The circuitry of an inverter is designed to convert direct current (DC) into alternating current (AC) when the grid AC supply is not available. The source of DC is a bank of batteries that can supply a specified amount of power (Amp Hours) from DC to AC.
Power inverters are fitted with a rectifier circuit that can convert AC from the grid power to DC at the required voltage and current strength to charge the battery bank. The rectifier circuit uses the same cable connections to the battery used by the inverter circuit to charge the batteries.
Inverters play the role of both supplying AC from DC when the AC power supply is down and converting AC to DC to recharge the batteries when the AC power supply is available. The inverter thus comprises dedicated circuits for:
- DC to AC inversion
- AC to DC rectification
- Battery charging and current draw from common cables
- Managing battery state of charge during discharge and charging cycles
Incorporating the AC to DC rectification into the inverter assembly eliminates the need for a separate rectifier with its cables from the AC power supply and the battery bank.
Let’s look at how an inverter is designed to achieve both functions of supplying AC and DC under different conditions.
A UPS Is A Combination Of An Inverter And Rectifier
Uninterrupted Power Supplies (UPS) systems are commonly used to ensure that the AC power supply to critical electrical devices is never disturbed. A UPS Inverter consists of four main components, namely:
- An inverter circuit
- A rectifier circuit
- A battery bank
- A static bypass switch
The inverter circuit performs the device’s primary function to convert DC from the battery bank into a pure sine-wave AC.
The AC from the power grid often contains voltage spikes and instability that can harm the AC devices powered by it. With a UPS Inverter, the quality of the output AC is a pure sine-wave and free of any fluctuations.
The UPS Inverter can only function when the battery bank is sufficiently charged to provide DC to be converted to AC. The rectifier circuit is connected to the grid AC and converts this to DC of the correct voltage and current to charge the battery bank.
The size of the UPS Inverter will determine whether the rectifier and battery charger are two separate systems or whether they are combined into one. The rectifier circuit can cope with voltage fluctuations and thus protect the battery from damage.
The rectifier and battery can be seen as a protective buffer between the grid-supplied AC and the AC-powered devices in your home or office.
The rectifier is robust enough to absorb the voltage fluctuations from the power grid and convert this to DC of the optimal voltage to charge the battery bank.
The UPS Inverter circuit, in turn, converts the stable DC from the battery bank to a constant AC output with minimal Total Harmonic Distortion (THD) for the AC-powered appliances to use. The purified sine-wave AC allows the powered devices to operate optimally with minimal heat build-up and extended life cycles.
The batteries in the UPS battery bank are modular units that can be added to provide DC power to the inverter for extended periods. The battery bank can be configured only to provide backup power when the power grid fails or provide a continuous supply of DC to the inverter to provide consistent AC with minimal THD.
The UPS Rectifier circuit converts grid AC to DC at the optimal charge voltage for the batteries to charge. The rectifier will absorb all the surges and fluctuations in the grid AC and generate a smooth output DC.
A Static Bypass Switch is built into the UPS Inverter to protect the system in the event of a fault. The static bypass switch can disconnect the UPS if there is an internal fault and connect the AC grid supply to bypass the rectifier, battery bank, and inverter.
It is not ideal for the static bypass switch to activate as it allows for unfiltered AC power to be fed directly to the AC-powered devices. Still, it will enable the devices to continue functioning when the UPS Inverter is disabled.
An Inverter Is Designed To Provide DC to AC and AC to DC
The primary function of an inverter is to convert direct current supplied from a battery bank of solar panel to alternating current of 120V and 60Hz required by the AC-powered appliances and devices in the US. (230V and 50Hz elsewhere).
Using the connections to the AC grid and the DC battery bank, the inverter has a secondary circuit dedicated to rectifying the AC grid power to recharge the battery bank with DC.
The inverter acts as a filter that cleans up the spikes and surges in the AC grid power supply as these can be harmful to electrical devices and cause them to burn out or significantly shorten their lifespan.
The rectifier circuit converts “dirty” AC to pure DC that can be adjusted to the optimal charge voltage required by the batteries in the battery bank. The rectifier will also monitor the battery state of charge and manage the charging process to prevent overcharging and damage to the batteries.
The inverter circuit draws DC from the optimally charged battery bank and inverts and purifies the output current to a pure sine-wave AC. The inverter comprises several Insulated Gate Bipolar Transistors (IGBTs) that switch the DC polarity at a frequency of 60 Hz creating pulse waves with different pulse widths.
The process is called Pulse Wave Modulation (PWM) and helps turn the stable DC into a pure sine-wave AC. The grid-supplied AC contains a lot of spikes and potential interruptions and typically has a THD of 5%, whereas the AC output from a pure sine-wave inverter is 3% or less.
The Inverter/Rectifier/Battery Bank/Static Bypass system is often referred to as an inverter even though it has a much broader function than converting DC to pure AC. In actual fact, it is a filter between unstable grid AC with a high degree of fluctuations and interruptions into a reliable supply of pure and stable AC.