A power inverter, or inverter, is an electronic device or circuitry that changes direct current (DC) to alternating current (AC).
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.
What is an Inverter/Charger?
Many systems incorporate an inverter/charger, which is a combination of an inverter, battery charger and transfer switch in one. The inverter portion converts DC power from an energy source into AC Power. The battery charger processes incoming AC power into DC power and recharges batteries using a multi-stage process, which helps assure maximum battery life. Some models are also able to automate supplementary power production with automatic generator start and stop capabilities
Battery Backup System Overview
A battery backup system for a sump pump consists of two main components: an inverter/charger and one or more batteries. The inverter is responsible for converting the power stored in the batteries into a form that can be used by your sump pump. It is also responsible for keeping the batteries fully charged at all times. You plug the inverter/charger into your wall outlet and then plug your sump pump into the inverter/charger, like this:
Note the red arrows showing the flow of electricity. During normal operation, the inverter/charger just passes the electricity coming from your wall outlet straight through to the sump pump as though the sump pump was plugged directly into the outlet. While it is doing that, it will also automatically charge the batteries and keep them fully charged as long as it continues to receive power from the wall outlet.
When your power goes out, the system will automatically send power from the batteries to the sump pump, like this:
Inverter/chargers vs. UPS systems
The inverter/charger is the heart of the system, and is responsible for three jobs:
- Charging the batteries and keeping them fully charged at all times
- Sensing when the power has gone out and automatically switching to battery power
- Converting the power stored in your batteries from “direct current” (DC) into “alternating current” (AC), which runs your sump pump
This description of an inverter/charger sounds an awful lot like the description of an uninterruptible power supply (UPS), which might lead you to wonder, “Why can’t I just install a UPS and be done with it?” Well, it basically boils down to the fact that a UPS is just not designed to do the job.
Although inverter/chargers and UPS systems work in very similar ways and perform similar functions, the battery in a UPS is not large enough to run a sump pump for any significant length of time. Most UPS systems are not designed to provide enough power to start a sump pump. In addition, UPS systems have very small battery chargers.
An inverter/charger also allows you much more flexibility to decide how much power you need. You can think of it like buying stereo equipment: you can buy the “all in one” model, which is inexpensive and easy to assemble but offers less powerful sound (the UPS), or you can buy individual modular components, which gives you increased flexibility and more powerful sound (the inverter/charger). And if your needs change in the future, you can expand the system without having to start over again.
Input and output
A typical power inverter device or circuit requires a relatively stable DC power source capable of supplying enough current for the intended power demands of the system. The input voltage depends on the design and purpose of the inverter. Examples include:
- 12 VDC, for smaller consumer and commercial inverters that typically run from a rechargeable 12 V lead acid battery.
- 24 and 48 VDC, which are common standards for home energy systems.
- 200 to 400 VDC, when power is from photovoltaic solar panels.
- 300 to 450 VDC, when power is from electric vehicle battery packs in vehicle-to-grid systems.
- Hundreds of thousands of volts, where the inverter is part of a High voltage direct current power transmission system.
An inverter can produce a square wave, modified sine wave, pulsed sine wave, pulse width modulated wave (PWM) or sine wave depending on circuit design. The two dominant commercialized waveform types of inverters as of 2007 are modified sine wave and sine wave.
There are two basic designs for producing household plug-in voltage from a lower-voltage DC source, the first of which uses a switching boost converter to produce a higher-voltage DC and then converts to AC. The second method converts DC to AC at battery level and uses a line-frequency transformer to create the output voltage.
This is one of the simplest waveforms an inverter design can produce and is useful for some applications.
A power inverter device which produces a multiple step sinusoidal AC waveform is referred to as a sine wave inverter. To more clearly distinguish the inverters with outputs of much less distortion than the “modified sine wave” (three step) inverter designs, the manufacturers often use the phrase pure sine wave inverter. Almost all consumer grade inverters that are sold as a “pure sine wave inverter” do not produce a smooth sine wave output at all, just a less choppy output than the square wave (one step) and modified sine wave (three step) inverters. In this sense, the phrases “Pure sine wave” or “sine wave inverter” are misleading to the consumer. However, this is not critical for most electronics as they deal with the output quite well.
Where power inverter devices substitute for standard line power, a sine wave output is desirable because many electrical products are engineered to work best with a sine wave AC power source. The standard electric utility power attempts to provide a power source that is a good approximation of a sine wave.
Sine wave inverters with more than three steps in the wave output are more complex and have significantly higher cost than a modified sine wave, with only three steps, or square wave (one step) types of the same power handling. Switch-mode power supply (SMPS) devices, such as personal computers or DVD players, function on quality modified sine wave power. AC motors directly operated on non-sinusoidal power may produce extra heat, may have different speed-torque characteristics, or may produce more audible noise than when running on sinusoidal power.
Modified sine wave]
A “modified sine wave” inverter has a non-square waveform that is a useful rough approximation of a sine wave for power translation purposes.
The waveform in commercially available modified-sine-wave inverters is a square wave with a pause before the polarity reversal, which only needs to cycle back and forth through a three-position switch that outputs forward, off, and reverse output at the pre-determined frequency. Switching states are developed for positive, negative and zero voltages as per the patterns given in the switching Table 2. The peak voltage to RMS voltage do not maintain the same relationship as for a sine wave. The DC bus voltage may be actively regulated or the “on” and “off” times can be modified to maintain the same RMS value output up to the DC bus voltage to compensate for DC bus voltage variation.
The ratio of on to off time can be adjusted to vary the RMS voltage while maintaining a constant frequency with a technique called PWM. The generated gate pulses are given to each switch in accordance with the developed pattern and thus the output is obtained. Harmonic spectrum in the output depends on the width of the pulses and the modulation frequency. When operating induction motors, voltage harmonics are not of great concern; however, harmonic distortion in the current waveform introduces additional heating and can produce pulsating torques.
Numerous electric equipment will operate quite well on modified sine wave power inverter devices, especially any load that is resistive in nature such as a traditional incandescent light bulb.
Most AC motors will run on MSW inverters with an efficiency reduction of about 20% due to the harmonic content. However, they may be quite noisy. A series LC filter tuned to the fundamental frequency may help.
By definition there is no restriction on the type of AC waveform an inverter might produce that would find use in a specific or special application.
The AC output frequency of a power inverter device is usually the same as standard power line frequency, 50 or 60 hertz
If the output of the device or circuit is to be further conditioned (for example stepped up) then the frequency may be much higher for good transformer efficiency.
The AC output voltage of a power inverter device is often the same as the standard power line voltage, such as household 120 VAC or 240 VAC. This allows the inverter to power numerous types of equipment designed to operate off the standard line power.
The designed-for output voltage is often provided as a regulated output. That is, changes in the load the inverter is driving will not result in an output voltage change from the inverter.
In a sophisticated inverter, the output voltage may be selectable or even continuously variable.
A power inverter will often have an overall power rating expressed in watts or kilowatts. This describes the power that will be available to the device the inverter is driving and, indirectly, the power that will be needed from the DC source. Smaller popular consumer and commercial devices designed to mimic line power typically range from 150 to 3000 watts.
Not all inverter applications are primarily concerned with brute power delivery; in some cases the frequency and or waveform properties are used by the follow-on circuit or device.
The runtime of an inverter is dependent on the battery power and the amount of power being drawn from the inverter at a given time. As the amount of equipment using the inverter increases, the runtime will decrease. In order to prolong the runtime of an inverter, additional batteries can be added to the inverter.
When attempting to add more batteries to an inverter, there are two basic options for installation: Series Configuration and Parallel Configuration.
An inverter converts DC power from an external power source into AC power.