In conventional inverters which are now a day’s commonly used in Pakistan,
consumer faces many problems relating to their performance. The foremost problem is
their efficiency which is in between 85% to 90%. The efficiency mainly depends upon
the switching frequency of the transformer which is used in DC/AC inverter.
The large size of transformer also make the U.P.S very bulky. It also occupies a
large area which is also not good from the point of view of the customer.
The E.M.F induced in the transformer given by the formula relating its
frequency, area, number of turns and magnetic field to each other is
The formula clearly justify that if we use a transformer with high working
frequency then we will have to design a transformer with small area.
The efficiency problem can be solved by making such a inverter which will have
a output very close to sinusoidal shape. In short if the output shape have distortion, there
will be less efficiency and it will result as a power loss in the form of heat which will
affect the load devices.
In our project we are trying to solve the above mentioned problems by using a
transformer which will give a high switching frequency so that we can improve both the
efficiency and large area problem. The high frequency will make a output very close to
sinusoidal shape, hence high efficiency. We are using ferrite core transformer for this
purpose. This transformer has a switching frequency range from 25khz to 500khz.
The output of the U.P.S battery (12 volts dc) is given to a dc to ac bridge circuit
which is connected to the primary side of the transformer. So, we get 24 volt rail to rail ac
input at primary of the transformer.
This transformer will convert 12VDC to 380VDC with fast switching frequency.
This high dc input will be given to a full bridge ac to dc converter with a SPWM gating
sequence. The output of this ac to dc converter is then passed through a low pass filter to
get a sine wave output waveform of the U.P.S
―To make the U.P.S more user friendly, energetic and efficient‖ is the manifesto of our
IMPORTANCE OF UPS
Now a day use of electricity is increasing day by day. Devices are operated on electricity thus the importance of electricity can never be avoided. In a country like Pakistan role of electricity is vital but the recent crisis of electricity shows that there should be some alternate to get electricity instead of depending only on the WAPDA supply. The recent calculation for electricity provided and demand is given in table
Power in MW
The above table shows the shortage of power is almost 5000MW. So there is a large load shedding in order to protect the whole power system from damaging. The calculation shows that it is not comfortable to depend only on the utility or WAPDA. There is a need of remedy for that shortage. Alternate method that are commonly use to get the power is
Generators are available in the market with different rating having
prices depending on the rating of the generator. usually for domestic 1KVA,2KVA,3KVA,4KVA or 5KVA are used. For commercial large generators are used shown in the below figure having rating 10KVA or above
generators are expensive also diesel require to operate them is also costly so another source of power is UPS. UPS stands for Uninterruptible Power Supply which supplies the stored electrical power to the load in case of the power shortage or cut-off. Mostly UPS store energy in the battery and the inverter convert that stored DC electrical energy into AC when there is no main supply from the WAPDA.Battery can give a backup time that depend upon the number of appliances operated on the electrical power supplied by the UPS.
Once the Store energy in the battery is discharged up to certain level battery need to recharge. This can be done using a battery charger. A battery charger converts the AC main into DC by the rectifier circuit. The block diagram of mostly available UPS in Pakistan is
Here we present some analysis about the Ups company present in Pakistan. This is carried out through a market survey that most people are using the UPS of the following Tags Different ups available in Pakistan:
The above table shows the Ups companies and their price rates for the 1000 watts. The UPS of these companies are mostly designed on the same feature. The inverter of these Ups is either operated on square wave or modified square wave. Different types of inverters and Problem of these Inverter are discussed in the next chapter.
CLASSIFICATION OF INVERETRS
Inverter is device that converts the DC Power into AC power. The voltage and the frequency of the converted AC power can be set to required level depending upon the selection of the transformers and control circuit.
AC main is converted into DC through a rectifier circuit and battery is charged than the DC from the battery is converted to AC by an inverter circuitry.
2.2 Types Of Inverter
Inverter available in the market are usually provide three types of output
- Square wave
- Modified square wave
- Pure sine wave
Here is some description about the three types of inverter also there drawbacks are mentioned.
2.2.1 Square Wave:
Inverters are first invented using a square wave as the output. Square wave has a large number of problems related to the functionality of the devices as these devices are usually designed to operate for the sine wave. Due to the harsh corner of the square wave it is very harmful for the electrical equipments. Harmonic content of the square is very high that result in the overheat of the electrical equipment. So the inverter use the square wave technology tends to produce a lot of heat due to power loss. Efficiency of the square wave inverter is less. The price of these inverter define the efficiency of the inverter.
2.2.2 Modified Square Wave:
Some alterations made on the hardware of the inverter in order to remove the harsh corner of the square wave. Some time the supplier of the modified square wave inverter use the term modified sine wave in order to increase the demand of the that inverter but in reality it is only the modified square wave . It has better characteristics than the square wave. But the overheat factor is still present that result in the reliability of the equipments. So here is a need to make of sine wave inverter.
2.2.3 Pure Sine Wave:
All the home appliances and electrical devices are designed to operate for the sine wave. So there is a need to design a inverter that gives output very close or similar to the sine wave. Our project is also to make a sine wave inverter. It also reduce the noise interferences associated with other inverters. With the pure sine wave inverter our project fully assures that sensitive loads will be powered correctly with no noise interference. Depending upon the switching frequency inverter can be classified as
- Low Frequency Inverters
- High Frequency Inverters
Switching frequency related to the rate at which the DC is converted to the AC. Low frequency inverter oscillates a DC voltage at 50Hz. The voltage of that inverter is then step up to desire level using a large and heavy transformer. The transformers are iron core shown below
High frequency inverter on the other hand use small size transformer. Such high frequency inverter may have large harmonic content near the range of the switching frequency. But the advantage is that these harmonics are high in order than the
fundamental frequency 50HZ. So in order to separate these harmonics a low pass filter is use. High frequency inverter use ferrite core transformer shown below
AC TO DC CONVERSION
3.1 Type of chargers:
i). Ferro-Resonant Charger
They are very bulky because of low frequency magnetic control system in it. They are slowly recharged. Their magnetic field may disturb other components of the equipment.
ii). Linear Charger
They also use low frequency transformer in it to reduce the level of the ac voltage, then a combination of bleeding resistor, series resistor and a capacitor is used to control the current and voltage level of dc output.
iii). Switch Mode Charger
It is a newly technique, it directly rectify ac input, then this dc input is switched with high frequency through transistors and with the help of high frequency transformer, its level is lowered and rectified to desired dc output level
3.2 A/H Ratting of the Battery
Before designing the charger stage of the battery, we must see the A/H ratting of the battery, which determines the charger time as well as the backup time with respect to the charging current. The ampere-hour ratting ( A/H ratting ) is the indication of battery energy capacity, it is given by the relation:
A battery with a capacity of 10 amp-hour should be able to continuously supply a current of 10 amp to a load for exactly 1 hour, or 20 amps for 1/2 hour, or 1/3 amp for 30 hours, etc., before becoming completely discharged.
Typical A/H Rating of Some Batteries:
- Typical automotive battery: 70 amp-hours @ 3.5 A (secondary cell)
- D-size carbon-zinc battery: 4.5 amp-hours @ 100 mA (primary cell)
- 9 volt carbon-zinc battery: 400 milliamp-hours @ 8 mA (primary cell)
The most common type of battery available is lead acid battery. A lead-acid cell generates around 2 volts. Small batteries contain 6 cells in a container which add together to give 12 volts at the terminals
3.3 Battery Charger Selection:
The steps to calculate the charger requirement for a battery are
i) Determine the suitable A/H ratting of the battery required according to the load and Backup as well as the charging time required
ii) Determine the recharge time required.
iii).To get the charging current, divide the A/H rating by that time.
iv). Add 20% to allow for the battery inefficiency.
v). If there is any additional battery load current during its charging like control circuit, add it also to the charging current.
vi).Select the next biggest charger in the range
Battery Voltage = 12 Volts
Load Required = 700
Watt Efficiency Required = 90%
Total Battery Current = (700/0.9)/12 = 70 Amp
Estimated Additional Current = 1 Amp
Total Battery Output Current = 71 Amp
Battery Charging Current = 0.1 * 71 = 7.1 Amp
Backup Time Required= 2 Hour
A/H Ratting Of Battery = 2 * 71 = 141 A/H