How is the typical modern "low frequency" pure sine wave inverter built? - Page 1
Forum section: electronics / general

author: chimerahitman
author_rank: Contributor
timestamp: September 28, 2020, 06:47:51 pm
content: Hello,I have been searching online about the construction method of a low frequency pure sine inverter and I have had no luck. Most search results come back for high frequency puresine inverters.I have seen some internal pictures of the AIMS low freq with two transformers but they are not high detail enough to see how they wire them up.The only document I was able to find was one redacted by Trace and how they use stacked transformers to recreate a better than modified square wave, or what they call a medium frequency conversion and using the xfmr as a low pass filter.Are there any other techniques being used? Any source of information in how they achieve low frequency xfmr sinewave outputs would be appreciated.Thank you,

author: David Hess
author_rank: Super Contributor
timestamp: September 29, 2020, 08:01:06 pm
content: There are not all that many options. An output transformer may or may not be used, and the active stage can be linear or switching. The common solution today is a class-D amplifier using PWM to drive an LC output filter because of its much higher efficiency than a linear output stage and its better power density than anything which includes a low frequency transformer.

author: fourtytwo42
author_rank: Super Contributor
timestamp: September 29, 2020, 08:39:36 pm
content: A brave person opening a can of worms!! DIY sites generally do not like people discussing such things but here is a link to one who did and was sub sequentially bannedhttps://www.electro-tech-online.com/threads/grid-tie-inverter-schematic-2-0.105324/Another source is the oz-inverter described in an Australian website something to do with sheds that I have now unfortunately forgotten (I seem to remember they surpass 6Kw).I am a strictly high frequency man myself but am unable to discuss my workEdit :- Found ithttps://www.thebackshed.comThere are bigger than this to be found

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 30, 2020, 01:27:51 pm
content: The mains transformer coupled inverters are largely obsolete, but for educational purposes, they can make for an inverter design that's far safer to tinker with. It could also be a way to work around UL requirements for a DIY grid tie inverter, if you use an enclosed transformer that's already approved (e.g. an outdoor lighting transformer, although most of the modern ones are switchers) and keep voltages on the low voltage side below 50V DC and 30V AC.

author: mag_therm
author_rank: Frequent Contributor
timestamp: September 30, 2020, 05:48:11 pm
content: Years ago I worked on single and three phase inverters that ran in parallel with the utility mains, powered by 130 V battery banks.They were fundamental frequency pwm scr type.To get the sinewave, there was an output LC tank rated at about 5 times the kW rating.In those days, specs for ups inverters often specified the ability to clear fuses and circuit breaker on faulted branch circuits.The tank provided that short time energy, allowing the inverter to run through faults.The difficulty was the large AC inductor with its cost, size, and a number of problems due to the spill-out of magnetic field.That fundamental topology might still be feasible in small ratings up to a few kVA.I have been thinking of building a model one in retirement hobby!

author: chimerahitman
author_rank: Contributor
timestamp: September 30, 2020, 06:42:20 pm
content: Thank you for the replies.Quote from: NiHaoMike on September 30, 2020, 01:27:51 pmIn my country (Dom. Rep.), XFMR coupled inverters are the most used topology with modified square wave outputs for backup power in case of and outage, which is pretty often. They are easier to fix for the tech since most of the time its a generic control board and lots of FETs.Newer pure sine wave models are coming out, but are quite expensive. Last time I checked it was about 300 USD per output 1kw.Quote from: fourtytwo42 on September 29, 2020, 08:39:36 pmThank you for the links, I need some time to digest this. Although I am not interested in grid tie, I can probably get good information from this.Quote from: David Hess on September 29, 2020, 08:01:06 pmI thought about this but then I came about conflicting information about how the high frequency switched inverters are not able to supply enough surge power for motors/inductive loads. Indeed I don't have a workshop, but I do run a well pump, an occasional compressor, 2 fridges, 2 conventional AC and 1 inverter tech AC.So far, with the inverter/charger I run lights, ceiling fans, and other odds and ends (pc, router, tv, etc) all accompanied by the buzz of the modified square wave (MSW). I have come to have a hand me down 8kW XFMR that is enough to run most if not all loads since I have a 7kW backup gen that deals with longer than half day outages.XFMRs are not cheap, but are much cheaper than getting a beefy 8kW pure sine wave inverter and was looking into making a Trace SW series 'copy'.