Inverter
Decently well documented inverter system, 2 kW, using transformers:
- Disadvantage: about 70-80% efficient; appears that non-solid state inverters are less efficient than solid state.
- Commercial, solid-state inverters are 90-95% efficient (ex., [1])
Another, smaller-power, less documented example:
http://www.i4at.org/lib2/inverter.htm
DIY Inverters
Microwave Transfomers
I found this site, where they propose to use modified transformers from old microwave ovens, you’d need about 10 broken micros’ and 20 N-type power MOSFET’s for a 9kw unit.
http://electronics-diy.com/electronic_schematic.php?id=609
Open Source 100kW Electric Vehicle Controller/Inverter
This is a senior project for electrical vehicles. Its out is 3 phase. This could be of interest for the induction furnace. The design is very simple. Not sure if it was ever implemented. It is made mostly from IC's. Maybe these could be replaced to reduce some of the costs.
http://eet.etec.wwu.edu/ahmanna/project/docs/Prodject%20description.pdf
Solar Grid-Intertie Inverter
http://www.timnolan.com/index.php?page=solar-grid-intertie-inverter
Scalable Commercial Inverter
The company MGE UPS seems to have merged with another one named APC, but the concept is good. This inverter is scalable up to 20kw. It looks like the inverters are designed to share some resources and are just connected in series.
http://rep.mgeups.com/3ppdf/s4.pdf
From wikipedia on scalable 3 phase AC: To construct inverters with higher power ratings, two six-step three-phase inverters can be connected in parallel for a higher current rating or in series for a higher voltage rating. In either case, the output waveforms are phase shifted to obtain a 12-step waveform. If additional inverters are combined, an 18-step inverter is obtained with three inverters etc. Although inverters are usually combined for the purpose of achieving increased voltage or current ratings, the quality of the waveform is improved as well. 1
A FLEXIBLE INVERTER CONTROLLER FOR PROTOTYPES