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Old 14th Feb 2020, 9:49 am   #1
ScottBouch
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Join Date: Feb 2013
Location: Leicestershire, UK.
Posts: 161
Default SMITHS "servomotor" 1111SCM-2

Hi all,

I'm looking for help please with figuring out a bit of an unusual motor from an attitude indicator form an English Electric Lightning aircraft, there are several of these motors inside it.

Manufacturer: SMITHS
Type: 1111SCM-2
Stores ref:6TD/4551218

The Reference coil is 115Vac, and the Control coils are 60Vac, all 400Hz.

Regarding the "SCM" in the type, I'm tempted to guess it stands for "Servo Control Motor", but this is an assumption, and you know what they say about assumptions...

I'm told that in similar aircraft instruments it is normal to see quadrature phase motors with two windings, marked Ref and Control, which should have a 90 degree phase angle between applied voltages. In normal quadrature phase motors, the Reference coil has a constant AC signal applied, but the amplitude of the Control coil sets the motor speed, and by giving the Control coil a -90 degrees phase angle from Ref, reverses direction of rotation. This info was from an ex air force friend who has a vast experience in aircraft instrumentation, but he's not seen a motor like this with two control coils, he's used to seeing one.

My confusion here is that these motors have two Control windings, and I don't quite know what to do with them.

There is one clue in the instrument wiring, the two Control coils have one end of each connected together, so the two Control coils are connected in series, with three wires back to the control circuits (you can visualise it a bit like a centre-tapped transformer wiring).

Powering the Ref coil is straightforward, 115Vac 400Hz, but what do I do with the two control coils?

Just thinking out loud, and guessing, could one Control coil be for clockwise, the other for anticlockwise? Ie: could these motors be for applications where the control circuit can't flip the control signal through 180degrees to achieve -90degrees for reverse direction? ie: inside the motor they could be arranged opposite to each other.

Any ideas, experiences, datasheets, user manuals, other info, will be very much appreciated as I'm currently flying blind with reverse-engineering them. Thought about approaching Smiths, but I assume they will probably either not be interested, nor have records dating back to the early 60's.

Many thanks, Scott.
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Last edited by ScottBouch; 14th Feb 2020 at 10:18 am.
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