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Old 11th Feb 2018, 6:49 pm   #1
astral highway
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Default Single phase ex-avionics motor start up?!

I have a beautiful 1970's ex-avionic cooling fan for a klystron, made by the RCA corporation. RPM is an astonishing 9,700.

The make is RCA 1710618-2

It's a good candidate for taking the heat off on of various planned projects.

I'm happy with building a suitable 0.18A, 115V 400Hz power supply for it,
but the start-up and wiring are unfamiliar.

There are three wires terminating from it, which I thought could be phase, neutral and ground, but this doesn't seem to be the case. One is striped red , one blue and one is white. It occurred to me that this could also be an arrangement for adjusting the RPM, depending on the temperature.

It needs a 0.15uF start capacitor. There is a polarity-marked small sliced-cylinder shape attached to the back, which appears to be a thermocouple.
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Old 11th Feb 2018, 7:54 pm   #2
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Default Re: Single phase ex-avionics motor start up?!

Not three-phase, is it? Wouldn't explain the capacitor, mind...
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Old 11th Feb 2018, 8:10 pm   #3
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Default Re: Single phase ex-avionics motor start up?!

Wouldn't the supply be applied across two of the wires, and the cap connected between one of these and the third?
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Old 11th Feb 2018, 8:16 pm   #4
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Default Re: Single phase ex-avionics motor start up?!

Hey Russell , good idea but it clearly states Ph:1 on the case!
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Old 11th Feb 2018, 9:16 pm   #5
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Default Re: Single phase ex-avionics motor start up?!

I would measure the resistance of the windings and work out which is the common wire for the two windings, if the two windings measure the same resistance fit the capacitor between this common point and one of the other wires. Apply the power to the other wire and common.
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Old 11th Feb 2018, 9:29 pm   #6
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Default Re: Single phase ex-avionics motor start up?!

The photo of the motor plate has "Ph", but the number (of phases) is illegible. A better photo or a full "translation" of what the motor plate says. The answer will follow from that I imagine.
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Old 11th Feb 2018, 9:33 pm   #7
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Default Re: Single phase ex-avionics motor start up?!

Al,

They are called single phase "split phase" types. These fans sound like jet engines and because of the 400Hz they kind of act like a speaker too and produce a very loud tone. One was used in the avionic video monitor, you can see the wiring on page 3 and a picture of it on page 18 of this article:

http://worldphaco.com/uploads/The_19...o_monitor..pdf

Attached a diagram that might help, showing the circuit. A series resistor can be used to slow them down.
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Old 11th Feb 2018, 10:06 pm   #8
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Default Re: Single phase ex-avionics motor start up?!

As Argus says, its a split phase motor, the makers plate even says the capacity is .5 uF.
If the cap is put onto the "wrong" legs it will run backwards.

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Old 11th Feb 2018, 11:14 pm   #9
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Default Re: Single phase ex-avionics motor start up?!

I've come across small avionics fans, 400Hz induction motors, a couple of inches in diameter, but they've always been 3-phase. Naturally, they run mighty fast!

As Russel says though, this wouldn't explain the capacitor.

I'd be surprised if there was a need for a single-phase 400Hz fan. If 400Hz is available it would normally be 3-phase, from a 3-phase generator. A balanced 3-phase system is constant power, whereas a single-phase system isn't: the load imposed by the generator on the engine varies from zero to maximum twice each cycle, which gives rise to torsional vibration meaning bigger flywheels and possibly torsional dampers are needed, which is all extra weight. So, even if only low power, 3-phase generation is best.
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Old 11th Feb 2018, 11:52 pm   #10
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Default Re: Single phase ex-avionics motor start up?!

Quote:
Originally Posted by kalee20 View Post
I've come across small avionics fans, 400Hz induction motors, a couple of inches in diameter, but they've always been 3-phase. Naturally, they run mighty fast!

I'd be surprised if there was a need for a single-phase 400Hz fan. If 400Hz is available it would normally be 3-phase, from a 3-phase generator.
They can be deceptive, because these split phase single phase fans have three terminals on them, and on the face of it look like a 3 phase product, but many of the small sized ones made by this company used in Avionics equipment run off just one phase, so they need the capacitor.

In the article I posted above, pg 18 or so, you can see a photo of the one used in the video monitor. I have seen these used in aircraft static inverters too. They really sound quite amazing as they start up and reach their operating RPM, you would swear it was a jet engine starting up. This is why I had to replace the one in the video monitor with a quieter DC fan. I also found if you lower the supply voltage they slow down well, but even running this one from 40V 400Hz, it was still very loud.

If you look on one of their product lists for these about half of the ones listed are 3 phase and the other half single phase:

http://www.rotron.com/-/media/ametek...fmfv.pdf?la=en

The small tube-axial fans list two one phase models and one three phase model:

http://www.rotron.com/-/media/ametek...x22e.pdf?la=en


These fans of course being avionics parts are like mil spec quality. I don't think I have every seen a better made fan than the tube-axial fan in my video monitor.

Last edited by Argus25; 12th Feb 2018 at 12:06 am.
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Old 12th Feb 2018, 12:24 am   #11
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Default Re: Single phase ex-avionics motor start up?!

I've obviously led a sheltered life! Thank you Argus! And I've dealt with Ametek before, but as a supplier not a customer.

Quite agree about the quality, it looks amazing. And airflow is awesome too.

As for slowing down at low voltages, yes very much so have I noticed. I guess the induction motor 'slip' from synchronous speed hurts efficiency quite a bit, running from sub-optimal voltages.
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Old 12th Feb 2018, 1:10 am   #12
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Default Re: Single phase ex-avionics motor start up?!

I don't think a lot of these small fans ever get to synchronous speed, even on full voltage but I have never put a strobe on one to find out. I was surprised how smoothly they slowed down when reducing the voltage, but ideally at least for the three phase types the frequency would be controlled to alter the speed.

I found that one quick way to make an avionics three phase power supply was using a motor VFD, but it needs a good sinewave filter

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

Al,

There are quite a few low power single phase 12 or 24V DC to 115V 400Hz inverters for aircraft instrument use that would run your fan. Search "Static Inverter". To give you an example of these sorts of things here is one type ( please excuse the ebay reference):

https://www.ebay.com/itm/New-Arnold-...IAAOSwA3dYkmDd

Unfortunately a lot of these sorts of parts are expensive as they are for aircraft.
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Old 12th Feb 2018, 8:11 am   #13
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Default Re: Single phase ex-avionics motor start up?!

Quote:
Originally Posted by MotorBikeLes View Post
The photo of the motor plate has "Ph", but the number (of phases) is illegible.
Yes, I would agree it's not crystal clear, but I wouldn't go as far as saying it's illegible. I don't think there's really any doubt that what it says against "Ph" is "1".
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Old 12th Feb 2018, 2:11 pm   #14
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Default Re: Single phase ex-avionics motor start up?!

Hi folks, thank you for the replies so far. Thank you Hugo, you’ve provided a lot to digest and investigate here. I’ll comment on your very full and useful post ,once I’ve followed up the links and ideas. It will probably be later this week.

Just to emphasise Kalee20 and MotorbikeLes,, I took a high-resolution photo and it is just compressed here. There is no doubt from the very clear lettering on the actual motor that it is indeed 1 phase.

There is no urgency with this project , but I’ve kept the fan as an object of beauty for some years and I am keen to give it a spin (excuse the pun) as it is exquisitely engineered and probably sounds like a small turbofan! It may be integrated into my ongoing major project , although overkill in terms of the huge forced air flow it undoubtedly provides!
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Old 12th Feb 2018, 4:27 pm   #15
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Default Re: Single phase ex-avionics motor start up?!

It's a split-phase motor, in which one winding is fed 90 out-of-phase; but that phase shift is created using a local capacitor, which is part of the motor's control gear. So the complete system consisting of the motor and all its control gear does only require a single phase from the mains supply.

If the capacitor is only used during starting (both to force the motor to start in the right direction and to provide the extra torque needed to accelerate from rest), there will be some sort of switching mechanism to disconnect it once the motor is up to speed. This may be a centrifugal switch, a fancier arrangement with some sort of speed sensor operating a relay contact, or just a simple timer.
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Old 12th Feb 2018, 9:25 pm   #16
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Default Re: Single phase ex-avionics motor start up?!

Quote:
Originally Posted by Argus25 View Post

Attached a diagram that might help, showing the circuit. A series resistor can be used to slow them down.
Very helpful and clear, thank you, Hugo.

Julie: also thank you. Easy enough, I imagine, to disconnect the cap with a simple relay and timer.

At the moment this curio is under inspection for runaway cost evaluation!

Among other challenges, I imagine some pretty heavy-weight and vibration-tolerant/absorbing mountings would be needed, but that's not a concern for now, only if I actually go ahead!
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Old 12th Feb 2018, 11:56 pm   #17
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Default Re: Single phase ex-avionics motor start up?!

That capacitor will need be of fairly decent quality, as it does do a bit of work.
Traditionally they are oiled paper, but that has probably been replaced with some fancy plastic. Mind you the old oiled paper caps just dry out, not explode like some caps do today.

Hmm I dont want to start an argument, but I believe that the cap must stay put after starting, i.e. the motor requires the phase shift all the time.

Isolation is always a good idea, but that motor will be so beautifully balanced, probably some rubber grommets around the mounting bolts will be sufficient.

Best wishes
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Old 13th Feb 2018, 5:23 am   #18
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Default Re: Single phase ex-avionics motor start up?!

Quote:
Originally Posted by joebog1 View Post
Hmm I dont want to start an argument, but I believe that the cap must stay put after starting, i.e. the motor requires the phase shift all the time.
No argument at all, spot on, it is a "motor run" capacitor not a "motor start" capacitor. It provides the phase shift required to create a partial 2 phase rotating magnetic field.

Another way this can be done for a single phase AC motor to acquire field rotation is to shade a pole with a thick copper shorted turn. Most small AC motors you see in ceiling ventilation fans are like this.

(use a 400 or 600V 0.47uF poly cap will be fine and you will have no trouble with it)

This type of thing fine (the ebay reference is just for the image and type of part that would work):

https://www.ebay.co.uk/itm/FILM-CAPA...oAAOSwvtFZ16Qp

Last edited by Argus25; 13th Feb 2018 at 5:34 am.
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Old 13th Feb 2018, 7:19 am   #19
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Default Re: Single phase ex-avionics motor start up?!

Avionics gear is usually built to a very high density and a lot of things require forced air cooling. In small aircraft this amounts to an outside pick-up scoop piped into a distribution piping bus with something along the way to (try to) remove water droplets. In bigger aircraft you'll find high pressure blowers.

Without forced air cooling some pieces of gear will cook themselves. They are required to be tested with cooling air failed to ensure they do not get into a dangerous condition. Some of the fancy new glass panel units produce a surprising amount of heat from all the processing power behind them. It's not just transmitters that go on the cooling air bus. lab testing of military avionics needs not just power, but also cooling air.

So cooling air movers for aviation are wanted to be very reliable. Ccentrifugal switches are avoided. That motor will be capacitor-run and designed for high slippage, acting as a torque generator... rather like those Papst motors driving the spools on a Revox.

At high altitude, the cooling capability of air diminishes a lot and fan revs go up. What's needed 'up there' becomes a tornado at ground level. At altitude the voltage at which air gaps break down also fall and you need to take precautions against outbreaks of St Elmo's fire inside equipment.

In pressurised cabins, things aren't actually any easier. In normal service the inside pressure is reduced as the plane climbs. Typical cabins being rated to 8psi pressure over what's outside. On descent, the overpressure lags the outside air pressure so the passengers don't get quite such fierce ear-popping, and the rate of change of stress on the structure is managed. Like in the compressor thread, there need to be facilities to remove condensate. However, the avionics have to be designed to live without such coddling. They get tested to prove they operate correctly if pressurisation fails, and also for sudden decompression. When they suddenly have to grab oxygen masks and dive, the pilots want the radios and instruments working. They will be too worried about what the structural failure was. Those little fans have to perform.

Some planes run (relatively) constant speed generators making 400Hz 3-phase. Some smaller planes run a more car-like system with an alternator and rectifier charging a 12 or 24v battery. There may be an inverter making 400Hz for any instruments and equipment needing it. This could be either 3 phase or single phase. The inverter could be either a rotary machine, or a static inverter done by electronics.

So there are parallel markets for 400Hz stuff in both single and 3 phase forms. So there are two versions of that little fan.

With care, a lot of avionics gear can be made to pass all the altitude tests without needing forced-air cooling. This is seen as a very big plus point by people choosing equipment. As an equipment designer, not having wet air and muck forced in has its attractions, but you have to arrange enough conduction cooling to handle 40,000 ft or so.

No wonder that little fan shifts so much air at sea level?

David
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Old 13th Feb 2018, 7:53 pm   #20
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Default Re: Single phase ex-avionics motor start up?!

Hugo: thank you for the link. I have experience of this excellent series of capacitors and use them in the development of my pulse project - I've selected them in series/parallel for the tank circuit because of their high dV/dT rating and ability to handle very high currents. I wouldn’t have known to select this as a candidate for this motor-start/ run function, so much appreciated.

David: really fascinating insight into why this thing appears to a layman like me to be mysteriously over-powered. It was in a snug, 50kg radar system by Racal and its duty was to get some serious airflow to a Klystron, although it probably doubled as the main cooling unit for the entire electronics . The magnetron had a forced-air arrangement with what I think are called muffin fans.

I think I will get it going, next in the queue after my current project. It would be interesting to see how much heat it could take off a bank of IGBT’s hard-switching something, or just a big old bit of hot glass and vacuum.

Everyone: Newton's third law is highly relevant here, right? It might take some back-of-the envelope calculation to work out a suitable mounting and attached to a suitable anchor for such a little power-house. It obviously isn't completely trivial.
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Last edited by astral highway; 13th Feb 2018 at 8:05 pm.
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