If its a military aircraft they are usually 26 VAC or 110 VAC 400 Hz and the core is tuned for a specific frequency. A few are 60 Hz.
This one is very definitely 115V 400Hz - I had the cover off the ADF indicator and it says so right on the synchro:
SYNCHRO TORQUE RECEIVER
CODE ACS 2/AF
SMITHS
MADE IN ENGLAND
And on the other side:
400 C/S
[wiring diagram]
115/90V
You'll need 3 phase power at a a few tens of mA and one signal with a controled phase relationship to spin the dial. Ie the phase angle is the position control.
[...]
Find US NAVY NEETS and read up on synchros and resolvers. They are not a dead technology, Many car instrument panels use resolvers to move the dials.
Ahh - that makes sense! The stuff I'd read said things like "the voltage on the stator determines the position", but that didn't make any sense. Phase angle vs. the rotor winding makes a lot more sense, thinking in terms of magnetic fields!
Thanks very much for the reference to the NEETS -- chapter 15 (
http://jacquesricher.com/NEETS/14187.pdf). I read through that a few times and it's pretty much clicked now. Looks like you're right, I need a 3-phase supply and a single-phase AC supply, and some way of varying the phase between them. This is at least simpler than my last idea, which involved a board full of analog multipliers and the ominous thought of "what the hell am I thinking here? This can't possibly be right!"
Looks like I'll have to dig through my box of ferrites and see if I have any suitable RM-cores and bobbins to make the transformers. I doubt I'll find 400Hz-capable transformers off-the-shelf. I guess that means I need to find some references on transformer design...
Tacho is driven with a nominal 26VAC signal derived from a tacho transmitter on the engine reduction gearbox, typically it is a 3 phase signal with variable frequency ( what you want) generated by a rotating magnet with a saturating shunt to provide a coarse voltage limit with increasing rotor speed in the 3 phase coils. You can excite it with 20VAC variable from 100 to around 500Hz, and it will indicate.
Thanks! I figured it was something along those lines. Small 3-phase generator on the gearbox runs small 3-phase motor in the tacho, which runs the tacho mechanism (the aluminium eddy-current cup, spring and needle). The only things I was missing were the drive voltage and frequency.
Time to bash together a transistor driver circuit and mate it to a PIC micro, I guess!
OK, don't hide it, where was this shop? Geetting quite rare now.
Birkett's, at the top of the hill in Lincoln. Absolutely full of aircraft instruments (broken and not so broken), ex-RAF parts, and all manner of other things.
I really need to do another Saturday day-trip with my rucksack and some bubble-wrap and buy out half the contents of his window!
A synchro transmits an angular position from somewhere, the engine or flap etc, to somewhere else, the cockpit. There are two basic types, one that doesn't use amplification called torque transmitter and receiver, and one that does need amplification. The torque type just move pointers in instruments.
Simply work by feeding a 115V 400Hz signal to the rotors of both synchros, wire the three stator wires together and that is it. There are 26V versions but somewhat rarer. Easiest way to generate the voltage is an old audio amp feeding a suitable transformer.
Best book is Synchro Engineering by Upson, seem to be £20 on abebooks.
Is it even possible to wind a single-phase to three-phase transformer? I can't even get my head around how you'd do such a thing (though admittedly transformer design is one of many facets of electronic engineering which is currently on my "black arts I really should learn more about" list!)
Thanks for the book reference - I might have a look for a copy if I get further into this.
Thanks,
Phil.