What is a Power Inverter?
An inverter is a device that performs the power
transformation from direct current voltage
(VDC) to alternate current voltage (VAC). Normally, the DC
power comes from a battery or bank of batteries which
is used as the inverter input. The inverter delivers AC
power in its output similar to the AC outlets in a house.
Generally a house AC outlet is connected to the electrical
power grid which carries a sine wave of 120 Vrms
(root mean square volts)/60 Hz what is just called 120 VAC.
Note: 120 VAC generates on a resistive load an average power equal to that a 120 VDC would generate.
What the inverter does is to deliver AC power with close characteristics to the electrical grid.
The cheapest inverters do not offer a sine wave but a quasi sine wave with positive and negative flat tops of around +/- 145V. They are called "modified sine wave inverters." That wave is generated by switching 145 VDC at 60 Hz.
That can be done using four synchronized switches arranged as a full bridge.
The positive flat top is obtained when SW1 and SW2 are ON, and SW3 and SW4 are OFF. The negative flat top is obtained when SW1 and SW2 are OFF, and SW3 and SW4 are ON.
The flat tops do not occupy the whole period of the 60 Hz wave. For some time the modified sine wave has a value of zero. So the duty cycle (ratio between both flat tops duration and the period) is less than one. The combination of 145V positive and negative flat tops and duty cycle produces on a resistive load an average power equivalent to 120 Vrms sine wave would produce. That does not hold for partial reactive loads. Those modified sine wave inverters work fine with loads like toasters or incandescent lights. They are not so good for TV's, microwaves, and electrical motors. In contrast, the most sophisticated inverters deliver 120 Vrms sine wave. They are called "true sine wave inverters." These inverters work fine with any kind of loads but they are more expensive.
The sine wave is generated by a train of pulses. The pulses have all the same amplitude (around 170 V like the peak of the 120 Vrms sine) and a width proportional to the instantaneous sine amplitude. That is called "pulse width modulation" (PWM). The number of pulses per cycle depends on how smooth the sine wave is wanted. By filtering the pulses, the result is a 120 Vrms/60 Hz sine.
Both types of inverters have a converter (DC to DC conversion) as a first stage:
- In the modified sine wave inverter the conversion is from battery DC to around 145 VDC
- In the sinusoidal inverter the conversion is from battery DC to around 170 VDC.
Both converters comprise a high frequency input inverter, a step-up transformer, a rectifier, and a filter.
Note:
To change the DC voltage from battery level using a
transformer, it is necessary to switch the
battery voltage using an input inverter. That
switched voltage is applied to the transformer
primary (transformers do not work with DC). At the
transformer secondary it is going to be a
switched voltage of high amplitude that is converted to
DC high voltage using a rectifier
(diodes) and a filter.
The purpose of using high frequency in the converter is to reduce the transformer size.