The
input voltage, output voltage and frequency, and overall
power handling, are dependent on the design of
the specific device or circuitry.
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.
Typical
applications for power inverters include:
- Portable consumer devices that allow the user to connect a battery, or set of batteries, to the device to produce AC power to run various electrical items such as lights, televisions, kitchen appliances, and power tools.
- Use in power generation systems such as electric utility companies or solar generating systems to convert DC power to AC power.
- Use within any larger electronic system where an engineering need exists for deriving an AC source from a DC source.
Input and output
Input voltage
A
typical power inverter device or circuit will require a relatively stable DC
power source capable of supplying enough current for the intended overall
power handling of the inverter. Possible DC power sources include: rechargeable
batteries, DC power supplies operating off of the power company line, and solar
cells. The inverter does not produce any power, the power is provided by the DC
source. The inverter translates the form of the power from direct current to an
alternating current waveform.
The
level of the needed input voltage depends entirely on the design and purpose of
the inverter. In many smaller consumer and commercial inverters a 12V DC input
is popular because of the wide availability of powerful rechargeable 12V lead
acid batteries which can be used as the DC power source.
An
inverter can produce square wave, modified sine wave, pulsed sine wave, 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.
A
power inverter device which produces a smooth sinusoidal AC waveform is
referred to as a sine wave inverter. To more clearly distinguish from
"modified sine wave" or other creative terminology, the phrase pure
sine wave inverter is sometimes used.
In
situations involving power inverter devices which substitute for standard line
power, a sine wave output is extremely desirable because the vast majority of
electric plug in products and appliances are engineered to work well with the
standard electric utility power which is a true sine wave.
At
present, sine wave inverters tend to be more complex and have significantly
higher cost than a modified sine wave type of the same power handling.
Modified sine wave
The
terminology "modified sine wave" has come into use and refers to an
output 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 transition, which only needs to cycle
through a three-position switch that outputs forward, off, and reverse output
at the pre-determined frequency. 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.
Harmonic spectrum in the output depends on the width of the pulses and the
modulation frequency. When operating induction motors, voltage harmonics is 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.
Other waveforms
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.
Output frequency
The
AC output frequency of a power inverter device is often the same as the
standard power line frequency, for example 60 or 50 cycles per second.
If
the output of the device or circuit is to be further conditioned (say stepped
up by a follow on transformer) then the frequency may be much higher for good
transformer efficiency.
Output voltage
The
AC output voltage of a power inverter device is often the same as the standard
power line voltage, such as household 120VAC or 240VAC. 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 output voltage
change from the inverter.
In
a sophisticated inverter, the output voltage may be selectable or even
continuously variable.
Output power
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.
Applications
DC power source utilization
Inverter
designed to provide 115 VAC from the 12 VDC source provided in an automobile.
The unit shown provides up to 1.2 amperes of alternating current, or enough to
power two sixty watt light bulbs.
An
inverter converts the DC electricity from sources such as batteries or fuel cells to AC electricity. The electricity can
be at any required voltage; in particular it can operate AC equipment designed
for mains operation, or rectified to produce DC at any desired voltage.
Uninterruptible power supplies
An
uninterruptible
power supply (UPS) uses batteries and an inverter to supply AC power
when main power is not available. When main power is restored, a rectifier supplies DC power to recharge the
batteries.
Electric motor speed control
Inverter
circuits designed to produce a variable output voltage range are often used
within motor speed controllers. The DC power for the inverter section can be
derived from a normal AC wall outlet or some other source. Control and feedback
circuitry is used to adjust the final output of the inverter section which will
ultimately determine the speed of the motor operating under its mechanical
load. Motor speed control needs are numerous and include things like:
industrial motor driven equipment, electric vehicles, rail transport systems,
and power tools. (See related: variable-frequency
drive)
Power grid
Grid-tied
inverters are designed to feed into the electric power distribution system.
They transfer synchronously with the line and have as little harmonic content
as possible. They also need a means of detecting the presence of utility power
for safety reasons, so as not to continue to dangerously feed power to the grid
during a power outage.
Solar
A
solar inverter can be fed into a commercial electrical grid or used by an
off-grid electrical network. Solar inverters have special functions adapted for
use with photovoltaic arrays,
including maximum power point tracking and anti-islanding protection. Micro-inverters convert direct current from
individual solar panels into alternating current for the electric grid. They
are grid tie designs by default.
Induction heating
Inverters
convert low frequency main AC power to higher frequency for use in induction heating. To do this, AC power is first rectified to provide DC power. The inverter then
changes the DC power to high frequency AC power.
Basic
designs
In
one simple inverter circuit, DC power is connected to a transformer through the center tap of the primary
winding. A 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.
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.
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.
Early inverters
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.
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