JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III POWER SEMICONDUCTOR DRIVES Previous Question Paper (February/March - 2022 - R18)
Answer any five questions. All questions carry equal marks.
Main
Explain the operation of single phase fully-controlled converter connected to DC separately excited motor.
Answer
A single-phase fully-controlled converter uses four thyristors (T1, T2, T3, T4) in a bridge configuration to convert AC supply to variable DC voltage for a separately excited DC motor. During the positive half-cycle of the supply, T1 and T2 are triggered at a firing angle α, applying the supply voltage to the armature. The motor current flows through T1, the armature, and T2. In the negative half-cycle, T3 and T4 are triggered at (π + α), again applying the supply voltage to the armature in the same polarity. The armature voltage is controlled by varying the firing angle α, which in turn adjusts the motor speed. Since the converter is fully controlled, it allows for two-quadrant operation, enabling both motoring and regenerative braking modes. The output voltage is given by V₀ = (2Vm/π) cos α, where Vm is the peak supply voltage.
A 230 V, 1000 rpm, 10 A separately excited DC motor is fed from a single phase full converter with AC source voltage of 200 V, 50 Hz, Ra = 5 Ω. The armature current is continuous. Calculate the firing angle for rated motor torque at 750 rpm.
Answer
Given: V = 230 V, N1 = 1000 rpm, Ia = 10 A, Ra = 5 Ω, Vs = 200 V, f = 50 Hz, N2 = 750 rpm. First, calculate the back EMF constant (Keφ) at rated conditions:
At 750 rpm, the back EMF is:
The required armature voltage at 750 rpm for the same torque (same Ia = 10 A) is:
For a single-phase full converter, the average output voltage is:
Where Vm = 200 × √2 = 282.84 V. Substituting the values:
Since cos α cannot exceed 1, this indicates that with the given supply voltage and circuit parameters, the motor cannot reach the rated torque at 750 rpm in continuous conduction mode; the firing angle α would be 0 degrees to provide the maximum possible voltage, which is still insufficient to maintain 10 A at that speed.
Explain the operation of three phase fully-controlled converter connected to DC series motor.
Answer
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A 440 V, 1000 rpm, 15 A separately excited DC motor has armature resistance and inductance of 1 Ω and 10 mH. The motor is controlled by a three phase full converter fed with source voltage 230 V, 50 Hz. Calculate the developed torque for α = 30° and speed 800 rpm.
Answer
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Explain the differences between plugging, dynamic breaking and regenerative breaking.
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A 200 V, 50 A DC series motor having combined resistance of 0.4 Ω is controlled in regenerative braking with supply of 220 V. The motor constant is 0.04 V-s/A-rad for a duty ratio of 50%. Determine the power returned to supply.
Answer
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Discuss the operation of chopper fed four quadrant separately excited DC motor.
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Explain the four quadrant operation of DC motors by three phase dual converters.
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Explain the control of induction motor using AC voltage controllers.
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A 440 V, 3 phase 50 Hz, 6-pole cage induction motor has the following equivalent circuit parameters: Rs = 0.2 Ω, Xs = 0.5 Ω, Rr = 0.3 Ω, Xr = 0.5 Ω and Xm = 20 Ω, all quantities referred to stator side. The motor is operating on full load slip of 0.04. If the two stator terminals are suddenly interchanged, calculate the primary current and the braking torque immediately after application of plugging.
Answer
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Explain the variable frequency control of induction motors using cyclo converter.
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A star connected squirrel cage induction motor has the following ratings and parameters: 440 V, 50 Hz, 6-pole, 1600 rpm, R1 = 1 Ω, X1 = X2' = 4 Ω, R2' = 3 Ω. The motor is controlled by a voltage source inverter at constant V/f ratio. The inverter allows a frequency of 60 Hz. Calculate the starting torque and current.
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Explain the static rotor resistance control of induction motors in detail.
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Explain in detail about the working of static krammer drive.
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Explain in detail about the self-control of synchronous motors.
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Explain the variable frequency control of synchronous motors using cyclo converter.
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