CHAPTER 1 1.1 INTRODUCTION
1.1.1 Backup Power
Electrical power interruptions are quite common in every place especially after a heavy downpour or a severe storm. To counter these power outages a search has been done to find a reliable and economic alternate source of backup power. Backup power is power which kicks in when power from the electrical grid is not available. Backup power systems may be designed to provide a seamless transition so that people never experience an interruption in power, or they can be designed to kick on when the power goes off, restoring power during an outage. Such systems are critical for businesses which want to retain functionality, and they can be very useful at home, especially if people rely on power to run lifesaving devices such as ventilators at home. There are a couple of ways to design a backup power system. Some systems are designed to provide people with a window of time which allows them to safely shut down equipment when the power goes down. In this case, the backup power is not designed to be used for working, but rather remains on for a few minutes after the power goes out to allow people to stop what they are doing, save, and safely shut down. These systems can be important when power interruptions might damage or corrupt data and computer systems. A UPS is an example of this type of system. Uninterruptible power supplies(U.P.S), as they are known, will provide a seamless transition. When they sense that the power is going out, they kick into action so that people do not lose power at any point. Other backup power systems last for a longer period of time on stored energy. They can recharge through the electrical grid when it is working, or through the use of things like windmills and solar panels. These systems may last for hours or days, depending on how they are designed. An example is a power inverter. Other backup power systems will generate energy when the power is out to maintain power until the grid is restored. Gas powered generators are an example, as are solar and wind systems. 1
In other instances, a backup power system may need a few minutes to activate, or it may be activated manually as needed. In addition to being used to provide power in an outage, such systems can also be used to save money on energy; for example, people can use backup power during peak periods, and recharge the backup system during off-peak periods, for lower electrical bills. It is a good idea to research power needs well before they become critical. Some reasons to invest in backup power include: being a business which wants to stay open during outages; using electronic medical devices; working on computers and wanting to avoid data loss; or simply wanting to enjoy the convenience of electrical power even when the grid is down. A power inverter is a type of backup power system. It is a device that is used to convert direct current to alternating current. It works by converting 12 Volt D.C. power into 220 or 240 volts A.C.. It¶s typical application is to convert a battery voltage into conventional household A.C voltage allowing users to use electronic devices when an A.C voltage is not readily available.. There are three basic type of D.C inverters: square wave, modified sine wave, pure sine wave. The square is the simplest and the least expensive type, but nowadays it is practically not used commercially because of low quality of power. The modified sine wave topologies ( which are actually modified square waves) produce square waves with some dead spots between positive and negative half-cycles. Modified sine-wave inverters are economical, but may present certain compromises with some loads such as microwave ovens, laser printers, clocks and cordless tool chargers. Pure sine-wave inverters produce A.C voltage with low total harmonic distribution. They are more expensive than square wave and modified sine-wave inverters. When used, there is need for clean sinusoidal output for some sensitive devices such as medical equipment, laser printers, stereos. e.t.c. Simple inverters make use of oscillators driving a transistor or mosfet to create a square wave, which in turn is fed through a transformer to produce the required output voltage. While Advanced inverters have started using more advanced forms of transistors or similar devices such as thyristors. Inverters are used in a wide range of applications, from small power supplies for a computer to large industrial applications to transport bulk power, from small car adapters to high power converters in solar powered systems. A few of the most widely used
2
applications of Power Inverters include running television sets, microwaves and other household appliances and charging cell phones, laptops from a car¶s cigarette lighter outlet and running power tools from a 12 vo lt battery on jobsites where electricity isn¶t available.
1.1. OBJECTIVES OF STUDY
y
To design and construct a 500VA inverter which is suitable as backup for household power supply by inverting a 12V D.C input to 240V D.C.
y
Design and construct a battery protection circuit to prevent over discharge of the inverter¶s battery in order to lengthen the lifespan of the battery.
1.2. AIM OF STUDY
An inverter needs to be designed to handle the requirements of an energy hungry household yet remain efficient during periods of low demand. The efficiency of the inverter is highly dependent on the switching device, topology and switching frequency of the inverter. The aim of this project is to produce an efficient DC to Single Phase 240 Volt AC inverter.
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CHAPTER 2: Literature review 2.1. INVERTERS
An inverter is an electrical device that converts direct current (DC) to alternating current (AC); the resulting AC can be at any required voltage and frequency with the use of appropriate transformers, switching, and control circuits.The electrical inverter is a high-power electronic oscillator. It is so named because early mechanical AC to DC converters were made to work in reverse, and thus were "inverted", to convert DC to AC. The inverter performs the opposite function of a rectifier.From the late nineteenth century through the middle of the twentieth century, DC-to-AC power conversion was accomplished using rotary converters or motorgenerator sets (M-G sets). In the early twentieth century, vacuum tubes and gas filled tubes began to be used as switches in inverter circuits. The most widely used type of tube was the [1][3]
thyratron.
The origins of electromechanical inverters explain the source of the term inverter. Early AC-to-DC converters used an induction or synchronous AC motor direct-connected to a generator (dynamo) so that the generator's commutator reversed its connections at exactly the right moments to produce DC. A later development is the synchronous converter, in which the motor and generator windings are combined into one armature, with slip rings at one end and a commutator at the other and only one field frame. The result with either is AC-in, DC-out. With an M-G set, the DC can be considered to be separately generated from the AC; with a synchronous converter, in a certain sense it can be considered to be "mechanically rectified AC". Given the right auxiliary and control equipment, an M-G set or rotary converter can be "run [7][8]
backwards", converting DC to AC. Hence an inverter is an inverted converter.
There have been a large number of articles written concerning power conversion in recent years. This can be attributed in part to the rise in popularity of high voltage DC transmission systems - and their integration with existing AC supply grids. There is also a consistent demand for high efficiency inverter devices for lower power applications - like houses, boats, caravans, UPS and remote areas of the world. This chapter will discuss and contrast recent literature concerning high power inverters and their t heir control.
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2.2. TYPES OF INVERTERS
Just as there are types of engines (gas, diesel, etc.), there are types of inverters. There are three basic types of inverters: 1. square wave 2. modified sine wave(modified square wave) 3. true sine wave (pure sine s ine wave)
2.2.1. Square Wave Inverters
Square wave inverters were the first types of inverters made. Square wave inverters result in uneven power delivery that is not efficient for running most devices.Square wave inverters simply reverse the polarity of the DC voltage 120 times per second. (Each cycle consists of two polarity reversals.) Their output is basically either +120V or -120V. Square wave inverters tend to produce a lot of "hum" in equipment connected to them and cause motors to run hotter than normal. They are no longer in common use in solar electric systems.Square wave inverters consist of a DC source, four switches, and the load. The switches are power semiconductors that can carry a large current and withstand a high voltage rating. The switches are turned on and off in correct sequence, at a certain frequency. The square wave inverter is the simplest and the least expensive type of inverter, but it produces the lowest quality of power.
2.2.2. Modified Sine Wave Inverters
Modified sine wave inverters can also be considered "modified square wave inverters." Their output consists of 4 voltage changes per cycle -- 0, +peak, 0, -peak, and back to 0. Most of these vary the percentage of the cycle that voltage is either +peak or -peak depending on the load (this is called pulse width modification or PWM). The peak voltage (+ or -) is usually set so that the average value of the voltage is approximately 120V under normal conditions. Modified square wave (modified sine wave) inverters deliver power that is consistent and efficient. This type of inverter is probably the most popular. They are relatively inexpensive and will run most
5
appliances. However, they may produce some hum or static and certain AC motors will tend to run a little hotter than they would with normal utility power or on a sine wave inverter. Also, certain battery chargers for cordless tools, photocopy machines, and laser printers have been reported to be incompatible with this type t ype of inverter.
2.2.3. Pure Sine Wave Inverters
Sine wave inverters produce power very similar to that produced by the utility companies in some situations, even better quality power.Sine wave inverters are more expensive than "modified sine wave" inverters but will run virtually any AC equipment just as well as utility power will (as long as the equipment's power requirements do not exceed the continuous and surge ratings of the inverter).Some sensitive equipment require a sine wave, like certain medical equipment and variable speed or rechargeable tools.True Sine Wave Inverters will: y
Allow inductive loads like microwave oven and motor driven products to run faster, quieter and cooler.
y
educe educe R
or eliminate audible and electrical noise in fans, florescent lights, audio
amplifiers etc.
y
Allow accessories that are sensitive to higher harmonic distortion levels found in modified sine wave inverters to function function seamlessly with pure sine wave inverters
2.2.3.1 Interference
The electronic circuitry in inverters may, in some cases, cause problems with radio and television reception, noise on telephones and buzz in audio equipment. Sine wave inverters cause the least amount of interference. Interference can be minimized by locating the inverter very close to the batteries, twisting together the cables that connect the inverter to the battery and locating the inverter away from appliances that are susceptible to interference. All inverters [4]
cause interference on AM radio.
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2.2.4. INVERTER WAVEFORMS
Here, we would look at the inverter waveforms and how the d ifferent types of loads behave when operating from them. In the figure on the next page is a graph representng the three different types of waveforms.
From the diagram, as time progresses from left to right, t he three different waveforms rise at different rates. For example, the sine wave smoot hly increases to its peak and smoothly decreases. Modified sinewave and square waves sho ot straight up, level off peak vo ltage and then drop straight down. The T he modified sinewave also sits at zero for a short period. This is the main difference between it and the square wave. Most devices with variable speeds such as electric e lectric drills, drills, or devices with chargers such as cordless drills or screwdrivers , can behave irrationally when o perating with modified sine or square wave inverters. These types t ypes of units use one of two types of o f solid state ³switches´ in them, SCR ¶s ¶s (Silicon Controlled R ectifiers) ectifiers) or Triac¶s. The basic theory is that a timing circuit looks at the point where the wave form crosses zero volts and uses this point as a reference to start its clock. On a drill for example, depend ing on how 7
much the trigger has been moved, a certain amount of o f electricity will be allowed through based on this time. Think of it as a water wheel that has been mounted mou nted under a faucet that t hat you control. If the water always comes out in smooth rising and falling surges, you could count when the surge hit zero pressure and then by delaying how much of each surges gets to the wheel, you can control its speed. If you let the ent ire surge from zero up to peak pressure and back to zero pressure through the faucet, the wheel win spin fastest. If you only let the po rtion of the surge from zero pressure through peak pressure through the faucet, the wheel will run about half [9]
speed . If there was no way to t ime this process it would be impossible to know exactly when to turn the faucet on and off to achieve the desired speed. Since a sinewave has a sloped ³zero crossing´(the point where the voltage passes thro ugh zero volts), the timing circuit will work. It knows when to turn the switch on or off. However if the wave passes thro ugh zero too fast or sits sits at zero for a period period of time, the timer gets confused. It doesn¶t know where zero is. The reason is that it looks for the rate of change cha nge or the slope of the zero crossing point. A modified sine or square wave has no slope. Therefore the timing circuit can¶t figure out when to let power through and when not to. Since the timer never starts, no power passes through to t he device at all. When the trigger on the drill is pulled all the wa y to high speed, the t he whole SCR and timer circuit is bypassed and the drill dr ill runs at full speed. This explains why all or no ne is available in some drills Battery chargers experience the same pro blems. The Triacs (Switches) dont know when to let electricity through since they can¶t find the zero crossings. crossings. Therefore inadequate (if any) charging will take place. So me chargers will react opposite and allow full power throug h, possibly overheating them. Small wall based charg ers (called wall warts) will often have overheating problems with modified sine or square wave inputs. Some computers and stereo equipment use switching power supplies that utilize utilize SCR ¶s and Triacs as well. These pieces of ¶s equipment might experience the same t roubles as in the examples given previously. Unfortunately it is hard to predict what exact models of equipment will have problems with pure sinewave, modified sinewave and square wave forms. forms. The only o nly way to know know for sure is to try it. If it doesn¶t work, take it back and a nd try another.
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2.3. PRINCIPLE OF OPERATION OF AN INVERTER
Most inverters do their job by performing two main functions: first they convert the incoming DC into AC, and then they t hey step up the resulting resu lting AC to mains voltage level using a transformer. The goal is to have the inverter perform these functions as efficiently as possible-so that as much as possible of the energy drawn from the battery is converted into mains voltage AC, a nd as little as possible is wasted as heat 2.3.1 DC-AC Inversion
The DC-AC inversion stage will be the most critical. With With quali qua lity ty design de sign of the DC-DC stage it should have a stable DC supply supp ly to work with, but it will have to cope with other issues such as reactive power correction and maintain a good level of vo ltage regulation in the most efficient manner possible. The most important part of the DC-AC Convers ion process is in the generation of the sinusoidal input signals to the gat es of the MOSFETs. Modern inverters use a basic circuit scheme like that shown in Fig. F ig. 1. below.
Fig.1: The basic circuit scheme used in many modern DC-AC inverters. MOSFETS Q1 and Q2 are used to convert the battery.s low voltage DC into AC, which is then stepped up to 230V R MS MS by the th e transformer.
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The DC from the battery is converted into AC very simply, by using a pair of power MOSFETs (Q1 and Q2) acting as very efficient electronic switches. The positive 13.8V DC from the battery is connected to the centre-tap of the transformer primary, while each MOSFET is connected between one end of the primary and earth (battery negative). So by switching on Q1, the battery current can be made to flow through the .top. half of the primary and to earth via Q1. Conversely by switching on Q2 instead, the current is made to flow the opposite way through the .lower. half of the primary and to earth. Therefore by switching the two MOSFETs on alternately, the current is made to flow first in one half of the primary and then in the other, producing an alternating magnetic flux in the transformer¶s core. As a result a corresponding AC voltage is induced in the transformer¶s secondary winding, and as the secondary has about 24 times the number of turns in the primary, the induced AC voltage is much higher: around 650V peak to peak. MOSFETs are used as the electronic switches, to convert the DC into AC, it¶s because they make the most efficient high-current switches. When they are off, they are virtually an open circuit, yet when they are on they are very close to a short circuit. So very little power is wasted as heat. In DCAC inverters designed to deliver high power, there are actually quite a few MOSFETs connected to each side of the transformer primary, to share the heavy current. However because they are essentially connected in parallel, they behave very much like the single transistors shown in Fig.1. They just behave like very high-power MOSFETs, able to switch many tens of amps. Note that: because the switching MOSFETs are simply being turned on and off, this type of inverter does not produce AC of the same .pure sine wave. type as the AC power mains. The output waveform is essentially alternating rectangular pulses, as can be see n from Fig.2. However the width of the pulses and the spacing between them is chosen so that the ratio between the
MS MS R
value of the output waveform and its peak-to-peak value is actually quite
similar to that of a pure sinewave. The resulting waveform is usually called a modified sinewave, and as the R MS voltage is close to 230V many AC tools and appliances are able to operate from MS such a waveform without problems. It¶s true, though, that this kind of waveform is not close enough to a sinewave for some appliances. That is mainly because the rectangular pulses contain not just the fundamental mains frequency, but quite a lot of its harmonics as well. So if the inverter is operating at the mains frequency of 50Hz, the output will also contain components at 100Hz, 150Hz, 200Hz, 250Hz 10
and so on. These harmonics can disturb the operation of some appliances. It¶s because of this shortcoming that manufacturers have come up with a more complex type of inverter, which does deliver a pure sinewave output.[9]
Fig.2: In most DC-AC inverters the switching MOSFETs are turned on alternately for short pulses, as shown, at the same repetition rate as the AC power mains. As a result the transformer produces a .modified sinewave. output (blue), rather than a true sinewave (red).
2.3.2. Output regulation
We take for granted the fact that our mains power is very well regulated so we can plug almost any appliance into a standard point outlet, and it will operate correctly. That is because 11
the electricity supplier has enormous generating plants, with automatic regulation systems to keep the mains voltage and frequency very close to constant, despite load variations of many megawatts. Inevitably you can¶t get this kind of performance from a much smaller electronic inverter, connected to a modest battery or solar panel as the energy source. However most modern inverters can provide reasonably good regulation for loads of up to their rated capacity (given in watts) assuming of course that they are running from a well-charged battery. In this type of inverter it isn¶t feasible to control the peak-to-peak output, because this is largely fixed by the battery voltage and the transformer¶s step-up ratio. So in most cases the regulation is achieved in a different way: by varying the width of the rectangular pulses, to control the form factor and hence the
MS MS R
value of the output voltage. This is called pulse width modulation
(PWM), and is usually done by having a feedback system which senses the inverter¶s output voltage (or load current). When this feedback senses that the load on the inverter¶s output has increased, the inverter¶s control circuitry acts to increase the width of the pulses which turn on the MOSFETs. The MOSFETs turn on for longer each half-cycle, automatically correcting the
MS MS R
value of the output to compensate for any droop in peak-to-peak output. The resulting regulation is usually capable of keeping the
MS MS R
value close to constant, for loads up to the inverter¶s full
rated output power. However this approach does have limitations, mainly because it can generally only increase the pulse width to a certain point. (In the extreme, the output becomes a square wave.) This may not be sufficient to allow the inverter to deliver enough
MS MS R
output
voltage in short-term overload or surge conditions. When many types of appliances are first turned on, for example, they draw a startup current which is many times greater than the current drawn when they are running. This type of surge can overload the inverter, and its protection circuitry may shut it down to prevent pr event damage to the transformer and MOSFETs. Some types of inverter incorporate special µsoft start¶ circuitry, to allow the inverter to cope with this type of short load current surge. The output voltage and power may drop, but at least the inverter keeps operating and allows the appliance to start up. Even so, there are some appliances and tools that are simply not compatible with inverters, because of their tendency to draw an extremely high startup current. Examples are refrigerators, freezers, air conditioners or any other appliance where a motor is driving a compressor or pump. As the motor in these appliances often has a very heavy load right at switch-on (with the compressor near .top dead centre.), it can need 12
to draw a huge current simply in order to start rotating. This type of appliance and tool should really be powered using a suitably su itably rated engine-driven alternator, not a DC-AC inverter.[9]
2.3.3. Voltage spikes
Another complication of the fairly high harmonic content in the output of modified sinewave. inverters is that appliances and tools with a fairly inductive load impedance can develop fairly high voltage spikes due to inductive back EMF.. These spikes can be transformed back into the primary of the inverter¶s transformer, where they have the potential to damage the MOSFETs and their driving circuitry. The risk of damage is fairly small during the actual power pulses of each cycle, because at these times one end of the primary is effectively earthed. Transformer action thus prevents the .other. end from rising higher than about twice the battery voltage. However as you can see from Fig.2, there are times during every cycle of operation when neither of the switching MOSFETs is conducting: the µflats¶ between the rectangular pulses. It. is at these times that the spikes can produce excessive voltage across the MOSFETs, and potentially cause damage. It¶s for this reason that many inverters have a pair of high-power Zener diodes connected across the MOSFETs, as shown in Fig.1. The Zeners conduct heavily as soon as the voltage rises excessively, protecting the MOSFETs from damage. Another approach is to have high-power standard diodes connected from each end of the primary to a large electrolytic capacitor, which becomes charged up to twice the battery voltage. When the ends of the primary attempt to rise higher than this voltage, the diodes conduct and allow the capacitor to absorb the spike energy. Thanks to this type of protection, most inverters are fairly tolerant of moderately inductive loads. However they may not be able to cope with heavy loads that are also strongly inductive . like heavy duty tools and machinery, or more than one or two fluorescent lights. Quite apart from the generation of voltage spikes, heavily inductive loads tend to demand current which is strongly shifted in phase relative to the inverter¶s output voltage pulses. This makes it hard for the inverter to cope, because the only energy available to the t he load between the pu lses is that stored in the transformer.[9]
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2.3.4.Power Switching
Designing power inverters can be summarised into attempting to meet two primary objectives:
y
Supply harmonic free electrical power at a constant voltage for a variety of loads, and have the
ability to cope with a non-ideal supply.
y
Supply this power as efficiently as possible with negligible electro-magnetic interference.
Mohan, Underland and
obbins obbins R
[11]
describe the two main sources of loss as switching power
loss and conduction loss. These losses can be simply explained from the following standard switching waveform taken from [11]:
Figure 3: Switching and Conduction Power Loss
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From this figure, the following equations relating to power loss can be obtained:
Several important conclusions can be drawn from these two equations:
y
Switching Loss varies linearly with switching frequency and rise/fall times. Hence switching
frequency can be increased provided devices with small rise and fall times are used.
y
Conduction loss is directly proportional to the on-state voltage. Clearly, a device needs to be
chosen that minimizes the on-state voltage (directly due to the MOSFET µOn-R esistance¶) esistance¶) to reduce conduction loss. Clearly then, the switching device chosen should meet the above criteria as closely as possible so as to maximise total efficiency of the device.
2.3.5. Ideal Characteristics of a Gate Drive
The ideal gate drive is one that: y
Provides sufficient drive power to keep the power switch in the on-state over the complete
range of operating voltages vo ltages and currents. y
Ensure that the switch remains in its off-state and is not triggered by noise from other
switching devices. y
Drive circuitry circuitry should be d irectly coupled between the switching device
y
Ensure that the switch turn on and turn off times are kept to a minimum. In a standard
MOSFET, the switching speed is directly related to the rate of supply of charge to the gate-input capacitance
[12]
. The resulting drive circuitry, while remaining as simple as possible, should
attempt to use this fact to maximise the switching speed of the MOSFET's.
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2.3.6. Soft Switching Techniques
With recent advances in inverter design, engineers have begun pushing developers to produce switching devices that will meet their needs. This however, is not the only alternative. By ensuring that all switching occurs with zero-voltage or zero-current in the switch many of the limitations inherent in the switching device can be overcome - or perhaps even utilised - to improve converter performance. This type of circuit design is called `soft-switching', and is easy to contrast to `hard-switching'. `hard-switching'. 2.3.7 Zero-Voltage and Zero-Current Switching
Looking back on figure 3 in Section 2.1.1 it becomes clear what is implied by zerovoltage or zero-current switching. It is a means by which additional circuitry is added external to the switch to either speed up or delay voltage or current rise/fall times to ensure that they cross at, ideally, zero voltage and current. An idealised zero-voltage zero-current switching pattern is shown in the foll fo llowing owing figure:
2.3.8. Transformer Selection
Transformer selection is influenced by the following factors ± switching speed, po wer loss and size. As switching frequencies are increased, generally a reduction of core size or minimum number of turns can be realized
[10]
. A point exists however, where increasing
frequency increases core losses to the point that there can be no further reduction in either core [10]
size or minimum number of turns
. This point is dependent on the type of material chosen for
the transformer core. The other factor in a transformers efficiency ± particularly at high frequency ± is eddy current losses in the transformer windings. This can be reduced by using bundles of numerous
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conductors or copper foil in higher current windings. From this optimization the ideal transformer for the situation can be constructed and utilised to increase overall system efficiency.
2.4. INVERTER POWER HANDLING CAPACITY AND EFFIENCY
It¶s helpful to know how much load an inverter can handle. Its power output is rated in watts (watts = amps x volts). There are three levels of power rating²a continuous rating, a limited-time rating, and a surge rating. Continuous means the amount of power the inverter can handle for an indefinite period of hours. When an inverter is rated at a certain number of watts, that number generally to its continuos rating. The limited-time rating is a higher number of watts that it can handle for a defined period of time, typically 10 or 20 minutes. The inverter specifications should define these ratings in [2]
relation to ambient temperature (the temperature of the surrounding atmosphere) . When the inverter gets too hot, it will shut off. This will happen more quickly in a hot atmosphere. The third level of power rating, surge capacity, is critical to its ability to start motors.
Some inverters are designed to be interconnected or expanded in a modular fashion, in [2]
order to increase their capacity. The most common scheme is to ³stack´ two inverters.
A cable
connects the two inverters to synchronize s ynchronize them so they perform as one unit. un it. 2.4.1. Inverter Efficiency
It is not possible to convert power without losing some of it. Power is lost in the form of heat. Efficiency is the ratio of power out to power in, expressed as a percentage. If the efficiency is 90 percent, 10 percent of the power is lost in the inverter. The efficiency of an inverter varies with the load. Typically, it will be highest at about two-thirds of the inverter¶s capacity. This is called its ³peak efficiency.´ The inverter requires some power just to run itself, so the efficiency of a large inverter will be low when running very small loads. In a typical home, there are many hours of the day when the electrical load is very low. Under these conditions, an inverter¶s efficiency may be around 50 percent or less. The full story
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is told by a graph of efficiency vs. load, as published by the inverter manufacturer. This is called [2]
the ³efficiency curve.´
Because the efficiency varies with load doesn¶t mean that an inverter with 93 percent peak efficiency is better than one with 85 percent peak efficiency. If the 85 percent efficient unit is more efficient at low power levels, it may waste less energy through the course of a typical day.
2.5. INTERNAL INVERTER COMPONENT PROTECTION
An inverter¶s sensitive components must be well protected against surges from nearby lightning and static, and from surges that bounce back from motors under overload conditions. It must also be protected from overloads. Overloads can be caused by a faulty appliance, a wiring fault, or simply too much load running ru nning at time. An inverter must include several sensing circuits to shut itself off if it cannot properly serve the load. It also needs to shut off if the D.C. supply voltage is too low, due to a low battery state-of-charge or other weakness in the supply circuit. This protects the batteries from overdischarge damage, as well as protecting the inverter and the loads. These protective measures are all standard on inverters that are certified for use in buildings.
2.6. INDUCTIVE LOADS & SURGE CAPABILITY OF A DC-AC POWER INVERTER
Some loads absorb the A.C. wave¶s energy in a regimented fashion, other loads will take all available power until they reach sufficient start up speed. This produces a time delay within the system and can create shocks, like towing a car with a rubber strap. These loads, if not sufficiently fed, create lags within the system, and can have big effects of the surrounding electrical system. If you have a central A.C. and your lights dim, that is the result of a motor or other inductive load drawing all available power, starving the lights momentarily. Motors are the most severely inductive loads. They are found in well pumps, washing machines, refrigerators,
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power tools, etc. TVs and microwave ovens are also inductive loads. Like motors, they draw a surge of power when they the y start. If an inverter cannot efficiently feed an inductive load, it may simply shut down instead of slowly starting the device. If the inverter¶s surge capacity is marginal, its output voltage will dip during the surge. This can cause a dimming of the lights in the house, and can crash a computer, ruin LCD TV screens, Plasma TVs and other sensitive electronics. Any weakness in the battery and cabling to the inverter will further limit its ability to start a motor as the increased amperage draw will result in further voltage det erioration. A battery bank that is undersized, has corroded connections, or is in poor condition, can be a weak link in the power chain. The inverter cables and the battery interconnect cables must be big, perhaps the size of a large thumb. The spike of D.C. through these cables is many [2]
hundreds of amps at the instant of motor starting.
2.7. IDLE INVERTER POWER
Idle power is the consumption of the inverter when it is on, but no loads are running. It is ³wasted´ power, so if the inverter is expected to be o n for many hours during which there is very little load, this ³idle power´ is to be as low as possible. Typical idle power ranges from 15 watts to 50 watts for an inverter sized for a home system.
y
Phantom Loads and Idle Loads Wasted Power
Another concern is ³idling loads.´ These are devices that must be on all the time in order to function when needed. These include smoke detectors, alarm systems, motion detector lights, fax machines, and answering machines. Central heating High-tech consumers are stuck with gadgets that draw power whenever they are plugged in. Some of them use power to do nothing at all. An example is a TV with a re mote control. Its electric eye system is on day and night, watching for your signal to turn the screen o n. Every appliance with an external wall-plug wa ll-plug transformer uses power even when the appliance is turned off. These type of loads are called ³phantom loads´ [2]
because their power draw is unexpected, unseen, and easily forgotten.
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y
Coping With Wasted Power
There are several ways to cope with phantom and idling loads: 1. You may be able to avoid them. 2. You can minimize their use and disconnect them when not needed, using external switches. 3. You can work around them by modifying certain equipment to shut off completely. 4. You can use some DC appliances. 5. You can pay the additional cost for a large enough power system to handle the extra loads plus the inverter¶s idle current.
2.8. DIFFERENCE BETWEEN GENERATORS AND INVERTERS
Before discussing the difference between an inverter and generator, it is first necessary to understand a little about electrical currents. Electricity is divided into two types of currents, alternating current (AC) and direct current (DC). AC, a more common current for home use, works by allowing electrons to flow in two different directions. In DC currents, electrons flow only one way. An inverter takes existing power that comes in the form of DC current and converts it to AC current. This is a popular option for those wanting to run home electronics in automobiles. Such cars often produce on DC current, which is not compatible with most electronics meant to run off standard outlets. Therefore, an inverter becomes necessary. A generator, on the other hand, is a machine that converts mechanical energy into energy in an electrical form. In most cases, electric generators are responsible for the energy a home receives. Large-scale electrical generators may be powered by coal, natural gas or nuclear energy. A portable generator commonly uses gasoline, which is burned to create electrical energy. Generators usually produce AC electricity. electricity. Simply stated, the difference between the two is that an inverter is only effective if there is already a source of electrical energy. It cannot generate its own. It can simply convert
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electrical energy that is already there. On the other hand, a traditional generator cannot make AC current into DC current. On the other hand, there are things known as inverter generators. These are like traditional generators in that they convert some other form of energy into electrical energy. However, they produce AC power, which is then converted to DC power before being converted back to AC. The reason for this conversion is that the power gained during the process. It allows the generator to be more fuel efficient, as well as operate more quietly than standard generators. [5]
Some people also confuse an inverter with a power converter, even using the terms interchangeably. However, a converter is used to change voltage from one level to another. For example, in Europe, a converter may be used to convert the voltage from 220 to 120, for electrical components meant to run o n a lower voltage, such as those in the United States.
2.9. APPLICATIONS AND ADVANTAGES OF INVERTERS
The following are the different applications app lications of inverters; y
DC power source utilization
.An inverter converts the D.C. electricity from sources such as batteries, solar panels, or fuel cells to A.C. electricity. The electricity can be at any required voltage; in particular it can operate A.C. equipment designed for mains operation, o peration, or rectified to produce D.C. at any desired voltage. Grid tie inverters can feed energy back into the distribution network because they produce alternating current with the same wave shape a nd frequency as supplied by the d istribution istribution system. They can also switch off automatically in the event of a blackout. Micro-inverters convert direct current from individual solar panels into alternating current for the [1]
electric grid .
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Uninterruptible power supplies
An uninterruptible power supply (U.P.S.) uses batteries and a n inverter to supply A.C. power when main power is not available. When main power is restored, a rectifier rectifier is used to supply D.C. power to recharge the t he batteries.
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Induction heating
Inverters convert low frequency main A.C. p ower to a higher frequency for use in induction heating. To do t his, A.C. power is first rectified rectified to provide D.C. power. The inverter then changes the D.C. power to high frequency A.C. power.
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HVDC power transmission
With HVDC power transmission, A.C. power is rectified and high vo ltage D.C. power is transmitted to another location. At the receiving location, an inverter in a static inverter plant converts the power back to A.C..
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Variable-frequency drives
A variable-frequency drive controls the operating o perating speed of an A.C. motor by controlling the frequency and voltage vo ltage of the power supplied to the motor. An inverter provides the controlled power. In most cases, the variable-frequency drive includes a rectifier so that D.C. power po wer for the inverter can be provided from main A.C. power. Since an inverter is the key component, variable-frequency drives are sometimes called inverter drives or just inverters.
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Electric vehicle drives
Adjustable speed motor control inverters are currently used to power the traction motor in some electric locomotives and diesel-electric locomotives as well as some batt ery electric vehicles and 22
hybrid electric highway vehicles such as the t he Toyota Prius. Various improvements in inverter technology are being developed specifically for electric vehicle applications[6]. In vehicles with regenerative braking, the inverter also takes power from the motor (now acting as a ge nerator) and stores it in the batteries.
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Inverter (air conditioning)
An air conditioner bearing the inverter tag uses a variable-frequency drive to control the speed of the motor and thus the compressor.
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The general case
A transformer allows A.C. power to be co nverted to any desired voltage, but at the same frequency. Inverters, plus rectifiers for D.C., can be designed to convert from any vo ltage, A.C. or D.C., to any other voltage, also A.C. or D.C, at any desired frequency. The output power can never exceed the input power, but efficiencies can be high, with a small proportion of the power dissipated as waste heat.[3]
2.9.1 Advantages Of Inverters
Can automatically switches from mains power to inverter power in less than 10milli-seconds and vice versa depending on if the inverter is automatically or manually operated. 1. No Petrol nor Diesel needed 2. No Noise 3. No Fumes 4. Very Portable 5. Cheaper to operate than generators 6. No special skills needed to operate 7. Quick and Easy to R echarge echarge 23
REFERENCES
1.
http://www.greentechmedia.com/articles/enphase-raises-15m-for-micro-inverters1373.html
2. ³How to Choose an Inverter for an Independent Energy System´ By Windy Dankoff Apr/May (2001) (#82) pp. 74-78
3. .http://en.wikipedia.org/wiki/Inverter-(electrical). 4. http://www.nooutage.com/inverter1.htm#waveforms. 5. http://www.wisegeek.com/inverters. Vehicles". Oak Ridge National 6. "Power Electronics: Energy Manager for Hybrid Electric Vehicles". Laboratory Review (U.S. Department of o f Energy) 33 (3). 2000. ndustry ndustry 7. Owen, Edward L. (January/February 1996). "Origins of the Inverter". IEEE I Applications Magazine: History Department (IEEE) 2 (1): page 64±66
8. "Inverter FAQ". FAQ". PowerStream. 2006. 9.
aycar aycar lectronics lectronics ³J E
Reference Data Sheet´: Sheet´: INVER TE TER .PDF (1) (Copyright © Jaycar .PDF
Electronics, 2000).
10. B. Andreycak, ³1.5Mhz Current Mode IC Controlled 50 Watt Power Supply,´ Unitrode application Note, no. U-110. obbins and T. Undeland, ³Power Electronics ± Converters, Applications obbins 11. N. Mohan, W. R and Design (2nd Edition),´ Wiley and Sons, New York, 1995. aciti, A. Testra, A. Galluzo, and M. Melito, ³Switching Behaviour aciti, 12. S. Musumeci, A. R Improvement of Insulated Gate-Controlled Devices,'' IEEE Trans. Po wer. Electron., vol. 12, no. 4, pp. 645-653, 1997.
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