01-11-2012, 05:10 PM
How an Inverter Works
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So how can an inverter give us a high voltage alternating current from a low voltage direct current.
Let's first consider how an alternator produces an alternating current. In its simplest form, an alternator would have a coil of wire with a rotating magnet close to it. As one pole of the magnet approaches the coil, a current will be produced in the coil. This current will grow to a maximum as the magnet passes close to the coil, dying down as the magnetic pole moves further away. However when the opposite pole of the magnet approaches the coil, the current induced in the coil will flow in the opposite direction.
As this process is repeated by the continual rotation of the magnet, an alternating current is produced.
Now lets consider what a transformer does. A transformer also causes an electric current to be induced in a coil, but this time, the changing magnetic field is produced by another coil having an alternating current flowing through it. Any coil with an electric current flowing through it will act like a magnet and produce a magnetic field. If the direction of the current changes then the polarity of the field changes.
Now, the handy thing about a transformer is that, the voltage produced in the secondary coil is not necessarily the same as that applied to the primary coil. If the secondary coil is twice the size (has twice the number of turns) of the primary coil, the secondary voltage will be twice that of the voltage applied to the primary coil. We can effectively produce whatever voltage we want by varying the size of the coils.
If we connected a direct current from a battery to the primary coil it would not induce a current in the secondary as the magnetic field would not be changing. However, if we can make that direct current effectively change direction repeatedly, then we have a very basic inverter. This inverter would produce a square wave output as the current would be changing direction suddenly.
This type of inverter might have been used in early car radios that needed to take 12 volts available in the car and produce the higher voltages required to run radio valves (known as tubes in America) in the days before transistors were widely used.
A more sophisticated inverter would use transistors to switch the current. The switching transistors are likely to be switching a small current which is then amplified by further transistor circuitry. This will still be a square wave inverter.
The Sine Wave Inverter
To get a sinusoidal alternating current from the output of our transformer, we have to apply a sinusoidal current to the input. For this we need an oscillator.
An amplifying transistor can be made to oscillate by feeding some of the amplified output back to its input as positive feedback. We will all have heard this effect at sometime when someone is setting up a PA or microphone system. If the microphone is too close to the speaker, some of the output from the speaker is fed back to the microphone and inputted to the amplifier again. The result is a howling sound.
The positive feedback in an electronic circuit can be tuned using extra components to produce the frequency we require (generally either 50 or 60 cycles per second to mimic mains electricity). If a crystal is used to control this frequency, as in a battery watch or clock, the frequency can be very accurately
Grid Tied Inverters
If the above example were a grid tied inverter, ie able to feed power back into the national grid, it would need to use a sample of the mains voltage to then be amplified within the inverter, or to synchronise the oscilator with that sample.
Grid tied inverters will also sense if there is a "power cut" and disconnect themselves from the grid. If they did not have this facility, in the event of a power cut, your inverter would be attempting to power all your neighbours houses and would present an electrocution risk to anyone working on power lines that had supposedly been turned off.
Inverter.
An inverter is used to produce an un-interrupted 220V AC or 110V AC (depending on the line voltage of the particular country) supply to the device connected as the load at the output socket.The inverter gives constant AC voltage at its output socket when the AC mains power supply is not available.
Lets look how the inverter makes this possible.To grasp the functioning of an inverter,we
should consider in the following situations.
When the AC mains power supply is available.
when the AC mains power supply is not available.
When the AC mains power supply is available.
When the AC mains supply is available,the AC mains sensor senses it and the supply goes to the Relay and battery charging section of the inverter.AC main sensor activates a relay and this relay will directly pass the AC mains supply to the output socket.The load will by driven by the line voltage in this situation.Also the line voltage is given to the battery charging section where the line voltage is converted to a DC voltage(12V DC or 24V DC usually),then regulated and battery is charged using it.There are special circuits for sensing the battery voltage and when the battery is fully charged the charging is stopped.In some inverters there will be a trickle charging circuit which keeps the battery constantly at full charge.
When the AC mains power supply is not available.
When the AC mains power supply is not available,an oscillator circuit inside the inverter produces a 50Hz MOS drive signal.This MOS drive signal will be amplified by the driver section and sent to the output section.MOSFETs or Transistors are used for the switching operation.These MOSFETs or Transistors are connected to the primary winding of the inverter transformer.When these switching devices receive the MOS drive signal from the driver circuit,they start switching between ON & OFF states at a rate of 50 Hz.This switching action of the MOSFETs or Transistors cause a 50Hz current to the primary of the inverter transformer.This results in a 220V AC or 110V AC (depending on the winding ratio of the inverter transformer) at the secondary or the inverter transformer.This secondary voltage is made available at the output socket of the inverter by a changeover relay.
Inverter Function and Benefits
Batteries produce power in direct current (DC) form, which can run at very low voltages but cannot be used to run most modern household appliances. Utility companies and generators produce sine wave alternating current (AC) power, which is used by most commonly available appliances today. Inverters take the DC power supplied by a storage battery bank and electronically convert it to AC power.
An inverter used for backup power in a grid connected home will use grid power to keep the batteries charged, and when grid power fails, it will switch to drawing power from the batteries and supplying it to the building electrical system. For an business or home office, a reliable power source is invaluable for preventing lost data on computer systems. Most modern inverters also include overvoltage and undervoltage protection, protecting sensitive equipment from dangerous power surges as well.
In some areas, grid connected homes can use inverters and alternative energy generators to sell power back to the utility company. With the inverter attached to solar, wind or water generators, the inverter can use the utility grid as its battery bank. Utility power will be used when alternative power sources are insufficient, but when power needs are low, excess alternative energy can be sent to the utility grid. Sellback power will then be credited to the user's utility bill.
In a stand-alone renewable power system, whether residential, industrial, marine or RV, the inverter allows AC electrical appliances to be run from the storage battery bank. When the battery bank becomes discharged, the inverter can automatically start a generator to power the system while the batteries recharge.