what is an invertter ,how to choose right inverter?

What's an Inverter?

An inverter converts DC voltage into an AC voltage. Most cases the input DC voltage is lower than the output AC voltage. This is because the grid supply voltage can be either 110VAC/120VAC, 220VAC, 230VAC or 240 Volts, depending on where you are located.

An inverter can be used as a standalone device for solar power applications or as a backup power supply to batteries that are separately charged.

Another configuration is when the inverter is part of a larger circuit, such as a power supply or UPS. In this example, the inverter input DC is drawn from the rectified mains AC of the PSU. The UPS's rectified AC is used when power is available. Batteries are used in case of power loss.

 

Different types of inverters are available depending on the form of the switching waveform. They have different circuit configurations, efficiencies and advantages.

Inverters provide ac voltage from DC power sources. They are useful for powering electronic equipment and other electrical equipment that is rated at the AC mains voltage. They are also used extensively in the switching mode power supplies inverting stage. Circuits can be classified by switching technology, switch type, waveform, frequency, and output waveform.

Basic inverter operation

Basic circuits consist of an oscillator and control circuit. Drive circuits for power devices, switching devices, and the drive circuit for them.

Conversion of DC to alternating Voltage is accomplished by using energy from the dc source, such as the battery or rectifier output into an alternating current. Switching devices are used to convert dc into alternating voltage. The transformer is then used to step up the power. There are many configurations that do not require a transformer. However, they are rarely used.

The power devices, such as MOSFET and power transistors, switch the DC input voltage on or off and pulses are fed to the primary side. Variable voltage in the primary causes an alternating voltage at the secondary winding. The transformer can also be used as an amplifier, increasing the output voltage in a proportion determined by the turn's. The output voltage rises from the 12 volts supplied in the batteries to 120 volts AC or 240 V.

Three common Inverter output stages include a push pull with centre tap transformer and push-pull halfway-bridge. Popular is the push pull with centre tap. It has simple, predictable results and is more popular. However, it requires a heavier transformer, and has lower efficiency.

The figure below shows a simple push-pull DC to AC Inverter with a centre tap transformer circuit.

 

Inverter output wave forms

Inverters can be classified according to the output wave forms they produce. The three most common types are the square wave, pure sine wave, and modified sine waves.

Figure 1: square wave

   

 

Although the square wave is simpler and more affordable, it does not have the same power quality as the other two. Modified square waves have a higher power quality (THD 45%) and are suitable for most electronic equipment. These rectangular pulses have dead spots that are between the positive and negative half cycles (THD approximately 24%).

Figure 2: Modified sine waveform

Figure 3: Pure sine wave

 

 

True sine wave inverters have the best waveform and the lowest THD (about 3%).  Output voltage clean power like utility-supplied electricity,This inverter is however the most costly and is used in medical equipment, laser printers, and other applications that require sinusoidal wave forms. They are also used in grid ties inverters, grid connected equipment and other applications.

 

Applications

There are many applications for inverters, from small car adapters to large grid systems and household applications.

Uninterruptible power supply

As standalone inverters

Solar power systems

As the building block for a switched-mode power supply

 

What is the difference between a Low- frequency inverter and a high frequency inverter?

Features of low frequency inverter:

 

1. When the low- frequency inverter is low power, the cost is higher than that of the high frequency inverter.

 

2. The weight and volume of the Low frequency inverter are larger than that of the high frequency inverter of the same power.

 

3. The efficiency of the LOW  frequency inverter is lower than that of the high frequency inverter, and the iron loss is basically unchanged when running under full load and light load, so the no-load loss of the LOW frequency inverter running under light load is relatively large .

 

4. The reliability of the low- frequency inverter is higher than that of the high frequency inverter, and it is not easy to break.

 

5. The Low- frequency inverter's load capacity, especially the impact load capacity, is better than that of the high-frequency inverter, and it can suppress the high-order harmonic components in the waveform.

Features of high frequency inverter:

 

1. The no-load loss of the high-frequency inverter is very small, so the inverter efficiency is high.

2. The high-frequency inverter is small in size and light in weight.

3. The high-frequency inverter has low no-load load, relatively poor overload capacity and cannot connect to full-load inductive load such as:

· Laser printers, photocopiers, magneto-optical hard drives

· Some fluorescent lights with electronic ballasts

· Power tools employing "solid state" power or variable speed control

· Some battery chargers for cordless tools

· Some new furnaces and pellet stoves with microprocessor control

· Digital clocks with radios

· Sewing machines with speed/microprocessor control

· X-10 home automation system

· Medical equipment such as oxygen concentrators

 

Performance comparison of power frequency inverter and high frequency inverter:

1. In terms of reliability, LOW frequency inverters are better than high frequency inverters

The Low-r frequency inverter uses a thyristor (SCR) rectifier, which has a very strong resistance to current impact. Since the SCR is a semi-controlled device, there will be no faults such as shoot-through and false triggering. In contrast, although the IGBT high-frequency rectifier used in the high-frequency inverter has a high switching frequency, the IGBT has a strict voltage and current working area during operation, and the impact resistance is low. Therefore, in terms of overall reliability, power frequency inverters are more stable than high frequency inverters

 

Low frequency inverter

 

High frequency inverter