One of the key components in a solar electric system is the DC to AC inverter, a device that changes the direct current from the solar panels or batteries into the same alternating current electricity that is available from your wall outlet.
Pure Sine Power Inverter by Xantrex
inverters are rated in terms of how much power they can handle. Small inverters may supply from 175 to 300 Watts, enough to run a couple of CFL lamps and portable CD player or radio. Large models can convert several kilowatts, enough to run an entire home.Low power inverters are often fitted with a 12V accessory, or “cigarette lighter”, plug while inverters rated at more than 300W typically have screw terminals for attaching connector cables directly to the battery. Smaller inverters usually feature one or two, 3-prong style AC receptacles. High power inverters are wired directly into the electrical distribution system.
Inverters will often list two capacity numbers, continuous and surge. Continuous capacity is the power the unit can deliver for a prolonged duration. The surge capacity can only be supplied for a short time. Electric motors, for example, require more energy to get started then a lesser amount to continue running. This type of usage justifies knowing the surge capacity of the inverter.
A special kind of DC to AC inverter is used to feed energy back onto the power grid. These “grid tie” inverters synchronize with the grid and are designed to go offline automatically in the event that the grid fails, this is a requirement to protect utility workers from electrical shock.
The shape of the AC waveform from most inexpensive invterters is, rather generously, referred to as a “modified sine wave”, it’s really closer to a square wave. The higher end models generate a pure sine wave shape. The power quality of pure sine wave inverters is as good or better than that of grid delivered electricity in terms of distortion and tolerances.
Actual Modified Sine Waveform Viewed On Oscilloscope
Most appliances and devices will run fine with the modified sine wave power, however, they will tend to run warmer and with lower efficiency. Poorly designed, cheap consumer electronics suffer from the lower power quality of modified sine inverters. The modified sine wave power is rich in harmonic energy which can cause interference if device’s power supply doesn’t provide adequate filtering. Also, some devices derive timing information from the AC waveform by detecting when the voltage swings from positive to negative or vice-versa, this is called zero-crossing.
A modified sine wave spends a fairly long time at zero volts, making it hard to detect zero crossing for timing. Pure sine inverters are almost always a better choice, but they are more expensive than modified sine wave units.
Pure Sine Inverter Waveform Viewed on an Oscilloscpe
A useful feature commonly found on inverters is the low voltage disconnect (LVD). The LVD shuts down the inverter when the supply voltage drops below a set value, usually 10.5 or 11 volts for a 12V inverter. The inverter can only make AC when the input voltage is within the designed range. The LVD also protects the battery from damage due to excessive discharge.