A power inverter, inverter or invertor is a power electronic device or circuitry that changes direct current (DC) to alternating current (AC). The resulting AC frequency obtained depends on the particular device employed. Inverters do the opposite of rectifiers which were originally large electromechanical devices converting AC to DC.
The input voltage, output voltage and frequency, and overall power handling depend on the design of the specific device or circuitry. The inverter does not produce any power; the power is provided by the DC source.
A power inverter can be entirely electronic or may be a combination of mechanical effects (such as a rotary apparatus) and electronic circuitry. Static inverters do not use moving parts in the conversion process.
Power inverters are primarily used in electrical power applications where high currents and voltages are present; circuits that perform the same function for electronic signals, which usually have very low currents and voltages, are called oscillators. Circuits that perform the opposite function, converting AC to DC, are called rectifiers.
In one simple inverter circuit, DC power is connected to a transformer through the center tap of the primary winding. A relay switch is rapidly switched back and forth to allow current to flow back to the DC source following two alternate paths through one end of the primary winding and then the other. The alternation of the direction of current in the primary winding of the transformer produces alternating current (AC) in the secondary circuit.
The electromechanical version of the switching device includes two stationary contacts and a spring supported moving contact. The spring holds the movable contact against one of the stationary contacts and an electromagnet pulls the movable contact to the opposite stationary contact. The current in the electromagnet is interrupted by the action of the switch so that the switch continually switches rapidly back and forth. This type of electromechanical inverter switch, called a vibrator or buzzer, was once used in vacuum tube automobile radios. A similar mechanism has been used in door bells, buzzers and tattoo machines.
As they became available with adequate power ratings, transistors and various other types of semiconductor switches have been incorporated into inverter circuit designs. Certain ratings, especially for large systems (many kilowatts) use thyristors (SCR). SCRs provide large power handling capability in a semiconductor device, and can readily be controlled over a variable firing range.
The switch in the simple inverter described above, when not coupled to an output transformer, produces a square voltage waveform due to its simple off and on nature as opposed to the sinusoidal waveform that is the usual waveform of an AC power supply. Using Fourier analysis, periodic waveforms are represented as the sum of an infinite series of sine waves. The sine wave that has the same frequency as the original waveform is called the fundamental component. The other sine waves, called harmonics, that are included in the series have frequencies that are integral multiples of the fundamental frequency.
Fourier analysis can be used to calculate the total harmonic distortion (THD). The total harmonic distortion (THD) is the square root of the sum of the squares of the harmonic voltages divided by the fundamental voltage:
There are many different power circuit topologies and control strategies used in inverter designs. Different design approaches address various issues that may be more or less important depending on the way that the inverter is intended to be used. For example, an electric motor in a car that is moving can turn into a source of energy and can, with the right inverter topology (full H-bridge) charge the car battery when decelerating or braking. In a similar manner, the right topology (full H-bridge) can invert the roles of “source” and “load”, that is, if for example the voltage is higher on the AC “load” side (by adding a solar inverter, similar to a gen-set, but solid state), energy can flow back into the DC “source” or battery.
Based on the basic H-bridge topology, there are two different fundamental control strategies called basic frequency-variable bridge converter and PWM control. Here, in the left image of H-bridge circuit, the top left switch is named as “S1”, and others are named as “S2, S3, S4” in counterclockwise order.
For the basic frequency-variable bridge converter, the switches can be operated at the same frequency as the AC in the electric grid (60 Hz in the U.S.). However, it is the rate at which the switches open and close that determines the AC frequency. When S1 and S4 are on and the other two are off, the load is provided with positive voltage and vice versa. We could control the on-off states of the switches to adjust the AC magnitude and phase. We could also control the switches to eliminate certain harmonics. This includes controlling the switches to create notches, or 0-state regions, in the output waveform or adding the outputs of two or more converters in parallel that are phase shifted in respect to one another.
Another method that can be used is PWM. Unlike the basic frequency-variable bridge converter, in the PWM controlling strategy, only two switches S3, S4 can operate at the frequency of the AC side or at any low frequency. The other two would switch much faster (typically 100 KHz) to create square voltages of the same magnitude but for different time duration, which behaves like a voltage with changing magnitude in a larger time-scale.
These two strategies create different harmonics. For the first one, through Fourier Analysis, the magnitude of harmonics would be 4/(pi*k) (k is the order of harmonics). So the majority of the harmonics energy is concentrated in the lower order harmonics. Meanwhile, for the PWM strategy, the energy of the harmonics lie in higher-frequencies because of the fast switching. Their different characteristics of harmonics leads to different THD and harmonics elimination requirements. Similar to “THD”, the conception “waveform quality” represents the level of distortion caused by harmonics. The waveform quality of AC produced directly by H-bridge mentioned above would be not as good as we want.
The issue of waveform quality can be addressed in many ways. Capacitors and inductors can be used to filter the waveform. If the design includes a transformer, filtering can be applied to the primary or the secondary side of the transformer or to both sides. Low-pass filters are applied to allow the fundamental component of the waveform to pass to the output while limiting the passage of the harmonic components. If the inverter is designed to provide power at a fixed frequency, a resonant filter can be used. For an adjustable frequency inverter, the filter must be tuned to a frequency that is above the maximum fundamental frequency.
Since most loads contain inductance, feedback rectifiers or antiparallel diodes are often connected across each semiconductor switch to provide a path for the peak inductive load current when the switch is turned off. The antiparallel diodes are somewhat similar to the freewheeling diodes used in AC/DC converter circuits.
Fourier analysis reveals that a waveform, like a square wave, that is anti-symmetrical about the 180 degree point contains only odd harmonics, the 3rd, 5th, 7th, etc. Waveforms that have steps of certain widths and heights can attenuate certain lower harmonics at the expense of amplifying higher harmonics. For example, by inserting a zero-voltage step between the positive and negative sections of the square-wave, all of the harmonics that are divisible by three (3rd and 9th, etc.) can be eliminated. That leaves only the 5th, 7th, 11th, 13th etc. The required width of the steps is one third of the period for each of the positive and negative steps and one sixth of the period for each of the zero-voltage steps.
Changing the square wave as described above is an example of pulse-width modulation. Modulating, or regulating the width of a square-wave pulse is often used as a method of regulating or adjusting an inverter’s output voltage. When voltage control is not required, a fixed pulse width can be selected to reduce or eliminate selected harmonics. Harmonic elimination techniques are generally applied to the lowest harmonics because filtering is much more practical at high frequencies, where the filter components can be much smaller and less expensive. Multiple pulse-width or carrier based PWM control schemes produce waveforms that are composed of many narrow pulses. The frequency represented by the number of narrow pulses per second is called the switching frequency or carrier frequency. These control schemes are often used in variable-frequency motor control inverters because they allow a wide range of output voltage and frequency adjustment while also improving the quality of the waveform.
Multilevel inverters provide another approach to harmonic cancellation. Multilevel inverters provide an output waveform that exhibits multiple steps at several voltage levels. For example, it is possible to produce a more sinusoidal wave by having split-rail direct current inputs at two voltages, or positive and negative inputs with a central ground. By connecting the inverter output terminals in sequence between the positive rail and ground, the positive rail and the negative rail, the ground rail and the negative rail, then both to the ground rail, a stepped waveform is generated at the inverter output. This is an example of a three level inverter: the two voltages and ground.
From the late nineteenth century through the middle of the twentieth century, DC-to-AC power conversion was accomplished using rotary converters or motor-generator sets (M-G sets). In the early twentieth century, vacuum tubes and gas-filled tubes began to be used as switches in inverter circuits. The most widely used type of tube was the thyratron.
The origins of electromechanical inverters explain the source of the term inverter. Early AC-to-DC converters used an induction or synchronous AC motor direct-connected to a generator (dynamo) so that the generator’s commutator reversed its connections at exactly the right moments to produce DC. A later development is the synchronous converter, in which the motor and generator windings are combined into one armature, with slip rings at one end and a commutator at the other and only one field frame. The result with either is AC-in, DC-out. With an M-G set, the DC can be considered to be separately generated from the AC; with a synchronous converter, in a certain sense it can be considered to be “mechanically rectified AC”. Given the right auxiliary and control equipment, an M-G set or rotary converter can be “run backwards”, converting DC to AC. Hence an inverter is an inverted converter.
Since early transistors were not available with sufficient voltage and current ratings for most inverter applications, it was the 1957 introduction of the thyristor or silicon-controlled rectifier (SCR) that initiated the transition to solid state inverter circuits.
The commutation requirements of SCRs are a key consideration in SCR circuit designs. SCRs do not turn off or commutate automatically when the gate control signal is shut off. They only turn off when the forward current is reduced to below the minimum holding current, which varies with each kind of SCR, through some external process. For SCRs connected to an AC power source, commutation occurs naturally every time the polarity of the source voltage reverses. SCRs connected to a DC power source usually require a means of forced commutation that forces the current to zero when commutation is required. The least complicated SCR circuits employ natural commutation rather than forced commutation. With the addition of forced commutation circuits, SCRs have been used in the types of inverter circuits described above.
In applications where inverters transfer power from a DC power source to an AC power source, it is possible to use AC-to-DC controlled rectifier circuits operating in the inversion mode. In the inversion mode, a controlled rectifier circuit operates as a line commutated inverter. This type of operation can be used in HVDC power transmission systems and in regenerative braking operation of motor control systems.
Another type of SCR inverter circuit is the current source input (CSI) inverter. A CSI inverter is the dual of a six-step voltage source inverter. With a current source inverter, the DC power supply is configured as a current source rather than a voltage source. The inverter SCRs are switched in a six-step sequence to direct the current to a three-phase AC load as a stepped current waveform. CSI inverter commutation methods include load commutation and parallel capacitor commutation. With both methods, the input current regulation assists the commutation. With load commutation, the load is a synchronous motor operated at a leading power factor.
As they have become available in higher voltage and current ratings, semiconductors such as transistors or IGBTs that can be turned off by means of control signals have become the preferred switching components for use in inverter circuits.
Rectifier circuits are often classified by the number of current pulses that flow to the DC side of the rectifier per cycle of AC input voltage. A single-phase half-wave rectifier is a one-pulse circuit and a single-phase full-wave rectifier is a two-pulse circuit. A three-phase half-wave rectifier is a three-pulse circuit and a three-phase full-wave rectifier is a six-pulse circuit.
With three-phase rectifiers, two or more rectifiers are sometimes connected in series or parallel to obtain higher voltage or current ratings. The rectifier inputs are supplied from special transformers that provide phase shifted outputs. This has the effect of phase multiplication. Six phases are obtained from two transformers, twelve phases from three transformers and so on. The associated rectifier circuits are 12-pulse rectifiers, 18-pulse rectifiers and so on…
When controlled rectifier circuits are operated in the inversion mode, they would be classified by pulse number also. Rectifier circuits that have a higher pulse number have reduced harmonic content in the AC input current and reduced ripple in the DC output voltage. In the inversion mode, circuits that have a higher pulse number have lower harmonic content in the AC output voltage waveform.
The large switching devices for power transmission applications installed until 1970 predominantly used mercury-arc valves. Modern inverters are usually solid state (static inverters). A modern design method features components arranged in an H bridge configuration. This design is also quite popular with smaller-scale consumer devices.
They are electronic devices that can turn DC (Direct Current) to AC (Alternating Current). It is also responsible for controlling speed and torque for electric motors.
Electric motors are found in most devices we use to do work such as small electronics, transportation, and office appliances. These motors need electricity to run. Matching the motor’s speed to the required process is essential to avoid wasting energy. In factories, wasted energy and materials could put the business at risk, and so inverters are used to control electric motors, boosting productivity and saving energy.
Fan and pump applications benefit significantly from AC drives. Superior to dampers and on/off controls, using AC drives can reduce energy consumption by 20 to 50 percent by controlling motor rotation. It is similar to reducing the speed of a car. Instead of using breaks, the speed of the car can be reduced by lightly pressing on the accelerator.
2. Soft Starters
An AC Drive starts the motor by delivering power at a low frequency. It gradually increases the frequency and motor speed until the desired speed is met. Operators can set the acceleration and deceleration at any time, which is ideal for escalators and conveyor belts to avoid dropouts of loads.
3. Controlled Starting Current
It takes seven to eight times of the AC motor’s full-load current to start the motor. The AC drive reduces the starting current, resulting in fewer motor rewinds, and this extends motor life.
4. Reduced Power Line Disturbances
Starting an AC motor across the line can place an enormous drain in the power distribution system, causing a voltage sag. Sensitive equipment such as computers and sensors will trip when a large motor starts. The AC drive eliminates this voltage sag by removing the power from the motor instead of tripping.
5. Easily Changes the Direction of Rotation
AC drives can handle frequent start and stop operations. It only needs a small current to change the direction of rotation after changing the rotation command. Stand mixers can produce the right output as the direction of rotation, and the number of revolutions can be controlled with an inverter drive
6. Simple Installation
AC drives are pre-programmed. Control power of auxiliaries, communication lines, and motor leads are already factory wired. The contractor only needs to connect the line to the power source that will supply the AC drive.
7. Adjustable Torque Limit
AC Drives can protect motors from damage by accurately controlling the torque. For example, in a machine jam, the motor will continue to rotate until the overload device opens. An AC drive can be set to limit the amount of torque applied to the motor to avoid exceeding the torque limit.
8. Elimination of Mechanical Drive Components
An AC drive can deliver low or high-speed required by the load without speed-increasing or reduction devices and gearboxes. This saves maintenance costs and floor-space requirements.
Billed as the world’s smallest 1100W power inverter, the Krieger is more than powerful enough to handle small appliances and electronics. Packed away in a heavy-duty aluminum casing that’s durable enough to last for years, the Krieger provides all of its relevant info on an LCD display, including output wattage, input voltage and battery level. It’s ideal for keeping power tools charged up or powering televisions, gaming consoles or small appliances such as a microwave (it features 2200 watts of peak power). Plugging any of those appliances into the KRIEGER is easy, thanks to the two standard AC 12V outlets on either of the dual USB charging ports that push out 2.1A, making it equally suitable for charging smartphones or tablets. It’s backed by KRIEGER’s three-year warranty.
BESTEK is one of the most highly regarded and well-known names in the power inverter space and their 300W product is a standout choice for buyers looking at compact choices (it’s about the length of a smartphone). The BESTEK includes two 110V AC outlets for charging electronics like a laptop and has dual USB charging ports for charging smartphones or tablets. The included two-foot cigarette adapter plug combines with the BESTEK’s small footprint to fit into nearly any type of vehicle, making it a great choice to pack while camping or on vacation. A built-in 40-amp fuse and the integrated cooling fan helps protect both a car and plugged in devices from overheating, overloading or overcharging. The strong metal housing ensures the BESTEK can handle a few drops and bumps.
The AMPEAK 2000-watt power inverter offers a maximum power output of 4000 surge watts. The available three AC outlets and single 2.1A USB outlet will work with everything from cell phones, digital cameras, electrical fans, freezers, floodlights, microwaves and other electronics you might find in an RV.
To protect against damage for any of your devices, the AMPEAK adds some welcome protections, including three cooling fans and an audible alarm to quickly alert you to overloads, over-voltage or overheating. Another major benefit to the AMPEAK 2000W is the opportunity to connect to a 12V battery system, allowing the AMPEAK to assist with providing additional power for your needs during events such as a hurricane or storm where power outages can be frequent.
You can never go wrong by having too much power on you while driving across town or the country. The POTEK 500W is an ideal option for buyers looking at power inverters that are properly sized for a car and keep the energy flowing to a slew of electronics and hand-held devices. The POTEK 500W and its dual 110V AC outlets and two USB ports will have no problem keeping a laptop, Kindle, iPad or multiple smartphones going with power to spare. Keeping all of these charges going simultaneously without causing any damage is the work of the built-in cooling fan that ensures no devices or the POTEK itself will get overheated or overloaded. Measuring 7.1 x 4.1 x 2.2 inches in size, the POTEK fits in most glove compartments or center consoles, which makes it a must-have for anyone who spends a good chunk of time commuting in a car.
The Renogy 2000W is a jack-of-all trades pure sine wave power inverter. It’s optimized for 12 VDC system and offers overload protection for both DC input and AC output. It has protection for under-voltage, over-voltage, over-temprature, overload, and short circut, with LED indicators for each item. There’s even high-speed ventilation fans built into the unit to keep it from overheating and there’s a ground-fault circuit interrupter to prevent short circuits. The power the Renogy 2000W produces is good enough for tools, fans, lights, and other electronics. As a final selling point, it has a built-in 5V/2.1A USB port and an AC hardwire port to cover all your needs and devices.
When it comes to keeping things powered up while on a boat, few electronics do as good a job as the Power TechON 3000 Pure Sine Wave power inverter. Equipped with dual AC inputs, one USB port and a hardware terminal, the Power TechON can reach a maximum of 6000 watts in surge power. When traveling on the ocean, safety and security are critical, so you can cruise easily knowing that it’s equipped with a multitude of different protection features to prevent overvoltage or overloaded devices. Capable of converting DC to AC power, the Power TechON comes with both black and red starter cables, as well as a wired remote that can reach up to 15 feet away. Once plugged in, the inverter will have no problem producing power to a slew of devices, including computers, laptops, smartphones and other handheld electronics whether you’re on the water or docked.
Companies and regular consumers strive to conserve energy. This has propelled the development of inverters in machineries and regular appliances. Inverters are hidden and stored in rooms with adequate ventilation. Nonetheless, they play a great role in energy saving. The ability to accurately control office devices depending on demand can significantly reduce energy consumption and production waste.
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