1.Why power electronics circuits are efficient? In practice, which types of losses occur in power electronics circuits?

Power electronics is the application of solid-state electronics to the control and conversion of electric power.The first high power electronic devices were mercury-arc valves. In modern systems, the conversion is performed with semiconductor switching devices such as diodes, thyristors, and power transistors such as the power MOSFET and IGBT. In contrast to electronic systems concerned with transmission and processing of signals and data, in power electronics substantial amounts of electrical energy are processed. An AC/DC converter (rectifier) is the most typical power electronics device found in many consumer electronic devices, e.g. television sets, personal computers, battery chargers, etc. The power range is typically from tens of watts to several hundred watts. In industry a common application is the variable speed drive (VSD) that is used to control an induction motor. The power range of VSDs start from a few hundred watts and end at tens of megawatts.

Types of Power Electronics Circuit

• Diode Rectifiers
• AC-DC Converters (Controlled Rectifiers)
• AC-AC Converters (AC Voltage Controllers)
• DC-DC Converters (DC Choppers)
• DC-AC Converters (Inverters)
• Static Switches

Rectifiers

It converts AC Voltage into a FIXED DC Voltage

AC-DC Converters

• A single-phase converter with two natural commutated thyristor is shown
• Average value of the output voltage can be controlled by varying the conduction time of thyristors
• This converters are also known as controlled rectifiers

AC-AC Converters

• Used to obtain variable AC output voltage from a fixed AC source
• a single-phase converter with a TRIAC is shown below

DC-DC Converters

• Is also known as Chopper or Switching Regulator
• The average output voltage is controlled by varying the conduction of transistor, t1.
• If T is the chopping period, then t1+deltaT
• Delta is called as the duty cycle of chopper

DC-AC Converters

• Is also known as Inverter
• If transistor Q1 and Q2 conduct for one-half period Q3 and Q4 conduct the other half, the output voltage is of alternating form

Static Switches

Power devices can be operated as static switches or contactors, the supply to these switches could be either AC or DC and the switches are called as AC static switches or DC switches

Power Electronics for Efficiency

Power electronics deals with the conversion and control of electrical energy with the help of power semiconductor devices that operate in a switching mode, and, therefore, efficiency of power electronic apparatus may approach as high as 98–99%. This technology uses electronic components such as inverters and transformers to convert the electricity from wall outlet into the right voltage and current to power each of handy or handheld devices. It is also used more widely to distribute electricity throughout the power grid to connect renewable energy systems, and to charge up electric vehicles. Electrical energy has already become our lifeline so we want to get a regulated form of this energy which in itself is not usable until it is converted into a tangible form of energy such as motion, light, sound, heat etc. In order to regulate these forms of energy, an effective way is to regulate the electrical energy itself and this forms the content of the subject power electronics. Power electronics is the study of switching electronic circuits in order to control the flow of electrical energy, its generation, transportation and applications. Power Electronics is the technology behind switching power supplies, power converters, power inverters, motor drives, and motor soft starters. Power electronics is an enabling technology, its development, together with internal developments, such as wide bandgap semiconductors, will be driven externally by applications in the future.

Power electronics is very important in modern high-efficiency energy processing systems, such as HVDC, SVC, flexible ac transmission system (FACTS) for active and reactive power flow control, uninterruptible power supply (UPS), and industrial process control with variable-frequency drives for improving productivity and quality of products in modern automated factories. Variable-frequency drives are now used extensively in pumps and compressor drives, paper and textile mill drives, subway and locomotive propulsion, electric and hybrid vehicles, elevators, metal rolling and textile mills, home appliances, machine tools, and robotics, variable-speed air conditioners, wind generation systems, ship propulsion. Power electronics plays a very important role in solving or mitigating our global warming problem, which is a very serious concern in our society.

• High reliability of power electronic converter system.
• Long lite and less maintenance due to absence of any moving parts.
• Flexibility in operation
• Fast dynamic response compared to electromechanical converter system.
• Small Size and less weight, thus low installation cost

Losses in Power Electronic Circuit:

The above figure shows the losses occurs in Switches during ON & OFF. Conduction losses mainly depend on the duty cycle, load current and junction temperature, whereas, switching losses depends on the load current, dc link voltage, junction temperature and switching frequency. If the switching frequency is higher, then the losses will be higher.

( i ) Conduction Losses:

Conduction losses are the losses that occur while the IGBT or freewheeling diode IS on and conducting current, the total power dissipation during conductionis computed by multiplying the on-state voltage (Vce_sat) and the on-state current (lc). In PWM applications the conduction loss must be multiplied by the duty factor to obtain aver -age power dissipatiom A first order approximation of conduction losses can be obtained by multiplying the IGBT\'s rated (Vce_sat) by the expected average device current Conduction loss is the on-state loss or steady state loss. The average power dissipated by the IGBT is given by below,

( ii ) Switching Losses:

In power electronics switching losses typically contribute a significant amount to the total system losses. Switching losses occurred because the transitions from on state to off-state and vice versa do not occur During the transition interval both the current through and the voltage across the device are substantially larger than zero which leads to large instantaneous power loss. The curves show the simplified current and voltage waveforms and the dissipated power during one switching cycle of an IGBT in an inverter leg.If one plans to calculate the junction temperature time behaviour to improve reliability of the design, it is necessary to calculate accurately the switching losses.

2.Using a suitable MATLAB model, show how duty cycle control signal can be generated for a simple power converter circuit?

What is boost converter?

A boost converter (step-up converter) is a DC-to-DC power converter that steps up voltage (while stepping down current) from its input (supply) to its output (load). It is a class of switched-mode power supply (SMPS) containing at least two semiconductors (a diode and a transistor) and at least one energy storage element: a capacitor, inductor, or the two in combination. To reduce voltage ripple, filters made of capacitors (sometimes in combination with inductors) are normally added to such a converter\'s output (load-side filter) and input (supply-side filter).

A boost converter is one of the simplest types of switch mode converter. As the name suggests, it takes an input voltage and boosts or increases it. All it consists of is an inductor, a semiconductor switch (these days it’s a MOSFET, since you can get really nice ones these days), a diode and a capacitor. Also needed is a source of a periodic square wave. This can be something as simple as a 555 timer or even a dedicated SMPS IC like the famous MC34063A IC.

As you can see, there are only a few parts required to make a boost converter. It is less cumbersome than an AC transformer or inductor.They’re so simple because they were originally developed in the 1960s to power the electronics systems on aircraft. It was a requirement that these converters be as compact and as efficient as possible.The biggest advantage boost converters offer is their high efficiency – some of them can even go up to 99%! In other words, 99% of the input energy is converted to useful output energy, only 1% is wasted.

How Does a Boost Converter Work?

It’s time to take a really deep breath, we’re about to plunge into the depths of power electronics. I’ll say at the outset that it is a very rewarding field.To understand the working of a boost converter, it is mandatory that you know how inductors, MOSFETs, diodes and capacitors work.With that knowledge, we can go through the working of the boost converter step by step.

STEP – 1

Here, nothing happens. The output capacitor is charged to the input voltage minus one diode drop.

STEP – 2

Now, it’s time to turn the switch on. Our signal source goes high, turning on the MOSFET. All the current is diverted through to the MOSFET through the inductor. Note that the output capacitor stays charged since it can’t discharge through the now back-biased diode.The power source isn’t immediately short circuited, of course, since the inductor makes the current ramp up relatively slowly. Also, a magnetic field builds up around the inductor. Note the polarity of the voltage applied across the inductor.

STEP – 3

The MOSFET is turned off and the current to the inductor is stopped abruptly.

The very nature of an inductor is to maintain smooth current flow; it doesn’t like sudden changes in current. So it does not like the sudden turning off of the current. It responds to this by generating a large voltage with the opposite polarity of the voltage originally supplied to it using the energy stored in the magnetic field to maintain that current flow.

If we forget the rest of the circuit elements and notice only the polarity symbols, we notice that the inductor now acts like a voltage source in series with the supply voltage. This means that the anode of the diode is now at a higher voltage than the cathode (remember, the cap was already charged to supply voltage in the beginning) and is forward biased.

The output capacitor is now charged to a higher voltage than before, which means that we have successfully stepped up a low DC voltage to a higher one!

I recommend that you go through the steps once again very slowly and understand them intuitively.These steps happen many thousands of times (depending on the frequency of the oscillator) to maintain the output voltage under load.

MATLAB MODEL USING THE BOOST CONVERTER

The above model consists of DC Voltage Source of (5 V), Inductor of (1 mh), Mosfet Switch is used in it after then then the Pulse generator is operated as the Duty cycle which is of 70% of pulse width then Diode is used, Capacitor of (33e-6 farad), Resistor of (100 ohm) and then Voltage measurement and current measurement are used. The simulation is continuous and operates as Boost Converter which increases the voltage of input one according to the duty cycle.

OUTPUT:

The Boost converter which increases the input voltage to 70% of duty cycle to output voltage. In the Result, we get the output voltage as approx 16V. BY varying the duty cycle, the output voltage also varies but more than the input voltage which is also called as Step_up Voltage.

3.Make a comparison of square wave inverter output voltage with sine modulated one

PURE SINE WAVE VS SQUARE WAVE INVERTERS

a) Output voltage wave form of the true wave inverter is a pure sine wave which we get through utility usage in our home for the electric IN pure wave of sine distortion is approximately zero just like electricity obtain from supplied by company. But in square wave harmonics produces distortion which can destroy or damage our equipment while running on square wave. So pure sine wave is better than square wave.

b) Most of the time we used inductive loads in our homes while we are using such loads we have to use pure sine wave. Because it run faster with pure sine wave than square Inductive load can be microwave oven, Motors, Cooler and etc. so again proved that pure sine wave inverter must be used for more efficiency.

c) Sine wave reduces audioable and noise pollution while running electric Even when we use square wave audio and noise pollution are in very large quantity.

d)Sine wave inverter output is very close to the AC current It is very pure form compared to square wave output Many devices gets heated up if you use on Square wave inverter and make humming noise too.

MODIFIED SINE WAVE VS SQUARE WAVE INVERTERS

A Modified Sine Wave inverter actually has a waveform more like a square wave, but with an extra step. A modified sine wave inverter will work fine with most equipment, although the efficiency or power of the equipment will be reduced with some.

1.Motors, such as retrigerator motor, pumps, fans etc will use more power trom the inverter due to lower efficiency. Most motors will use about 20% more power. This is because a fair percentage ot a modified sine wave is higher frequencies — that is, not 60 Hz — so the motors cannot use it.

2.Some fluorescent lights will not operate quite as bright, and some may buzz or make annoying humming noises.

3.Appliances with electronic timers and/or digital clocks will often not operate correctly. Many appliances get their timing from the peak of the line power — basically, the modified sine has a flat top rather than a peak this may cause the occasional double trigger Because the modified sine wave is noisier and rougher than a pure sine wave, clocks and timers may run taster or not work at all.

4.Items such as bread makers and light dimmers may not work at all — in many cases appliances that use electronic temperature controls will not control. The most common is on such things as variable speed drills will only nave speeds — on and off.

5.most equipment will operate without any noticeable difference, and because the lower cost, makes this the most common inverter sold and generally the only type found at your local retailer

CONCLUSION

Sine wave inverter output is very close to the AC current. It is very pure form compared to square wave output. Many devices gets heated up if you use on Square wave inverter and make humming noise too. Because of this reason square wave inverters in few European countries is banned.

4.Explain the difference between VFD and Vector control drive. Can we use VFD method for induction motor in EV application?

VFD [Variable Frequency Drive]

VFDs are used in applications ranging from small appliances to large compressors. About 25% of the world\'s electrical energy is consumed by electric motors in industrial applications. Systems using VFDs can be more efficient than those using throttling control of fluid flow, such as in systems with pumps or fans. However, the global market penetration for all applications of VFDs is relatively small. Pulse Width Modulated Variable Frequency Drives. When operated from a constant frequency power source (typically 60Hz), AC induction motors are fixed speed devices. A variable frequency drive controls the speed of an AC motor by varying the frequency supplied to the motor.

1. A standard VFD will sometimes be referred to as a \"V/Hz\" drive to differentiate it from a vector drive. It maintains a certain V/Hz ratio to the motor at all times. For example, a 230 Vac, 60 Hz motor wants to see a V/Hz ratio of 3.83 (230/60 = 3.83). When a V/Hz drive changes speed (frequency), it also changes the output voltage to keep the ratio constant. So, at 30 Hz (half speed), the same 230 Vac motor will only see 115 Vac output from the drive.
2. For most applications this works great, as long as you operate from about 6-60 Hz (10:1 speed range). Below 6 Hz, a motor on a V/Hz type VFD can\'t generate much torque, because at those low speeds, the V/Hz ratio to achieve maximum torque is different than at higher speeds. If you try to run at those low speeds with a V/Hz drive, you will typically see the motor shaft \"cogging\" as it tries to turn.

Vector Control Drive

Blaschke introduced the principle of Vector Control to realize the dc motor characteristics in an induction motor drive. For the same, he has used decoupled control of torque and flux in the motor and gives the name transvector control. In DC machine particularly separately excited dc motor, the field flux is perpendicular to the armature flux. Being orthogonal, these two fluxes produce no net interaction to each other. Adjusting the field current can therefore control DC machine flux, and the torque can be controlled independently of flux by adjusting the armature current. An AC machine is not so simple because of the interactions between the stator and rotor fields, whose orientations are not held at 900 but vary with operating conditions. We can obtain the DC machine like performance in holding a fixed and orthogonal orientation between the fields and armature fields in an AC machine by orienting the stator current with respect to rotor flux so as to attain independently controlled flux and torque. Such a scheme is called Vector Control or Field Oriented Control. Vector Control is the most commonly used method of speed control technique of an induction motor because of its superior transient response compared to other methods and its ability to enable independent torque and flux control. Generally, Variable frequency drives employ Pulse Width Modulation technique (PWM).

1. Vector drives can separately manipulate the voltage and frequency to always produce the optimum V/Hz ratio for maximum torque. In this way, vector drives can generate more starting torque, and can provide full torque down to 1 Hz or less, while V/Hz drives are limited to about 5-6 Hz.
2. There are basically 2 types of vector drives: closed-loop, and open-loop or sensorless.

Closed-loop vector drives require special motors with encoder feedback (hence the term \"closed-loop\") that provide motor shaft position info to the drive. The drive uses this information to constantly alter the V/Hz ratio to produce maximum torque. This type of VFD is very high performance and very expensive.

Sensorless vector drives work with standard motors, but because there is no encoder feedback, the performance is not quite as good as closed-loop, but still much better than V/Hz drives. Instead of getting real-time info from an encoder, a sensorless vector drive estimates what is going on by creating a motor model within it\'s software. This model is created using motor info that the user manually enters, along with other info that the drive measures when it is connected to the motor.

Vector drives (more accurately referred to as \"Field Oriented Control\" drives) do not explicitly solve for a V/Hz relationship. Rather, they solve for two specific currents within the stator simultaniously. The V/Hz relationship is solved implicitly according to flux density from the vector sum of current components described below:

The first CURRENT is the Magnetizing current. Since there is no magnetic field within the rotor of the motor, it must be induced using electromagnetism (Hence Induction Motor). A DC current is controlled in the stator to induce an electromagnetic field in the rotor. As the rotor is turned (eg motor spinning) this magnetic field is rotated synchronously with the rotor to maintain a static orientation of the magnetic field relative to the rotor.

The second CURRENT is the Torque Producing Current. This current is controlled in the stator to produce another electromagnetic field within the stator itself that is oriented 90 degrees to the magnetic field simultaniously induced in the rotor as described above. As the intensity of the torque producing current increases, the torque applied to the rotor (ex motor shaft) increases.

NOTE:It is the VECTOR SUM of these two currents in the stator that is solved for by the drive in a Vector drive application. Hence \"Vector Drive\"

Can we use VFD method for induction motor in EV application!

variable frequency control method is most applicable tor pumps and other small applications. In the variable frequency drive the flux and the current are coupled to each other and the stator drop compensation were also their due to the coupled flux and the current the efficiency of the battery is low but In the Space vector Pulse Width Modulation, the torque and flux are decoupled and also it can achieve to reference voltage and small switching losses. It can be most usable in Induction motor for better performance. For Electric Vehicle, Induction motor are mostly used based on the performance and drive. VFD method may not prefer for the electric drive. The best method is Space Vector Control Method for Electric vehicle.