Saturday, 16 September 2017

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Design And Construction Of 2kva Pure Sine Wave Inverter



An inverter is a device that changes D.C. voltage into A.C. voltage. A direct current (D.C) is a current that flows in only one direction, while an alternating current (A.C.) is that which flows in both positive and negative directions. Inverters are used to operate electrical equipment from the power produced by a car or boat battery or renewable energy sources, like solar panels or wind turbines. DC power is what batteries store, while AC power is what most electrical appliances need to run. So, an inverter is necessary to convert the power into a usable form.

The output voltage of a sine-wave inverter has a sine wave-form like the sine wave-form of the mains / utility voltage. In a sine wave, the voltage rises and falls smoothly with a smoothly changing phase angle and also changes its polarity instantly when it crosses 0 Volts.

Pure sine wave inverters are used to operate sensitive electronic devices that require high quality waveform with little harmonic distortion. In addition, they have high surge capacity which means they are able to exceed their rated wattage for a limited time. This enables power motors to start easily which can draw up to seven times their rated wattage during start up. Virtually any electronic device will operate with the output from a pure sine wave inverter.

Sine wave inverter has the following characteristics:
  1. High efficiency
  2. Low standby losses
  3. High surge capacity
  4. Low harmonic distortion
All grid tied inverters are pure sine (true sine) inverters, hence the grid, by nature, is a pure sine 
wave electricity source. The importance of pure sine wave inverters may be apparent especially for off grid applications such as RV, boat or cabins. Off grid inverters are used for connecting a battery source or a solar PV system to an AC load such as a home appliance, a laptop charger, a TV.


Pulse Modulated Wave Inverters

The most common generally used inverters available are the “modified square wave or pulse modulated wave inverters” variety, usually available at more moderate prices compared to pure wave models. The modified square wave or PWM output inverters are designed to have somewhat better characteristics than square wave units, while still relatively inexpensive.

Although designed to emulate the pure sine wave output, pulse modulated wave inverters do not offer the same perfect electrical output. As such, a negative by-product of the modified output units is electrical noise (hum) which can prevent these inverters from powering certain loads properly. For example, many televisions and stereos are sensitive to supplied power, use power supply incapable of eliminating common mode noise.
As a result, powering such equipment with a pulse modulated wave inverter may cause a small amount of “snow” on your video picture, or “hum” on your sound system, likewise most appliances with timing devices, battery chargers and variable speed devices may not work well or sometimes may not work at all.

Since the width of the square wave can be adjusted, the rate at which pulse is produced can be controlled using “pulse width control”, and since the pulse in not in ON position at all times (i.e. it has a dead time) the heat produced is almost one quarter that of the square wave inverter. The “dead time” in the pulse width modulator can be said to be the time at which the transistor is not ON thereby reducing the heat produced by the transistor. (Doucet et al, 2007)

Pulse width modulation (PWM) is a powerful technique for controlling analogue with a processor’s digital outputs. . It is also known as pulse duration modulation (PDM). The leading edge of the carrier pulse remains fixed and the occurrence of the trailing of the pulses varies. PWM signals find a wide application in modern electronics. Some of the reasons for this are:

1. Reduced Power Loss – switched circuits tend to have lower power consumption because the switching devices are almost always off (low current means low power) or hard-on (low voltage drop means low power). Common circuits that utilize this feature include switched-mode power supplies, Class D audio power amplifiers, power inverters and motor drivers. Frequently, these circuits use semi-analogue techniques (ramps and comparators) rather than digital techniques, but the advantages still hold.

2. Easy to Generate – PWM signals are quite easy to generate. Many modern microcontrollers include PWM hardware within the chip; using this hardware often takes very little attention from the microprocessor and it can run in the background without interfering with executing code. PWM signals are also quite easy to create directly from a comparator only requiring the carrier and the modulating signals input into the comparator.

3. Digital to Analogue Conversion – pulse width modulation can function effectively, as a digital to analogue converter, particularly combined with appropriate filtering. The fact that the duty cycle of a PWM signal can be accurately controlled by simple counting procedures is one of the reasons why PWM signals can be used to accomplish digital-to-analogue conversion.
The desired PWM technique should have the following characteristics.

  • Good utilization of DC supplies voltage possibly a high voltage gain.  
  • Linearity of voltage control.
  • Low amplitude of low order harmonic of output voltage to minimize the harmonic content of output currents.
  •  Low switching losses in inverter switches.
  •   Sufficient time allowance for proper operation of the inverter switches and control system.

There are many types of PWM techniques used in sine wave inverters. The commonly used techniques are:

Single or 2 level PWM; 

It’s the simplest way of producing the PWM signal. It’s through comparison of a low-power

reference sine wave with a triangle wave as shown in figure 3. Using these two signals as input to a
comparator the output will be a 2-level PWM signal as shown in figure 2.2.1. It is the most common
and popular technique of pulse-width-modulation (PWM).
 A Two-Level PWM

The harmonic content can be reduced significantly by using several pulses in each half- cycle of the output voltage. There exist different levels of multiphase PWM producing an improved output with increase of the level of the PWM used. The most common ones are: 3 levels PWM, 5 levels PWM, 7 levels PWM and 9 levels PWM. The choice of which PWM level to use is determined by the cost of the inverter and the quality of the output. To balance between cost and quality of the inverter, a 3level PWM is commonly used.
A Three level PWM.

Comparing the 3-level PWM to the 2-level PWM, the harmonics plot shows no higher level harmonics of significant magnitude. This represents the 3-Level signal following much more closely the desired sine wave. However, the primary frequency has a much lower voltage magnitude than that of the 2-Level design. The reason for this is the presence of other frequencies which are not harmonics of the 50Hz signal, which are caused by the switching of the signal from one polarity to the other, and back.

In electronic power converters and motors, PWM is used extensively as a means of powering alternating current (AC) devices with an available direct current (DC) source or for advanced DC/AC conversion. Variation of duty cycle in the PWM signal to provide a DC voltage across the load in a specific pattern will appear to the load as an AC signal, or can control the speed of motors that would otherwise run only at full speed or off. The pattern at which the duty cycle of a PWM signal varies can be created through simple analogue components, a digital microcontroller, or specific PWM integrated circuits.

Analogue PWM control requires the generation of both reference and carrier signals that feed into a comparator which creates output signals based on the difference between the signals. The reference signal is sinusoidal and at the frequency of the desired output signal, while the carrier signal is often either a saw tooth or triangular wave at a frequency significantly greater than the reference. When the carrier signal exceeds the reference, the comparator output signal is at one state, and when the reference is at a higher voltage, the output is at its second state. This process is shown in Figure 3 with the triangular carrier wave in black, sinusoidal reference wave in blue, and modulated and unmodulated sine pulses.

A digital microcontroller PWM requires a reference signal, sometimes called a modulating or control signal, which is a sinusoidal in this case; and a carrier signal, which is a triangular wave that controls the switching frequency. Microcontroller modules are used to compare the two to give a PWM signal.

The applications of PWM are wide variety used like ranging from measurement and communications to power control and conversion. In PWM inverter harmonics will be much higher frequencies than for a square wave, making filtering easier.

In PWM, the amplitude of the output voltage can be controlled with the modulating waveforms. Reduced filter requirements to decrease harmonics and the control of the output voltage amplitude are two distinct advantages of PWM. Disadvantages include more complex control circuits for the switches and increased losses due to more frequent switching. (Mburu, 2014)


Square, Modified and Pure Sine Wave Inverters

On the market today are two different types of power inverters, modified sine wave and pure sine wave generators. These inverters differ in their outputs, providing varying levels of efficiency and distortion that can affect electronic devices in different ways.

A modified sine wave is similar to a square wave but instead has a “stepping” look to it that relates more in shape to a sine wave. This can be seen in Figure 2.2.2 below, which displays how a modified sine wave tries to emulate the sine wave itself. The waveform is easy to produce because it is just the product of switching between 3 values at set frequencies, thereby leaving out the more complicated circuitry needed for a pure sine wave. The modified sine wave inverter provides a cheap and easy solution to powering devices that need AC power. It does have some drawbacks as not all devices work properly on a modified sine wave, products such as computers and medical equipment are not resistant to the distortion of the signal and must be run off of a pure sine wave power source. (Doucet et al, 2007)

The sine wave inverters provide electrical power similar to the utility power received. It is highly reliable and does not produce noise interference associated with other types of inverters. With its “perfect” sine wave output, the power produced by the inverter fully assures that sensitive loads will be correctly powered with no interference. Some appliances which are likely to require pure sine wave include computers, battery chargers, variable speed motors, and audio /visual equipments. (Doucet et al, 2007)

Square Modified and Pure sine wave


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