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Experiment No: Date:

1- INVERTER USING POWER MOSFET

AIM: To study 1- inverter using power MOSFET.

APPARATUS:

THEORY:The bipolar junction transistors are current controlled devices. Field effecttransistors on the other hand do not normally require any input current and areunipolar devices. They involve a single conducting channel which can be eitherN or P type material. The FETs offer advantages in switching service since theydo not suffer the delays associated with minority carrier storage. Because theyalso demand no input current, they are easier to drive. They are also lesstemperature sensitive and less susceptible to second breakdown in high powerapplications. All JFETs are operated with reverse bias on their gate leads toprevent gate current. However a large input signal may momentarily overcomethe reverse bias and turn on gate diodes drawing appreciable current from thesource. These disadvantages are overcome by insulating the gate terminal fromthe channel with a thin layer of silicon dioxide. Those FETs that use thistechnique are known as metallic oxide semiconductor field effect transistor orMOSFETs.

MOSFETs are operated in depletion mode as do JFETs with negative voltageon the gate terminal for N channel device. Depletion mode operated devices are

normally in ON condition. MOSFETs may also be operated in the enhancementmode. In this mode device is normally in OFF condition and a sufficiently largepositive voltage on the gate terminal can turn on the device. Generally powerenhancement MOSFETs are used in power electronic circuits. Its structuralwith normal biasing of N channel enhancement MOSFET is shown in figure 1with the circuit symbol. A metallic gate is deposited on the thin layer of metal


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oxide which is deposited on the channel opposite to the substrate. Due to theinsulated gate, negligible gate current flows.The power enhancement MOSFET has an anti parallel fast turn on diode whichpermits reverse current on the same magnitude as that of the main MOSFET, sothat drain substrate junction will not be damaged when drain and source hasreverse biasing.

Application:

The enhancement MOSFET is used as a switch in power electronics sufficientgate voltage (Vgs) so that it conducts in the constant resistance region. Theconduction lost of the MOSFET is high due to large value of device resistanceis ON state. The MOSFET can be triggered directly from the CMOS or othergates due to high input impedance. Switching times (Turn ON and Turn OFF) is

very low and hence switching loss is almost nil. The gate drive power is alsonegligible. They have larger gains and simple and cheaper triggering circuits. Ithas only one disadvantage i.e. higher conduction drop generally five times morethan the power transistor of the same rating.

Description of the set up:

This set up is designed to demonstrate the working of a typical single phaseinverter using power MOSFETs in bridge configuration. The inverter works as

both low frequency (typically 50 Hz or so) and around 500 Hz. This frequencyis variable. For low frequency output, i.e. 50 Hz, 230 volt, 40 watt lamp load isprovided through set up transformer. For high frequency application only aresistance load of 25 ohms is provided. The panel layout is shown in figure 2.

The set up has built in d.c. power supply at 18 volts, 3 amp and this d.c. poweris converted into higher voltage at 50 Hz. There are three other d.c. powersupplies 5 volts for logic gates, 12 volts, 12 volts and 12 volts (all isolated) foroptoisolator operated driver circuits. TR1 is main step down transformer, TR2for auxiliary power supplies and TR3 is step up output transformer. The power

MOSFETs are arranged in bridge configuration and driven by appropriatedriver stages. The entire system is mounted on a neatly labeled anodized plateindicating various controls very clearly. Appropriate test points are provided.

Operation of the set up:


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Figure 3 shows the power circuit of the MOSFET driven single phase inverter.The MOSFETs are in bridge configuration. Figure 3A shows the gate drainvoltage waveforms (Vg1,Vg2, Vg3 and Vg4) required by the power MOSFETs foe

necessary operation. When T1 and T3 are turned ON by the gate signals Vg1and Vg3, the current flows through point B to A and results in a positive halfcycle of the output.

In the next half cycle, the drive for T2 and T4 is provided by Vg2 and Vg4waveforms. The transistor T1 and T3 are already turned off by their gatedrivers. Now with T2 and T4 ON, current flows for A to B forming negativehalf cycle output waveform.

In order to avoid the shorting of the d.c. power supply with T1,T2,T3 and T4

power MOSFETs in ON condition, a special trick is used to remove the gatedrives before 15 millisecond just before the transition takes place i.e. at the timeof expected turn ON, all the gate drives are removed. This delay is generated bymonostable IC 74121. The logic generated by IC 7486 Exclusive OR gateprovides the necessary gate drive for all the power MOSFETs. Refer figure 4for details of gate driven circuitry. By changing the timing capacitor of astablemultivibrator (555) we can gate two frequencies (around 50 Hz & 500 Hz)variable by potentiometer P1. Switch SW2 offers changeover from highfrequency (upward position), with simultaneous switching of load also with 40

watt lamp is placed on the output of the inverter. For high frequency onlyresistance load is applied. You can observe basic clock signal at TP1 withrespect to the ground and gate drives for T1 & T3 at TP2 and the same for T2 &T4 at TP3. If you observe waveforms across TP2 & TP3, low voltage version ofthe output waveform is seen on the C.R.O.

PROCEDURE:

1.Get yourself conversant with the various blocks of the entire system. Try tounderstand the function of the various controls on the panel board of thesystem.

2.Ensure that the link between the binding posts marked (LINK ORAMMETER) is open. This link provides main d.c. power as input to theinverter.


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3.Keep the switch SW2 in downward position (high frequency position).Connect the C.R.O. between TP1 and ground.

4.Now switch ON the mains supply and observe the waveforms for transistorsT1 & T3 and T2 & T4 at test points TP2 and TP3 respectively.

5.Now keep the switch SW2 in upward position and connect the C.R.O. acrossthe binding post for output.

6.Now place the link across the binding post marked (LINK OR AMMETER).The d.c. power is supplied to the inverter and inversion operation startsindicated by the output waveforms across the C.R.O. screen. Note thatalready a load resistance of 25 ohms/ 25 watts is internally connected acrossthe output.

7.Change the frequency of the output by operating the potentiometer P1.Observe the discontinuity in the waveform at zero transition.

8.Now remove the link and switch ON the SW2 in lower position. Ensure thatthe lamp 40 watt/ 230 volts is in place. Now if the d.c. link is put ON, thelamp glows with full intensity. You may observe the waveforms across theoutput terminals marked lamp load. Lamp load is in operation only for lowfrequency operation.

9.Study the operation of the optoisolator circuitry for isolation of triggeringsource and the gate and drain circuits of transistor T1 & T2. Note that the 12volts supplies for T1 & T2 drives are completely isolated as S1 & S2 (drainsfor T1 & T2) are completely isolated with respect to the main ground. For T3& T4, we note that triggers source ground and main supply ground are thesame.

PRECAUTIONS:

1. Do not start the d.c. power supply by placing a link across the input buildingpost (Red and Black). Do not short the test points by the links provided.


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2. While changing theSW2, always disconnect the d.c. power input to the

inverter and only then effect the change.

3. Note that waveform on low voltage side are to be observed for protection ofC.R.O.

CONCLUSION:


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SERIES INVERTER

AIM: To study about series inverter.

APPARATUS:

THEORY:The series inverter uses class A type commutation. Commutating components Land C are applied in series with the load to form an under damped circuit, sinceSCRs turns off by themselves when the current becomes zero. This inverter isclassified as a self commutating inverter. Figure 1 indicates the schematicarrangement of a simple series inverter. Let the initial voltage across the capacitorbe with the polarity as shown in fig. When TH1 is turned ON, the waveforms for

current I will be as shown in fig 1B.The necessary condition to obtain this loadcurrent is that the series circuit consisting of commutating components C and Land R must be under damped. Therefore R must be less than 4L/C.

At point A load current I is zero and TH1 will be turned off and also the capacitorC will be charged to a voltage Vc in the reverse direction. Duration AB should bemore than the turned off time it requires. Capacitor C will be now dischargedthrough TH2 and the under damped circuit. Load current I will be in the oppositedirection and again becomes zero and at point C and TH2 will then turn off. TheOutput frequency will then be given by,

Toff2

T

1


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The main limitations of series inverter here are:

1. The TH2 can be triggered only after TH1 is turned off, otherwise there willbe short circuit on the d.c. supply and commutations of SCR will not takeplace. This limits the maximum operating frequency.

2. For output frequencies much lower than the ringing frequency the distortionin the load voltage waveforms is high. This is because the off time is large incomparison with the duration of conduction of SCRs.

3. The commutating components are required of high rating because they carrythe load current continuously and the capacitor supplies the load current inevery alternate half cycles.

4. The power flow from d.c. source is intermittent. This is because the currentis supplied to the load only when TH1 is fired.

5. Output regulation is poor.6. Out of this limitation 2, 3, 5 are inherent in all types of series inverters andcan not be overcome. The limitations one and two can be relaxed by

modified series inverter.

Fig.2 shows two modifications for the series inverter configuration describedearlier. In fig 2A inductors L1 and L2 have the same inductance and are closelycoupled. Therefore when TH1 is fired and current I1 begins to rise during the firstquarter of the cycle, the potential across L1 will be +ve with respect to polarity asshown in fig. though induced voltage in L2 will now add to the capacitor voltage inreverse biasing TH2. For this mode of operation the circuit has no special

advantages over that in fig.1, except if SCR in the former circuit will experiencereverse voltage for longer time. However the important feature of this circuit is thatTH2 can be triggered even before TH1 has been turned off. This mode of operationis possible because of the induced voltage in commutating inductor L1 and L2,where as in the series inverter described earlier the same operation results in shortcircuit in the d.c. supply. In this circuit as shown in fig. 1A, the power flow fromthe d.c. source is intermittent. This drawback is overcome in the circuit shown infig 2B where during both half cycles of the output power is drawn from the inputsupply. One half of the load current is supplied by the capacitor C1 and C2 and the

other half flow from d.c. supply. The inductance L1 and L2 are identical and alsoare the capacitor C1 and C2. The circuit arrangement i.e. demonstrated in this setup is based on figure 2B.

PROCEDURE:


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1. Ensure that control knob is in minimum position and load lamp (230 volts

25watts) is in place.

2. Connect a link (patch cord provided) across the terminals marked LINK.Effectively this provides d.c. supply for the inverter.

3. Immediately after connecting a link, the load lamp glows. If it does not,remove the link and try again. This means inverter has started functioning.Even after two or three trials, if you find that inverter is not starting, thencheck for open fuse or a loose connection etc.

4. You may observe various waveforms on CRO.5.

You may note this series inverter is basically producing a low voltage a.c.supply (about 12volts a.c) and the same is stepped up to feed a 230 voltlamp.

PRECAUTIONS:

1. Do not tamper with the components of UJT firing circuit.2. If the series inverter does not start functioning as indicated by glowing load

lamp immediately disconnect the link.

CONCLUSION:



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PARALLEL INVERTER

AIM:To studyabout parallel inverter.

APPARATUS:

THEORY:The SCR inverter is designed to convert the 230 volt 50 Hz in to 24 volts d.c. andthen by making use of a parallel inverter the d.c. voltage is inverted in toappropriate a.c wave at 230 volts a.c lamp load at 30 watts can be connected to thevarious blocks which go in to the making of this inverter.

1. A.C to D.C. conversion: 230 volts 50 Hz a.c. is converted in to 24 volts d.c.by making use of a rectifier and a suitable capacitive filter.

2.

Parallel inverter: The d.c. voltage generated from above is again inverted into an a.c. voltage with the help of parallel inverter shown in fig.1. Theparallel capacitor commutated inverter is one of the oldest forcedcommutated inverter. SCR 1 and SCR 2 are alternately turn ON to connectthe d.c. source voltage to one half of the load. Assuming SCR 1 is ON, thecommutating capacitor C1 is also charged to a voltage E.D.C., neglecting theinfluence of inductor L1. SCR 1 is commutated when SCR 2 is triggered asthis connects the charged commutating capacitor C 1 across SCR 1 toreverse bias this SCR causing it to turn OFF. During the SCR 2 conductingperiod, the capacitor is charged to the opposite polarity ready to commutate

SCR 2 when SCR 1 is triggered. The major purpose of L1 is used such thatthe d.c. source current is almost constant.


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ADVANTAGES:

1.

It is one of the simplest force commutated inverters and provides a feasibleapproach when load power factor is near unity and where the magnitude ofthe load is relatively constant.

2. A reasonably sinusoidal voltage can be produced when suitable output filteris used.

DISADVANTAGES:

1. Reliable starting and stopping is a fairly difficult design problem to avoidcommutation failure and/or excessive current due to transformer saturation.

2. The d.c. inductor L1 is and commutating capacitor C1 are relatively large forcircuits operating at normal power frequencies. This increases size, weightand cost of the equipment.

SCR Triggering Circuit:

An astable multivibrator circuit is incorporated which gives a square wave outputat about 50 Hz. In order to achieve reliable firing of SCR 1 and SCR 2 acontinuous train of sharp pulses is provide to the gates of the SCRs by making useof UJT relaxation oscillator as shown in fig.2.

PROCEDURE:

1. Ensure that connecting link is across the terminals marked LINK isremoved. Confirm that the load lamps are in place.

2. Now switch ON the main power supply. With the help of CRO observe thewaveforms at terminals marked TP1 and TP2 with respect to groundterminal.

3. Now place connecting link across the terminals marked LINK and observethat output load lamp glow observe the waveform at the point marked TP3

and TP4 with respect to ground.


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PRECAUTIONS:

1. After placement of link across the terminals marked LINK the lampshould immediately glow. If the output lamps do not glow, it indicatesfailure of the parallel inverter to get started. Remove the link immediatelyunder such condition.

2. You may try once again by placing the connecting cable across the terminalsmarked LINK.

3. Do not use this connecting link to effect connections any where else on thisboard.

4. Do not connect CRO to observe waveform across the final output terminalsbecause the voltage is 230 volts which may damage the CRO. Properprecaution regarding range of the input, isolating transformer etc must betaken while making this observation.

CONCLUSION:



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3-PHASE INDUCTION MOTOR CONTROLLER (V\F SCHEME)

USING IGBT(PWM)

AIM:To study the 3-phase induction motor controller using IGBT.

APPARATUS:

THEORY:Induction motors are relatively cheap and rugged machines because they can bebuilt without slip rings and commutations. Consequently much attention is given tothe use of induction motor control for starting, braking, speed reversals, speedcontrol etc. We present here a very sophisticated system of complete and full proofcontrol of 3-phase induction motor using V/F scheme and IGBT with PWM

techniques, an industrial version. The induction motor is inherently a constantspeed motor when supplied form a constant voltage and constant frequency.The synchronous speed of an induction motor is a speed of mmf waveform rotating

in the air gap i.e. Ns=p

fr.p.s. Where f is supply frequency. For fixed no. of poles a

change in supply frequency would bring about a proportioned change insynchronous speed and actual speed and would follow roughly in the same manner.An induction motor is designed to work at a particular flux density and aselectromagnetic. Torque is proportional to the magnetic flux density, it is necessaryto have a high value of flux density. If the applied voltage can be said to be equal

to the induced emf, then from the equation of the induced emf V=k, where k isthe constant involving form factor, no. of turns etc and = maximum flux per pole with V = RMS voltage across the motor terminals andf is the excitation frequency of the supply, the applied voltage to the inductionmotor must be adjusted in proportion to the frequency i.e. V/f ratio must bemaintained constant.


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Principle of Operation:

The unit supplied involves a dc link three phase bridge inverter using IGBT inpulse width modulation techniques. It also includes two numbers of

microprocessors for sophisticated controls the excitation frequency of the inverteroutput and the same potentiometer is also controlling the pulse width of thewaveform in the individual half cycles. For fixed dc excitation (around 330 volts),the output voltage A.c. is changing because of change in the pulse width of thechopped waveform for each positive and negative half cycle. T5he carrierfrequency can be changed over a wide range from 2 KHZ to14 KHZ. The powerfulsoftware is designed to keep V/f ratio almost constant, in addition to over currentprotection frequency of speed indication and many others.

PRECAUTIONS:

1. For observation of waveforms connect CRO to the 3 pin socket provided onthe right hand panel. Keep the sensitivity control in 20 volt/division range.Do not touch metallic parts of CRO. Isolating transformer is built in theequipment provided.

2. Under no circumstances the CRO should be connected to the external mainsother than the one provided on the right hand side panel of the set up.Earthing pin of CRO can damage the unit if isolating transformer is not

used.

3. For voltage measurements, keep patch cords in the terminals provided belowthe ac voltmeter and connect them to the motors terminals carefully.

4. Take care to see that the motor terminals are not shorted while observing theoutput voltage.

5. Do not apply sudden loads on the motor.6. Use only CRO to measure the output frequency of the inverter.7. Please try to understand all the operating controls and instructions given

separately and only then proceed to operate the set up.

PROCEDURE:


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1. After switching on the unit, you may use button MON on the main panel

for selection of various parameters. (Press just once)

2.

You have to use the Arrow keys (up and down) browsing variousparameters.

3. For changing the value of any of the parameterspress Enter and then againby using ARROW keys you can change the value accordingly.

4. Press MON twice for coming back to initial status.5. Selecting the functions first press MON and browse for. Here press

ENTER and you will get F100. Then browse for desired function number

by using arrow keys.

6. Before you press RUN keep FREQUENCY pot in minimum position.7. After finishing the experiment press STOP and make sure that speed and

output voltage is zero, and then only turn off the mains power supply.

OBSERVATION TABLE:

Sr.No.

Speed (NS)

in rpmFrequency

in HZVoltagein Volt

V / Fratio

V / Nratio

12345


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6789

101112131415

CONCLUSION:

Experiment No.: Date:


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Aim: To study the performance of three phase squirrel cage induction motor drives withthree phase voltage source inverter using PSIM.

Circuit Diagram:

Waveform for Gate pulse:


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Waveform for inverter input d.c. voltage, speed and torque:

Waveform for three phase motor supply current:


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Conclusion:


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CIRCUIT DIAGRAM:


Aim: Simulation of three phase current source inverter using PSIM.

Circuit Diagram:


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Wavefom for Phase voltage Va, phase current Ia,Ib,Ic :


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Conclusion:


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Aim: To study the performance of three phase voltage source inverter with PWM controlwith selected harmonic elimination technique using PSIM.

Circuit Diagram:


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Waveforms for output a.c. current:

Waveform for input voltage, output of PWM control and line to line voltage Vab


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Conclusion:

Date post:	03-Apr-2018
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