Thursday, January 8, 2009
Monday, January 5, 2009
An inverter is an electronics device that converts battery DC to an AC signal it is the same thing as an oscillator. The AC signal can be of various waveforms; sinusoidal, rectangular, square, saw tooth, etc. The type of waveform that we use in our industrial and domestic homes is a sinusoidal wave; this is the best waveform that can run our appliance with out any problem of over heat.
Most model produce a modified square wave. This waveform allow home owners to run 98% of the typical loads in a house such as fluorescent lights, TVs, stereos, vacuums and power tools. The few limitations include some type of electronic controls like dimmer, switches, sensitive electronics like laser printers and photocopiers, and some small rechargeable devices. Occasionally some of these products will not work, or even fail, with modified square wave power. Some appliances like microwave may be noisier and stereo equipment and TVs may have a slight hum or buzz with this type of inverter power.
Inverters that produces pure sine wave to mimic convectional grid power eliminates background noise so that all appliance, including electronics, work without problem. They are particularly suited for sensitive electronics found in some computers and higher quality sound equipment.
INVERTER FOR HOMES…
In large remote residences, particularly those using auxiliary generators, inverters can reduce the cost of power generator by up to 90%. Most inverters include a stand-by battery charger, so that when the generator is on, the batteries are automatically recharged. Once the generator is turned off, the inverter system powers the same AC circuits. Not only do you have quiet power available 24 hours a day, but in most cases the fuel savings alone can pay for the complete cost of the inverter system in less than a year!
WHY DO I NEED AN INVERTER?
Inverter converts DC battery power to standard AC power. They allow you to run regular 120V, 220VAC appliances; including TVs, computers, microwaves and power tools. With an inverter your AC loads are run off your batteries and they can be used any time of day and night - without a generator – and definitely during a utility power failure.
When designing an inverter the power rating of the load is taken into consideration at maximum capacity. Choose a size that can power the appliance you plan to use. Typical sizes installed in our home systems are 1000W to 2500W. Larger inverters from 4KW to 11KW are used in large power systems and industrial applications. Inverters are rated according to the continuous power that they can produce; however, they are also designed to deliver large amounts of current for short period of time – a feature called surge capacity
DESIGN OF AN INVERTER
When designing an inverter, the first thing that comes to mind is an oscillator circuit. An oscillator is an electronic device that converts battery DC to an AC signal. The AC signal produce are non sinusoidal – they are AC signals that shows a great deviation from sine/cosine waveform; square wave, rectangular wave, saw tooth wave, trapezoid wave, quasi sine wave, etc (are all complex waves). However, sine wave can be generated by using special kind of oscillators such as Wien Bridge, Hartley oscillator, RC oscillator, and other Radio frequency oscillators. This kind of wave is not very easy to generate.
The use of relaxation oscillators can produce square wave, quasi sine wave etc. Examples of these are the Multivibrators; Astable, Monostable, and the Bistable (flip- flop).
It is note worthy that it is the oscillator that produces the waveform signal for the inverter. So, the choice of oscillator matters.
> A transformer may be defined as a piece of apparatus
> without a continuously moving part which employs the
> principles of electromagnetic induction to transform
> alternating or intermittent voltage or current in one
> winding into alternating voltage or current in one or more
> windings, usually of different value of voltage or current.
> A transformer used to increase voltage of an a.c is called
> step up transformer while the one used to decrease voltage
> of an a.c is called step down transformer.
> Moreover, transformer can also be used to isolate one
> circuit from another, which ensures safety and guide against
> electric shock when working on the low side of an a.c.
> A transformer consists of laminated sheets made of silicon
> steel insulated from one another. This reduces eddy current
> loss. The vertical positions of the core are referred to as
> limb and the top and bottom positions are the yoke. Both the
> primary and secondary coils are wound on the limbs. The
> primary coil is connected to the a.c supply while the
> secondary is usually connected to the load.
> 2.15 TYPES OF TRANSFORMERS
> There are different types of transformers:
> 1. Single phase transformers
> 2. Three phase transformers
> 3. Auto-transformers
> 4. Current transformers
> 5. Voltage (or potential) transformers
> 6. Power transformers.
> Power transformers have a high utilization factor, that is,
> they are designed to run with almost constant load, which is
> equal to their rating or capacity.
> The maximum efficiency is designed to be at full load, this
> means that the full load winding losses must be equal to
> core losses.
> 2.16 TRANSFORMER COOLING
> When transformer is supplying power to a load, heat is
> generated in the winding and core due to losses. The heat
> dissipated due to losses should be transferred away from the
> winding and core of the transformer to avoid damage.
> The cooling methods used are:
> 1. Air cooled system: - The transformer casing is
> perforated on both sides. Air circulates through the unit by
> convention. This is usually confined to small transformer of
> the range 1KVA – 5KVA.
> 2. Oil –filled cooling system
> 3. Water cooled system
> 4. Air blast (force air) cooling system.
In principle, the parameters describing the resulting A.C voltage (frequency, amplitude, phase number, e.t.c.) can be arbitrarily chosen. In practice, single phase and three phase realization are most common. Frequency is limited by the dynamic behavior of the electric element chosen.
The pole changes required to turn out an A.C voltage is performed by means of suitably chosen electronic switching elements usually transistors or thyristors. Depending on the switching elements used, we have thus, a thyristor switching inverter and a transistor switching inverter.
Furthermore, due to elimination of mechanical contacts, transistor and /or thyristor inverters are not subject to wear, and there is no soot deposit. They are faster in term of switching speed, they are portable and are maintenance free.
Moreover, in designing inverter, the basic components; the simple diodes, transistors, the power MOSFETS, the thyristors, the transformers, and Battery are effectively utilized.
2.2 THE DIODE
A diode is a rectifying device, which permits current flow in one direction only, being able to withstand a potential difference without current flow in the opposite direction.
The active material from which the semi-conductor power diodes is formed is silicon, a semi-conducting material, that is, a material of which the conductivity is classified as being between insulating and conducting; its resistance decreases with temperature rise. The diode is a P-N junction device from which all other semiconductor components are made.
The N-type semiconductor has electrons (Negative charges) and P-type semiconductor has holes (Positive charges).
2.3 Operation of the P – N Diode
When the P-type is more positive with respect to the N-type by the application of voltage, the electrons in N-type are pulled to the side of P-type and hole is pulled to the side of N-type. In this way, the electric current flows through the semiconductors. And the diode is said to conduct. Conversely, when the N-type is made more positive with respect to the P-type, by the application of potential difference, electrons in N-type are pulled with positive voltage on the side of N-type and holes in the P-type are pulled with the negative voltage on the side of P-type. In this case, the electrons in the semiconductor do not move an so, the diode will not conduct and is said to be reverse biased.
The concept of Uninterruptible Power Supply (UPS) comes to mind because of the challenging problems we encounter in the Nigeria power system.
In Nigeria where we experience unpredicted power cut, voltage fluctuation, low voltage, etc, efforts has been made to substitute for existing power with an alternative power which is usually generator. This ensures that businessmen and other business organizations will be able to meet their customer's day to day needs and requirements in order to efficiently maximize profit.
However, the use of private generators has posed more problems to business men because of the inherent disadvantages associated with maintenance cost, repair due to wear of machine parts and cost of fueling the generator. This makes it exorbitantly expensive to run. Moreover, the noise associated with its operation is a nuisance to the neighborhood, a major source of pollution. Another more dangerous pollutant is the fume released to the atmosphere when the generator is in normal working operation. This is hazardous to the environment and society at large.
Consequently, because of the aforementioned disadvantages associated with generators own by individuals, the inverter provides a clean, noiseless, maintenance free power supply to run our domestic and commercial loads when there is utility failure. By incorporating automatic switching mechanism between utility and inverter, the UPS is defined. The UPS ensures that there is constancy of power to the system in use even if there is utility failure and supplies rated voltage and frequency to load even if utility voltage is low.
The inverter converts DC battery power to standard AC power. They allow us to run regular 230Vac appliances, including TVs, computers, microwave and power tools. Inverter can be used any time of the day and night.
Inverters are rated according to the continuous power that they can produce.. Moreover, for the purpose of this project, a 1KVA uninterruptible power supply (UPS) will be designed and constructed.
The UPS consist of the following incorporated systems:
1. Inverter system
2. Automatic battery charger with controller
3. battery monitor( to automatically disconnect load when battery is low)
4. Automatic voltage control(AVC)
5. Indicators/display to signal 1,2,&3
6. Auto transfer between utility power and inverter.
1.1 THE INVERTER SYSTEM
This converts DC battery voltage to alternating voltage AC. The AC voltage is an approximation of sine wave; it is the pulse width modulation type.
1.2 AUTOMATIC BATTERY CHARGER
This charges the battery immediately when utility power is restored. The charger is a two stage charger which provides a constant current until the battery reaches its rated capacity and then switches to a float voltage. The current then reduces as necessary to maintain the battery at the float voltage (trickle current).
1.3 THE BATTERY MONITOR
This is achieved by use of comparator. The comparator compares the output battery voltage with the input and when battery voltage goes below the set point, it automatically disconnects the load from battery so that battery is not completely flat.
It indicates normal working condition of the above features.
1.5 AUTO TRANSFER SWITCH
These are relay operated switches to automatically switch on/off load from utility to inverter or vice versa. When there is utility failure it switches automatically to inverter and when utility power is restored, it automatically switches from inverter to utility power.
1.6 AUTOMATIC VOLTAGE CONTROL (AVC)
This functions like a voltage regulator. It ensures that there is no much voltage variation from the preset desired value of 230V when load of its rated capacity is impressed on it. This makes the UPS unique.
Moreover, the design of the UPS requires a careful selection of semiconductor parts from the electronic data sheet, the power switching semiconductor device so chosen is the MOSFET. The power MOSFET has a positive temperature coefficient for resistance; hence, paralleling of the device is relatively simple. The popular IRF 150N is selected because it is more versatile, rugged and has a maximum drain current rated 40A, Vds 100V maximum. This component will be carefully mounted on an aluminium heat sink for heat dissipation during working condition.