Page No : 1/ 25 SUMMER – 12 EXAMINATION Subject … Describe with neat sketch the principle of...

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MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION (Autonomous) (ISO/IEC - 27001 - 2005 Certified) ___________________________________________________________________________________ Page No : 1/ 25 SUMMER – 12 EXAMINATION Subject Code : 12232 Model Answer Q1 a) (i) State the meaning of holding current and latching current. Label them on the VI characteristic of SCR. -4 marks 1 marks for meaning of holding current 1marks for meaning of latching current 2 marks for VI characteristics of SCR Ans: 1) Holding Current (I H ) :- This is the minimum anode current required to maintain the thyristor in the on- state. To turn off a thyristor, the forward anode current must be reduced below its holding current for a sufficient time for mobile charge carriers to vacate the junction. 2) Latching current (I L ):- This is the minimum anode current required to maintain the thyristor in the on-state immediately after a thyristor has been turned on and the gate signal has been removed. If a gate current, greater than the threshold gate current is applied until the anode current is greater than the latching current I L then the thyristor will be turned on or triggered. VI Characteristics of SCR Q1a) ii) Draw construction of SCR using two transistor model. Explain its operation. -4 marks. 2 marks for construction of SCR using two transistor model 2marks for explanation Ans:

Transcript of Page No : 1/ 25 SUMMER – 12 EXAMINATION Subject … Describe with neat sketch the principle of...

Page 1: Page No : 1/ 25 SUMMER – 12 EXAMINATION Subject … Describe with neat sketch the principle of working of single phase to single phase cycloconverter with resistive load. -4marks

MAHARASHTRA STATE BOARD OF TECHNICAL EDUCATION(Autonomous)

(ISO/IEC - 27001 - 2005 Certified)___________________________________________________________________________________

Page No : 1/ 25SUMMER – 12 EXAMINATION

Subject Code : 12232 Model Answer

Q1 a) (i) State the meaning of holding current and latching current. Label them on the VI characteristic of SCR. -4marks

1 marks for meaning of holding current 1marks for meaning of latching current 2 marks for VI characteristics of SCR

Ans:1) Holding Current (IH) :- This is the minimum anode current required to maintain the thyristor in the on- state. To

turn off a thyristor, the forward anode current must be reduced below its holding current for a sufficient timefor mobile charge carriers to vacate the junction.

2) Latching current (IL):- This is the minimum anode current required to maintain the thyristor in the on-stateimmediately after a thyristor has been turned on and the gate signal has been removed. If a gate current,greater than the threshold gate current is applied until the anode current is greater than the latching current IL

then the thyristor will be turned on or triggered.

VI Characteristics of SCR

Q1a) ii) Draw construction of SCR using two transistor model. Explain its operation. -4 marks.

2 marks for construction of SCR using two transistor model 2marks for explanation

Ans:

Page 2: Page No : 1/ 25 SUMMER – 12 EXAMINATION Subject … Describe with neat sketch the principle of working of single phase to single phase cycloconverter with resistive load. -4marks

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Page No : 2/ 25SUMMER – 12 EXAMINATION

Subject Code : 12232 Model Answer

The basic p-n-p-n SCR structure is analogous to a pair of complementary n-p-n and p-n-p bipolar transistors.

The n-p-n and p-n-p transistors in the equivalent model are interconnected so that regenerative action occurswhen a proper gating signal is applied to the base of the n-p-n transistor.

If a positive pulse is then applied to the base of the n-p-n transistor, the transistor turns on and forces thecollector (which is also the base of the n-p-n transistor) to a low potential; as a result, a current Ia begins toflow. Because the p-n-p transistor Q1 is then in the active state, its collector current flows into the base of then-p-n transistor (Ic1=lb2) and sets up the conditions for regeneration. If the external gate drive is removed, themodel remains in the on state as a result of the division of currents associated with the two transistors,provided that sufficient principal current (Ia) is available

Q1 a) iii) Differentiate between 1Φ & 3Φ controlled converter on the basis of number of SCRs, currentcapacity, application, cost -4 marks

1 marks each for given points to differentiate.Ans:

1 Φ controlled converter 3 Φ controlled converter

1) No. of SCR - Four

2) Current capacity - low

3) Application – Low power DC load

4) Cost- less expensive

5) PIV across thyristor: low

6) Ripple factor: low

7) Filter required at dc output

1) No. of SCR :Six

2) Current capacity :High

3) Application :Used for high power D.C. load

4) Cost :More expensive

5) PIV across thyristor: high

6) Very low

7) No need of filter at dc output

Q1 a) iv) Draw circuit diagram of 3 Φ series inverter and describe its operation-------4 marks

2 marks for circuit diagram of 3Φ series inverter 2marks for explanation of operation of 3Φ series inverter

Ans:

Page 3: Page No : 1/ 25 SUMMER – 12 EXAMINATION Subject … Describe with neat sketch the principle of working of single phase to single phase cycloconverter with resistive load. -4marks

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Page No : 3/ 25SUMMER – 12 EXAMINATION

Subject Code : 12232 Model Answer

A 3Φ series inverter can be developed using three separate 1Φ series inverter. Each phase functions asindependent series inverter. The value of L, R &C are chosen such that the current response is under damped.

The thyristors are triggered in sequence T1,T6,T2,T4,T3&T5.The firing frequency is six times the outputfrequency. the interval between successive firing is T/6. If coupled inductors are used then T4 can be turnedOn before turning OFF of T1. Similarly thyristor in other phase can be turned ON. if coupled inductor is notused only one thyristor in one phase remains ON.

Q.1 b) i) list different applications of thryistor in power electronics. Describe with neat sketch working ofbattery charger using SCR. -6 marks

2 marks for any four applications of thyristors. 2marks for neat sketch of battery charger using SCR. 2marks for explanation of operation battery charger using SCR

Ans:

Different applications of thryistor in power electronics : phase controlled converters, inverters, choppers,Static VAr Compensators, V/f drives, Dc drives, Heating and welding applications, HVDC applications, Temp.controllers, Fan speed regulators, Static switches, battery chargers

Working of battery charger:

When switch S1 is closed, the single phase 230V supply is stepped down by step –down center tappedtransformer. D1 and D2 form fullwave rectifier. Due to this pulsating DC appears across P and Q. WhenSCR is off, its cathode is held at the potential of the discharged battery. During each positive half cyclewhen potential of O rises to sufficient level so as to forward bias diode D3 and gate cathode junction ofSCR, the gate pulse is provided to SCR1 and its turned on.

When SCR1 is ON, charging current flows thr. Battery. Due to zener diode, the potential of point ‘O’ isheld at 12V. When the battery is fully charged cathode remains at 12V and D3 can not be forwardbiased and prevents SCR from turning ON and further charging is stopped.

Page 4: Page No : 1/ 25 SUMMER – 12 EXAMINATION Subject … Describe with neat sketch the principle of working of single phase to single phase cycloconverter with resistive load. -4marks

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Page No : 4/ 25SUMMER – 12 EXAMINATION

Subject Code : 12232 Model Answer

Q1b (ii)Draw circuit diagrams of basic series inverter and parallel inverter. State any two limitations ofeach. - 6marks

2 marks for circuit diagrams of basic series inverter. 2marks for circuit diagrams of basic parallel inverter. 1marks for for any two limitation of basic series inverter. 1marks for for any two limitation of basic parallel inverter.

Ans:

Circuit Diagram of basic series and parallel inverter :

Circuit diagram of basic series inverter Circuit diagram of parallel inverter

Disadvantages of Basic series inverter:

1. High rating of commutating components are required.

2. The Max. Output frequency is limited to value of commutating components.

3. The load voltage waveform is distorted

4. Load current is drawn from D.C. source only during positive cycle only.

Disadvantages of Parallel inverter: -

1. High value commutating capacitor is required.

2. The output wave form is square wave and requires filter to get sinusoidal voltage.

3. Center tapped transformer is required.

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Page No : 5/ 25SUMMER – 12 EXAMINATION

Subject Code : 12232 Model Answer

Q2 a) State any four methods of SCR triggering. Draw a circuit diagram of SCR triggering by UJT.

-4 marks

2 Marks for four methods of SCR triggering 2 marks for circuit diagram of SCR triggering by UJT

Ans:

Methods of SCR triggering

1. Thermal triggering (Temp. triggering)2. Radiation triggering (Light triggering)3. dv/dt triggering4. Gate triggering5. Voltage ttriggering

Q2b) List the factors required for selection of heat sink used for SCR. -4 marks.

4marks for listing any six factors required for selection of heat sink used for SCR.

Ans:

Thermal conductivity of metals, the surface area of heat sink, Thickness of metal, Attachment with case ofdevice, It is surface condition, SCR current rating, SCR casing structure.

Page 6: Page No : 1/ 25 SUMMER – 12 EXAMINATION Subject … Describe with neat sketch the principle of working of single phase to single phase cycloconverter with resistive load. -4marks

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Page No : 6/ 25SUMMER – 12 EXAMINATION

Subject Code : 12232 Model Answer

Q2.c) Describe with neat sketch the principle of working of single phase to single phase cycloconverter withresistive load. -4marks

2 marks for diagram of 1Φ to 1Φ cycloconverter with resistive load. 2marks for explanation of 1Φ to 1Φ cycloconverter with resistive load.

Cycloconverter is device for converting input at one frequency to output at different frequency with onestage conversion. Two types of configurations are used for single phase to single phase cycloconverter1) midpoint type 2) bridge type (any one is enough)In midpoint type,During +ve half cycle of supply voltage, P1 is gated to produce positive going output.During –ve half cycle of supply voltage, P2 is gated to producve positive going output.During +ve half cycle of supply voltage, N2 is gated to produce negative going outputDuring –ve half cycle of supply voltage, N1 is gated to produce negative going output.

In bridge type,P1 P2 and P3P4 thyristors are gated to produce positive going load voltage for positive and negative halfcycles of supply voltage respectivelyN1N2 and N3N4 are gated to produce negative going load voltage for positive and negative half cycles ofsupply voltage respectively

Page 7: Page No : 1/ 25 SUMMER – 12 EXAMINATION Subject … Describe with neat sketch the principle of working of single phase to single phase cycloconverter with resistive load. -4marks

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Page No : 7/ 25SUMMER – 12 EXAMINATION

Subject Code : 12232 Model Answer

Q2.d) With the help of neat circuit diagram explain working of sinusoidal pulse width modulation.4 marks

2 marks for diagram of sinusoidal PWM. 2marks for explanation of sinusoidal PWM.

Ans:

In sinusoidal pulsewidth modulation, the pulse width is varied according to the amplitude of a sine waveevaluated at the center of the same pulse. By comparing a sinusoidal reference signal with a triangular carrierwave of frequency fc, the gating signals are generated. the frequency of inverter i.e fo is determined byfrequency of reference signal fr & its amplitude controls the modulation index ’M’.(M= ER/Ec)The no. of pulses per half cycle depends on the carrier frequency. There are largest harmonic amplitudes inthe o/p voltage of order fc/fr+-1 or 2Np+-1 where Np is the no of pulses per half cycle.

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Page No : 8/ 25SUMMER – 12 EXAMINATION

Subject Code : 12232 Model Answer

Q.2 e) State different methods to control the output voltage of inverter. Explain PWM method.-4 marks

2 marks for listing two different methods to control the output voltage of inverter. 2 marks for explanation of sinusoidal PWM.

Ans:The various methods to control the output voltage of inverter are as following.

1) External control of A.C. output voltage.

2) External control of D.C. input voltage.

3) Internal control of inverter

i) Series inverter control

ii) PWM control.

Pulse width modulation: PWM control method:

This is most efficient method of controlling the output voltage. In this method PWM is incorporatewithin the inverters. In this method a fixed d.c. voltage is supplied to inverter & controlled a.c.output voltage is applied to inverter & controlled a.c. output voltage is obtained by adjusting theON-OFF periods of inverter devices.

The commonly used PWM control techniques are .

1. Single pulse width modulation

2. Multiple pulse width modulation

3. Sinusoidal pulse width modulation

Advantages:-

1. Additional components are not required.

2. Lower order harmonics can be eliminated.

Disadvantage:-

The main drawback of this method is that the SCRs used in this method must have very lowturn -ON & turn -OFF times, therefore they are very expensive.

Page 9: Page No : 1/ 25 SUMMER – 12 EXAMINATION Subject … Describe with neat sketch the principle of working of single phase to single phase cycloconverter with resistive load. -4marks

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Page No : 9/ 25SUMMER – 12 EXAMINATION

Subject Code : 12232 Model Answer

Q.2 f) Describe the operation of speed control of Dc servomotor. -4 marks

2 marks for block diagram of speed control of DC servomotor. 2 marks for explanation of speed control of DC servomotor.

Figure 1: Torque speed characteristics Figure 2: Block diagram of DC servo control

Figure 3 Full bridge drive for DC servo motor

Explanation of speed control of DC servomotor:D.C servomotors are separately excited D.C motor with linear torque speed characteristics. These

motors are used in feedback control system for position control applications. The control system forDC servo adjusts the average armature voltage so that the output position is equal to the inputposition command. The servo amplifier is a full bridge chopper amplifier.

Page 10: Page No : 1/ 25 SUMMER – 12 EXAMINATION Subject … Describe with neat sketch the principle of working of single phase to single phase cycloconverter with resistive load. -4marks

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Page No : 10/ 25SUMMER – 12 EXAMINATION

Subject Code : 12232 Model Answer

Q3. (a): State the meaning of commutation. Explain class B method of commutation. -4marks

1 marks for meaning of commutation 1 marks for diagram of class B commutation. 1 marks for waveform of class B commutation . 1 marks for explanation of diagram of class B commutation.

Ans:Commutation :- Definition-The process of turning off the conducting SCR is called as commutation.Class –B commutation.

Figure shows circuit for class B commutation. It is also called as self commutation by LC circuit when supplyV is applied, capacitor C starts charging with upper plate +Ve & charges to supply voltage. As soon as SCRS1 is triggered, the circuit current flows in two directions.1) The load current IL flows through V, SCR S1, load RL & back to V.2) Commutating current IC.As S1 is triggered, capacitor C now starts discharging through + ve plate of capacitor C. L, SCR S1 & -ve plateof capacitor C. When capacitor C is fully discharged, it starts getting charged with reverse polarity. Due tothe reverse voltage, a commutating current Ic starts flowing which opposes load current IL. When thecommutating current IC is greater than load current IL SCR S1 turns off.Capacitor C gets charged to the supply voltage again with upper plate +ve.

Q3. (b): Draw and explain working of single phase half controlled bridge inverter. -4 marks

2 marks for circuit diagram of single phase half controlled bridge inverter 1 marks for waveform single phase half controlled bridge inverter . 1 marks for working of single phase half controlled bridge inverter .

Page 11: Page No : 1/ 25 SUMMER – 12 EXAMINATION Subject … Describe with neat sketch the principle of working of single phase to single phase cycloconverter with resistive load. -4marks

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Page No : 11/ 25SUMMER – 12 EXAMINATION

Subject Code : 12232 Model Answer

Ans:

Single phase half bridge inverter consists of two thyristors (T1,T2) and two diodes (D1,D2) and a three wiresupply. For 0 < t < T/2, T1 conducts and the load is subjected to a voltage Vs/2. At t=T/2, T1 is commutatedand T2 is gated on. During, T/2 < t <= T, T2 conducts and the load voltage is –Vs/2. The load voltage is thusalternating waveform of amplitude Vs/2 and frequency 1/T Hz. D1, D2 are feedback diodes required forinductive/ capacitive load. If load is inductive, current lags voltage. The diode conducts load current whenSCRS are off. Load reactive energy is fed back to supply.

Q3.(c): List drawbacks of harmonics at the output of inverter. explain filter method of harmonicreduction. -4marks

2 marks for two drawbacks of harmonics at o/p of inverter. 1 mark for diagram of filter method. 1 mark for explanation of filter method.

Ans:Drawbacks of harmonics at the output of inverter:-Presence of harmonics at the inverter output causes

1) Overheating of conductors2) Torque pulsations in induction motor.3) Malfunctioning of relays4) Increase in the KVA demand5) Third harmonic current increases the neutral current in case of three phase inverter6) Decreases overall efficiency of system.

Harmonic filters are employed at the output of the inverters. Low pass filters, resonant filters arecommonly used to filter harmonics of the inverter output. They are mainly LC circuits which block or by passthe particular harmonic component and reduce harmonic content of the supply going to load. For low passfilter, as the frequency increases, the capacitive impedance will decrease and inductive impedance willincrease and thus the filter will attenuate higher i.e. harmonic frequencies. In case of resonant filter, The LCparallel branch will provide low impedance path for the harmonic frequency to which its tuned. The values ofL and C are found from the expression for resonant frequency,

LCfc 2

1

Page 12: Page No : 1/ 25 SUMMER – 12 EXAMINATION Subject … Describe with neat sketch the principle of working of single phase to single phase cycloconverter with resistive load. -4marks

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Page No : 12/ 25SUMMER – 12 EXAMINATION

Subject Code : 12232 Model Answer

Q.3(d): State the working principle of chopper. State control techniques used for chopper.-4 marks

1 marks for working principle of chopper 1 marks for diagram of chopper 2 marks for stating control techniques used for chopper.

Ans:Chopper principle.-

A chopper is a high frequency semiconductor switch. It connects source to load and disconnectsload from source at high speed. In this way, a chopped dc voltage appears across the load. In achopper, the switch may be any switching device such as SCR, BJT, MOSFET,GTO etc. Diode D isfreewheeling diode. It conducts only when switch is in off state for highly inductive load. L is filterinductance. Vs is fixed D.C. voltage.

dcdcon

dcoffon

onoavg VV

T

TV

TT

TV

is called as duty ratioControl techniques used for chopper are as follows:1) Constant frequency systemIn this method, the on time Ton is varied keeping chopping frequency (f= 1/T) constant2) Variable frequency systemIn this method, chopping frequency is varied and either Ton or Toff is kept constant.

Page 13: Page No : 1/ 25 SUMMER – 12 EXAMINATION Subject … Describe with neat sketch the principle of working of single phase to single phase cycloconverter with resistive load. -4marks

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Page No : 13/ 25SUMMER – 12 EXAMINATION

Subject Code : 12232 Model Answer

Q. 3(e): List any six advantages of AC drive over DC drive. - 4marks 4 marks for six advantages of AC drive over DC drive.

Ans:

Advantages of A.C. drive over D.C. drive are as follows.1) The maximum speed of AC drive is higher because of brushes are absent.2) Large power to weight ratio.3) Less maintenance needed as the brushes are absent.4) Higher order harmonics are present in the input current of AC drives and can be easily filtered unlike DC

drives which draw low order harmonics due.5) Can’t be used in petrochemical, or chemical industrial environment because of sparking at brushes.6) AC drives are smaller, lighter, more commonly available, and less expensive than DC drives.

Q.4a (i): Draw circuit diagram of 1Φ half controlled converter using RL load. Sketch waveforms of outputvoltage and output current. --4 marks

2 marks circuit diagram of 1Φ half controlled converter using RL load 2 marks for waveforms of output voltage and output current.

Page 14: Page No : 1/ 25 SUMMER – 12 EXAMINATION Subject … Describe with neat sketch the principle of working of single phase to single phase cycloconverter with resistive load. -4marks

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Page No : 14/ 25SUMMER – 12 EXAMINATION

Subject Code : 12232 Model Answer

Q.4a (ii): State the effect of source impedance on the performance of single phase fully controlledconverter. --4marks

2 marks circuit diagram of 1Φ fully controlled converter. 2 marks for explanation of effect of source impedance on the performance of single phase fully

controlled converter.Ans:

The effect of source inductance is that it causes both incoming and outgoing thyristors to conduct together.The duration for which both incoming and outgoing thyrsitors are in conduction is called as commutationperiod ot overlap period. For single phase fully controlled converter, during overlap period, the outputvoltage is zero.

Q.4a (iii): Differentiate between cycloconverter and cycloinverter( any four points) – 4 marks

1 mark for each point

Ans: Cycloconverter Cycloinverter

Output frequency Less than input frequency Higher than input frequency

Device used SCR SCR

Type ofcommutation

Natural or line commutation Load commutation

Acts as Frequency divider Frequency multiplier

Applications Low speed, high torque ac drives Electrical heating, speed control ofmotors

Page 15: Page No : 1/ 25 SUMMER – 12 EXAMINATION Subject … Describe with neat sketch the principle of working of single phase to single phase cycloconverter with resistive load. -4marks

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Page No : 15/ 25SUMMER – 12 EXAMINATION

Subject Code : 12232 Model Answer

Q4 a iv): Describe working of closed loop control of stepper motor

2 marks for explanation 2 marks for figure

Ans:In a closed-loop stepping motor system the rotor position is detected and fed back to the control unit. Eachstep command is issued only when the motor has responded satisfactorily to the previous command and sothere is no possibility of the motor losing synchronism. A schematic closed-loop control is shown in Fig.Initially the system is stationary with one or more phases excited. The target position is loaded into thedowncounter and a pulsed START signal is applied to the control unit, which immediately passes a stepcommand to the phase sequence generator. Consequently there is a change in excitation and the motorstarts to accelerate at a rate dictated by the load parameters.As the first step nears completion the position detector generates a pulse which is sent to both thedowncounter and the control unit. The downcounter is decremented and therefore contains the position ofthe load relative to the target. The typical drive circuit for unipolar drive is shown in Fig b.

Q4b i) Describe operation of 3ph fully controlled bridgeconverter with RL load with neat circuit diagram. Sketch different input output waveforms. – 6 marks

2 marks for circuit diagram of 3ph converter with RL load 2 marks for waveforms with RL load 2 marks for explanation

Ans:

Fig. a Fig. b

Three phase fully controlled bridge converter with RL load

Page 16: Page No : 1/ 25 SUMMER – 12 EXAMINATION Subject … Describe with neat sketch the principle of working of single phase to single phase cycloconverter with resistive load. -4marks

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Page No : 16/ 25SUMMER – 12 EXAMINATION

Subject Code : 12232 Model Answer

The three phase fully controlled converter is a 6-pulse converter.

Each thyristor conducts in two pairs and each pair conducts for 600.

At any instant, two thyristors can conduct and there are six such pairs: (T6T1), (T1T2), (T2T3),(T3T4), (T4T5),(T5T6).

The output voltage waveform for any α is asix pulse wave with ripple frequency of 300Hz.

The firing angle α is measured at t = 300

For α > 60, the waveforms are different. The voltage goes negative due to the inductive nature of the load.

For α > 900, the load voltage is negative.

The max value of α is 1800

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Page No : 17/ 25SUMMER – 12 EXAMINATION

Subject Code : 12232 Model Answer

Q4b ii): Classify choppers based on quadrants of operation. Sketch only circuit diagrams of each.

6 marks

3marks classification of choppers based on quadrants 3 marks for circuit diagram of each type.

Ans:

First Quadrant Type-A chopper Quadrant diagram

Second Quadrant Type-B chopper

Two Quadrant type A or Type-C chopper

Page 18: Page No : 1/ 25 SUMMER – 12 EXAMINATION Subject … Describe with neat sketch the principle of working of single phase to single phase cycloconverter with resistive load. -4marks

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Page No : 18/ 25SUMMER – 12 EXAMINATION

Subject Code : 12232 Model Answer

Two quadrant type B or type D chopper

.

Q5.a. Draw neat sketch of speed control method of dc series motor using chopper circuit. Explain itsworking. State advantages and disadvantages of this method. ---- 8 marks

2 marks for circuit diagram of chopper circuit 2 marks for waveforms 2 marks for explanation 2 marks for advantages and disadvantages (one point each)

Ans:

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Page No : 19/ 25SUMMER – 12 EXAMINATION

Subject Code : 12232 Model Answer

DC chopper series motor drive a) circuit diagram b) quadrant diagram c) waveforms

D.C. series motor produces high starting torque. They are capable of constant power operation at all speedsand are therefore suitable for traction purpose. The chopper based DC series motor drive can be operated in1) current limit control and 2) frequency control.

A chopper drive can operate in two quadrants. A chopper drive is easily adaptable for regenerative braking ofdc motors and thus it results in energy saving which is mainly required for battery operated vehicles.

Advantages: 1) Simple circuit, 2) High efficiency, 3) Light weight, 4) Energy saving by regenerative braking

Disadvantages: 1) A separate Dc source is needed, 2) More suitable for battery operated vehicles

Q5b. Draw a neat sketch of speed control of 3 phase IM using variable frequency square VSI. Explain itsworking. List any four advantages of VF drive. –8 marks

4 marks for circuit diagram of 3 phase IM using variable frequency square VSI 2 marks for explanation 2 marks for advantages of VF drive. (Four points one mark each).

Ans:

The speed of induction motor can be expressed by,

P

fn s

s

120

Page 20: Page No : 1/ 25 SUMMER – 12 EXAMINATION Subject … Describe with neat sketch the principle of working of single phase to single phase cycloconverter with resistive load. -4marks

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Page No : 20/ 25SUMMER – 12 EXAMINATION

Subject Code : 12232 Model Answer

By changing frequency, speed can be controlled. Apart from frequency, the applied voltage needs to becontrolled, to keep the air gap flux constant and avoid saturation.

The dc voltage is obtained from a phase controlled rectifier. This dc voltage is fed to a force commutatedbridge inverter which generates the required variable voltage variable frequency output connected to thedelta connected stator of 3ph induction motor. The output of the inverter is stepped square wave. Thephase controlled converter and inverter are interconnected by a link inductor which smoothens the dcvoltage ripples.The closed loop control adjusts the firing angle of the converter to maintain the required dc voltage atthe input of the voltage source inverter and adjusts the firing of thyristors of the inverter to maintain therequired frequency.

Q5 c: List commutation methods of choppers with neat sketch. Explain working of auxiliary commutationmethod. Draw necessary waveforms. --- 8 marks.

2 marks for listing commutation methods for chopper. 2 marks for circuit diagram 2 marks for working details –explanation of auxiliary commutation method. 2 marks for waveforms.

Ans:

Commutation methods for choppers:(1) Forced commutation: a) voltage commutation b) current commutation((2) Load Commutation

Auxiliary commutated chopper

This type of chopper uses Auxiliary commutation. In this commutation method, the auxiliary thyristor T2 isrequired to commutate main thyristor.

To start the circuit, C is initially charged with polarity shown by triggering aux thyristor T2. As shargingcurrent decays to zero, T2 is turned off.

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Subject Code : 12232 Model Answer

T1 is turned on. Source current flows in two paths: load current constitutes one path and the commutationcurrent ic in another path. When T1 is on, load gets connected to supply and the load current flows through+Edc-T1-load—Edc

Oscillatory commutation current flows through C+-T1-L-D-C-. The capacitor votage reverses its polarity.Further oscillations are stopped by D.

When auxiliary thyristor T2 is gated on, reverse charged capacitor is placed across T1 and turns it off.

The capacitor gets charged to supply voltage through T2-load and T2 is off when C is fully charged.

Auxiliary commutation chopper waveforms

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Subject Code : 12232 Model Answer

Q6 (a). Explain working of modified series inverter

2 marks for circuit diagram of modified series inverter 1 marks for explanation 1 marks for waveforms.

The values of L and C are such that it forms under damped RLC circuit (C

LR

42 )

When T1 is turned on , it starts conducting and sinusoidal current starts flowing through load(T1-L1-C-load) .The capacitor voltage reverses its polarity and gets charged upto (Ec+Edc). The current reaches zero and T1

commutated.

When T2 is turned on, capacitor C gets discharged and negative sinusoidal current pulse flows through C –L2-T2-load. When current reaches zero, T2 is turned off. The capacitor voltage reverses to a voltage whichdepends upon R,L, C

The main drawbacks of the basic series inverter are:

1) The load voltage waveform has more distortion due to time delay

2) The maximum inverter frequency is limited to slightly less than the ringing frequency to avoid short circuitacross supply.

These drawbacks are overcome in modified series inverter.

The inductance is replaced by a mutually coupled inductance. L1 and L2 have same inductance are closelycoupled. Because of this modification, thyristor T2 can be triggered before T1 is turned off. When T2 istriggered little before T1 is turned off, the capacitor voltage is will appear across L2. Since L1 and L2 are closelycoupled, same voltage will appear across L1 and the cathode voltage will be greater than the anode voltagefor T1 and turn it off.

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Subject Code : 12232 Model Answer

Q6 (b). Explain working of Jones chopper with neat circuit diagram- 4marks

2 marks for circuit diagram of Jone’s Chopper 1 marks for explanation of basic principle 1 marks for waveforms.

Jones chopper is an example of class D commutation in which a charged capacitor is switched by an auxilarySCR to commutate the main SCR.

In this circuit SCR1 is the main switch and SCR2 is the auxilary switch which is of lower capacity than SCR1and is used to commutate SCR1 by a reverse voltage developed across the capacitor C.

“The special feature of the circuit is the tapped autotransformer T through a portion of which the loadcurrent flows”.

When T1 is ON, capacitor C discharges resonantly through T1, L1, D1 . This discharge current does not flowthrough L2 and back to the battery because of transformer action of T. The load current is picked up by T1and the freewheel diode D1 is reverse biased. As the capacitor voltage swings negative, the reverse bias ondiode D2 decreases.

When T2 is on the negative voltage on capacitar C is applied across T1 and it becomes OFF. The load currentwhich is normally constant starts to flow in T2 and capacitor C. The capacitor C charged positively at first uptoa voltage equal to supply voltage Vdc. The freewheel diode become forward bias and begins to pickup load

current. And capacitor current starts to reduce. After this the energy 2

21

LI in the inductance L2 is forced in

to the capacitor C. Charging is positively to 2

21

CV the capacitor current continues to decrease as a result

current through T2 decreases gradually become OFF.

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Subject Code : 12232 Model Answer

Q6 (b). Explain working of Jones chopper with neat circuit diagram- 4marks

2 marks for circuit diagram of Jone’s Chopper 1 marks for explanation of basic principle 1 marks for waveforms.

Jones chopper is an example of class D commutation in which a charged capacitor is switched by an auxilarySCR to commutate the main SCR.

In this circuit SCR1 is the main switch and SCR2 is the auxilary switch which is of lower capacity than SCR1and is used to commutate SCR1 by a reverse voltage developed across the capacitor C.

“The special feature of the circuit is the tapped autotransformer T through a portion of which the loadcurrent flows”.

When T1 is ON, capacitor C discharges resonantly through T1, L1, D1 . This discharge current does not flowthrough L2 and back to the battery because of transformer action of T. The load current is picked up by T1and the freewheel diode D1 is reverse biased. As the capacitor voltage swings negative, the reverse bias ondiode D2 decreases.

When T2 is on the negative voltage on capacitar C is applied across T1 and it becomes OFF. The load currentwhich is normally constant starts to flow in T2 and capacitor C. The capacitor C charged positively at first uptoa voltage equal to supply voltage Vdc. The freewheel diode become forward bias and begins to pickup load

current. And capacitor current starts to reduce. After this the energy 2

21

LI in the inductance L2 is forced in

to the capacitor C. Charging is positively to 2

21

CV the capacitor current continues to decrease as a result

current through T2 decreases gradually become OFF.

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Page No : 23/ 25SUMMER – 12 EXAMINATION

Subject Code : 12232 Model Answer

Q6 (b). Explain working of Jones chopper with neat circuit diagram- 4marks

2 marks for circuit diagram of Jone’s Chopper 1 marks for explanation of basic principle 1 marks for waveforms.

Jones chopper is an example of class D commutation in which a charged capacitor is switched by an auxilarySCR to commutate the main SCR.

In this circuit SCR1 is the main switch and SCR2 is the auxilary switch which is of lower capacity than SCR1and is used to commutate SCR1 by a reverse voltage developed across the capacitor C.

“The special feature of the circuit is the tapped autotransformer T through a portion of which the loadcurrent flows”.

When T1 is ON, capacitor C discharges resonantly through T1, L1, D1 . This discharge current does not flowthrough L2 and back to the battery because of transformer action of T. The load current is picked up by T1and the freewheel diode D1 is reverse biased. As the capacitor voltage swings negative, the reverse bias ondiode D2 decreases.

When T2 is on the negative voltage on capacitar C is applied across T1 and it becomes OFF. The load currentwhich is normally constant starts to flow in T2 and capacitor C. The capacitor C charged positively at first uptoa voltage equal to supply voltage Vdc. The freewheel diode become forward bias and begins to pickup load

current. And capacitor current starts to reduce. After this the energy 2

21

LI in the inductance L2 is forced in

to the capacitor C. Charging is positively to 2

21

CV the capacitor current continues to decrease as a result

current through T2 decreases gradually become OFF.

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Page No : 24/ 25SUMMER – 12 EXAMINATION

Subject Code : 12232 Model Answer

Q6 (c). State principle of induction heating. Draw the block diagram of it using thyristor circuit.—4marks

1 marks for principle of induction heating. 2 marks for block diagram. 1 marks for explanation.

Basic principle of induction heating:

Induction heating operates on the principle of ‘induction’ in order to heat an electrically conductingmaterial. The material to be heated is known as the work piece and the coil wound around it is known aswork coil. The work coil acts as primary winding and the work piece act as a short circuited secondarywinding of a transformer.

When a high frequency ac voltage is applied across the work coil, a magnetizing current flowsthrough it. This will generate flux in the work piece and induce voltage into work piece. Since the work pieceis closed onto itself, eddy current flows into it. Work piece will get heated up due to finite resistance offeredby work piece to the flow of the eddy currents. The heat is confined to surface because of skin effect.

In voltage source series resonant inverter, power controlled by a variable switching frequency control

Q6 (d). State the importance of freewheeling diode is case of 1ph fully controlled converter. Draw thecircuit diagram 1ph. Fully controlled half wave converter with r-L load and free wheeling diode.(any foureach). -------------4 marks

2 marks for stating importance of freewheeling diode. 2 marks for circuit diagram.

The importance of freewheeling diode:

1) It prevents the output voltage from becoming negative.2) The load current is transferred from the main thyristors to the freewheeling diode, thereby

allowing all of its thyristors to regain their blocking states.3) Input power factor is improved.4) Load current waveform is improved and thus the load performance is better.

Page 25: Page No : 1/ 25 SUMMER – 12 EXAMINATION Subject … Describe with neat sketch the principle of working of single phase to single phase cycloconverter with resistive load. -4marks

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Page No : 25/ 25SUMMER – 12 EXAMINATION

Subject Code : 12232 Model Answer

Q6 (e): List specifications and ratings of SCR (any four each)—4marks.

1/2 mark for each parameter (any eight)Voltage ratings:

Peak Working Forward OFF state voltage (VDWM) ,Peak non-repetitive off state forward voltage (VDRM)

Peak repetitive off state forward voltage (VDSM), Peak working reverse voltage (VRWM)

Peak repetitive reverse voltage (VRRM), Peak non-repetitive reverse voltage (VRSM)

Current Rating:

Maximum RMS current (Irms), Average current rating (ITAV), Latching Current (IL), Holding Current (IH)

Surge Current rating, I2t rating, di/dt rating, Average Power rating Pav, Maximum junction temperature Tjmax