I need help designing a 400hz 115vac 3phase power supply -- Help

If you are borrowing mil spec gear, they will not want it damaged, so you'll need to read the relevant spec and understand how to contain your supply within the spec.  Too many harmonics and you'll most likely damage the power supplies in the borrowed gear. 

5A 3 phase 115V may not sound too bad compared with a domestic supply, but for an inverter 3 x 5 x 115V = 1.7kVA.  The VA is important.  It's only watts if the AC power factor is 1.  It usually isn't. So to produce 1.7kW at a typical PF of 0.7, you'll need 2.5kW. This assumes an ideal inverter, and the load is balanced. Is this 3 or 4 wire 3-phase?

If you can live with say 0.5A, then an oscillator with 120 degree separated outputs (arduino pwm), 3 x 100W class D audio amp and a 3 phase home-made step up transformer (and a feedback loop) would do it.

Don't start at 28V or the first challenge will be to produce 115V x root 2 (1.4) = 161VDC at 15A (well more like 22A as the inverter will be say 70% efficient) 22A x 161 = 3.5kW @ 161V, or a whopping 126A @ 28V.

I've over simplified the maths a bit and there is a root 3 (1.7) in there too for 3 phase power, but it gives you a feel for scale.

Borrow a scope and start producing the waveforms - you'll need simple filters to see the sine waves. Then create the feedback loop with the ADCs allowing you close the loop and regulate the output under small loads. Add a current transformer or 3 so that you can measure the current draw to protect the inverter. When that's working well, you can move onto the 3 phase H bridge & hi/lo side drivers to give the current gain. I'd strongly advise the purchase of a variable transformer to produce the 161VDC. The H bridge FETS/IGBT/ whatever you choose will never be protected adequately by a fuse, so the first time you power up, you start at say 10V and a few lamps as loads. If the FETs mis-fire and short the supply, at 10V they will survive. A mis-fire at 161V means you're buying more fets.

It will be a fun, interesting project and you will learn loads. Start simple and build up. Good luck.

No thank god I am not borrowing anything..  It's all my gear.  It's for my flight simulator. The first four are my ADI, HSI, 2x Fan Speed which I am using as my RPM but reading as a percent. These 4 require 400hz 25 - 100vac Single cycle for power. The synchros that move the cards (dial) are 3 phase motors. They are sort of like you cars alternator which btw can be used to generate 400hz 3 phase.

which is rather easy. Just pull the diodes, and then put a 400hz signal on the drive lead, and then spin it. Crude but effective you just need a motor, and an alternator. Flight Simulator 101, or Back to College, Part 7: Understanding Synchros.  Anyway thats it.. I know the army/airforce used to build a oscillator and then put it through an audio amp..  Not to bore you with a lot of math you are looking between phase difference or angle  0, and the rotor. Synchros where normally wired in parallel. Where one of devices was in the cockpit displaying some angle, and that angle was input to the other device or transmitter.  now we are replacing the transmitter with electronics.

No thank god I am not borrowing anything..  It's all my gear.  It's for my flight simulator. The first four are my ADI, HSI, 2x Fan Speed which I am using as my RPM but reading as a percent. These 4 require 400hz 25 - 100vac Single cycle for power. The synchros that move the cards (dial) are 3 phase motors. They are sort of like you cars alternator which btw can be used to generate 400hz 3 phase.

which is rather easy. Just pull the diodes, and then put a 400hz signal on the drive lead, and then spin it. Crude but effective you just need a motor, and an alternator. Flight Simulator 101, or Back to College, Part 7: Understanding Synchros.  Anyway thats it.. I know the army/airforce used to build a oscillator and then put it through an audio amp..  Not to bore you with a lot of math you are looking between phase difference or angle  0, and the rotor. Synchros where normally wired in parallel. Where one of devices was in the cockpit displaying some angle, and that angle was input to the other device or transmitter.  now we are replacing the transmitter with electronics.

Hi Cris!

One interesting way might be to use a microcontroller and an excellent IC called MC33035 which is intended for driving three-phase motors. It can handle up to 30V supply, so this is at the low-end of your 25-115V requirement.

Basically, it uses hall sensors to determine shaft position when driving a motor, but that could be replaced with a microcontroller generating 400Hz on three 120degree-phased digital outputs since there is no motor or sensors

(or a logic circuit could be used, but a microcontroller may be a bit easier).

The MC33035 is available in DIP package, but would also need three PNP and three N-ch MOSFETs for driving the power, and would generate a 'stepped' voltage output (with 6 steps over 360 degrees) rather than smooth sine-wave,

but I think (never done this before : ( that this might still function. Basically, the circuit in figure 36 of the datasheet, but without the sensors. It is under-using the MC33035, since it would only be used as a glorified set of H-bridges,

but at least ensures correct timing to avoid shoot-through on the H-bridges and has other bits of protection too.

It would be an interesting experiment, but I have no synchro's : (

EDIT: There is a diagram in the datasheet that shows how to power the H-bridge portion from a higher DC supply, using an optocoupler).

Guys I am going to post 2 or 3 links to pdfs there good read, and you will understand what im up to...

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thanks to all.

and Petri no thanks for the Drive.

Moogs Handbook

DDC handbook

a cool little training guide

Thanks for the synchro explanation. Interesting stuff.

As it's your own gear, can you dismantle the single phase gear and see if there is a switch mode power supply in there.  If they are SMPS, regular 60Hz, or DC will probably be fine.  SMPS have a rectifier as the first component, so they don't really care what you give them.

I read a few specs for synchros (Moogs).  They need a small amount of power say 100mA - do you have the spec.  This makes life safer and easier.  The other thing I read is you can half the frequency and half the volts without affecting the accuracy too much. I'm out on a limb here, but it looks like they are tolerant of less than perfect 3 phase, so if you didn't have exactly 120 deg separation, the error is minimal as both RX & TX cancel the error.

Staying with 3 phase, I think you can do it with a stereo class D amp (say 20W) ie 2 channels. Class D simply as you'll have less heat to dissipate and you can probably dispense with filters on the arduino output & amp input. Create 2 pwm outputs 120 deg apart, 1 wired to each channel. The output from channel 1 is put through a handmade step up transformer with 2 x 115V output windings.  Wire these back to back to create 115V 2 phase, separated 180 deg. Output 2 is wired through a similar transformer (easier to make 2 identical, but not important) and wire the secondary up to the first in the correct sense.

As your current demand is small, I imagine the single phase loads, distributed over the 3 phases, will not cause too much voltage influence, so you may not even need feedback. I seem to remember aero stuff keeps going under tough conditions.

If it wasn't for the 3 phase loads, 25V/400Hz would be quite easy - wouldn't even need a step up transformer, or a PWM signal - just a couple of logic chips.

I have a gut feeling the synchros wouldnt work with MC33035 stepped output, or would run warm. Good idea all the same.

A hacked low frequency ie heavy UPS would be very lossy in the transformer (or make a new one).  A high frequency UPS might work.  Swap the main crystal out and change the output filter.