In one simple inverter circuit, DC power is connected to a transformer through the centre 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. A similar mechanism has been used in door bells.
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
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
There are many different power circuit 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.
Many inverters are provided with a power switch, and must be turned on before they supply AC power. However some models are provided with ‘auto turn-on, so they stop working when the AC load is removed, but turn on again automatically when a load is connected. This allows the power switch of an appliance or tool to be used to control the inverter’s operation as well, conserving battery energy while still allowing the appliance to be operated in exactly the same way as when it is connected to the mains.
As negative sequence harmonics is most dangerous one, therefore measures should be taken to reduce them.
Actually there is a different kind of problem with many kinds of fluorescent light assembly: not so much inductive loading, but capacitive loading. Although a standard fluoro light assembly represents a very inductive load due to its ballast choke, most are designed to be operated from standard AC mains power. As a result they’are often provided with a shunt capacitor designed to correct their power factor when they are connected to the mains and driven with a 50Hz sinewave. The problem is that when these lights are connected to a DC-AC inverter with its ‘modified sinewave’ output, rich in harmonics, the shunt capacitor doesn’t just correct the power factor, but drastically reduces the stability, because its impedance is much lower at the harmonic frequencies , since frequency increases when harmonics are present and thus capacitance decreases which further decreases impedence. As a result, the fluoro assembly draws a heavily capacitive load current, and can easily overload the inverter.
The most common type of pure sinewave inverter operates by first converting the low voltage DC into high voltage DC, using a high frequency DC-DC converter. It then uses a high frequency PWM system to convert the high voltage DC into chopped’ AC, which is passed through an L-C lowpass filter to produce the final clean 50Hz sinewave output.