle for voltage control. Plot the fundamental component, distortion factor, and THD against the modulation index M.
Answer
For a uniform PWM inverter with 4 pulses per half-cycle (p=4), the fundamental component is given by V1 = (4Vs/π) * sin(pδ) * sin(ωt), where δ is the pulse width. As the modulation index M increases, the fundamental component V1 increases linearly with M. The Distortion Factor (DF) is defined as the ratio of the harmonic content to the fundamental, which decreases as the number of pulses increases and as M approaches 1. The Total Harmonic Distortion (THD) is calculated as:
At low modulation indices, the THD is high because the fundamental component is small relative to the harmonic content. As M increases toward 1, the fundamental component grows significantly, causing the THD to decrease rapidly. The plot of these parameters against M shows V1 as a linear rising curve, while both DF and THD exhibit a downward trend, with THD reaching its minimum value near M = 1.
A single phase full bridge inverter uses a uniform PWM with seven pulses per half cycle for voltage control. Plot the fundamental component, distortion factor and THD against the modulation index M.
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Give the differences between CSI and VSI.
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What is the principle of vector control and explain the direct vector control scheme.
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Find the relationship between the dc link voltage and the stator frequency for the closed implementation of a volts/Hz inverter fed induction motor drive. The motor parameters are as follows: 5hp, 60 Hz, star connected, 4-pole, 0.86pf and 0.86 efficiency, Xs = 0.227 Ω, Rr = 0.183 Ω, Xm = 20.30 Ω, Xr = 0.841 Ω respectively.
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What is field weakening mode on induction motor explain with its characteristics?
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Describe the speed control of Inverter driven induction motor.
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Briefly explain the flux weakening operation of Synchronous motors.
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Explain: i) constant torque angle control, ii) unity power factor control.
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A 3phase, 400V, 50Hz, 6 pole, star connected round-rotor synchronous motor has Zs = 0+j2Ω. Load torque, proportional to speed squared, is 340N-m at rated synchronous speed. The speed of the motor is lowered by keeping V/f constant and maintaining unity pf by field control of the motor. For the motor operation at 600rpm, Calculate i) supply voltage ii) armature current.
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What is the advantage of CSI fed control of synchronous motor drive?
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Discuss the working of a three phase BLDC motor drive when fed from VSI on closed loop operation. Draw neat circuit diagram.
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Explain the working of PMSM drive.
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Give the advantages of a DSP motion control drive.
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Give some basic blocks used in designing a model in DSP.
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