An inverter is a system which converts
Direct Current (DC) into AlternatingCurrent (AC) for use in AC devices.
When an electrical appliance that isdesigned to operate solely from
alternating current at 60 Hz is to be usedwith a DC storage system
(i.e., deep cycle batteries,) an inverter is necessary.The inverter
takes the DC output from the battery storage system and througha
conversion process changes the waveform to alternating current at
thedesired voltage and frequency.
The direct current (DC) from the batteries may be used for many generalpurposes, however, some devices require AC power for correct and safe operation.Depending on the overall requirements and uses of the wind system, either,none, some or all of the power is inverted to AC Since all inverters useup some power to operate themselves, only those appliances which operatesolely on AC should operate through the inverter. (Note: many devices operateon either AC or DC)
There are three basic categories of inverters available and each differswidely in cost and efficiency ratings. The three categories are: Rotary,Vibratory, and Solid State.
This inverter system utilizes a DC motor to operate a governed AC generatorto produce the desired AC power. The system generally exhibits excellentwaveform output and voltage, but its efficiency may run as low as 50%, usingconsiderable power to operate the inverter. Furthermore, since the rotaryinverter is a rotating device with brushes, it requires regular maintenanceand repair.
These inverters are also electromechanical devices but contain less movingparts than the rotary inverter. The vibrator assembly is driven by a DCpower source which commutates the waveform through a transformer-filternetwork into an AC signal at constant frequency. Efficiencies run up toapproximately 75%. These inverters generally require little maintenance.The vibrator assembly is replaced once every 1,000 to 1,500 hours. Vibrator-typeinverters are a good choice for low power applications and are relativelyinexpensive, compared to the solid state inverters.
Solid State Inverters
Solid state inverters are generally either constant frequency output invertersor synchronous inverters. Constant frequency output inverters provide anominal 120V 60 hertz signal when used independently of the utility grid.A synchronous inverter matches the voltage level, frequency and power outputto the utility lines to provide AC output. A synchronous inverter allowsthe wind plant to interface with the utility line to sell excess electricityback to the utility company. Because it is interfaced with the utility grid,there is no need for a storage system.
There are two general types of solid state inverters, one of which producesa square (or semi-square) wave output, and the other a sine wave. The square(or semi-square) wave units are less expensive and in many applicationswork very well. However, they should not be used to power devices wherea good quality sine wave is required (stereos, television, etc.)
Modern, high quality sine wave units are generally more expensive but willbe very sensitive to sine wave characteristics where a high quality AC signalis essential to performance of the load.
Though the true-sine-wave, solid state inverter is generally more expensivethan the other two types, it has the highest efficiency rating (up to 90%)when operated at rated capacity, and is virtually maintenance-free.