abtechnosolutions
Contents
1 INTRODUCTION AND BACKGROUND.. 2
1.1.1 Short Range Radiative. 3
3 ELEMENTS OF WIRELESS POWER TRANSFER SYSTEM… 10
4 HARDWARE PROTOTYPE DESIGN RECOMMENDATIONS. 31
4.2 Power Inverter Circuit Prototype. 34
CHAPTER 1
1 INTRODUCTION AND BACKGROUND
Wireless power transfer (WPT) or wireless electricity (WiTricity) is defined as the capability to transfer electrical power from a power source to an electrical load without the use of wires. Hertz is the first one who demonstrated the propagation of radio waves. Nicola Tesla proposed the first idea of WPT via ionised air. Tesla used a conductive coil (which is the form of air-core inductor) connected in series with a Leyden jar (which is a form of capacitor) to form a loop resonator. He excited one loop (primary coil) as the power transmitter and used a second loop resonator (secondary coil) as a power receiver as shown
In most applications, achieving high power efficiency is extremely challenging because the magnetic coupling decreases as the distance increases as shown in Figure 1.2. Therefore, power transmission range need to extend for the WPT to be more useful. Several methods previously proposed to accommodate power pickups, which are located at various lateral positions. Tesla showed that using magnetic resonance of the coupled coils could achieve optimal energy transfer. Before 1930, there was no serious interest in wireless power transmission duo to the knowledge of the people realized that the efficient point to point transmission of power depends upon concentrating the electromagnetic energy into narrow beam. Recently, attentions to wireless power transfer have been increasing dramatically since Soljacic has published WPT using magnetic resonances. Many researches are going on the inductive coupling, which is the basic core of WPT.
Generally, WPT can be classified into two types radiative and nonradiative power. Radiative power transfer relies on high-frequency excitation of a power source, and radiative power emitted from an antenna and propagates through a medium (such as vacuum or air) over long distance (i.e., many times larger than the dimension of the antenna) in the form of electromagnetic wave. Nonradiative wireless power transfer relies on the near-field electromagnetic coupling of conductive loops [6]. In the last few years, many applications were proposed via WPT such as autonomous electronic devices cellphones [7], laptops, iPods [8], or electric cars, whose batteries need to be recharged, and for lighting and TV sets which do not have secondary batteries and require to be powered continuously.
1.1 WPT Classification
WPT is classifying with respect to distance into three radiative ranges: short-range radiative, midrange radiative and long range radiative.
1.1.1 Short Range Radiative
Transformer based inductively coupled near-field wireless links have been studied extensively in the past three decades as a means of transferring power to implanted biomedical devices. Even with deliberative optimizations, the reported maximum transferred power is limited to 275 mW over one cm [9]. In addition, in [10] wireless power transfer link is described that uses a rotating magnet to transfer power. The rotating magnet based power transfer link is able to deliver 678 mW over a 1 cm distance wirelessly.
1.1.2 Mid-Range Radiative
WPT technology using magnetic resonance coupling has received much attention due to its potentials toward convenient noncontact electric energy supplying manners for various modern electric devices from low-power biomedical implants to high-power electric vehicles. In [11], they use impedance matching (IM) networks to adjust the resonance frequency of a pair of antennas at a certain distance to 13.56MHz. The simulations and experiments show that the IM circuits can change the frequency to 13.56MHz for different air gaps, improving the power transfer efficiency. Experiments also show that IM achieved just by observing and minimizing the reflected wave.
In [12] two-coil wireless power transmission system was analyzed, including the driving amplifier, and a demonstration system was built and characterized. The system achieves 76% efficiency for a distance of 1 meter for 40W-transferred power.
1.1.3 Long Range Radiative
Wireless power transfer in long range radiative is radiated as electromagnetic waves so a transmitter antenna is used to transfer electromagnetic waves. Long range radiative has the advantages of human safety, small size and long distance (in Km) while its disadvantage is low efficiency.
1.2 Motivation
Through the years, technology has allowed the cellular phone, laptops ,iPods, etc. to shrink not only the size of the ICs, but also the batteries. New combinations of materials have made possible the ability to produce batteries that not only are smaller and last longer, but also recharged easily. However, as technology has advanced and made our electronic devices smaller and easier to use, we still have one of the original problems: we must plug it into the wall in order to recharge the battery. Most people accept this as something that will never change, so they might as well accept it and carry around either extra batteries with them or a charger. Either way, it is just something extra to weigh a person down. There has been research done in the area of shrinking the charger in order to make it easier. The market for electric power becomes more competitive. Improving accuracy and effectiveness of transfer capability computations for all areas of power systems would prove a very strong economic incentive.
1.3 Objective
The objectives of this thesis are to design magnetic coupled resonators for wireless power transfer. In this work, we will design two models of power transfer systems using the electromagnetic professional program EMPRO. The first model is single transmitter single receiver where we will study the effect of five parameters which are the height of coil (H), the number of turns (N), the radius of source loop (r), the radius of coil (R), and the angle between the transmitter and the receiver (Ɵ) as shown in Figure 1.3. The second model has two transmitters and one receiver to improve efficiency with angle of receiver.
In the second model with two transmitter and single receiver we will study the effect of four parameters which are the distance between loop and transmitter coils (dl), the distance between the two transmitter coils (ds),the radius of transmitter loop (r) and the angle between the transmitter and the receiver (Ɵ) as shown in Figure 1.4. In addition, we are going to fabricate the two models and compare their practical results with the simulation results by EMPRO.
CHAPTER 2
2 LITERATURE REVIEW
Inductive Power Transfer (IPT) technology is now widely regarded as an efficient and effective method for transferring power wirelessly from one system to another through weak magnetic coupling and across an air-gap. It is possible to use Wireless Power Transfer for electric vehicles in the parking area and electric bicycles. Furthermore, it is proposed that electric vehicles and electric trains in motion as well as and robots, can be charged wirelessly. This technology is indifferent to equipment size. However, in researchers have focused on improving the wireless power transfer’s distance and efficiency by reducing the ohmic loss of self-resonators by using metamaterials. As more studies and researches introduced for further development of WPT in the world, it is expected that the need for a power line, the “last wire” left in the world, will eliminate and can lead the way to a “truly” wireless world. In WPT systems, impedance matching techniques are commonly used for high power efficiency. In, transmitting coil carefully wound to generate uniform magnetic field distribution. The work in introduced multiphase system to obtain the wider power delivery zone. Two-transmitters in achieve the wide power transmission area. However, these methods do not extend transmission distance in coil axis directions. In [24], the transmission distance is adjusted by controlling the loading effect of main transmitter (Tx) and receiver (Rx) coils, or by changing the drive frequency of power source. However, the range adaptation intended to improve the performance at short distances where the resonant frequency splitting occurs.
The work in [25] reduces the coil losses and increases the transmission distance by utilizing superconductor, which is very expensive. Employing very high-Q Tx/Rx coils could somewhat compensate the effect of the low coupling. However, the achievable Q-factor of practical resonator is limited due to the conductor losses of wire and loading effects of source/load resistances. Moreover, very high-Q means excessive reactance per given load. Higher reactance translates to the higher magnetic field, which may cause adverse effects to human body. Placing intermediate resonators, which receive the magnetic field from transmitter and, then, relay the field to receiver, would easily enhance the magnetic coupling at the longer distance. Such field repeaters operate in passive manner, unlike the base station repeaters of communication systems. Works [26], [27] and the patents [28]-[29], [30], [31] also propose such intermediate field repeaters. The idea of using intermediate resonators studied also in the field of magneto-inductive (MI) waveguide research [32]-[33] to transfer communication signals. Although these works were successful to prove the usefulness of repeaters, further works, which provide the guidelines for optimum use of repeaters needed. In [34] they use magnetic coupled circuit model to investigate the effects of the magnetic coupling of nonadjacent resonators for wireless power transfer applications. They used six resonators and achieved 2-meter air gap. In [35] band-pass filter design theory employed to implement WPT by considering a 2-pole bandpass filter with characteristic of Butterworth type.
Researchers measured S-parameters by using Vector Network Analyzer as presented in [36], analytical expressions of the resonant peaks of input impedance and the frequencies of maximum transferred power in the WPT systems in the case of tight magnetic coupling. The measurement with fabricated coil TVs and the 3D EM simulation result validate that the analytical expressions successfully predict the frequencies of power source where the maximum power transferred in both cases of the constant AC voltage source and the constant AC current source in the case of tight magnetic coupling. The analytical expressions show that the frequencies at which the transferred power maximized coincide with the resonant frequencies of input impedance in the WPT systems. Throughout the presented analytical expression, it becomes easier to determine and optimize the source type and the frequency of power source for a more power efficient WPT systems. Optimal wireless power transfer could occur at the resonance frequency of the resonators. In fact, short-range wireless energy transfer via magnetically coupled coils has been widely used in electric machines. since the dc and ac electric machines were invented For low-power applications, wireless power transfer has found applications in battery charging for portable electronic products such as (fixed-positioning charging) electric toothbrush and medical equipment [37], [38] and (free-positioning) Qi-compatible wireless charging system for portable electronic products such as mobile phones. Medium- and high power wireless power transfer also resurfaced since 1990 when inductive power transfer techniques investigated for industrial robots and electric vehicles. An extra relay resonator inserted between the transmitting coil and the receiving coil in order to increase the energy efficiency in the wireless power transmission has been proposed [39]. The energy efficiency of wireless power transfer will decrease rapidly as the transmission distance increases. For power transfer by induction, high operating frequency will increase the ac winding resistance and, therefore, decrease the energy efficiency of the resonator system. Such high-frequency winding loss is particularly relevant when high-power applications, such as induction heating, are involved [40], [41].
CHAPTER 3
3 ELEMENTS OF WIRELESS POWER TRANSFER SYSTEM
When a current carrying conductor is formed into a loop or several loops to form a coil, a magnetic field develops that flows through the center of the loop or coil along its longitudinal axis and circles back around the outside of the loop or coil [1]. The magnetic field circling each loop of wire combines with the fields from the other loops to produce a concentrated field down the center of the coil [2]. Most loop coils are wound as skinny donuts, using multiple turns of insulated wire. When current passing through a coil, a magnetic field will be generated as shown in figure 2.1. Two types of current may be passing through a coil: Direct current (DC) and alternating current (AC). DC will generate a static magnetic field, and AC will generate an alternating magnetic field. Through this thesis we are interested the AC field, which we will call an electromagnetic field for simplicity.
Figure 2.1 Magnetic fields around a coil that is carrying current
When we bring a second coil close to the first one, the signal from the first coil will couple into the second coil via induction. Induction is incredibly useful, and is how transformers work. Inductive coupling efficiency depends on several things, like coil placement and the core material they are wound on. Transformers use iron or steel to improve coupling efficiency, while metal detector coils use air [3].
3.1 Coils Types:
There are several types of coils, which developed since the past times. The types according to the physical structures are helical coils, spiral coils, and inverse conical coils as shown in Figure 2.2 [4]. These structures have different calculations for fields. Through this thesis, a helical structure will present.
3.2 Coils Parameters
Electromagnetic waves denoted as a type of electromagnetic radiation, which organized according to the frequency (f) of its waves. Frequency counts the number of incidences that a repetition of an event occurs per unit of time. Usually, frequency is given in Hertz (Hz), which means the number of cycles per second. Each cycle also mentioned as a period (Τ) [5].Therefore, frequency is the reciprocal of period:
f =1/T
In this section, a brief discussion of the different calculations of helical coil according to their physical structures will present.
3.2.1Inductance
Inductance is the characteristic of an electrical circuit that opposes the starting, stopping, or a change in value of current [6]. Inductance has the same effect on current in an electrical circuit as inertia has on the movement of a mechanical object. It requires more energy to start or stop current than it does to keep it flowing [7]. Faraday’s law of induction is a basic law of electromagnetism that predicts how a magnetic field will interact with an electric circuit to produce an electromotive force (EMF). It is the fundamental operating principle of transformers, inductors, and many types of electrical motors, generators and solenoids. Eletromotive force (emf) developed whenever there is relative motion between a magnetic field and a conductor. Electromotive force is a difference of potential or voltage, which exists between two points in an electrical circuit. In generators and inductors, the emf developed by the action between the magnetic field and the electrons in a conductor.as shown in figure 2.3[8].
Self-inductance: Even a perfectly straight length of conductor has some inductance current in a conductor produces a magnetic field surrounding the conductor. When the current change, the magnetic field will change. This causes relative motion between the magnetic field and the conductor, and an electromotive force (emf) induced in the conductor [9]. This emf called a Self-inductance emf because it induced in the conductor carrying the current. The emf produced by this moving magnetic field also referred to as counter eletromotive force (cemf) [10]. The polarity of the counter electromotive force is in the opposite direction to the applied voltage of the conductor. The overall effect will be to oppose a change in current magnitude.
This effect is summarized by Lenz’s law which states that: the induced emf in any circuit is always in a direction to oppose the Effect that produced it [11]. Since all circuits have conductors in them, we can assume that all circuits have inductance. However, inductance has its greatest effect only when there is a change in current. Inductance does not oppose current, only a change in current. Where current is constantly changing as in an ac circuit, inductance has more effect.
To increase the property of inductance, the conductor can formed into a loop or coil. A coil also called an inductor. Figure 2-4 shows a conductor formed into a coil. Current through one loop produces a magnetic field that encircles the loop in the direction as shown in Figure 2-4(a). As current increases, the magnetic field expands and cuts all the loops as shown in Figure 2.4(b). The current in each loop affects all other loops. The field cutting the other loop has the effect of increasing the opposition to a current change [12].
Figure 2.4 a conductor formed into a coil.
Inductors classified according to core type. The core is the center of the inductor just as the core of an earth is the center of an earth. The inductor made by forming a coil of wire around a core. The core material is normally one of two basic types: soft iron or air. The air-core inductor may be nothing more than a coil of wire, but it is usually a coil formed around a hollow form of some nonmagnetic material such as cardboard. This material serves no purpose other than to hold the shape of the coil [12].
There are several physical factors, which affect the inductance of a coil. They include the number of turns in the coil, the diameter of the coil, the coil length, the type of material used in the core, and the number of layers of winding in the coils. Doubling the number of turns in the coil will produce a field twice as strong, if the same current is used [13].
The second factor is the coil diameter; larger diameter requires more wire to construct a coil than one of small diameter with an equal number of turns. Therefore, more lines of force exist to induce a counter emf in the coil with the larger diameter. Actually, the inductance of a coil increases directly as the cross sectional area of the core increases. The third factor that affects the inductance of a coil is the length of the coil. Doubling the length of a coil while keeping the same number of turns halves the value of inductance. The fourth physical factor is the type of core material used with the coil. Inductance of a coil increases directly as the permeability of the core material increases [14]. The inductance of a long solenoid readily shown by applying Faraday and Ampere’s Laws to be [15]:
The number of turns and the coil length are constrained by the fixed length of wire that we have.
Let s = total length of wires.
Then:
n = total turns in coil
But also n Nb
So
It indicates that maximum inductance will obtained when b is as small as
possible. In the limit, this will occur when the loop has a single turn, as reasoned above.
However, from physical considerations, this argument is flawed. Equation (2.2) no longer holds. It derived assuming that the magnetic field, B, is constant over the surface area of the coil, and inside it, and is zero everywhere outside. These assumptions are approximately true for a long solenoid, but are incorrect for a single turn loop.
The field around a single loop is not negligible. Furthermore, B varies across the loop. The axial magnetic field at the Centre of a single circular loop of radius R carrying current I, due to all sections of the wire, is [16]:
Off axis, as we approach very close to the wire, only the nearest portion of the wire influences the field. At a distance r from the wire, it becomes:
Where r = the radial distance from center of the wire.
Equation (2.2) never used to describe the behavior of real coils it is too simpleminded. The standard one conventionally used is Wheeler’s 1928 formula, given below. We will see that this does not predict the answer reasoned above [17].
Where: L= inductance in microhenry
d= coil diameter, meters.
l = coil length, meters.
n= total number of turns.
In 1928, Wheeler published another formula, which he maintained “a relative error of less
than 0.001” this [18]:
3.3 Resonant Circuits
Any system having at least a pair of complex conjugate poles has a natural frequency of oscillation. If the frequency of the system driving force coincides with the natural frequency of oscillations, the system resonated and the system response becomes maximum. This phenomenon is knows as resonance and the frequency at which this phenomenon occurs is known as resonant frequency [20].
In electrical systems, resonance occurs when the system contains at least one inductor and one capacitor. In this system, the phenomenon of cancellation of reactance when the inductor and capacitor are in series or cancellation of susceptances they are in parallel. The circuit under resonance is purely resistive in nature.
Electrical resonance is broadly classified into two categories
1- Series resonance
2- Parallel resonance
3.3.1 Series Resonance
Circuits containing resistance (R), inductance (L), and capacitance (C) elements often have special characteristics useful in many applications because their frequency characteristics (impedance, voltage, or current vs. frequency) may have a sharp maximum or minimum at certain frequencies. For series resonance, the condition of resonance is straightforward and it is characterize by minimum impedance and zero phases [22].
A coil of self-inductance ‘L’ connected in series with a capacitance ‘C’ and a resistance R form a circuit called a series LCR circuit as shown in Figure 2-7.
The impedance Z of the circuit is given by:
Where:
Z and R in ohms
L is in Henrys
C is in Farads
and ω is the angular frequency of the applied voltage in radians/sec.
Resonance occur when average magnetic energy stored in the inductor L (Wm) equal average
electric energy stored in the capacitor C (We):
Quality factor, Q, Quality factor is a measurement of the loss of a resonant circuit. Reactive components such as capacitors and inductors often described with a figure of merit called Q.
While it can be define in many ways [23], its most fundamental description is:
3.3.2 Parallel Resonance
The parallel resonant circuit has the basic configuration of Figure 2.8. This circuit often called the tank circuit due to the storage of energy by the inductor and capacitor. A transfer of energy similar to that discussed for the series circuit also occurs in the parallel resonant circuit
In the ideal case (no radiation losses, and so on ) ,the capacitor absorbs energy during one half-cycle of the power curves at the same rate at which it is released by the inductor .during the next half-cycle of the power curves ,the inductor absorbs energy at the same rate at which the capacitor releases it. The total reactive power at resonance is therefore zero.
3.4 Equivalent circuit
Electromagnetic resonance coupling involves creating an LC resonance, and transferring the power with electromagnetic couplings without radiating electromagnetic waves. Hence, the magnetic coupling and electric coupling can be represented as mutual inductance and mutual capacitance respectively as shown in Figure 2-9 [25]. Zsource in Figure 2.9 represents the characteristic impedance, and Zload is the impedance of the load. In this system, they both
considered to be the same at Z0, 50Ω the default characteristic of most high frequency systems. The ohm loss and the radiation loss of the coils are represented by R. The power is transferred via magnetic coupling. Therefore, the coupling can be represented by mutual induction Lm. Next, the resonance frequency is calculated based on the equivalent circuit. To satisfy the resonance condition, the reactance of Figure 2.9 must be zero, as in equation (2.21). The
condition in (2.22) can be satisfied by two resonant frequencies as calculated in equation (2.22) and (2.23). The coupling coefficient (k) can be calculated from equations (2.22) and (2.23) to become equation (2.24). It represents the strength of the magnetic coupling between the coils, which is closely related to factors such as the air gap between the coils and the obstacles between them.
EQUIVALENT CIRCUIT
3.5 Efficiency
The efficiency of the power transfer is calculated based on the equivalent circuit [26]. The ratio of power reflection η11 and transmission η21 can be defined by equations (2.25) and (2.26), where S11 is the reflection coefficient at port 1 with port 2 terminated in a matched load and S21 is the transmission coefficient from port 1 to port 2 with port2 terminated in a perfect match. It gives a measure of the amount of signal that transmitted from port 1 to port 2. To simplify the calculations, R is considered to be 0 Ω. Here, S21 can be calculated using equation (2.27).
3.6 Ohmic Resistance
The total ohmic resistance for an N-turn circular-loop antenna with loop radius a, wire radius b, and loop separation 2c, shown in Figure 2.10 is given by
3.7 PWM Inverter
With the increasing use of renewable energy, particularly grid-tied photovoltaics, the need to have a good quality single-phase inverter has become important. The conventional technology used for single phase inverters typically consists of either square-wave or pwm inverters. The square wave type is the simplest method to produce AC from DC; however, it suffers from low frequency harmonics which causes difficulty in filtering out the noise to prevent these harmonics to return back to the primary side of the transformer. The pwm inverter, on the other hand, forces the harmonics to be way up higher than the fundamental (line) frequency; thus, easing up the filtering requirement of the inverter. However, the major drawback of the pwm inverter is the increased switching losses due to the frequent switching actions of the electronic switches within the inverter. This project proposes an improved version of the square-wave inverter by increasing the number of steps per one period of the desired frequency with the use of additional electronic switches. With the conventional square wave inverter, there are only two pulses generated by the controller to mimic an AC output.
This is in fact what is causing the low harmonic distortion of the inverter. To lessen these low harmonic distortions in square wave technique, the approach is then to increase the number of steps per period. This is the approach taken by the proposed inverter by adding the number of steps to seven; hence, the name seven-level H-bridge interconnected inverter. In essence, the seven-step AC output voltage generated by the inverter will push the low harmonic to above seven times the line frequency. For a 60-Hz AC system, this means the harmonic content of the inverter output will be at 420 Hz and above. This is a significant improvement over 180 Hz and above offered by the conventional square wave technology.
As previously mentioned, the purpose of an inverter is to convert DC power to AC power. Inverters are an integral part of many technologies including uninterruptable power supplies, induction heating, high-voltage direct current power transmission, variable frequency drives, electric vehicle drives, and multiple renewable energy applications. All of these technologies use inverters to achieve different goals, but all produce AC power from a DC input.
There are many varieties of inverter designs. The most common topology uses what is referred to as the H-bridge topology. Its basic configuration is shown in Figure 2-1.
This topology is used in conjunction with either the square wave, or pulse width modulation (PWM) switching schemes. The square-wave switching scheme is a method for controlling the switches (labeled S1through S4) in order to achieve a square wave AC output signal. The AC output is achieved by using a control signal with a 50% duty cycle wired to S1 and S4. An inverted copy of the same signal is also wired to S2 and S3. This switching scheme ensures that S1 and S4 are always on when S2 and S3 are off. It should be easily seen how such a switching scheme creates the square wave output shown in Figure 2-2. The advantage of using an H-bridge inverter is that only a single, simple control signal is required to control four transistors. The disadvantage, however, is that the squarewave output is a low quality AC signal that injects many harmonics into any loads to which it is powering.
As mentioned previously, PWM control signals can be used with the same H-bridge topography. The disadvantage of the PWM switching scheme is that it is more complicated than the square-wave switching scheme. Multiple, relatively complex control signals are needed to control the transistors of the PWM inverter. The advantage, however, of the PWM switching scheme is that it is able to generate a more perfect sinusoidal AC output, which some loads prefer. An example PWM inverter output is shown in Figure 2-3.
This paper presents a proposed new single-phase interconnected H-bridge inverter (or multistep inverter). One advantage of a multistep inverter is that it provides a more sinusoidal output voltage than an inverter with a basic square-wave switching scheme. Another advantage is that its control signals are less complicated than that of the PWM inverter. However, the multistep inverter does not generate as high of quality of sinusoidal output voltage as that of the PWM switching scheme inverter. It does, though, provide an inverted signal of sufficient quality for most loads. Furthermore, the control signals required by the multistep inverter are relatively complicated compared to the single control signal of the square wave H-bridge inverter. In other words, the multistep inverter is a compromise between a complicated, but high quality PWM inverter, and a simple, but low quality square wave inverter. Figures 2-4 and 2-5 below show the circuit diagram and the ideal output voltage for the multistep inverter.
The nine MOSFETs of the multistep inverter of Figure 2-4 are switched on and off in a controlled manner in order to control the flow of power through the circuit and to generate an inverted stair step output (Vout) from a DC input voltage (Vin), as shown in Figure 2-5.
After studying the inverter topology and determining the control signals, the next step of the design was to simulate the interconnected H-bridge inverter circuit. The PSPICE schematic of the inverter circuit is shown in Figure 3-1. SBREAKS were used in order to simulate the switching characteristics of near ideal transistors. An input voltage (Vin) of 10 V was chosen arbitrarily for the simulation. The expected output will be an AC voltage with a peak-to-peak voltage of 20 Volts (10V to -10 V) Figure
I decided to simulate the control signals using piece-wise linear voltage sources. By using piecewise linear sources, one can precisely control exactly how a source behaves. The specific piece-wise linear voltage source used is called “VPWL_F_RE_FOREVER.” This voltage source creates its output based upon the input it receives from a csv file (comma separated value). This file includes a series of pairs of numbers denoting time and voltage value. This series of pairs define one period of the piece-wise linear source. The period defined by the csv file is then repeated indefinitely. In this case, every piece-wise linear source represents a separate control signal. Using piece-wise linear sources to implement the control signals is far easier to both implement and understand, than attempting to implement the control signals with vpulses in series, as was my initial implementation. A sample period of the nine control signals are shown in Figure 3-2.
The above logic used for the above control signals was derived from performing circuit analysis on the inverter circuit. That analysis resulted in the table of control signals seen in Table I. In Table I a ‘1’ denotes a transistor that should be “on” in order to achieve given output voltages. I also simulated a simpler five-step inverter as well as a square wave inverter so that I also simulated a simpler five-step inverter as well as a square wave inverter so that
CHAPTER 4
4 HARDWARE PROTOTYPE DESIGN RECOMMENDATIONS
After successfully simulating the inverter circuit using PSpice, the next step in the design was to choose a method of implementing those control signals. Drawing upon school experience, it was decided that the best way to generate these control signals was to design a finite state machine (FSM) which would be implemented using a complex programmable logic device (CPLD). In this case, a CPLD can be programmed to implement a FSM with 13 different states. One state for each of the output levels of one period of the inverter output waveform and a 13th state for dead time. The “dead time” state briefly sets all transistors to the “off” position. This is necessary to prevent undesired draining of the capacitors due to crossover, resulting from the non-ideal rise and fall times of the transistors. As the state machine changes from one state to the next, the CPLD changes its nine outputs to the different transistor switches. Figure 4-1 illustrates the state machine. In the figure below, switches with a value of ‘1’ denote the transistors of the inverter that will be set to the “on” position. All other transistors would be set to “off.”
The Digilent XC2-XL system board, which includes the CoolRunner 2 CPLD was chosen to control the MOSFETs of this inverter due to the author’s familiarity with its use. Additionally, CoolRunner 2 CPLD’s are good choice because they combine very low power with high speed, high density, and high I/O counts in a single device. In summary, the CoolRunner 2 was chosen for the prototype due to its ease of use, cost effectiveness, low power consumption and high-speed operation. A dead time state lasting 542 ns was chosen for a few reasons. 542 ns is a relatively short period of time compared to the period of the 60 Hz output. Additionally, it is a sufficient length of time to prevent the unwanted crossover state between two transistors that occurs due to the non ideal rise and fall times of transistors. Lastly, 542 ns is the smallest dead time that can be implemented
using the 1.8432 MHz clock included with the Digilent system board. Implementing the dead time requires the creation of two clocks within the synchronous portion of the VHDL code. One clock for the dead time, and another clock to time the length of the 12 different output states. The code used to generate this timing is shown in Figure 4-2 below.
Figure 4-2. VHDL Code to Implement Delay and Output State Clocks
The logic design for the hardware prototype uses the Coolrunner 2 CPLD to generate two different clock signals, one for the approximately 1.389 ms output states (1/12 of the AC cycle), and one for the 542 ns dead time state. The Coolrunner 2 also implements the state machine as well as the logic that determines the next state the machine should transition to. The CPLD also outputs the control signals to the amplification portion of the design through header B shown in the CPLD overview of Figure 4-3. All of the prototypical code for the CPLD state machine implementation can be found in the appendices at the end of this paper.
Amplification Circuit- As previously mentioned, the CoolRunner 2 CPLD simultaneously outputs the nine control signals to the amplification stage of the prototype design through header B shown above. An amplification stage is necessary because the transistors of the inverter circuit require gate voltages higher than that which is outputted by the Digilent board. In order to provide this gain in the control signals, TL081 operational amplifiers can be used in a non-inverting amplifier configuration.
4.2 Power Inverter Circuit Prototype
It was necessary to select capacitors, diodes and transistors for the inverter design. MOSFETs were chosen for a few reasons. First, MOSFETs contain an anti-parallel schottky diode which will eliminate the need for the purchase of additional diodes for this prototype. Second, MOSFETs have fast switching characteristics and high efficiency. Finally, MOSFETs are turned on by a voltage source rather than a current source like BJTs. Since the Coolrunner 2 generated control signals are voltage sources, it was appropriate to use MOSFETs.
The IRFZ34N N-Channel MOSFET was chosen for the prototype design. It is thru-hole rather than surface mount, which will make building a prototype much simpler. It has a maximum current capacity of 29 A and a maximum Vds of 55 V, which far exceeds what’s necessary for this prototype at a minimal cost of $1.20 apiece. It has a very low Rds(on) of .040 ohms which will lead to low resistive losses and higher efficiency. Also, the IRFZ34N MOSFET has a rise time of 49 ns and a fall time of 40 ns, both of which are very low and more than sufficient for this prototype. For the purpose of the prototype, 4.7 micro Farad capacitors with a maximum voltage of 25 Volts were chosen. These were chosen because I had a surplus of them available so they could be used at no cost. By putting two of these capacitors in parallel, a capacitor equivalence of 9.4 microFarads is achieved. A capacitor of this size will provide a sufficient hold-up time for this prototype. Lastly, 25 Volts is more than adequate for this prototype since the most voltage that should ever be across a capacitor is 3.34 Volts for a 10 Volt DC input.
4.3 TESTING
As mentioned earlier in this paper, the goal of this senior project was to simulate a seven-step inverter and draw conclusions on whether it produces less lower harmonics than a square wave inverter. For that reason, I simulated three different inverters, a square wave inverter, a five-step inverter and a seven-step inverter. Table II shows the amplitudes of the lower harmonics for each of those inverters.
For ease of comparison, Figure 5-1 is a graph comparing the Fourier analysis of each of the three simulated inverter circuits. By looking at Figure 5-1, one can tell that the harmonics of the multistep inverters are lower than the harmonics of the square wave inverter, especially for the lower harmonics. The harmonics of the seven-step inverter are nearly nonexistent compared to the harmonics of the square wave inverter. This is what we expected. Since the multistep inverter designs were able to decrease the lower harmonics of the output voltage, those harmonics will be easier to filter out than the large harmonics of the square wave inverter. In this project, the research, design and simulation of an interconnected H-bridge single phase inverter was explored. The inverter was simulated and recommendations were made for implementing a hardware prototype.
The purpose of this senior project was to draw conclusions on whether the inverter produces less low harmonics than a square wave inverter. For that reason, three different inverter circuits were simulated, a square wave inverter, a five-step inverter and a seven-step inverter. The simulations showed that interconnected H-bridge multistep inverters do succeed in diminishing the lower harmonics of the outputted AC voltage. All harmonics were decreased and most notably the 180 Hz harmonic was reduced from 4.22 V for the square wave inverter to .769 V for the seven-step inverter. Additionally, the seven-step inverter provided a “cleaner” output voltage than the five-step inverter. The conclusion can be drawn that the more steps that an inverter output voltage contains, the cleaner (less harmonics) the AC output voltage. The more steps of an inverter output voltage, the more accurately it approximates a pure sinusoidal AC voltage. The control signals for the multistep inverter designs are more complicated than the square wave inverter’s control signals, but the cleaner voltage output of the multistep inverter designs make them better suited for modern inverter applications such as converting the DC power of solar panels to clean AC power.
If one were to commercially produce this interconnected H-bridge inverter for sale, modifications could be made to the prototype discussed in order to achieve: reduced size, cost avings and higher efficiency. A commercial version should not use a separate CPLD development board to create the control signals. A smaller CPLD board would be used that draws its power from the DC power inputted to the inverter rather than drawing power from separate batteries as the prototype CPLD board does. Additionally, surface mount integrated circuits would be used instead of thru-hole circuits in order to reduce size and increase efficiency. Also, MOSFET drivers can be used in the control signal amplification stage of the design rather than non-inverting operation amplifiers. Lastly, the MOSFETs and capacitors chosen for a commercial design would not be as robust as those chosen for the prototype. Cost would be reduced by using MOSFETs and capacitors that meet but not exceed the voltage and current ratings required.
4.4 TLP250
FEATURES:
- Transistor Inverter.
- Inverter for Air Conditioner.
- IGBT Gate Drive.
- Power MOS FET Gate Drive.
The TOSHIBA TLP250 consists of a GaAlAs light emitting diode and a
Integrated photo detector.
This unit is 8−lead DIP package.
TLP250 is suitable for gate driving circuit of IGBT or power MOS FET.
• Input threshold current: IF=5mA(max.)
• Supply current (ICC): 11mA(max.)
• Supply voltage (VCC): 10−35V
• Output current (IO): ±1.5A (max.)
• Switching time (tpLH/tpHL): 1.5μs(max.)
• Isolation voltage: 2500Vrms(min.)
• UL recognized: UL1577, file No.E67349
• Option (D4) type
VDE approved: DIN VDE0884/06.92,certificate No.76823
Maximum operating insulation voltage: 630VPK
Highest permissible over voltage: 4000VPK
4.5 Diode
Diodes allow electricity to flow in only one direction. Diodes are the electrical version of a valve and early diodes were actually called valves. The schematic symbol of a diode is shown below. The arrow of the circuit symbol shows the direction in which the current can flow. The diode has two terminals, a cathode and an anode as shown in Figure 1. If a negative voltage is applied to the cathode and a positive voltage to the anode, the diode is forward biased and conducts. The diode acts nearly as a short circuit. If the polarity of the applied voltage is changed, the diode is reverse biased and does not conduct.
The diode acts very much as an open circuit. Finally, if the voltage vD is more negative than the Reverse Breakdown voltage (also called the Zener voltage, VZ), the diode conducts again, but in a reverse direction. The voltage versus current characteristics of a silicon Diode
Forward Voltage Drop
Electricity uses up a little energy pushing its way through the diode, rather like a person pushing through a door with a spring. This means that there is a small voltage across a conducting diode, it is called the forward a diode is almost constant whatever the current passing through the diode so they have a very steep characteristic (refer to current-voltage graph).
4.5.1 Reverse Voltage
Though we say that a diode does not conduct in the reverse direction, there are limits to the reverse electrical pressure that can be applied. The manufacturers of diodes specify a peak inverse voltage (PIV) that the diode can safely withstand. If this is exceeded, the diode will fail and allow a large current to flow in the reverse direction. This voltage is also called the Reverse Breakdown voltage.
Ideal Diode
For most practical applications, the operating voltage is high, and the forward voltage drop is negligible in comparison. The voltage-current characteristics of a diode (shown in figure 3) suggest that we can use the following model of an ideal diode for all practical purposes (i.e., ignoring the forward voltage drop).
The ideal diode acts as a short circuit for forward currents and as an open circuit with reverse voltage applied.
EXPERIMENTAL RESULTS
CHAPTER 5
5 CONCLUSIONS
The paper has presented the step-by-step design of the WPTBC for an electric city-car. The basic functioning of an SS resonant WPTS has been reviewed and the procedure followed in designing the power converters and the coil-coupling has been thoroughly described. A systematic analysis of the power losses in the WPTBC component has been also carried out. Afterwards, a prototypal WPTBC has been set up according to the design results and tested in different working conditions. A number of measurements obtained from the prototype have been reported that fully confirm the soundness of both the design procedure and the loss analysis of the WPTBC.
FUTURE WORK
The concept of Microwave Power transmission (MPT) and Wireless Power Transmission system is presented. The technological developments in Wireless Power Transmission (WPT), the advantages, disadvantages, biological impacts and applications of WPT are also discussed. This concept offers greater possibilities for transmitting power with negligible losses and ease of transmission than any invention or discovery heretofore made. Dr. Neville of NASA states “You don’t need cables, pipes, or copper wires to receive power. We can send it to you like a cell phone call – where you want it, when you want it, in real time”. We can expect with certitude that in next few years’ wonders will be wrought by its applications if all the conditions are favourable.
REFERENCES
- W. Brown, “The History of Power Transmission by Radio Waves,” IEEE Transactions on Microwave Theory and Techniques, vol.32, No.9, pp. 1230- 1242, September 1984.
- N. Tesla, “Apparatus for transmitting electrical energy,” US patent number 1,119,732, issued in December 1914.
- R. Lomas, The Man Who Invented the Twentieth Century—Nikola Tesla—Forgotten Genius of Electricity. London, U.K.: Headline 1999, pp.146.
- A. Kurs, A. Karalis, R. Moffatt, J. D. Joannopoulos, P. Fisher, and M. Soljačić, “Wireless Power Transfer via Strongly Coupled Magnetic Resonances,” Science, vol. 317, No. 5834, pp. 83-86, 2007.
- A. Karalis, J. Joannopoulos, and M. Soljačić, “Efficient wireless nonradiative mid-range energy transfer,” Annals of Physics, vol. 323, No. 1, pp. 34 -48, 2008.
- W. Zhong, C. K. Lee, “General Analysis on the Use of Tesla’s Resonatorsin Domino Forms for Wireless Power Transfer,” IEEE Trans. vol.60, no.1,pp. 261-270, January 2013.
- J. Yungtaek and M. M. Jovanovic, “A Contactless Electrical Energy Transmission System for Portable-Telephone Battery Chargers,” IEEE Trans. Ind. Electron., vol. 50, No. 3, pp. 520–527, June 2003.
- K. Hatanaka, F. Sato, H. Matsuki, S. Kikuchi, J. Murakami, M. Kawase, and T. Satoh, “Power Transmission of A desk with A cord-Free Power Supply,” Magnetics, IEEE Transactions on , vol.38, no.5, pp.3329,3331, September 2002.
- G. Wang, W. Liu, M. Sivaprakasam, and G. Kendir, “Design and Analysis of an Adaptive Transcutaneous Power Telemetry for Biomedical Implants,” IEEE Trans. on Circuits Syst. I, Reg. Papers, pp. 2109–2117, Octaber 2005.
- H. Jiang, J. M. Zhang, S. Y. Zhou, S. S. Liou, H. Shahnasser” A Rotating-Magnet Based Wireless Electrical Power Transfer for Biomedical Implants,” 2010 3rd International Conference on Biomedical Engineering and Informatics, pp. 1409 – 1411 , Octaber 2013.
Project Center Tirunelveli 