This description of the inverter is preliminary and incomplete. Although working, the inverter still has some way to go, esp. in the area of waveform synthesis. The Inverter has been tested and is now in fact in standby use awaiting the next power failure (Of course since the inverter is ready there won't be one ... :)
The project presented is a 2kVA power inverter designed for backup during utility failures. The inverter is affectionately called the 'I2K' for two reasons: It has a 2kVA output capacity and some have suggested it might have been useful when Y2K hits (remember the Y2K paranoia/fiasco :) Duh.
This inverter is of a traditional Ferro resonant design using large step-up transformers however thepulse-width modulated signals which feed the transformers are generated in real-time by a PIC RISC microcontroller. The microcontroller senses AC output voltage via an 8-bit ADC then adjusts the pulse width accordingly using a closed control loop algorithm. The functions of waveform timing, pulse width adjustment, and error control (shutdown when overload occurs) are all done in software makingupgrades easy to accomplish.
Many commercial inverters tested, including those used for PC UPS usages, will not provide the enormous start-up currents required by many motors -- especially single phase motors under load such as those found in the average natural gas furnace. This design provides those required currents and, during one test, was observed to convert over 2kVA of power (The inputcurrent exceeded 200A at 12VDC for several seconds until the motor started). Software control of the inverter allows the inverter to be overloaded for a period of time before shutdown occurs giving the motor time to start.
The inverter has been tested on 1/3hp single phase motor loads as well as with large resistive loads of over 1500W and has performed well. Power factor correction was found to berequired with large motor loads and has been included in the design.
Efficiency of the inverter (Power Output / Power Input) driving a variety of both resistive and inductive loads ranges between 71% and 82%. With an improved sinewave algorithm it is felt that the inverter can approach the theoretical efficiency limit of 92%.
Disclaimer and Warning
The design presented employs highcurrents on the primary side and high voltages on the secondary side. High currents of over 1000A were observed during circuit fault and despite the fact this design has safety systems included to deal with such problems, high currents can cause spectacular -- and dangerous -- effects such as component explosion. The output of the inverter is 120VAC and must be treated with the same respect as anyother AC source. In addition, connection of the inverter to any device must be in accordance with local/national electrical rules (in Canada, the Canadian Electrical Code) which specifies grounding, neutral bonding, and other isuues. Do not attempt construction of a device such as this unless entirely familiar with high-power electronics as well as proper safety procedures. The author specificallydisclaims any and all liabilities associated with the construction and use of such devices. The design is presented here in the interests of providing information on operational principles only.
DANGEROUS HIGH VOLTAGES AND HIGH CURRENTS ARE PRESENT IN A PROJECT OF THIS TYPE ... DO NOT ATTEMPT CONSTRUCTION UNLESS SKILLED IN SUCH AREAS
Power Drivers and Electronics
The design is of a standardferroresonant type using large step-up transformers to convert 12 V into 120 V. 12 VDC from large, high capacity batteries is chopped into alternating square waves which are fed to opposite windings of a transformer. The batteries can be seen in the photo to the left. I use a bank of twelve cells, each rated at 2V, 300Ah. The cells were surplus and were originally used to backup a large...