2 4.1 INTRODUCTIONThe category of converters, which converts dc power into ac power popularly known as the inverters.The application areas for the invertors includes theUninterrupted Power Supply the ac motor speedcontrollers,etc.The inverters can be classified based on number of factors like:the nature of output waveform (sine, square, quasi square, PWM etc),the power devices being used (thyristor, transistor, MOSFETs, IGBTs),the configuration being used (series, parallel, half bridge, full bridge).
3 4.2 Basic Series Inverters (Self Commutated Inverter) The size and the cost of the circuit can be reduced to some extent if theoperating frequency is increased but then the inverter grade thyristorswhich are special thyristors manufactured to operate at a higherfrequency must be used, which are costly.4.2 Basic Series Inverters(Self Commutated Inverter)The series inverter uses a class A type commutation. The commutating components L1, C1 are connected in series to form an under damped tuned circuit.Since the SCRs turn off themselves this circuit is known as commutated inverters .
4 Fig. (a )Basic series configuration Operation :At instant t0 SCR1 is turned on. Let the initial voltage capacitor be “vc” with its left plate negative w .r. t. right plate and the sinusoidal load current starts flowing.
5 Fig (b) Mode 1 (t0 to t1)The capacitor C1 start charging in the opposite direction as shown in fig B.The load current eventually comes to zero at instant t1 and SCR1 comes out of conduction due to natural commutation.The voltage on the capacitor C1 at instant t1 is greater than V with its left plate positive w.r.t. its right plate.As there is no discharge path for the capacitor, this voltage will be held constant up to instant t2 where SCR2 is triggered.
6 Fig ( c ) Mode 2 (t1 to t2)At instant t2, SCR2 is turned on and the load voltage and current both becomes negative.The capacitor now discharges resonantly through SCR2, R, L1, as shown in fig (c)At instant t3 the discharge current goes to zero and SCR2 turned off again due to natural commutation. The voltage on C1 is equal to vc.Off time :During the time interval between t1 and t2 both the SCRs are in the off state. Load voltage as well as load current are zero. Therefore this interval is known as off time of the circuit.
7 Limitation on the maximum operating frequency Disadvantages:Limitation on the maximum operating frequencyDistortion in the output wave formHigh rating of commutating componentsThe peak amplitude and duration of output current depends on the load parameters resulting in poor regulation for the inverter.The power flow from the dc source is intermittent. Therefore, the dc supply must have a large peak current rating and the input current contains high percentage of harmonics.
8 Modified Series Inverter The operation can be divided into two modes.Mode 1:At the instant when SCR T2 is triggered, the voltage across the capacitor will be slightly less than (E c + E dc)and the load voltage and current will be closed to zero. Hence the voltage across the capacitor minus the load voltage will appear across L2.Since L1 is closely coupled to L2, the same voltage will appear across L1.Mode 2:The voltage across L1 will tend to increase the cathode potential of SCR T1 more than its anode potential and therefore, SCR T1 will be reverse biased and turn-off. Thus, even if SCR T2 is turned on before SCR T1 is switched off, it will not result into short circuiting of the d.c. source. A similar operation will take place if SCR T1 is triggered before SCR T2 is turned off.
12 Basic Parallel Inverter A parallel inverter is used to produce a square- wave from a d.c. supply.In this inverter, the commutating capacitor comes in parallel with the load during the operation of the inverter. Hence it is called as ‘parallel inverter’.OperationMode 1:This mode begins when T1 is fired and current flows through the inductance L and the thyristor T1.When SCR is turned on, a d.c. voltage E dc appears across half the transformer primary, which means the total primary voltage is 2 E dc, hence the capacitor is charged to 2 E dc.
13 Mode 2:This mode begins when thyristor T2 is fired. When T2 is turned on, the commutating capacitor applies voltage -2 Edc to appear across T1, it will be turn off.SCR T2 will now be conducting and the voltage of 2 E dc will appear across the transformer primary and commutating capacitor, but with reverse polarity.Mode 3:During mode 3, this SCR is again turned on. Commutating capacitor applies a voltage -2 E dc to appear across T2.when this reverse voltage is applied for sufficient time across T2, it will be turned off. If trigger pulses are applied periodically to alternate thyristors, an approximately rectangular voltage waveform will be obtained at transformer output terminals.
15 Circuit Diagram Parallel Inverter With Feedback Diodes
16 Parallel Inverter With Feedback Diodes The circuit operation can be divided into differentoperating modes.Thyristor T1 and T2 are the main load carrying thyristors.Inductor L and capacitor C are the commutating components. Diodes D1 and D2 are the feedback diodes. Which permit the load reactive power to be fed back to the d.c. supply.
17 Mode 1 :-During this mode, thyristor T1 is triggered at instant R.Battery voltage forces the current to the primarysection through path Edc -C-A-T1-L- E dc . Terminal C ispositive with respect to A.The flux produced due to this current induces thevoltage in all sections of transformer winding. Theload voltage is nearly equal to Ede and is in suchdirection so as to force current into the dot atterminal P. Due to autotransformer action, voltageEdc is induced in CE section of primary winding.
18 Mode 2:This mode begins with thyristor T2 switched on at instant S. When T2 is turned on, capacitor C will immediately apply a reverse voltage of 2 E dc across SCR T1 and turned off. When SCR T1 is turned off, the capacitor will discharge through SCR T2, inductor L, diode D1, and a portion of a transformer primary winding BA.Thus, the energy stored in the capacitor will be fed back to the load through the transformer coupling of windings BA and PQ.The load current which earlier flowing through SCR T1, will now flow through CB and diode D1 to negative input terminal. This can happen only if diode D1 is forward biased and capacitor discharge current is more than the load current.
19 The current through inductance L will now flow through diode D2, DE and SCR T2, and the trapped energy in inductor L will be fed back to the load. D point is now connected to the negative supply terminal, the load voltage polarity will be reversed.Also capacitor C will be charged in the opposite direction slightly more than supply voltage. Thyristor T2 will stop conducting. Energy is transferred from the capacitor and inductor to the load.Mode 3 :This mode begins with when load current becomes zero, diode D2 will be blocked and SCR T2 will have to triggered again at instant U to reverse the direction of the load current. When thyristor T2 starts conducting, the load voltage will again become equal to Edc.
20 Waveforms For parallel Inverter with Feedback Diode
21 Single Full Bridge Inverter Introduction –A serious drawback of the half bridge inverter is that, it requires a 3-wire dc supply. This is overcomed by the commonly full bridge inverter.
23 Construction: freewheeling diode. It has consist of four thyristor and fourfreewheeling diode.Two thyristor T1 and T2 must be gatedsimultaneously at frequency F=1/T and thyristor T3and T4 must be gated 180 out of phase frequency can be controlled by varying the periodic time T.
24 Operation-When we apply positive load voltage Edc then thyristor T1 and T2 conduct.When we apply negative voltage –edc then thyristor T3and T4 is conduct.Diode D1to D4 serve to feed the load reactive power back to the dc supply.In place of SCRT1, hear two thyristor T1 and T2 conduct similarly in place of SCR2 thyristor T3 and T4 conduct and in place of D1 and diodeD1, D2 conduct, where as instead of D2, hear D3and D4 conductThe load voltage wave form is fairly rectangular and is not affected by the nature of the load.
26 The current rating of power device is equal to the load current. Advantage :No need of an output transformer.Efficiency is high.The current rating of power device is equalto the load current.Disadvantage :Number of four transistors are requiredCosts is highApplication :Used in commutation circuit for bridgeinverter
28 Construction :- It is a complementary impulse commutated inverter. This means that if two inductors are tightly coupled, triggering of one thyristor , turns off another thyristor .Main thyristors T1,T2 .Feedback diodes D1,D2.two capacitors C1,C2 .magnetically coupled inductors L1 and L2.inductance L .
29 OperationMode- 1Thyristor t1 is triggered, then SCR T1 is turn on, upper d.c. source load current Il to the load.As the load current is constant. Voltage drop across L1 is negligible. With zero voltage drop across L1 , T1 , C1 and across C2 is Edc load.Mode -2When SCR T2 is triggered ,turn off the SCR T1voltage across C1 and C2 cannot be changeequal voltages is induced across L1Voltage across thyristor T1 is ET1 = EdcIc1 = Ic2 KCL at node ‘o’ in fig.Ic1 + Ic2 = Il + Il ; IC1 = IC2 =Ic3
30 Mode -3At instant t1 , where capacitor C1 is charged to supply voltage Edc , i.e. Ic 1 =0 at t 1, Vc 2 =0. Just after t 1, current( Il + Im/2) through C1 tends to charge it with bottom plate .At t1 ,the energy stored in inductor L2 is dissipated.This energy is dissipated at instant t 2, therefore I2 decays to zero & a result SCR T2 is turned off at T2.Mode -4When the current IT2 through L2 & T2 has decayed to zero . the load current IL=ID2 still continues flowing through the diode D2 as ID2 during (t3-t2 ) interval.
31 Mode -5As soon as iL equal to ID2.tend to reverse , diode D2 is blocked.Thyristor T2 already gated during the interval (t3-t2)gets turn ON to carry the load current in the reverse direction .The capacitor C1 , now charged to the source voltage Edc is ready for commutating the main thyristor T2 .
34 Operation :-Mc Murray Bedford full bridge inverter circuit can be realized by connecting two half bridge inverters.for Mode 1, thyristors T1 and T2 are conducting and load current flows through Edc, T1, L1 load Zl , L2, and T2.Voltage across C1, C2 is zero but capacitors C3, C4 are charged to voltage Edc.For initiating communication of T1, T2 thyristors T3, T4 are triggered.This reverse-biases T1, T2 by voltage (-Edc) and makes them turned-off.
35 4.3 PULSE –WIDTH MODULATION Pulse-width modulation technique is a control within the inverter & is also known as a variable-duty-cycle regulation.This method of regulation employs variation of the conduction time per cycle to alter the rms output voltage of the inverter. In order to accomplish this regulation technique.
37 0peration:- In fig, SCR1&SCR2 - Two main loads carrying SCRs SCR3, SCR4 - Two auxiliary SCRs which are ofsmaller ratingC1 & C2 - Two separate commutatingcapacitors.When, SCR1 - ONPower is delivered to the load at the same time, C1 ischarged to the voltage of the transformer section ABwith a polarity as shown above fig,SCR1 - OFF
38 At any desired instant by triggering SCR3 After interval, SCR2 – ONTo deliver power in the negative half-cycle.C2 charged at the same time by the voltage of transformationsection CD.SCR2 - OFF by firing SCR4In this method produces a quasi-square-wave outputas below in fig,
40 Single pulse width modulation There is one pulse per half-cycle, and its width is varied
41 Advantages: The modulation index is: The rms output voltage is: Less effect of noisesynchronization between the transmitter & receiveris not essential..
42 Disadvantages: Application: In order to avoid any wave form distortion, thebandwidth required for the PWM communications large ascompare to BW of PAMAverage power transmitted can be as low as 50% ofmaximum powerApplication:voltage regulators.class D audio amplifiers ,which are highly effectively.The following fig, shows the harmonic reduction profile withvariation of the modulation index M
43 The domain harmonic is the third and DF decreases significantly at a low output voltage.
44 Multiple pulse modulation The harmonic contents can be reduced by using several pulses ineach half cycle of output voltage. This type of modulation is alsoknown as uniform-pulse – width-modulationThe number of pulses per half cycle is:Here, = modulation frequency ratiomfThe rms output voltage is:
46 The following waveform shows the harmonic reduction against variation of the modulation index & P=5
47 Sinusoidal pulse modulation Instead of maintaining the width of all pulses the same, the widthof each pulse is varied in proportion to amplitude of a sine waveThis kind of modulation is known as SPWM.The rms output voltage is:
52 INTRODUCTION Chopper is a static device. A variable dc voltage is obtained from a constant dc voltage source.Also known as dc-to-dc converter.Widely used for motor control.Also used in regenerative braking.Thyristor converter offers greater efficiency, faster response, lower maintenance, smaller size and smooth control
53 Definition of ChopperA chopper is an electronic switch that is used to interrupt one signal under the control of another.In other words we can say that, a chopper is a kind of switch which allows the power flow in the circuit for a required duration.Thus, the chopper is also known as a d.c. to d.c. converter.
54 Types of Choppers Step-down choppers: Step-up choppers: In step down chopper output voltage isless than input voltage.Step-up choppers:In step up chopper output voltage is more than input voltage.
55 Step-up chopper Definition : The chopper can be use to produced higher voltages at the load than the input voltage (i.e., E0 ≥ Edc ). This is called as Step-up chopper
56 WorkingCase 1 : When the chopper is on the current IL flows through inductor and it stores the energy during on period.Case 2 : When the chopper is on the inductor force to permit the flow of current through the diode and the load. Therefore the load voltage becomes E0 = Edc + EL .E0 = Edc + L. d/dt .ILWhen the inductor current decreases to zero, the polarity across inductor gets reversed and the process is repeated.
57 Step-Down chopperDefinition :When the output voltage is less than the input dc voltage (E0 < Edc ) then it is called as Step-down copper.i.e., E0 < Edc
58 Working :Case 1 : During the ON period of chopper the input voltage Edc connected to the load and the inductor stores the energy across it.Case2 : During the OFF period of chopper the inductor current and load current because of freewheeling diode get short circuited through the load and inductor. Therefore the load voltage is zero during the off period.
59 The output voltage E0 is expressed as, E0 = Edc × (TON / TON +TOFF ) (1)Hence,E0 = Edc (TON / T ) (2)Here,TON +TOFF = T = Chopping PeriodTON = ON period of chopperTOFF = OFF period of chopperα = TON / TThe ratio of TON / T is called as duty cycle and represented as ‘α’.E0 = Edc . αWhere,the chopping frequency control the output voltage.E0 = Edc . TON .FWhere, F = 1/T = Chopping frequency
61 Chopper Configuration Figure shows the quadrant based chopper classification according to the nature of voltage and current .The dc chopper circuit is the combination of all these quadrants , in which the dc motor has to be operated as a load.
62 In quadrant second and fourth, the direction of energy flow is reversed and the motor is used as a generator rather than drive (motor).In quadrant first and third , the motor is used as a drive in clockwise and anticlockwise direction respectively.
63 Classification Of Choppers Power ElectronicsClassification Of ChoppersChoppers are classified as:Class A ChopperClass B ChopperClass C ChopperClass D ChopperClass E ChopperProf. T.K. Anantha Kumar, E&E Dept., MSRIT
64 Class A Chopper Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Power ElectronicsClass A ChopperProf. T.K. Anantha Kumar, E&E Dept., MSRIT
65 Power ElectronicsWorkingWhen chopper is ON, supply voltage V is connected across the load.When chopper is OFF, vO = 0 and the load current continues to flow in the same direction through the FWD.The average values of output voltage and current are always positive.Class A Chopper is a first quadrant chopper .Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
66 It is used to control the speed of dc motor. Power ElectronicsClass A Chopper is a step-down chopper in which power always flows form source to load.It is used to control the speed of dc motor.The output current equations obtained in step down chopper with R-L load can be used to study the performance of Class A Chopper.Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
68 Class B Chopper Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Power ElectronicsClass B ChopperProf. T.K. Anantha Kumar, E&E Dept., MSRIT
69 During the ON period of the chopper, the inductance L stores energy. Power ElectronicsWhen chopper is ON, E drives a current through L and R in a direction opposite to that shown in figure.During the ON period of the chopper, the inductance L stores energy.When Chopper is OFF, diode D conducts, and part of the energy stored in inductor L is returned to the supply.Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
70 Average output voltage is positive. Power ElectronicsAverage output voltage is positive.Average output current is negative.Therefore Class B Chopper operates in second quadrant.In this chopper, power flows from load to source.Class B Chopper is used for regenerative braking of dc motor.Class B Chopper is a step-up chopper.Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
71 Power ElectronicsWaveformsProf. T.K. Anantha Kumar, E&E Dept., MSRIT
72 Class C Chopper Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Power ElectronicsClass C ChopperProf. T.K. Anantha Kumar, E&E Dept., MSRIT
73 Class C Chopper is a combination of Class A and Class B Choppers. Power ElectronicsClass C Chopper is a combination of Class A and Class B Choppers.For first quadrant operation, CH1 is ON or D2 conducts.For second quadrant operation, CH2 is ON or D1 conducts.When CH1 is ON, the load current is positive.Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
74 The output voltage is equal to ‘V’ & the load receives power from the source. When CH1 is turned OFF, energy stored in inductance L forces current to flow through the diode D2 and the output voltage is zero.
75 Current continues to flow in positive direction. Power ElectronicsCurrent continues to flow in positive direction.When CH2 is triggered, the voltage E forces current to flow in opposite direction through L and CH2 .The output voltage is zero.On turning OFF CH2 , the energy stored in the inductance drives current through diode D1 and the supply.Output voltage is V, the input current becomes negative and power flows from load to source.Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
76 Average output voltage is positive Power ElectronicsAverage output voltage is positiveAverage output current can take both positive and negative values.Choppers CH1 & CH2 should not be turned ON simultaneously as it would result in short circuiting the supply.Class C Chopper can be used both for dc motor control and regenerative braking of dc motor.Class C Chopper can be used as a step-up or step-down chopper.Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
77 Power ElectronicsWaveformsProf. T.K. Anantha Kumar, E&E Dept., MSRIT
78 Class D Chopper Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Power ElectronicsClass D ChopperProf. T.K. Anantha Kumar, E&E Dept., MSRIT
79 Class D is a two quadrant chopper. Power ElectronicsClass D is a two quadrant chopper.When both CH1 and CH2 are triggered simultaneously, the output voltage vO = V and output current flows through the load.When CH1 and CH2 are turned OFF, the load current continues to flow in the same direction through load, D1 and D2 , due to the energy stored in the inductor L.Output voltage vO = - V .Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
80 Average output voltage becomes negative if tON < tOFF Power ElectronicsAverage load voltage is positive if chopper ON time is more than the OFF timeAverage output voltage becomes negative if tON < tOFFHence the direction of load current is always positive but load voltage can be positive or negative.Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
81 Power ElectronicsWAVEFORMSProf. T.K. Anantha Kumar, E&E Dept., MSRIT
82 Class E Chopper Prof. T.K. Anantha Kumar, E&E Dept., MSRIT Power ElectronicsClass E ChopperProf. T.K. Anantha Kumar, E&E Dept., MSRIT
83 Four Quadrant Operation Power ElectronicsFour Quadrant OperationProf. T.K. Anantha Kumar, E&E Dept., MSRIT
84 Class E is a four quadrant chopper Power ElectronicsClass E is a four quadrant chopperWhen CH1 and CH4 are triggered, output current iO flows in positive direction through CH1 and CH4, and with output voltage vO = V.This gives the first quadrant operation.When both CH1 and CH4 are OFF, the energy stored in the inductor L drives iO through D2 and D3 in the same direction, but output voltage vO = -V.Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
85 Therefore the chopper operates in the fourth quadrant. Power ElectronicsTherefore the chopper operates in the fourth quadrant.When CH2 and CH3 are triggered, the load current iO flows in opposite direction & output voltage vO = -V.Since both iO and vO are negative, the chopper operates in third quadrant.Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
86 Power ElectronicsWhen both CH2 and CH3 are OFF, the load current iO continues to flow in the same direction D1 and D4 and the output voltage vO = V.Therefore the chopper operates in second quadrant as vO is positive but iO is negative.Prof. T.K. Anantha Kumar, E&E Dept., MSRIT
88 Working :Figure shows the basic power circuit of Jones chopper. This chopper circuit is an example of Class D commutation. In this circuit, SCR T1 is the main thyristor , whereas SCR T2 , capacitor C , D2 , and autotransformer (T) forms the commutating circuit for the main thyristor T1 .Therefore , the special features of this circuit is the tapped autotransformer T through a portion of which the load current flows. Here , L1 and L2 are closely coupled so that the capacitor always gets sufficient to turn off the main SCR T1.
89 If the main thyristor T1 is on for a long period , then the motor will reach the maximum steady-state speed determined by the battery voltage , the motor and the mechanical load characteristics. If thyristor T1 is off , the motor will not rotate. Now , if thyristor T1 is alternatively on and off in a cyclic manner , the motor will rotate at some speed between maximum and zero.Let us assume that initially capacitor C is charged to a voltage Edc with the polarity shown in figure . As shown in fig. , SCR T1 is triggered at time t = t1 , current flows through the path CA – T1 – L2 – CB and capacitor C charges to opposite polarity , i.e. Plate B positive and plate A negative.
90 However , diode D1 represents further oscillation of the resonating L2 C circuit. Hence capacitor C retains its charge until SCR T2 is triggered. The capacitor voltage waveform are drawn at bottom plate B of capacitor.Now , at time t = t3 , SCR T2 is triggered. Current flow through the path CB – T2 – T1 – CA . Therefore, discharge of capacitor C reverse biases SCR T1 and turns it off. The capacitor again charges up with plate A positive and SCR T2 turns off because the current through it falls below the holding current value when capacitor C is recharged.
91 The cycle repeats when SCR T1 is again triggered The cycle repeats when SCR T1 is again triggered. The use of autotransformer insures that whenever current is delivered from dc source to the load , a voltage is included in L2 in the correct polarity for changing the commutating capacitor to a voltage higher than Edc . Thus , the autotransformer measurably enhances the reliability of the circuit.At t5 , the bottom plate (B) of capacitor C reaches a peak value. Since at t5 , the capacitor is charged to a voltage greater than Edc , diode D1 is again forward biased . Capacitor C now discharges to a value lower than Edc . The time duration t3 to t4 is the circuit turn off time presented to SCR T1.