The trigger circuit operates at low power levels volts whereas thyristors operate at high voltage levels volts. Hence if the Thyristor acts as a short the entire volts get applied across the firing circuit causing damage. Hence isolation is needed. How is a pulse transformer different from other transformer? A pulse transformer is one in which the input at the primary is current which is transformed into a pulse at the secondary. Thus it does not step-up and step-down as other transformers. What are the features of pulse transformer?
The primary magnetizing inductance is high, coupling efficiency is high, and interwinding capacitance is low and has greater insulation. What are the advantages of using pulse transformer? As the UJT works in a mode called as a relaxation oscillator i. UJT turns on or off depending on the charging and discharging of the capacitor. Time constant can be varied with Chance delay angle can be varied.
The UJT thus gives a firing angle range of 0- Why is the sneer diode used? The sneer diode provides a constant supply voltage for UJT. It enables synchronization with zero crossings. Sneer diode acts as a regulator. The sneer clamps the rectified voltage to vs. This sneer voltage acts as a supply for UJT relaxation oscillator. What is meant by ramp control, open loop control or manual control with respect to UJT firing circuit?
Ramp control-The graph of time period in milliseconds with the firing angle in degrees is a ramp. The ramp slope can be controlled by the potentiometer. Manual control-The potentiometer in the kit can be used to get various firing angles. This is manual control. What is a firing circuit? It is a circuit, which is used to trigger a device at various instants of time. Why a bridge rectifier is used? What is the load used? Load is high power dissipation resistor. What is time constant of a circuit?
What are the advantages of UJT pulse trigger circuit? The resistors, capacitors depend heavily on the trigger characteristics of the Thyristor used. The power dissipation is high due to prolonged pulse. But the pulse triggering can accommodate wide tolerances in triggering characteristics by instantaneously overdriving the gate. The power level in such circuits is lower as the triggering energy can be stored slowly and discharged rapidly when the triggering is required.
Why is UJT used as relaxation oscillator? The UJT is used as a relaxation oscillator to obtain sharp, repetitive pulses with good rise time. Also it has good frequency stability against variation in the supply voltage and temperature. What are the applications of UJT trigger circuits? What is valley voltage? It is the voltage at which the UJT turns off and the capacitor starts charging again. What is the discharging path if the capacitor?
The capacitor discharges through emitter, base and primary of the pulse transformer. What is relaxation oscillator? When the capacitor discharges to a valley voltage, the UJT turns off and capacitor starts charging again. This mode of working of UJT is called relaxation oscillator. Draw the static characteristics of UJT. What is negative resistance? During this period the voltage V decreases with increase in current, hence this portion of V-I characteristics is called negative resistance.
What is interring base resistance? Inter base resistance is the resistance between 2 bases. What is intrinsic stand off ratio? Its value ranges between. What is the width of the triggering pulse? Why are the capacitors CIF and C used? Capacitor CIF is used to minimize the ripples and C is used for charging and discharging so that the trigger is eventually formed.
O, Probes. Switch on the mains supply unit. Now set the o converter mode.
Connect Pm and Pa to input 1 and 2 of pulse transformer isolator. AC phase control. It generates pulses in synchronization with the AC supply. A pulse transformer is used to isolate the firing pulses. Required graphs are plotted. What is ac voltage controller? If a Thyristor switch is connected between ac supply and load, the power flow can be controlled by varying the rms value of ac voltage applied to the load and this type of power circuit is known as an ac voltage regulator 2.
What are the applications of ac voltage controllers? The most common applications of ac voltage controllers are: industrial heating, on-load transformer tap changing, light controls, speed control of polyphase induction motors and ac magnet controls. What do you mean by sequence control? The use of two or more stages voltages controllers in parallel for the regulation of output voltage.
Give the classification of ac voltage regulators. They are classified as: 1. What are the two types of control? Why are extra commutation components not required? Hence extra commutation components are not required. What is the difference between cycloconverters and ac voltage controllers? In cycloconverters ac to variable ac frequency of output can be varied. What is diac firing circuit? A diac firing circuit consists of a diac that is used to generate trigger pulses for the Thyristor diac can conduct in both directions and it does not have any control terminal in the form of a gate.
What are the merits and demerits of voltage controllers? The merits are that they are simple without commutation circuits, high efficiency and less maintenance. The demerits are that the load current is asymmetric phase control and hence harmonics are present and intermittent supply of power in on-off control. Why is the trigger source for the two Thyristor isolated from each other in a single-phase voltage controller?
When one Thyristor is on, the other should be off. Both the Thyristor should not conduct at a time. To observe load voltage on CRO. Pure R load. R-L load R load in series with L load. R-L loads with free wheeling Diode. Fig 8. Viva questions: 1. What is a full controlled rectifier? It has 4 thyristors and hence all of them can be controlled for rectification purpose. In a full converter the polarity of the output voltage can be either positive or negative but the output current has only one polarity.
What is a semi converter? A semi converter is a one-quadrant converter and it has only one polarity of output voltage and current. What is a dual converter? A dual converter can operate in all 4 quadrants and both output voltage and current can be either positive or negative. How can we control the output voltage of a single-phase full converter?
By varying the trigger angle. What is MCB? MCB-Miniature circuit breaker. This is used as switch, which opens or switches off when the voltage or current is above the rated value of that of MCB. How many lines are there in single-phase system? Two lines- 1line 1neutral 7. What is the type of commutation used? Line commutation. What is rectification mode and inversion mode? During the period alpha to the input voltage Vs and input current Is are positive and the power flows from supply to the load. The converter is said to be operating in rectification mode.
The converter is said to be operating in inversion mode. Where is full bridge converter used?
It is mainly used for speed control of dc motors. What is the effect of adding free wheeling diode?
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Free wheeling action does not takes place in single-phase full converter inherently as there are 4 thyristors and no diodes. It is more forward biased compared to T1 and T2. Hence freewheeling diode conducts. The freewheeling diode is connected across the output Vo. The energy stored in the load inductance is circulated back to the load itself. Connections are made as shown in the figure 9. Switch on DC toggle switch of chopper module.
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Switch on the trigger input by pushing- in pulsar switch. Observe the out put waveform across load on CRO. Keep the duty cycle at mid position and vary the frequency from minimum to maximum and record the output voltage VLDC readings. Keep the frequency at mid position, vary duty cycle from minimum to maximum and output voltage VLDC readings.
Note down the output waveform for mid value of frequency and duty cycle. Connections are made as shown in fig 9. Follow the same procedure as listed in steps 2 to 8 above. Readings and output waveform is to be recorded with and without free wheeling diode. First use DC toggle switch and then the pulsar.
What are choppers? A dc chopper converts directly from dc to dc and is also known as dc-dc converter. What does a chopper consist of? On what basis choppers are classified in quadrant configurations?
The choppers are classified depending upon the directions of current and voltage flows. These choppers operate in different quadrants of V-I plane. There are broadly following types of choppers: class a chopper first quadrant ; class B second quadrant Class C and class D two quadrant choppers , class C in II quadrant and I whereas class D in IV quadrants, and I class E is four quadrant operator. What are different control strategies found in choppers? The different control strategies are pulse width modulation, frequency modulation and current limit control, variable pulse width and frequency.
Explain the principle of operation of a chopper? A chopper acts as a switch, which connects and disconnects the load, hence producing variable voltage. What are the advantages of DC choppers? Define duty cycle. The duty cycle of chopper controls its output voltage. How can ripple current be controlled? Ripple current is inversely proportional to the frequency and hence can be controlled by having higher frequency. What is step up chopper? If the output average voltage is greater than the supply voltage, then the chopper is called step up chopper.
On what does the commutating capacitor value depend on? It depends on the load current. What are the disadvantages of choppers? How do they have high efficiency? DC choppers uses switching principle, hence they have high efficiency. What are the applications of dc choppers? Battery operated vehicles, switched mode power supplies, traction devices, lighting and lamp controls, trolley cars, marine hoists, and forklift trucks.
Mine haulers etc. Connect the circuit as shown in fig Adjust field supply to mid value. Vary the firing angle for SCR rectifier and note down the firing angle, Armature voltage, current and speed of the DC motor. Firing angle to be read from the oscilloscope. Keep the firing angle at mid value i. Vary the field voltage by the pot meter and note down the values of field voltages and speed.
Plot the relevant graph. What is principle of dc motor? An electric motor is a machine, which converts electrical energy into mechanical energy. When the field magnets of a multipolar dc motor are excited and its armature conductors are supplied with current from supply mains they experience a force tending to rotate the armature. These forces collectively produce a driving torque or twisting moment , which sets the armature rotating.
How can the speed of the series motor controlled? What are the advantages of field method? What arethe disadvantages of field method? Commutation becomes unsatisfactory. What are the factors controlling speed? Speed can be controlled by controlling flux,resistance,voltage. What is the significance of back emf?
Because of its opposing direction it is referred to as back emf Eb. V has to drive Ia against the opposition of Eb. The power required to overcome this opposition is EbIa.
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What is torque? Torque is twisting or turning moment of a force about an axis. The torque developed by the armature of a motor is armature torque. The torque available for useful work is known as shaft torque available at the shaft. How can dc motors be classified? What are the main losses in motors? Why are starter used in dc motor? Hence starters which is basically a resistance connected in series with the motor.
What is the parameter that is being varied by varying the firing angle? The armature voltage is varied which inturn varies the speed of the motor by varying the firing angle.
What are the operating modes of dc motor? Motoring, regenerative braking, dynamic braking, plugging. To control the speed of universal motor. RESULT: The theoretical values of current, voltage and firing angle are computed and compared with their practical counterparts.
The results are plotted and verified. Connect the circuit as shown in the figure. Switch on the mains V to Isolation Transformer and Triac firing circuit. Switch on the trigger on push button switch NOTE: Triac firing circuit potentiometer to be at approximately 90o. Vary the firing angle, note down the angle and speed of the induction motor. Shutdown the equipment by switching off the power supplies.
Plot the relevant graphs. The speed of induction motor is controlled using triac. Graph of firing angle versus speed of induction motor is plotted. The input and load is AC. The power is to be controlled in positive as well as negative half cycles. TRIAC is a bi-directional device that is it conducts in both directions. TRIAC turns-off when voltage is reversed. Single gate controls conduction in both directions. Refer figure 4. The sensitivity is less in mode III 3rd quadrant when MT 2 is negative and positive gate current and it requires larger value of latching current to trigger.
Why should we use TRIAC for bi-directional flow of current, instead we can use a ordinary conductor wire? The ordinary conductor does not provide the facility to switch on or off that is, we cannot control the flow of current. We have to manually switch on or off the supply. What happens when gate voltage is increased? The forward break over voltage starts decreasing. Switch on the power supply to the unit. It displays S Press INC key to set rpm.
Press ENT Press ENT. Step size Half or Full Then it displays n…….. Rpm set for speed mode. Switch on the power supply. Display shows S 00 3. Press SET. Display shows rpm. Follow similar steps as above from step 7 for setting the parameters for step mode. In the last display shows S……. Steps set.
What is stepper motor? A stepper motor is one in which the motor rotates step by step in steps of 1. Why is stepper motor also called digital motor? A stepper motor is driven by square pulses and hence does not require pulse width modulation technique. It can be driven by simple digital circuits and hence is called digital motor.
How many steps are covered in one revolution? What are merits of stepper motor? It is easy to interface with digital controller ,low cost solution for position control and also zero feedback error in servo system. What are its applications? Texts that will be displayed have been partly translated by an automatic translation system. To view any text in its original language English , leave the mouse pointer on this text or touch the text if you use a tablet. Product: Electricity and Electronics Training Systems. Prefered way of communication: Email Phone. Send me news about this product line.
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