Modding a cheap 240v/60hz to 115v/400hz - Page 1
Forum section: electronics / projects

author: georges shed
author_rank: Contributor
timestamp: June 08, 2020, 07:52:12 pm
content: Hi,Has anyone had experience with modding a cheap inverter to be 115v 400hz? Is it at least feasible? It’s to power aircraft aviaonics.

author: Mr. Scram
author_rank: Super Contributor
timestamp: June 08, 2020, 07:56:52 pm
content: Quote from: georges shed on June 08, 2020, 07:52:12 pmI can only assume you've considered this, but I wouldn't do any sketchy "home improvement" on avionics.

author: georges shed
author_rank: Contributor
timestamp: June 08, 2020, 07:59:54 pm
content: I have a few instruments such as a HSI. Unfortunately I don’t have £1000’s to spend on a proper inverter, so a diy solution is my only option.

author: langwadt
author_rank: Super Contributor
timestamp: June 08, 2020, 08:44:34 pm
content: how much power do you need?

author: georges shed
author_rank: Contributor
timestamp: June 08, 2020, 08:56:20 pm
content: Not much as this stage. No more than a couple of amps ...

author: georges shed
author_rank: Contributor
timestamp: June 08, 2020, 08:57:16 pm
content: The other option is using a class d audio amp and a transformer but I thought I’d try nodding an existing inverter first.

author: bdunham7
author_rank: Super Contributor
timestamp: June 08, 2020, 09:28:34 pm
content: Where are you getting this 240VAC 60Hz?I'm going to presume this is for bench testing, not installation into an aircraft.You can try a low-frequency sine inverter (the kind with a large transformer) or your amplifier with transformer, but for the power levels you are looking for, your efficiency and load regulation will be terrible unless you can get an actual 400Hz transformer--and those are going to not be priced to your liking. Some transformers are designed to work on 50-400 Hz, but those are typically low power.Repurposing a HF switching UPS inverter might seem a better solution, but I don't know if you can tolerate the noise and distortion that these typically have.One solution that might work pretty well is to use a bank of individual audio amplifiers, each running on their own isolating transformer, in series. 4 of them, putting out 30VAC RMS (which would be a fairly powerful amp) would do it. You would need to isolate the inputs of the amps, but 4 small audio transformers or linear optoisolaters would do that for you.Are you sure you can't just buy something? How cheaply are you trying to get by?

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 08, 2020, 10:12:30 pm
content: See if you can find a VFD. It can be programmed to output 115V at 400Hz and all you need to add is an output filter.Quote from: Mr. Scram on June 08, 2020, 07:56:52 pmMost likely he's building a flight simulator with authentic instruments. Not all gamers enjoy the "glass cockpit" look and feel.

author: trobbins
author_rank: Frequent Contributor
timestamp: June 08, 2020, 10:52:24 pm
content: There may be cheap solid state PA amplifiers around - the most common higher powered ones (eg. 120W TOA, with 180W max rating) often have a dedicated output transformer with 100V speaker distribution feed with negative feedback for pretty low distortion, and a wide frequency range (typically down to 100Hz at -3dB).If you have multiple devices to power, then each device may be under 120W. You may also find that the 120W rating can be upgraded with some awareness - the TOA schematics for example are usually available, and the output transformer should handle a noticeably higher power level at 400Hz.But it may also depend on what each device does with its 115VAC 400Hz input.If it internally uses an isolation transformer and typical rectifier/filter for DC rails, and you can internally access the device, then you may want to look at the DC rails and see if external DC feeds can be used, or look at the secondary ACV winding levels, and see if 60Hz power can be fed in to the device at the secondary level. If a power limited inverter or PA amp is used, the mains AC current draw from that type of rectifier/filter may cause significant flat-topping of the voltage waveform, which may or may not be an issue with either the power generator or the device itself.

author: Mr. Scram
author_rank: Super Contributor
timestamp: June 09, 2020, 01:31:20 am
content: Quote from: NiHaoMike on June 08, 2020, 10:12:30 pmI hadn't considered that. A flight simulator is another story completely.

author: syau
author_rank: Frequent Contributor
timestamp: June 09, 2020, 10:24:18 am
content: Get a surplus 28V to 115V 400Hz transformer and drive it with 400Hz via power amp may be the best solution.

author: georges shed
author_rank: Contributor
timestamp: June 09, 2020, 10:24:38 am
content: Thanks for the reply's, yes i should have mentioned its for a flight simulator not an actual aircraft so as long as the instruments work i dont mind how i achieve it. i was hoping for a very low cost solution no more than £50. I bought a cheap 300w inverter off ebay (still waiting for it to arrive) and wondered about the feasability of modding it to 115v 400hz. I thought i may be able to switch out the 60hx occilator for 400 hz or something...Even if i had a bigger budget of hundreds im still not getthing into the price range of buying even a cheap 115v 400hz inverter, Anything Aircraft related is immediatley 4x as expensive haha

author: georges shed
author_rank: Contributor
timestamp: June 09, 2020, 11:29:25 am
content: Quote from: bdunham7 on June 08, 2020, 09:28:34 pmMy mistake its 50hz not 60, that would be the output of the standard inverter...

author: Yansi
author_rank: Super Contributor
timestamp: June 09, 2020, 12:12:48 pm
content: Also make sure you understand that most if not all cheap inverters do NOT produce sinusoidal voltage output. It is fine for the typical loads like bulbs and other power supplies, but may not be enough to run some types of avionics.Using an old VFD (variable frequency drive) WITH a low pass filter is probably the easiest and most suitable of the all mentioned solutions. The output lowpass filter is a MUST and access to a DCbus- terminal on the inverter is required for proper operation.Audio power amplifiers will also work with step-up transformers, but I would recommend AGAINST using linear ones. Find a class D one, that will be absolutely fine with driving heavy AC loads for long period of time.

author: georges shed
author_rank: Contributor
timestamp: June 09, 2020, 12:14:27 pm
content: Thankyou very much for the advise!

author: Henrik_V
author_rank: Regular Contributor
timestamp: June 09, 2020, 12:23:23 pm
content: A couple of amps at 115V is still a couple of x00 Watts ...Do aircraft instrumentations (indicators) really need that much power?A cheap solution migth be a transformer with split secondary and build a royer converter tuned to 400 Hz ...

author: Yansi
author_rank: Super Contributor
timestamp: June 09, 2020, 01:16:15 pm
content: Royer converter is not tuned. Royer converter is a hard switching one, using core saturation for commutation. (But sure, I understand you mean the very similar resonant topology, but that is not a Royer)That is certainly not a cheap solution, that is an ugly solution. With a about 300W output and with a reasonable Q of the LC tank of about about a few (say 2 to 5), you would need to size the primary side of the transformer to at least 600 to 1500 VA. That is one badass transformer (even though at 400Hz it will be quite smaller).Then imagine the size of the resonant cap for 1500 VA. Probably not that bad, but now how about the input choke, that will have to pass the full input current at whatever low voltage and still have significant impedance at 400Hz?

author: georges shed
author_rank: Contributor
timestamp: June 09, 2020, 01:32:59 pm
content: Quote from: Henrik_V on June 09, 2020, 12:23:23 pmProbably not but at some stage i will want to run multiple instruments, I cant test their draw yet... I can always start with something small as a proof of concept

author: bdunham7
author_rank: Super Contributor
timestamp: June 09, 2020, 01:49:12 pm
content: OK, if the instruments will never be returned to actual service, the obvious solution to me is to open them up and see what the power supply actually does. I would guess that internally they very likely would run on a single 28VDC supply because that is what airplanes without a 115VAC 400Hz bus would have. The 115VAC 400Hz bus just allows you to both distribute power with small wires and use small transformers at the end--I don't think there are fans and gyro motors running on that. Converting them may be far simpler than rigging up a 400Hz supply.

author: Yansi
author_rank: Super Contributor
timestamp: June 09, 2020, 03:40:54 pm
content: Quote from: bdunham7 on June 09, 2020, 01:49:12 pmI very strongly doubt that, unless they are some very very simple instruments. Especially, when the OP can't help himself with obtaining 115V 400Hz in the first place, modding any kind of avionics is I'd say outta question. I think there are many options to skin the cat here and most of them are appropriate and easy solutions (unlike some). Just see back above.

author: schmitt trigger
author_rank: Super Contributor
timestamp: June 09, 2020, 04:02:14 pm
content: Over 30 years ago, we required to operate an electrodynamic shaker, which was purchased at a flea market without its controller.We used an old McIntosh MC275 vacuum tube amplifier, with its two channels fed with inverted polarity sinewave signals from a simple function generator as the power driver.It worked correctly, but as it was operating at open loop control, we had to monitor it closely to prevent overdriving the shaker.If it worked with an ultra-inefficient class B vacuum tube amp, I would also recommend as a quick solution, a class D audio amp with a step up transformer.I hope no auidiophile is watching this thread...............

author: bdunham7
author_rank: Super Contributor
timestamp: June 09, 2020, 04:55:19 pm
content: Quote from: Yansi on June 09, 2020, 03:40:54 pmThe instruments don't need to be simple, just the power supply. I'm guessing that they have a small transformer, a bridge rectifier, a filter cap and maybe a regulator. Remove the transformer and rectifiers, run two wires out and supply it with 28 VDC. Why would avionics be so special?

author: themadhippy
author_rank: Super Contributor
timestamp: June 09, 2020, 05:29:02 pm
content: Go old skool and build a rotary convertor,bog standard 240v motor driving a 115v 400hz genny,for 3 quid i'd be tempted to try using this motor as the gennyhttps://mullard.org/products/motor-115v-400hz

author: Emo
author_rank: Regular Contributor
timestamp: June 09, 2020, 06:30:06 pm
content: Are you sure you don't need three phase supply?I did build a small 3 phase 400Hz power supply to feed an artificial horizon. Just a few watts with three channel audio amplifier like setup and RC phase shifts between the inputs. Not sure though if the three phase was also needed for the slave instruments like compass etc.Eric

author: georges shed
author_rank: Contributor
timestamp: June 10, 2020, 09:28:24 am
content: Quote from: themadhippy on June 09, 2020, 05:29:02 pmGood find! bought it...

author: georges shed
author_rank: Contributor
timestamp: June 10, 2020, 09:30:18 am
content: Quote from: Emo on June 09, 2020, 06:30:06 pmNot sure, ill have to look into it, I currently dont have access to the instruments and was just testing the water really to see it its possible - which it seems to be! thanks everyone

author: georges shed
author_rank: Contributor
timestamp: June 10, 2020, 10:07:57 am
content: Just bought this bad boy for £35...Wish me luck!