A thought problem, not a trick question: Assume two identical 12 hour clocks set accurately to the current time. One runs normally and the other backwards. In a 24 hour period, how many times will the hand positions of the two clocks exactly match? (Answer is the same whether a second hand is included or not).
Well, that's an old issue with 100% symetrical synchronous motors as they can rotate and lock to 60Hz in either direction. I believe GE Telechron clock motors had sufficent armature asymetry to put the armature at rest slightly in the clockwise magnetic aligment with the stator poles to make them always start in the correct direction. As for some 1960's vintage turntables, I believe at least one model from AR (Acoustic Reseach) used two motors, a small induction motor to accelerate the platter in the correct direction and a synchronous motor to lock it to 60Hz.
Ampex used fairly powerful hysteresis synchronous motors in their commercial audio tape recorders designed in the 1950's. They'd always start up in the correct direction. And they had two sets of windings to provide two speeds to pull tape at 7.5 or 15 inches per second. The one idiosynchracy of these motors was a cogging problem. If you switched motor speed windings while it was energized and turning, you would get temporary magnetization of the rotor such that it would cog sufficiently to be heard as tape flutter. Merely powering down the motor would void the magnetic memory and the flutter would go away. Easy to do since a spring loaded tape guide lever (tape break switch) controlled the capstan motor to stop transport motion if the tape snapped or slipped off of the take-up reel.
I've got a cheap quartz alarm clock thats doing the same trick. I can actually get it to change direction at will by removing and inserting the battery at the time the alarm function is tripped. I don't dare take it apart to see how it got to this state, because knowing a thing or two about how these things are put together, I know it's designed to be a one-way process.
Back in 1972 at Cornell, we still had motors and electrical machinery courses. The text said cheap clock movements (no one had the ubiquitous battery operated quartz analog movements until the 80's?) used shaded pole synchronous motors - the shading was a loop of copper around one of the armatures.
I took my clock and moved the loop around the opposite armature and sure enough it ran backwards. I could not use the alarm function because of the way it engaged it would be going the wrong way. I pried the plastic face off and made a new face with the numbers labeled counterclockwise and I had that around for many years. At the time i just had drafting tools and made the face of paper, it looked a bit rough. Maybe I should make a new face now that I have CAD stuff... Have to dig the clock out, I'm sure its still somewhere around. Too bad the case is a hideous 1970's green color.
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