When thyristor is on, supply voltage appears across the load and when thyristor is off, the voltage across the load will be zero. A variablefrequency drive is a device used in a drive system consisting of the following three main subsystems. A chopper circuit is operating on trc at a frequency of 2 khz on a 460 v supply. This type of variable speed solution is often the preferred choice in high power applications, ranging from a few megawatts to over a hundred megawatts, 5, 6. The current source inverter is load commutated if the load power factor is leading whereas it must be force commutated if the load power factor is. If the maximum value of load current is 10 a, then the maximum current. Ac motor, main drive controller assembly, and driveoperator interface 210211 ac motor. In the below circuit, the inductor, capacitor and resistor form a second order under damp circuit. In the down mode, power is switched from a dc power source to a lower voltage energy storing load while in the up mode stored energy in the load is transferred to the higher voltage source. Both of these current source type of choppers can be designed to operate at chopping frequencies in excess of 5 kilohertz which provides for a fast response. Power electronics, smps, choppers, inverters, induction. A chopper is a static device which is used to obtain a. Jones chopper is the example for voltage commutated chopper. When v 0 v s the two choppers will be off but both the diodes d1 and d2 will start conducting.
The current source inverter is either load commutated or force commutated. The scr t1 is main scr whereas diode d1, capacitor c and saturable reactor sr makes commutation circuit for the scr t1. The circuit diagram of the type d chopper is shown in the above figure. Single phase fully controlled bridge converter fwr with r and rl loads. A chopper is a static device that converts fixed dc input voltage to a variable dc output voltage directly. Source, ujt triggering circuit of variable frequency, class d commutation and. It connects source to load and disconnects the load from source at highspeed. Voltage source inverter vsi voltage source inverter lci load commutated inverter typical applications pumps, fans, conveyors, extruders, mixers, compressors, grinding mills, suitable for retrofit of. Voltage impulse commutated chopper both constant frequency and. Commutation of thyristorbased circuits partii nptel. So, chopper serves the same purpose in dc circuit transfers in case of ac circuit.
To start the circuit, capacitor c is initially charged with. Since oscillating current through t1 turns it off it is called current commutated chopper. Electrical engineering mcqs for preparation of job test and interview, freshers, students, competitive exams etc. In this process, a charged capacitor momentarily reverse biases the conducting thyristor and turn it off. The average voltage across the load can be controlled by varying the. Fast responseload commutated choppers sciencedirect. Type d chopper or twoquadrant type b chopper two quadrant type b chopper or d chopper circuit. Power electronics online course diode circuit with free wheeling diode, scr circuit with free wheeling diode, fully controlled 1ph scr bridge rectifier, purely resistive load, operation with an rl load, rlc load and source inductance, battery charger, two quadrant dc drive, fully controlled 3 ph scr bridge rectifier, dc power supply, four.
The commutation process plays an important role in the operation and control of both. An scr is not suitable since once the device is latched on in this dc supply application, it remains on. Adtrons jones chopper is a selfcontained stand unit useful in the study and demonstration of the principle and working of a jones chopper. Thyristor forced commutated dc choppers springerlink. Also, the choppers are commutated independently of supply voltage and load variations. In the threephase pwm aa chopper design, the waveforms of the load currents and voltages generated from the aa chopper circuit connected to the threephase source voltage are obtained. Connect the rheostat as the load and the voltmeter across the load terminals. The basic circuit for the morgan chopper is shown in the figure a. After t 3 i c supplies load current io and the excess current. Voltage and current waveforms when the chopper is turned on the current through the inductance l will increase from i 1 to i 2. The second order underdamped circuit is formed by the inductor or ac resistor, capacitor and resistor. When the chopper is on, v 0 is zero but the load voltage e drives the current through the inductor l and the chopper, l stores the energy during the time t on of the chopper.
The system uses darlington transistor switches in a conventional connection. Class a is one of frequently used thyristor commutation techniques. The ac electric motor used in a vfd system is usually a threephase induction motor. An improved upanddown chopper circuit is provided which is useful for voltage regulation in a bidirectional dc power system. Pulse width adjust has no function in a class a chopper. Voltage commutated choppers are widely used in high power circuits where load fluctuation is not large.
Recent listings manufacturer directory get instant insight into any electronic component. Single phase uncontrolled half wave rectifier with. The output voltage and current waveforms are asshownin figure 2. In a chopper or inverter having power thyristors that are switched on a control energization of a load from a dc source, and having commutating capacitors to which current is diverted from the power thyristors when they are to be switched off, recycling means are provided for recharging each commutating capacitor to bring it back to its original potential after each commutation and. Dcchoppers with vcc,ccc load commutated and multiphase. The voltage v 0 exceeds the source voltage and hence the circuit acts as a stepup chopper and the energy which is stored in l is released to the load. Set the regulated power supply to give an output of 15v and set current limit to minimum. This paper describes two fast response load commutated choppers. The circuit consists of a semiconductor diode, resistor, and a load. The commutation circuit consists of an auxiliary thyristor t2, diode d1, inductor lm and capacitor c. Pdf ee6503 power electronics pe books, lecture notes.
Voltage commutated chopper mode1 and mode2 voltage. The output voltage can be varied by changing the duty ratio. Some types of singlephase motors or synchronous motors can be advantageous in some situations, but. If the load voltage is 350 volts, calculate the conduction. In electronics, a chopper circuit is used to refer to numerous types of electronic switching devices and circuits used in power control and signal applications. Class a is also called as selfcommutation and it is one of the most used technique among all thyristor commutation technique. Chopper can increase or decrease the dc voltage level at its opposite side. Types of chopper circuits type a, type b, type c, type d. This form of thyristor natural commutation, which is. Numerical problems a for a current commutated chopper, peak commutating current is twice the maximum possible load current.
Before proceeding further, it is recojpmmended to read about the principle of operation of dcdc chopper circuits. For all type of chopper circuit, the output voltage value is controlled by periodic closing and opening of the switches used in the circuit. Type b chopper or secondquadrant chopper chopper second quadrant. Hello hassan, before answering your question, lets show how an scr may be used in a chopper circuit. For a maximum load current of 200a, calculate i the values of. C for which the scr is reverse biased by the commutation circuit d for which the scr is reverse biased to reduce its current below the holding current year 2011 two marks mcq 9. Pdf pwm control of ac chopper fed by unbalanced 3phase. The exciting current of the saturable reactor is neglected. In chapters 11 and 12 thyristor devices were used in conjunction with an ac supply which causes thyristor turnoff at supply current reversal. The input dc voltage can be increased stepup or decreased stepdown in output side so a dc chopper circuit can be considered as dc equivalent to an transformer. T 1 main thyristor, t a auxiliary thyristor, l,c commutating components, r c charging resistor. Afetr t 4, a constant current equal to io flows through v s, c, l, d 2 and load. The load current remains almost ripple free at 10a. The work is motivated mainly by electricallydriven gas.
Chopper systems offer smooth control, high efficiency, faster response and regeneration facility 2. In a load commutated dc dc chopper, the capacitor has a. Stepdown and stepup choppercontrol strategyforced commutated choppervoltage commutated, current commutated, load commutated, switched mode regulators buck, boost, buck boost converter, introduction to resonant converters. In type b or second quadrant chopper the load must always contain a dc source e. When the two choppers are on the output voltage v 0 will be equal to v s. Close the switch, initially capacitor short circuited, after 4 5 time constants, v c v s at t 0, t 1 is on, load is connected across the supply v o v s tank circuit starts conduction diode forward bias.
What would be the disadvantage of using a thyristor. The principle of operation of chopper can be understood from the circuit diagram below. Commutation circuit of dc choppers can be broadly classified into 2 groups as given below. Two mode operation of chopper buck converter dcdc maximum and minimum values of the load current. When thyristor is on, supply voltage appears acrossthe load and when thyristor isoff, the voltage acrossthe loadwill be zero. If thyristor is triggered or turned on, then anode current will flow by charging capacitor c with dot as positive. Electrical engineering mcqs for preparation of job test and. Generation of firing signals for thyristorstriacs using digital circuit microprocessor. Ac voltage controller using triac diac combination. Essentially, a chopper is an electronic switch that is used to interrupt one signal under the control of another. The power flow from supply to load and vice versa is possible at different power factor due to low impedance of dc supply. Morgan chopper is the example for the current commutated chopper.
790 1206 1652 1628 846 2 1412 1108 180 429 1157 1152 1425 157 1430 1530 1224 977 1420 1255 1432 217 1103 1461 558 689 49 1044 1068 1528 1385 822 1445 407 808 915 567 172 308 1238 1449 1285 892 566