Monday, 23 April 2012

The Turntable Motor Controller Project - Intro


OK, so there are a few motor choices for turntables within the two major categories; direct drive and non direct drive.  We will not be dealing with the direct drive motors, they are not the most popular choice for turntable aficionados even if there may be good ones out there.  I want to address indirect drive turntables and these fit into two major categories, belt drive and idler wheel drive.  In driving the platter these models can use either an AC synchronous motor or a DC motor.

My turntables of preference use AC synchronous motors, with their benefit being that the rotational speed is strictly controlled by the AC frequency it is fed.  A typical AC synchro motor is displayed in the photo above from an Ariston RD11.  A DC motor has it's frequency determined by the voltage it is driven at (or pulse width driven, another method of altering average voltage).  So I find the use of the AC motor easier to control as one only needs to modify the frequency and needs not pay attention to controlling the level of voltage applied (usually the motor is driven at it's full specified voltage).  A DC motor can be prone to rotation speed drift should the voltage level not be correct for the desired speed.  It is not my desire to get into too much description of all these drive types, methods of control and benefits or limitations, there is plenty out there on the web if you search for the info.  Suffice it to say that I will be working with AC motors and my project will serve to drive these precisely and optimally for best speed control.

Most of the AC motor turntables are speed controlled by the frequency of the AC coming into your home from the electrical utility (60Hz here, 50Hz used in many other countries).  The electrical utilities are usually pretty good at maintaining a constant 60Hz for which these motors are typically rated for their speed spec, but there is no guarantee that it will not drift or change.  If it does change from 60 Hz, your motor speed will also change.  So delivering a good steady 60Hz is important.  However, depending on the precision of your turntable's other parts (pulley, platter diameter etc.) even with exact 60Hz driving your motor, these other components have an effect on the rotational speed and so your turntable may not be rotating at a precise 33.3 rpm.  Being able to slightly adjust the 60 Hz up or down would allow us to dial in the right frequency so that we are getting exactly 33.3 Hz.  An added benefit is also that by adjusting the AC frequency we could even get the motor-platter to spin at 45 rpm for us without switching the drive pulley of the turntable.

So this is where my project leads me, the ability to adjust the 60 Hz to my desired value for precise speeds of 33.33 rpm or 45 rpm.  Another reason for doing this project is that I also have in mind at some time to build my own turntable, having a motor controller that is adjustable will free me from having to have a very precisely dimensioned drive pulley for the platter.  As long as I am close, I should be able to adjust the motor speed for spot on platter rotation speed.

So in my next entry I will describe the goals and general design ideas I have for the motor speed controller.

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