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Old 6th Sep 2017, 9:07 am   #1
Argus25
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Default 120V 400Hz Three phase sine wave inverter.

Recently I acquired some vintage video monitors that run from 120V 400Hz 3Ph, which I am researching at the moment. These monitors have the interesting black level clamp circuit I posted on another thread and contain numerous other innovations that have impressed me beyond all measure.

I sheepishly looked around my workshop and had nothing even close that I could power them with, though I could have bypassed their power supplies and gone to multi DC supplies. So I knocked up an inverter out of "off the shelf parts" rather than designing something from scratch as the time commitment would have been very large. The article is here:

http://worldphaco.com/uploads/3phase.pdf
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Old 6th Sep 2017, 1:00 pm   #2
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Default Re: 120V 400Hz Three phase sine wave inverter.

What an amazing project I'm guessing you are a professional electronics engineer or something similar because I done imagine many amateur engineers doing a project like that. The big question why do the monitors need 3x120 volts input presumably they have 3 power,supply each doing a bit of the monitors circuitry. Talk about making it complicated!!
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Old 6th Sep 2017, 1:13 pm   #3
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Default Re: 120V 400Hz Three phase sine wave inverter.

An interesting way to do it, certainly the basic inverters are cheap enough these days and it saves the chore of selecting chips and mosfets.

Ed
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Old 6th Sep 2017, 1:21 pm   #4
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Default Re: 120V 400Hz Three phase sine wave inverter.

Ed,
Yes, it struck me that the basic hardware was in the motor drives and as I noted in the article, it really is a bit of a chore to make this sort of power architecture from scratch and I would think there would be a few accidents and destroyed mosfets and IGBT's along the way in the R&D process.
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Old 6th Sep 2017, 3:03 pm   #5
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Default Re: 120V 400Hz Three phase sine wave inverter.

A nice bit of lateral thinking.
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Old 6th Sep 2017, 9:24 pm   #6
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Default Re: 120V 400Hz Three phase sine wave inverter.

Quote:
Originally Posted by hannahs radios View Post
What an amazing project I'm guessing you are a professional electronics engineer or something similar because I done imagine many amateur engineers doing a project like that. The big question why do the monitors need 3x120 volts input presumably they have 3 power,supply each doing a bit of the monitors circuitry. Talk about making it complicated!!
The monitors have a three phase compact 400Hz power transformer. They have a wye configured primary. The secondaries , three are Delta connected, each passes to a three phase full wave rectifier (with six diodes just like an alternator rectifier but more compact) and one isolated winding associated with the CRT heater supply runs from one of the transformer phases. All 3 phases require power for it to work properly, and each phase is separated by 120 degrees.
The advantage is very low ripple out of the rectifiers, especially since its 400Hz, the power tranny is small, they did not need large electrolyics for the power supply. In fact the monitors do not contain a single electrolytic cap (much to my delight), it uses mil spec axial tantalums throughout.
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Old 6th Sep 2017, 10:21 pm   #7
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Default Re: 120V 400Hz Three phase sine wave inverter.

A lack of electrolytic must be a good thing I know from personal experience when they let go they really mean it! Thanks for the explanation of the circuit of the monitors PSUs I didn't even consider that they would have a transformer inside I assumed it would be a chopper supply like domestic TVs have although thinking about it properly now I can see the advantage one of which is less noise thats got to be a good thing. Cant wait to see a video of them working if possible
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Old 7th Sep 2017, 6:35 am   #8
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Default Re: 120V 400Hz Three phase sine wave inverter.

The only time I came across 120V at 400Hz, it was to power the scanner and rotating deflection coils of a marine radar which I was involved with getting running during my school sixth-form days at the end of the 1960s. This supply was derived from a fairly hefty motor-generator back then.
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Old 7th Sep 2017, 8:08 am   #9
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Default Re: 120V 400Hz Three phase sine wave inverter.

Hi Gents, for those interested in making an electronic inverter there are now numerous chips available to drive BLDC and similar motors at only a few pounds each; some even have moderate power output stages within the chip.
This would not be the solution for Argus as he wanted a fixed frequency, but if you want a range of frequencies this system works well.
There were designs published in WW several years ago that used simple TTL circuits driving FET output stages and could be used to power aircraft instrumentation.

3 phase full wave rectification; this gives a ripple of 4% without the use of capacitors and is much used on LV high current plating supplies. It does however put a very poor power factor back onto the supply.

Ed
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Old 7th Sep 2017, 8:51 am   #10
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Default Re: 120V 400Hz Three phase sine wave inverter.

Is there any way this approach could be extended to generating single phase 80 volts at 1600Hz, 500 to 1000 watts?

Andy
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Old 7th Sep 2017, 7:21 pm   #11
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Default Re: 120V 400Hz Three phase sine wave inverter.

Hi Andy, the answer is a cautious yes, but I suspect you will need to partly, at least balance the load on the other phases and losses might be a good deal higher at 1600hz.

Ed
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Old 7th Sep 2017, 9:24 pm   #12
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Default Re: 120V 400Hz Three phase sine wave inverter.

Quote:
Originally Posted by M0FYA Andy View Post
Is there any way this approach could be extended to generating single phase 80 volts at 1600Hz, 500 to 1000 watts?

Andy
Most of the VFD's I have seen appear to have a max base frequency setting of 400Hz, but there may be some that allow a higher frequency setting. You could probably get away with taking most of the power from one phase alone. The VFD normally protests and cuts out when it is overloaded, but it might detect imbalance in the load too, it could depend on the particular VFD design and how it was programmed.
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Old 10th Sep 2017, 11:30 am   #13
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Default Re: 120V 400Hz Three phase sine wave inverter.

3 phase 120V 400Hz was/is a standard power supply system for aircraft. The higher frequency reduces the weight of iron needed for power supply transformers etc. We had outlets for this in the Avionics lab when I was at Plessey, powered from a motor-generator set. It was only run up when needed, and on one occasion when it hadn't been used for some time, the guy who had to start it up found that the circuit breaker kept tripping. Assuming the circuit breaker was faulty, he tied it in the ON position with something, and that was when smoke started coming from behind the wall of the lab where the short circuit was!
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Old 10th Sep 2017, 6:36 pm   #14
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Default Re: 120V 400Hz Three phase sine wave inverter.

For trying out aircraft instrumentation I managed to aquire a rotary converter 28V DC in to 115V 3ph out. However, it needs to be in another room to what you are working in as it screams like a jet engine!
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Old 10th Sep 2017, 10:55 pm   #15
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Default Re: 120V 400Hz Three phase sine wave inverter.

I looked around for one of those converters and I couldn't find one.

The unit I made makes a subdued 400Hz whine, it comes from the 20mH line reactor, even though it is a professional part all varnished and painted etc, it still makes some noise. It would possible to shut it up completely if I put it in a rubber potted box, but on the whole I don't like potting things and making them difficult to get at later.
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Old 17th Sep 2017, 11:42 am   #16
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Default Re: 120V 400Hz Three phase sine wave inverter.

I have now used this supply to successfully power the Avionics video monitor.

Ran into an interesting snag though. The monitor has an extremely elaborate 3 phase power filter assembly in it which has filter capacitors from the phases to neutral and from the phases to ground as well. Small imbalances here result in enough earth current to trip the mains RCD powering the VFD. So I had to modify the system by adding an off the shelf isolating transformer, explained in the modified text.

I took some photos of the monitor's face with some images and a test pattern and added this to the article too. The monitor has a very high video frequency response extending over 6MHz in excess of the video signal its being fed with. The photo's don't really do it justice. Also the 14BAP4 is an industrial fine spot focus crt. Click here and scroll to the end of the article to see the screen photos:

http://worldphaco.com/uploads/3phase.pdf

Also I have attached one here.
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Click image for larger version

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Old 17th Sep 2017, 1:11 pm   #17
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Default Re: 120V 400Hz Three phase sine wave inverter.

A neat looking unit and some very good pictures.

Voyage to the bottom of the sea takes me back. The only submarine with a whopping great bay window at the front so you could really see the attacking squid/minefield/etc. As a kid I noticed that under attack the whole thing swung around (well the stagecrew swung the camera) Until a brave submariner made it up the sloping deck to turn off a large handwheel to stop the influx of water. I just wondered why they didn't just turn it off at the first sign of approaching squid/minefield/whatever?

Ah, Memories!

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