The Turntable Motor Controller Project - Part 2 - A Bit More About Synchronous Motors
Before describing my controller design goals let's talk a bit more about AC synchronous motors and what makes them spin. The AC motor needs voltage delivered in alternating current but unlike a DC motor which only needs to be fed a single DC current into two wires, an AC motor needs multiple sets of wires (coils) to be fed the AC current, most AC synchronous motors have either 2 or 3 sets of coils, 2 phase and 3 phase respectively. The thing is that these AC waveforms have to be fed into the motor out of phase from each other, ie, slightly shifted in time (technically in degrees) from each other in order for the motor to rotate.
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3 Phase AC |

The motors used in turntables can be either low voltage (20-30V AC) or line power driven (110V) so this has to be considered when I design the controller. The turntables I mentioned previously use line power level motors (110V here, 220V in other countries).
So I am going to be dealing with 2 phase motors for my project, they are easy to come by and are inexpensive and work well enough. Interestingly, stepper motors are basically AC synchronous motors that if driven by AC waveforms as above will also work. Stepper motors can be found in all kind of computer devices such as printers, scanners and some disk drives. So it is conceivable, with a good controller, to scavenge some of these motors and use them to drive a turntable ! Nice and cheap (usually free if you hunt for discarded printers by the curbside as I have been known to do) but maybe we will see how that works out some other time. Most of these steppers are low voltage ranging from 5-24V so that needs to be considered.
So this is not a very deep description on AC motors, but hopefully is enough to understand where I am heading on this project.
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