Power Electronics Lab ExperimentNo-1-5
Experiment No. 1
Objective
To study SCR/TRIC/ DIAC/ MOSFET/IGBT Characteristics
Objectives
1. To understand performance of SCR/TRIAC/DIAC/MOSFETS and IGBT.
2. To understand the latching current, holding current.
3. To identify the area of application.
Apparatus Required:
S.No.
|
APPARATUS
|
RANGE
|
QUANTITY
|
1
|
SCR, TRIAC, DIAC, MOSFETS and IGBTs Power Circuit kit
|
NA
|
1
|
2
|
Power Supplies,
|
0-30V
|
1
|
3
|
Wattage Resistors
|
---
|
NA
|
4
|
Ammeter
|
0—10A
|
1
|
5
|
Voltmeter
|
0-300V
|
1
|
Theory
An SCR is a three terminal, four layer latching device. The three terminals are anode, cathode and gate. When the anode is more positive with respect to the cathode. Junction’s j1, j3 are forward biased and the junction j2 is reverse biased. Only a small leakage current flows through the device. The device is said to be in the forward blocking state or OFF state. When the anode to cathode voltage is increased to break-over value, the junction j2 breaks down and device starts conducting. The anode current must be more than a value known as latching current in order to maintain the device in the ON state. Once SCR starts conducting, it behaves like a conducting diode and gate has no control over the device.
The device can be turned OFF only by bringing the device current below a value known as holding current. The forward voltage drop across the device in the ON state is around one volt. When the cathode voltage is made positive with respect to the anode voltage the junction j2 is forward biased and the junction’s j1 and j3 are reverse biased. The device will be in the reverse blocking state and small reverse leakage current flows through the device. The device can be turned ON at forward voltages less than break over voltage by applying suitable gate current.
STUDY OF SCR CHARACTERISTICS
I) latching current measurement
Adjust gate voltage to a slightly higher value than what is found in (a) set pot (P2) at the maximum position. The device must be in the off state. Note that by simply removing the anode supply patch cord, you can switch SCR into OFF state. With anode voltage applied and gate drive provided the SCR will switch ON and remain in ON condition only if the resultant anode current is above some min value that is latching current value. The gate voltage must be kept constant in this experiment. With the help of (P2) pot, gradually increase the anode current (Ia), in steps; each step may be of 1mA. Open and close the gate supply after each step. If anode current is greater than this latching current of the device, then the device stays ON even after the gate current is removed. Otherwise the device goes into the blocking state as soon as gate drive is removed. Note the value of the latching current. You may control anode voltage also to find out the value of latching current by slowly varying the dimmer control, while using fixed load resistance.
II) Holding current Measurement.
Increase the anode current from latching current level with pot or by increasing the supply voltage slightly. Remove the gate supply permanently. The thyristor must be fully ON. Now start decreasing the anode current gradually by adjusting pot (P2). If thyristor does not turn off even after pot is at the highest resistance value, then reduce anode supply voltage. Observe the current when the device turns OFF. The anode current through the device at this instant is the holding current (Ih) of the device.
STUDY OF TRIAC CHARACTERISTICS
A TRIAC is a bidirectional thyristor (it can conduct in both directions) with three terminals. It is used extensively for control of power in AC circuit. When in operation, a TRIAC is equivalent to two SCRs connected in anti-parallel. Its three terminals are usually designated as MT1, MT2 and gate.
The V-I characteristics of a TRIAC is based on the terminal MT1 as the reference point.
The first quadrant is the region wherein MT2 is positive with respect to MT1 and vice-versa for the third quadrant. The peak voltage applied across the device in either direction must be less the break over voltage in order to retain control by the gate. A gate current of specified amplitude of either polarity will trigger the TRIAC into conduction in either quadrant, assuming that the device is in a blocking condition initially before the gate signal is applied. The characteristics of a TRIAC are similar to those of an SCR, both in blocking and conducting states, except for the fact that SCR conducts only in the forward direction, whereas the TRIAC conducts in both the directions.
1) Select Triac as the device under test by referring to the top RHS diagram. Note MT1, MT2 & gate terminals properly.
2) You have to follow general test procedure as per SCR test. only difference is that you can check that Triac can turn on while MT2 is either +ve or –ve w.r.t MT1 & gate voltage polarity also may be reversed. You may use switch SW2 to find out reverse gate current.
STUDY OF MOSFET CHARACTERISTICS
a) Output characteristics:
It indicate the variation of drain current ID as a function of drain–source voltage VGS as a parameter. For low values of VDS, the graph between ID–VDS is almost linear; this indicates a constant value of on resistance RDS = VDS / ID. For given VGS, if VDS is increased, output characteristic is relatively flat indicating that drain current is nearly constant.
b) Transfer characteristics:
This characteristic shows the variation of drain current ID as a function of gate-source voltage VGS. Threshold voltage VGST is an important parameter of MOSFET. VGST is the minimum positive voltage between gate and source to induce n-channel. Thus, for threshold voltage below VGST, device is in the OFF-state. Magnitude of VGST is of the order of 2 to 3V.
STUDY OF IGBT CHARACTERISTICS
a) Output characteristics:
Output characteristics of an IGBT show the plot of collector current IC versus Collector-Emitter voltage VCE for various values of Gate-Emitter voltages. In the forward direction, the shape of the output characteristics is similar to that of BJT. But here the controlling parameter is Gate-Emitter voltage VGE because IGBT is a voltage controlled device. When the device is OFF, junction j2 blocks forward voltage and in case reverse voltage appears across collector and emitter, junction j1 blocks it. VRM is the maximum reverse breakdown voltage.
b) Transfer characteristics:
The transfer characteristics of an IGBT is a plot of collector current IC versus Gate-Emitter voltage VGE. This characteristic is identical to that of power MOSFET. When VGE is less than the threshold voltage VGET, IGBT is in the OFF-state.
The bipolar junction transistors are current controlled devices. Field effect transistors, on the other hand do not normal require any input current and are unipolar devices. They involve a single conducting channel which can be either N or P type material. The FETs offer advantages in switching service since they do not suffer the delays associated with minority carrier storage. Because they also demand no input current, they are easier to drive. They are also less temperature sensitive and less susceptible to second break down in high power applications. If JFETs are operated with reverse bias on their gate leads to prevent gate current. However a large input signal may momentarily overcome the reverse bias and turn on gate diodes drawing appreciable current from the source. These disadvantages are overcome by insulating the gate terminals from the channel with a thin layer of silicon dioxide (Metallic oxide). Those FETs that use this technique are known as metallic oxide semiconductor field effect transistor or MOSFETs.
MOSFETs are operated in depletion mode as do JFETs with negative voltage on the gate terminal for N channel device. Depletion mode operated devices are normally in ON condition. The MOSFETs may also be operated in the enhancement mode. In this mode the device is normally in off condition and a sufficiently large positive voltage on the gate terminal can turn on the device. Generally power enhancement MOSFETs are used in power electronics circuits. A metallic gate is deposited on the thin layer of metal oxide (insulator) which is deposited on the channel opposite to the substrate. Due to insulated gate, negligible gate current flows.
The power enhancement MOSFET has an anti parallel fast turn on diode which permits reverse current of the same magnitude as that of the main MOSFET, so that drain substrate junction will not be damaged when drain and source has reverse biasing.
Applications:
The enhancement MOSFET is used as a switch in power electronics by keeping sufficient gate voltage (VGS) so that it conducts in the constant resistance region. The conduction loss of the MOSFET is high due to large value of device resistance in the ON state. The MOSFET can be triggered directly from the CMOS or other gates due to high input impedance.
Switching times (turn ON and Turn Off) are very low and hence switching loss is almost nil. The gate drive power is also negligible. They have larger gains and simple and cheaper triggering circuits. If has only one disadvantage i.e. higher conduction drop generally five times more than the power transistor of the same rating.
STUDY OF INSULATED GATE BIPOLAR TRANSMISTOR
(IGBT) CHARACTERSTICS
IGBT is a new development in the area of power MOSFET technology. This device combines into it the advantages of both MOSFET and BJT. So as IGBT has input impedance like a MOSFET and low on-state power loss as in a BJT. Further, IGBT is free from second breakdown problem present in BJT. IGBT is also known as metal –oxide insulated gate transistor (MOSIGT), conductively –modulated field effect transistor (COMFET) or gain –modulated FET (GEMFET). It was also initially called insulated gate transistor (IGT ).
Basic structure and working:
IGBT is constructed virtually in the same manner as a power MOSFET. There is however; a major difference in the substrate. The n+ layer substrate at the drain in power MOSFET is now substituted in the IGBT by a p+ layer substrate called collector. Like a power MOSFET, an IGBT has also thousands of basic structure cells connected appropriately on a single chip of silicon.
When gate is positive with respect to emitter and with gate-emitter voltage more than the threshold voltage of IGBT, an n-channel is formed in the p – regions as in a power MOSFET. This n-channel short-circuits the n- region with n+ emitter regions. An electron movement in the n-channel, in turn causes substantial hole injection from p+ substrate layer into epitaxial n- layer. Eventually a forward current is established as shown in fig. The three layers p+, n- and p constitute a pnp transistor with p+ as emitter nas base and p as collector. Also n- , p and n+ layers constitute npn transistor as shown in figure .Here n- serves as base for pnp transistor and also as collector for npn transistor. Further p serves as collector for pnp device and also as base for npn transistor. The two pnp and npn transistors can , therefore be connected as shown in fig to give the equivalent circuit of an IGBT. Fig. is the circuit symbol for IGBT with gate (G) emitter (E) and collector (c) as its three terminals.
IGBT characteristics
Static V-I or output characteristics of an IGBT (N-channel type) show the plot of collector current IC versus collector emitter voltage VCE for various values of gate-emitter voltages. In the forward direction, the shape of the output characteristics is similar to that of BJT. But here the controlling parameter is gate –emitter voltage VGE because IGBT is a voltage –controlled device.
Applications of IGBT
IGBTS are widely used in medium power applications such as dc and ac motor drives, UPS systems; power supplies and drives for solenoids, relays and contactors. Though IGBTs are somewhat more expensive than BJTs, yet they are becoming popular because of lower gate-drive requirements. Lower switching losses and smaller snubber circuit requirements. IGBT converters are more efficient with less size as well as cost, as compared to converters based on BJTs. Recently IGBT inverter induction –motor drives using 15-20 KHz switching frequency are finding favour where audio –noise is objectionable. In most applications, IGBTs will eventually push out BJTs. At present, the state of the art IGBTs are available up to 1200 V, 500A.
DIAC CHARACTERISTICS
DIAC can be tested by applying about 32 volts across its terminals with both the polarities, including the limiting resistance of 2K in series with M3 meter. When the dimmer voltage is around 32 volts the DIAC starts conducting as indicated by M3 meter. You may reverse the terminals supply voltage & observe that DIAC still turns ON.
CHARACTERISTICS OF SCR:
Procedure:
1. Connections are made as shown in the circuit diagram.
2. The value of gate current IG, is set to convenient value by adjusting VGG.
3. By varying the anode- cathode supply voltage VAA gradually in step-by step, note down the corresponding values of VAK & IA. Note down VAK & IA at the instant of firing of SCR and after firing (by reducing the voltmeter ranges and increasing the ammeter ranges) then increase the supply voltage VAA. Note down corresponding values of VAK & IA.
4. The point at which SCR fires, gives the value of break over voltage VBO.
6. The on state resistance can be calculated from the graph by using a formula.
7. The gate supply voltage VGG is to be switched off
8. Observe the ammeter reading by reducing the anode-cathode supply voltage VAA. The point at which the ammeter reading suddenly goes to zero gives the value of Holding Current Ih.
9. Steps No.2, 3, 4, 5, 6, 7, 8 are repeated for another value of the gate current IG.
Circuit Diagram
Experimental kit diagram:
Ideal Graph:
Base Diagrams of 2N3669/70 & TY604: -
Note: - Follow the same design procedure for TRIAC connection sting
Measurement of Latching Current
Procedure
1. Connections one made as shown in the circuit diagram
2. Set VGG at 7 volts
3. Set VAA at particular value, observe IA, by operating the switch (on & off). if it goes to zero after opening of the switch, indicates IA < IL
4. Repeat step 3 such that the current IA should not go to zero after opening of the switch. Then IA gives the value of IL.
Circuit Diagram
TABULAR COLUMN
S.No.
|
IG =….(μA)
|
IG =….(μA)
| ||
VAK (V)
|
IA (mA)
|
VAK (V)
|
IA (mA)
| |
Result
Viva Questions
1. Explain the working operation of V-I characteristics of S.C.R.
2. Define Holding current, Latching current on state resistance, Break down voltage
3. Explain the working operation of S.C.R. characteristics by using two transistors analogy
4. Write an expression for anode current
5. Mention the applications of S.C.R.
6. What is thyristor?
7. What are the different families of thyristor devices?
8. What are the modes of an SCR?
9. Define Latching current (IL).
10. Define Holding current (Ih). Which will be larger either IL or Ih?
11. What are the different methods to turn ON the SCR?
CHARACTERISTICS OF TRIAC
Procedure
1. Connections are made as per the circuit diagram.
2. Keep in position minimum so IS and VA across MT1 and MT2 are zero.
3. Switch on the supply.
4. Allow low voltage between MT1 and MT2. Increase VA so IA increases. Repeat it till the device turn ON.
5. Slowly increases gate to MT1 voltage set particular IG = 18mA.
6. Keep IG constant and increase VA in step by step when VA increases. VA increases slightly when break over is reached voltage get decreases but current increases sharply.
7. For reverse characteristics, change the connection to make MT1 positive with respect to MT2 and repeat the same procedure.
Circuit Diagram
Base diagram of BT136:
Tabular Column
S.No.
|
IG =….(mA)
|
IG=….(mA)
| ||
VAK (V)
|
IA (mA)
|
VAK (V)
|
IA (mA)
| |
Nature of Graph
Result
Viva Questions:
1. What is TRIAC?
2. TRIAC is only used in AC circuits. Justify.
3. How does a TRIAC work?
4. Draw the equivalent circuit for TRIAC?
5. What are the differences between SCR and TRIAC?
6. Explain the different working modes of operations of a TRIAC?
7. Why i-mode is more sensitive among all modes?
8. What are the applications of TRIAC
9. Compare SCR , TRIAC & DIAC
CHARACTERISTICS OF MOSFET
Procedure
1) Make the connections as per the circuit diagram.
2) Switch on the supply.
3) Set the gate current at a fixed value by varying RPS on the gate-cathode side.
4) Vary the voltage applied across Gate and corresponding VDS ( VCE) and
ID ( IC ) is noted .
5) The above steps are repeated for different values of I G .
6) Vary the voltage across Collector and Emitter and noted down VGE and IC.
7) Draw the graph between V GS (VCE ) and ID (IC ) and VGS (VGE ) and ID (IC ).
Circuit Diagram
Tabular Column
S.No
|
VGS =….(V)
|
VGS =….(V)
|
V DS =….(V)
| |||
VDS (mV)
|
ID (mA)
|
VDS (mV)
|
ID (mA)
|
VGS (mV)
|
ID (mA)
| |
1
2
3
4
5
6
| ||||||
Nature of Graph
TRANSFER CHARACTERISTICS DRAINCHARACTERISTIC
CHARACTERISTICS OF IGBT:
Procedure
Collector Characteristics
1. Connections are mode as shown in the circuit diagram.
2. Initially set V2 to VGE1 = 5V (slightly more than threshold voltage)
3. Slowly vary V1 and note down IC and VCE
4. For particular value of VGE there is pinch off voltage (VP) between collectors and emitter
5. Repeat the experiment for different values of VGE and note down IC v/s VCE
6. Draw the graph of IC v/s VCE for different values of VGE.
Transconductance Characteristics
1. Connections are mode as shown in the circuit diagram.
2. Initially keep V1 and V2 at zero.
3. Set VCE1 = say 0.8 v
4. Slowly vary V2 (VGE) and note down IC and VGE readings for every 0.5V and
enter tabular column
5. Repeat the experiment for different values of VCE and draw the graph of
IC v/s VGE.
Circuit Diagram
Tabular Column
S.No
|
VGE =….(V)
|
VGE =….(V)
|
V CE =….(V)
| |||
VCE (mV)
|
IC (mA)
|
VCE (mV)
|
IC (mA)
|
VGE (mV)
|
IC (mA)
| |
1
2
3
4
| ||||||
Nature ofGraph
TRANSFER CHARACTERISTICS DRAIN CHARACTERISTICS
Result
Viva Questions
1. What are the different types of Power MOSFET?
2. Power MOSFET is a voltage controlled device? Why?
3. Name the three regions of operation in a MOSFET.
4. Define threshold voltage.
5. Define Pinch off Voltage.
6. Compare Power MOSFET’s with BJT’s.
7. What are the different types of Power IGBT?
8. Power IGBT is a voltage controlled device? Why?
9. Name the three regions of operation in an IGBT.
10. Define threshold voltage.
11. Define Pinch off Voltage.
12. Compare Power MOSFET’s , IGBT with BJT’s.
Experiment No. 2
Object
To study Triggering circuits/phase control.
Objectives:
1. To understand resistance, resistance capacitance and UJT triggering circuits of SCR
2. To know the application of triggering circuits.
3. To conduct the performance of triggering circuits
Apparatus Required:
S.No.
|
APPARATUS
|
RANGE
|
QUANTITY
|
1
|
TRIAC Power Circuit kit
|
Single Phase 230 V /
|
1
|
2
|
TRIAC Firing Circuit kit
|
Single Phase 230 V /
|
1
|
3
|
Isolation Transformer
|
230 / 115 V
|
1
|
4
|
Auto Transformer
|
230 / 230 V
|
1
|
5
|
CRO
|
20 MHZ
|
1
|
6
|
Loading Rheostat
|
100 Ω,
|
1
|
7
|
Patch Chords
|
10
|
Theory
Resistance Triggering:
Resistance trigger circuits are the simplest & most economical method. During the positive half cycle of the input voltage, SCR become forward biased but it will not conduct until its gate current exceeds Igmin . Diode D allows the flow of current during positive half cycle only. R2 is the variable resistance & R is the stabilizing resistance .R1 is used to limit the gate current. During the positive half cycle current Ig flows. Ig increases and when Ig= Igmin the SCR turns ON .The firing angle can be varied from 0 — 90° by varying the resistance R.
R —C Triggering:
By varying the variable resistance R, the firing angle can be varied from 0 —180° .In the negative half cycle the capacitance C charges through the diode D2 with lower plate positive to, the peak supply voltage Emax .This Capacitor voltage remains constant at until supply voltage attains zero value. During the positive half cycle of the input voltage, C begins to charge through R. When the capacitor voltage reaches the minimum gate trigger voltage SCR will turn on.
A synchronized UJT triggered circuit using an UJT is shown in the figure. Diodes ‘D1’ to ‘D4’ rectify ac to dc. Resistor R1 lowers Vdc to a suitable value for the zener diode and UJT. Zener diode ‘Z’ functions to clip the rectified voltage to a standard level, ‘Vz’ which remains constant except near the Vdc zero. The voltage Vz is applied to the charging circuit RC. Current ‘I’, charges capacitor ‘c’ at a rate determined by ‘R’ voltage across capacitor is marked by ‘Vc’ as shown. When ‘Vc’ reaches the unijunction threshold voltage Vz, the t-B1 junction of UJT breaks down and the capacitor ‘c’ discharges through the primary of pulse transformer sending a current ‘C2’ as shown.
As the current ‘i2’ is in the form of pulse, windings of the pulse transformer have pulse voltages at their secondary terminals. Pulse at the two secondary windings feeds the same in phase pulse to two SCRs of a full wave circuits. SCR with positive anode voltage would turn ON. As soon as the capacitor discharges, it starts to recharge as shown. Rate of rise of capacitor voltage can be controlled by varying ‘R’. The firing angle can be controlled up to above 150o. This method of controlling the output power by varying the charging resistor ‘r’ is called ramp control, open loop control (or) manual control.
TRIAC PHASE CONTROL CIRCUIT:
Triac is a bidirectional thyristor with three terminals. Triac is the word derived by combining the capital letters from the words TRIode and AC. In operation triac is equivalent to two SCRs connected in anti-parallel. It is used extensively for the control of power in ac circuit as it can conduct in both the direction. Its three terminals are MT1 (main terminal 1), MT2 (main terminal 2) and G (gate).
Circuit Diagram
Nature of graph:
Tabular column:
S.No.
|
Input voltage (V)
|
Input cycle time(ms)
|
Voltage across resisitor (V)
|
Voltage across zener diode(V)
|
Voltage across capacitor (V)
|
Voltage across load (V)
|
1
| ||||||
2
| ||||||
3
| ||||||
4
| ||||||
5
|
Procedure
R Firing
1. Connections are made as shown in fig.
2. Switch on the power supply to the CRO.
3. Set the CRO to the line trigger mode.
4. Switch on power supply to the SCR trainer.
Circuit diagram for RC triggering:
|
Wave forms:
Tabular column:
Sl.No
|
Input voltage (V)
|
Input cycle time (ms)
|
Resistance
Value(k Ω)
|
Out put voltage Vrms(V)
|
Voltage across (anode to cathode) Vrms
|
Procedure:
RC FIRING:
1. Connections are made as shown in fig.
2. Switch on the power supply to the CRO .
3. Set the CRO to the line trigger mode.
4. Switch on power supply to the SCR trainer.
5. Observe the waveform on the CRO.
6. Study the waveforms for various firing angle by varying the pot in R trigger circuit.
7. Observe the range of firing angle control. t u t e o f T e c h n o l o g y Page 53
8. For any one particular firing angle plot the waveforms of the ac voltage, voltage across the load and the SCR.
9. Measure the average dc voltage across the load and rms value of the ac input voltage using g' DMM
10. Calculate the dc output voltage using the equation
Circuit Diagram
Waveform:
Tabulation
Procedure
1. Connect a & k terminal of UJT triggering circuit to the gate cathode terminals of SCR.
2. Give a 24 V ac supply.
3. Observe the waveforms and plot it for one particular firing angle by adjusting the potentiometer and observe the range over which firing angle is controllable.
4. Observe that capacitor voltage is set at every half cycle.
VIVA QUESTIONS
1. Explain how synchronization of the triggering circuit with the supply voltage across SCR is achieved?
2. How can the capacitor charging be controlled?
3. What is the maximum value of firing angle which can be obtained from the circuit?
4. How is the output power to the triggering circuit controlled?
5. Compare UJT triggering circuit with RC firing circuit?
TRIAC PHASE CONTROL CIRCUIT
Procedure
1) Make the connections as per the circuit diagram.
2) Keep the multiplication factor of CRO’s probe at maximum position.
3) Switch on the TRIAC knob and firing circuit kit.
4) Vary the firing angle in steps and note the readings of waveforms in CRO.
5) Switch of the supply.
6) Draw the output waveforms.
Circuit Diagram
Tabular Column
S.No.
|
Firing angle a
|
Voltage (v)
|
Time
‘t ‘ (ms)
|
Current (A)
|
Practical value ao
|
WAVEFORM
Results
Viva Questions
1. What type of commutation is used in this circuit?
2. What are the effects of load inductance on the performance of AC voltage controllers?
3. What is extinction angle?
4. What are the disadvantages of unidirectional controllers?
5. What are the advantages of ON-OFF control?
Experiment No.3
Objective
To conduct the performance of single phase fully controlled bridge converter
Objectives
1. To study single phase fully controlled bridge converter.
2. To plot the output waveforms of single phase fully controlled bridge converter
Apparatus Required
Sl.no
|
Name of components
|
Rating /Ranging
|
Quantity
|
01
|
1- Ф fully controlled bridge converter(kit)
|
NA
|
01
|
02
|
DC motor
|
1HP, 1-Ф ,230V,10A
|
01
|
03
|
Tachometer
|
0-6000rpm
|
01
|
04
|
Connecting wires
|
0-5/10A
|
10
|
05
|
Multimeter
|
0-300/600V
|
01
|
Theory
A fully controlled converter or full converter uses thyristors only and there is a wider control over the level of dc output voltage. With pure resistive load, it is single quadrant converter. Here, both the output voltage and output current are positive. With RL- load it becomes a two-quadrant converter. Here, output voltage is either positive or negative but output current is always positive. Figure shows the quadrant operation of fully controlled bridge rectifier with R-load. Fig shows single phase fully controlled rectifier with resistive load. This type of full wave rectifier circuit consists of four SCRs. During the positive half cycle, SCRs T1 and T2 are forward biased. At ωt = α, SCRs T1 and T3 are triggered, then the current flows through the L – T1- R load – T3 – N. At ωt = π, supply voltage falls to zero and the current also goes to zero. Hence SCRs T1 and T3 turned off. During negative half cycle (π to 2π).
SCRs T3 and T4 forward biased. At ωt = π + α, SCRs T2 and T4 are triggered, then current flows through the path N – T2 – R load- T4 – L. At ωt = 2π, supply voltage and current goes to zero, SCRs T2 and T4 are turned off. The Fig-3, shows the current and voltage waveforms for this circuit. For large power dc loads, 3-phase ac to dc converters are commonly used. The various types of three-phase phase-controlled converters are 3 phase half-wave converter, 3-phase semi converter, 3-phase full controlled and 3-phase dual converter. Three-phase half-wave converter is rarely used in industry because it introduces dc component in the supply current. Semi converters and full converters are quite common in industrial applications. A dual is used only when reversible dc drives with power ratings of several MW are required. The advantages of three phase converters over single-phase converters are as under: In 3-phase
Procedure
1) Note that there are separate switches are provided for electronic circuit ON/OFF and control circuit ON/OFF.
2) Before you switch on the power, connect motor cable (12 pin johson cable) and proximity switch cable (3 pin socket) correctly.
3) Select proper desired mode of operation under closed loop/ open loop system.
4) Remove the Load on the motor.
5) Keep CHOKE /NO CHOKE mode as per desired mode of operation.
6) Then switch on the power for electronic and control circuit.
7) For motor to start running you have to take pot P1 to most anticlockwise position.
8) By doing this , you will be pressing limit switch of the shaft and motor will start with low speed at the start (soft start)
9) Now gradually load the motor and take ,speed, current voltage readings.
10) You can observe various test points as described.
11) Take down the readings in tabular form and analyze.
12) Do not load the motor beyond 3.5 Amp.
Circuit Diagram
Tabulation
SL.NO
|
α (0)
|
T (ms)
|
Output Vdc(V)
|
Idc(A)
|
FORMULAE USED:
Average output voltage, Vo = (2*Vm / Π ) *cosα
Where,
Vm = Peak phase voltage, Volts
α = Firing angle, degrees
Waveform
Result
Viva Questions
1. If firing angle is greater than 90 degrees, the inverter circuit formed is called as?
2. What is displacement factor?
3. What is Dc output voltage of single phase full wave controller?
4. What are the effects of source inductance on the output voltage of a rectifier?
5. What is commutation angle of a rectifier?
6. What are the advantages of three phase rectifier over a single phase rectifier?
Experiment No. 4
Objective
To study Three Phase Converter (Half Wave and Full wave Bridge).
Objectives
1. To study the operation of three phases SCR half controlled converter
2. To study the basic operation principle of a 3 phase half controlled converter
3. To study the characteristics of converter waveforms
Apparatus required
Sl.no
|
Name of components
|
Rating /Ranging
|
Quantity
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01
|
3- Ф fully controlled bridge converter(kit)
|
01
| |
02
|
DC motor
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1HP, 1-Ф ,230V,10A
|
01
|
03
|
Tachometer
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0-6000rpm
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01
|
04
|
Connecting wires
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0-5/10A
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10
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05
|
Multimeter
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0-300/600V
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01
|
Three Phase Half wave bridge convertor
Theory
Three phase half controlled bridge rectifier circuit consists of three SCRs in three arms and three diodes in the other three arms. The output voltage V0 across the load terminals is controlled by varying the firing angles of SCRs T1, T2, T3. The diodes D1, D2 and D3 provide merely a return path for the current to the most negative line terminal. For firing angle less than 30°, the output terminal voltage of the converter is always positive, and the freewheeling diode does not come into operation. As the firing angle is retarded beyond this point, so the load current starts to freewheel through the diode for certain periods, thus cutting off the input line current, and preventing the output terminal load voltage from swinging into the negative direction. Hence the effect of the freewheeling diode is to cause a reduction of ripple voltage of the output terminals and at the same time to divert the load current away from the input lines.
Procedure
1. Switch ON the power supply ON/OFF switch.
2. Switch ON the pulse ON/OFF switch.
3. Vary the firing angle step by step in the range 180° – 0°.
4. For each firing angle observe the output waveform through CRO.
5. Tabulate the readings.
Circuit Diagram
Tabulation
SL.NO
|
α (0)
|
Vdc(V)
|
Idc(A)
|
Vdc(V)obs
|
Waveforms
FORMULAE USED:
Average output voltage,
Vo = ( 3√3 Vm / 2Π ) * (1+cosα)
Where,
Vm = Peak phase voltage, Volts , α = Firing angle, degrees
Test points:-
a) TP1 (red) and Ground (Black) :- UJT Relaxation oscillator output
b) TP2(red) and Green( - ve of CRO):- Attenuated output for CRO.
c) TP3:- Output for single lamp load connection (no waveform)
d) TP4 :- Load Current waveform
Procedure
1. Connect the three phase 440 volt supply to the rear of the instrument using the terminals marked R,Y,B .Connect Load bank.
2. Initially connect for delta operation.. Keep load control pot in minimum position. Switch ON the supply.
3. Observe the various waveforms on CRO by appropriately connecting test points. You may connect 1HP D.C. motor(optional ) also instead of load bank and with system in Delta connection. Start the P1 from zero , you may need to adjust “P2’ slightly .Slowly increase the speed. You can observe waveforms TP2 and TP4 with Motor as load.
4. Now connect for star connection .Connect lamp load bank .Do not connects motor when system is in ‘Star connection’ In this case the line voltage will be 140 volts. You may have to adjust pot marked “ADJUST” to get DC output voltage between approximately 50 volts D.C. to 175 volt D.C. You may observe the waveforms at TP1, TP2 and TP4.
Calculation
By using time division setting of CRO.
You may calculate the delay angel of the firing .Refer to the formula given below.
Verify the output voltage on the basis of output voltage indication.
Sample calculations:
Assume delta connection._
Line to line voltage i.e. VL = 81 volts (RMS)
(i.e. voltage between K1and K2 ,K2 and K3 ,K3 and K1)
Where Emph is the peak value of phase to neutral voltage and α is the delay
angle.
Typical time /division for measurement is 1 msec/di.
If α = 36 degree i.e. 2 divisin on CRO
For 1 msec/div setting
(Please note 1 msec = 18 o (electrical)
Hence
Verify Edc as shown from the panel voltmeter for this setting.
For star connection one can perform the calculations using the line to line voltage measured between K1 and K2, K2 and K3 and K3 and K1. Other details remain the same as for delta connection calculations.
Result
Viva Questions
1. What is the condition for load current should be discontinuous?
2. What is the output ripple voltage frequency of three phase half wave converters?
3. What are the two modes of operation present in the three phase half controlled rectifiers?
4. What is the use of freewheeling diode present in the three phase half controlled rectifiers?
5. What is the condition for the output voltage should be negative?
Three Phase full wave bridge convertor
Theory
A three-phase fully-controlled bridge rectifier can be constructed using six SCRs. The bridge circuit has two halves, the positive half consisting of the SCRs T1, T3 and T5 and the negative half consisting of the SCRs T2, T4 and T6. At any time, one SCR from each half conducts when there is current flow. The SCRs are triggered in the sequence T1, T2, T3, T4, T5, T6 and T1 and so on. When the SCRs are fired at 0o firing angle, the output of the bridge rectifier would be the same as that of the circuit with diodes. For instance, it is seen that D1 starts conducting only after θ = 30o. In fact, it can start conducting only after θ = 30o, since it is reverse-biased before θ = 30o. The bias across D1 becomes zero when θ = 30o and diode D1 starts getting forward biased only after θ =30o.
For α = 0o, T1 is triggered at θ = 30o, T2 at 90o, T3 at 150o and so on. For α = 60o,
T1 is triggered at θ = 30o + 60o = 90o, T2 at θ = 90o + 60o = 150o and so on. Note that positive group of SCRs are fired at an interval of 120o. Similarly, negative group of SCRs are fired with an interval of 120o. But SCRs from both the groups are fired at an interval of 60o. This means that commutation occurs every 60o, alternatively in upper and lower group of SCRs. Each SCR from both groups conducts for 120o.
Procedure
1. Switch ON the power supply ON/OFF switch.
2. Switch ON the pulse ON/OFF switch.
3. Vary the firing angle step by step in the range 180° – 0°.
4. For each firing angle observe the output waveform through CRO.
5. Tabulate the readings.
Circuit Diagram
Tabulation
SL.NO
|
α (0)
|
T
|
Vdc(V)
|
Idc(A)
|
Calculations
FORMULAE USED:
Average output voltage, Vo = ( 3√3 Vm / Π ) *cosα
Where,
Vm = Peak phase voltage, Volts α = Firing angle, degrees
Waveforms
Result
Viva Questions
1. What is a three phase controlled rectifier?
2. What are the advantages of three phase controlled rectifiers?
3. What are the classifications of three phase controlled rectifier?
4. What are the advantages of six pulse converter?
5. Write down the expression for average output voltage of three phase full converter.
6. What are the effects of source impedance in the controlled rectifiers?
Experiment No: 5
Objective
To study the characteristics of single phase dual converter
Objectives
1. To study about single phase dual converter
2. To plot the output waveforms for single phase dual converter
Apparatus Required
Sl.no
|
Name of components
|
Rating /Ranging
|
Quantity
|
01
|
1- Ф fully controlled bridge converter(kit)
|
01
| |
02
|
DC motor
|
1HP, 1-Ф ,230V,10A
|
01
|
03
|
Tachometer
|
0-6000rpm
|
01
|
04
|
Connecting wires
|
0-5/10A
|
10
|
05
|
Multimeter
|
0-300/600V
|
01
|
Theory
The DC motors are undisputable electrical equipments in any application. Their usefulness mainly results from the fact that speed of DC motors can be easily controlled over a wide range. Before the advent of thyristors other simple methods were used for control of these motors. Well-known among them was Ward Leonard system. The introduction of thyristors has completely changed the picture and now thyristor controlled DC motors are dominating the field.
The present system makes use of armature voltage control for speed control of DC motor with direction reversal facility and dynamic braking system. It is an open loop system based on a dual converter using non circulating current mode of operation.
Dual Converter:
As the name indicates, a dual converter consists of two converters (single phase or three phases), both either fully controlled or half controlled connected to the same load. The purpose of a dual converter is to provide a reversible DC voltage to the load, typically a separately excited DC motor. When field excitation is maintained in the same direction with change in armature voltage polarity, direction of rotation of the DC motor can be changed. Figure 1 shows the schematic arrangement of a dual converter. The two modes of its operation are non-circulating current mode and the circulating current mode. In the former, only one of the bridges is triggered. When reversal of output is required, the firing pulses for the conducting bridge are stopped and the second bridge is triggered. When reversal of output is required, the firing pulses for the conducting bridge are stopped and the second bridge is gated. Since the conducting SCR in the first bridge will turn OFF only when the current goes to zero, a small dead time must be allowed before the second bridge is gated. Otherwise the AC input will be shorted through the two bridges. In order to avoid shorting of the bridges a sufficient dead band is included while transferring from one direction to the other so that thyristors get sufficient time to get turned OFF. In the circulating current mode, both bridges are gated simultaneously, one operating in the rectifying mode and other in the inverting mode to avoid short circuit. The logic and control circuitry required for circulating current is quite complex. The main advantage of the circulating scheme is the rapidity with which the phase reversal of the output current can be obtained. With need for less sophisticated system and consequent low cost of the system has made the non circulating current mode most popular and practically this approach is used in all dual converters. The present set up is also based on non circulating current mode.
Dynamic Breaking:
When the armature supply of a running DC motor is disconnected and the armature is shunted by a resistor, a generating torque is developed by the motor and it brakes on its own. The field supply during this process remains connected. The main advantage of this system lies in the smallest time interval during which motor can be brought to rest. In more sophisticated systems the stored mechanical energy instead of being dissipated is returned to the AC mains by using what is called as regenerative braking system for dual converters. In this system when the motor is required to be braked, the pulses for the bridge working as converter are blocked and the other bridge is operated in inverter mode. Thereby facilitating conversion of DC power onto AC. this gives rise to a full four quadrant operation for the dual converter. In this particular set up, we are using dynamic braking with the help of resistor (lamp load for visual effect).
Description of the set-up:
The system to demonstrate the functioning of this 1 phase dual converter is housed in a sturdy MS powder coated box. It houses transformers for control circuit & 1 no of power transformer (0-130 v AC secondary o/p, 2 Amps), for energizing the power circuit. All the eight thyristors (TYN 612) are grouped into two sections are for +ve o/p & for –ve output (4 no of each group). The necessary isolated DC supplies for opto isolators (8 no) are generated & an AC sensing circuit at inside is provided to achieve proper synchronization.
Microcontroller PIC 16F870 forms the heart of the system & generates all the control signals for proper function of the unit, with the help of pot P1.It ensures,
- Soft start arrangement
- Sufficient dead band for the generation of pulses for either +ve o/p (clockwise movement of P1) or –ve o/p (anticlockwise movement) of pot P1. In the middle (marked zero position) none of the pulses for triggering the thyristors are generated.
Trigger pulses TP1, TP2, TP3 & TP4 for all the four thyristors in each group are generated & can be observed on test points marked properly for the +ve & -ve channels.
The test point TP5 & TP6 display the ZCD waveform of the system. As the pot P1 is moved clockwise or anticlockwise, the phase angle for reference pulse & triggering pulse goes on varying. The system is provided with 2 no AC switches, one for electronic control circuit. If this switch (SW1) is ON, the triggering pulse only can be observed on CRO. When the other switch (SW2 marked power ON circuit) is switched on power transformer is brought into action & the system starts to work as dual converter.
You can either connect the motor load or lamp load. For motor load 12 pin socket is provided with 110 v DC motor.
For lamp load a separate load bank is provided. The attenuator o/p can be observed on CRO. As there is complete isolation of power circuit & mains supply, no isolating transformer is required for CRO.
The switch SW3 on the right hand side of main power is used to demonstrate the dynamic braking of the system. The switch SW3 is required to be kept in downward position for normal-dual converter operation. (Marked NORMAL). With motor load connected, the switch SW3 if then in upward position, connects the armature circuit to lamp load (ensuring that thyristor o/p is disconnected), the stored KE in motor rotor is converted into DC voltage which lights the lamp momentarily & dissipates all the stored energy in the motor rotor in the lamp load & motor comes to halt immediately, indicating the process of dynamic braking.
Procedure
1) Ensure pot P1 in zero position & SW3 is in downward (NORMAL) position.
2) Connect load bank to 12 pin socket
3) Turn ON the switch SW1 for electronic control circuit.
4) Connect CRO and observe all waveforms for TP1, TP2, TP3 & TP4 for both +ve & -ve outputs, by rotating pot P1 in clockwise and anticlockwise direction respectively. If you observe the waveform at TP5 & TP6 you can see phase shifted triggering pulses for thyristor.
5) Bring pot P1 to zero position & turn ON switch SW2 for power circuit, the thyristor circuit is energized & you can observe the lamp intensity control. When you move pot P1 in clockwise & anticlockwise direction, voltage polarity is indicated by voltmeter. You may observe o/p waveform across the attenuated test point for o/p.
6) You must not observe the waveforms for triggering circuit & output simultaneously on CRO.
7) Now turn OFF power circuit & connect the motor to the output socket with no load on the motor. Rotate the pot P1 to zero position & turn it in clockwise direction the motor starts rotating in clockwise direction. Load the motor & observe the effect on the o/p waveform across attenuated output test points.
8) Dynamic Braking: keep motor running at about 100 v with negligible load on the motor. Then throw the switch SW3 in upward position. The armature will be disconnected from power circuit & connected to lamp load which is located on the right hand side socket. Thus stored KE will be converted into electric energy & lamp will glow momentarily & motor comes halt immediately. If lamp is removed & the same process is repeated with motor running, you notice that motor take longer time to stop.
9) Now throw the switch SW3 in downward (NORMAL) position. This switch should be normally in downward position.
10) You can repeat the sequence when you rotate the motor in anticlockwise direction also.
Circuit Diagram
\Waveform
Result:
Viva Questions
1. Define Dual Converter?
2. Explain the operation of single phase Dual Converter
3. What are the application of single phase Dual converter
4. What is the difference between dual converter and Cycloconverter
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