Power Semi Conductor Drives

Set No. 1

Code No: RR410202

IV B.Tech I Semester Regular Examinations, November 2007 POWER SEMI CONDUCTOR DRIVES (Electrical & Electronic Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ⋆⋆⋆⋆⋆ 1. (a) Explain the concept of constant torque control and constant power control. (b) Explain how the speed control of a dc motor is achieved illustrating the triggering circuits of the thyristors. [6+10] 2. Describe the relative merits and demerits of the following types of braking for dc motors: mechanical braking, dynamic braking and regenerative braking with neat diagram. [16] 3. (a) List the advantages offered by dc chopper drives over line-commutated converter controlled dc drives. (b) A dc chopper controls the speed of dc series motor. The armature resistance Ra = 0.04Ω, field circuit resistance Rf = 0.06Ω, and back emf constant Kv = 35 mV/rad/s. The dc input voltage of the chopper Vs=600v. If it is required to maintain a constant developed torque of Td = 547N-m, plot the motor speed against the duty cycle K of the chopper. [6+10] 4. (a) Explain why stator voltage control is suitable for speed control of Induction motors in fan and pump drives. Draw a neat circuit diagram for speed control of scheme of 3 phase Induction motor using AC Voltage Controller. (b) A 440V, 3 phase, 50 Hz 6 pole 945 RPM delta connected Induction Motor has the following parameter referred to the stator. RS = 2.0Ω, Rr = 2.0Ω, XS = 3Ω, Xr = 4Ω. When driving a fan load at rated voltage it runs at rated speed. The motor speed is controlled by stator voltage control. Determine motor terminal voltage, current and torque at 800 RPM. [8+8] 5. With a block schematic diagram explain how the speed of the Induction motor can be controlled automatically (i.e., using closed loop scheme) with voltage source Inverters. Mention the applications of the above scheme. [16] 6. A SER controlled slip ring motor drives a fan load having a characteristic TL = kN 2 . The motor is rated at 440V, 50Hz, 100kW. The speed is to be controlled from a rated value of 1420 to 750rpm. The equivalent circuit has the following parameters: R1 = 0.052Ω, R2 ’ = 0.06Ω, X1 + X2 ’ = 0.29Ω, X0 = 10Ω. Stator / rotor turns ratio = 1.2. Determine the firing angle and draw the speed-torque characteristic. [16]

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Set No. 1

Code No: RR410202

7. A 6 MW, three phase, 11 kV, 50 Hz, unity power factor, 6-pole, star-connected synchronous motor has the following parameters: armature resistance = 0, synchronous reactance = 9 ohms, rated field current = 50 A. The machine is controlled by variable frequency at constant V/f ratio up to base speed and at constant V above base speed. Calculate the armature current and power factor for regenerative braking power output of 4.2 MVA at 800 rpm and rated field current. Draw motor characteristics and waveforms under the above method of control. [16] 8. Describe self-controlled and load-commutated inverter controlled synchronous motor drives in detail and compare them [16] ⋆⋆⋆⋆⋆

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Set No. 2

Code No: RR410202

IV B.Tech I Semester Regular Examinations, November 2007 POWER SEMI CONDUCTOR DRIVES (Electrical & Electronic Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ⋆⋆⋆⋆⋆ 1. Two independent single-phase semi-converters are supplying the armature and field circuits of the separately excited dc motor for controlling its speed. The firing angle of the converter, supplying the field, adjusted such that maximum field current flows. The machine parameters are: armature resistance of 0.25 Ω, field circuit resistance of 147 Ω, motor voltage constant Kv=0.7032 V/A-rad/s. The load torque is T=45 N-m at 1000 rpm. The converter are fed from a 208 V, 50 Hz ac supply. The friction and windage losses are neglected. The inductance of the filed and armature circuits are sufficient enough to make the armature and field currents continuous and ripple free. Determine (a) the field current, (b) the delay angle of the armature converter, (c) input power factor of the armature circuit converter.

[16]

2. Describe the relative merits and demerits of the following types of braking for dc motors: mechanical braking, dynamic braking and regenerative braking with neat diagram. [16] 3. (a) Explain with neat circuit diagram the basic principle of operation of a class A type of chopper. The chopper is connected to R-L-E load. Analyze the same for continuous current mode of operation. (b) A dc supply of 200v supplied power to separately excited dc motor via a class A thyristors chopper. The motor has an armature circuit resistance of 0.33Ω and inductance of 11mH. The chopper is fully on at the rated motor speed 1200rpm when the armature current is 20A. If the speed is to be reduced to 800rpm with the load torque constant, calculate the necessary duty cycle. If the chopper frequency is 500Hz, is the current continuous? [8+8] 4. What is an AC Voltage Controller? Explain with suitable diagrams the various types of solid state 3 phase AC Voltages Controllers which can be used for speed control of 3 phase Induction motors from stator side. Mention the advantages of the AC Voltages Controllers over the other methods of solid-state speed control techniques of 3 phase Induction motor. [16] 5. Discuss with suitable schematic diagrams the principle of operation of CSI fed Induction motor drives. Mention their merits and limitations. [16] 6. A 3-phase, 460V, 60Hz, 1164rpm, 6 pole, star connected, wound rotor induction motor has the following parameters referred to the stator: R1 = 0.4Ω, R2 ’ = 0.6Ω, 1 of 2

Set No. 2

Code No: RR410202

X1 = X2 ’ = 1.8Ω, X0 = 40Ω, Stator to rotor turns ratio = 2.5. The motor speed is controlled by static rotor resistance control. The filter resistance is 0.02Ω. The value of the external resistance is chosen such that α = 0 and the breakdown torque is obtained at Standstill: Determine the following: The value of the external resistance a for a speed of 60rpm at 1.5 times the rated torque. The speed for a = 0.6 and 1.5 times the rated torque neglect the friction and windage loss. [16] 7. A 6 MW, three phase, 11 kV, 50 Hz, unity power factor, 6-pole, star-connected synchronous motor has the following parameters: armature resistance = 0, synchronous reactance = 10 ohms, rated field current = 50 A. The machine is controlled by variable frequency at constant V/f ratio up to base speed and at constant V above base speed. Calculate the torque and field current for rated armature current, 750 rpm and 0.8 leading power factor. Draw motor characteristics and waveforms under the above method of control. [16] 8. Describe the open-loop and closed loop methods of speed control of a synchronous motor using VSI. [16] ⋆⋆⋆⋆⋆

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Set No. 3

Code No: RR410202

IV B.Tech I Semester Regular Examinations, November 2007 POWER SEMI CONDUCTOR DRIVES (Electrical & Electronic Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ⋆⋆⋆⋆⋆ 1. A dc series motor has Ra = 3 Ω, Rf = 3 Ω and Maf = 0.15 H. The motor speed is varied by a phase-controlled bridge. The firing angle is π /4 and the average speed of the motor is 1450 rpm. The applied ac voltage to the bridge is 330 Sin wt. Assuming continuous motor current find the steady state average motor current and torque. Sketch the waveforms for output voltage, current and gating signals. [16] 2. (a) Draw the circuit diagram and explain the operation of closed-loop speed control with inner-current loop and field weakening. (b) A single phase fully controlled double bridge converter is operated from 120v, 60Hz supply and the load resistance is 10 ohms. The circulating inductance is 40mH. Firing delay angle for converter I and II are 600 and 1200 respectively. Calculate the peak circulating current and the current through converters. [8+8] 3. (a) Derive the expressions for average motor current, current Imax and Imin and average torque for chopper-fed dc separately excited motor. (b) A dc chopper controls the speed of a separately excited motor. The armature resistance is Ra =0.05Ω. The back emf constant is Kv =1.527v/A-rad/s. The rated field current is If = 2.5A. The dc input voltage to the chopper is Vs= 600v. If it is required to maintain a constant developed torque of Td = 547N-m, plot the motor speed against the duty cycle k of the chopper. [8+8] 4. A 3 phase, 4 pole, 50 Hz squirrel cage Induction motor has the following circuit parameter. r1 = 0.05ohm, r2 = 0.09ohm, X1 + X2 = 0.55ohm. The motor is star connected and rated voltage is 400V. It drives a load whose torque is proportional to the speed and is given as T1 = 0.05 ω Nw-m. Determine the speed and torque of the motor for a firing angle of 450 of the AC Voltage Controller on a 400V, 50 Hz supply. [16] 5. With a block schematic diagram explain how the speed of the Induction motor can be controlled automatically (i.e., using closed loop scheme) with voltage source Inverters. Mention the applications of the above scheme. [16] 6. A 600V, 50Hz, 30kW, 3-phase induction motor is used as the drive motor in an SER system. It is required to deliver constant (rated) motor torque over the full range from 100rpm to the rated speed of 1000rpm. The motor equivalent circuit parameters are: 1 of 2

Set No. 3

Code No: RR410202

R1 = 0.05Ω, R′ = 0.07Ω, R0 = 53Ω, X0 = 23Ω, X1 + X2 ’ = 0.153Ω. Stator to rotor turns ratio is 1.3. Calculate the motor currents, efficiency and power factor at 300 rpm. [16] 7. Discuss the CSI method of speed control of synchronous motor and describe the operation of the converter with waveforms. [16] 8. Describe the open-loop and closed loop methods of speed control of a synchronous motor using VSI. [16] ⋆⋆⋆⋆⋆

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Set No. 4

Code No: RR410202

IV B.Tech I Semester Regular Examinations, November 2007 POWER SEMI CONDUCTOR DRIVES (Electrical & Electronic Engineering) Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks ⋆⋆⋆⋆⋆ 1. (a) Explain how four-quadrant operation is achieved by dual converters each of 3 φ full wave configuration for d.c. separately excited motor. (b) Distinguish between circulating current and non-circulating current mode of operation. [10+6] 2. Describe the relative merits and demerits of the following types of braking for dc motors: mechanical braking, dynamic braking and regenerative braking with neat diagram. [16] 3. (a) Derive the expressions for average motor current, current Imax and Imin and average torque for chopper-fed dc separately excited motor. (b) A dc chopper controls the speed of a separately excited motor. The armature resistance is Ra =0.05Ω. The back emf constant is Kv =1.527v/A-rad/s. The rated field current is If = 2.5A. The dc input voltage to the chopper is Vs= 600v. If it is required to maintain a constant developed torque of Td = 547N-m, plot the motor speed against the duty cycle k of the chopper. [8+8] 4. (a) Using 3 phase solid state AC Voltage Controllers explain clearly how it is possible to achieve 4 quadrant operation of 3 phase Induction motors. (b) Draw a closed loop block schematic diagram for the above speed control technique. Mention the merits of the above method of speed control. [8+8] 5. The equivalent circuit of a three phase Induction motor having a delta connected stator has the following parameters at rated frequency of 50Hz. r1 = 0.1Ω, r2 = 0.2Ω, x1 = 1.0Ω, x2 = 1.0Ω, xm = 20Ω The rated voltage of machine is 250V. Determine the voltage to be applied as a function of stator frequency for rotor frequencies of 0, 2, 3Hz. Consider only the fundamental. [16] 6. A fan with a load characteristic TL = kN 2 is to be driven by an SER drive incorporating a 440V, 50Hz, 100kW induction motor. It is required to deliver rated speed of 1440 rpm and to provide smooth speed control down to 750 rpm. The motor equivalent circuit parameters referred to primary turns are R1 = 0.052Ω, R2 ’ = 0.06Ω, Xm = 10Ω, X1 + X2 ’ = 0.29Ω, R0 = 100Ω. Stator to rotor turns ratio: 1.2 Calculate the motor efficiency and power factor at 750rpm. Friction and windage effects may be neglected. It is assumed that the motor is started from rest and run upto 750 rpm by the secondary resistance method. [16] 1 of 2

Set No. 4

Code No: RR410202

7. How is the output voltage of a VSI improved by PWM techniques? Explain how you will use this converter for speed control of a synchronous motor. [16] 8. Describe the open-loop and closed loop methods of speed control of a synchronous motor using VSI. [16] ⋆⋆⋆⋆⋆

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