Driving a Turntable with a Two-Phase, Bipolar Stepper

Question

I'm a relative newbie to electronics, and as an educational project I'm building a record turntable. I'm comfortable with the purely mechanical aspects of the project, but I need some help understanding how to drive the motor.

I have a two-phase, bipolar stepper-motor that I'm trying to use to drive the platter via a belt. The platter will turn once for every six turns of a wheel driven by the stepper. Using a stepper-motor controller in micro-stepped (1/8th-step) mode provides great torque and speed control, but makes the motor much too noisy for this application since the tone arm on a turntable is very sensitive to vibrations.

If the accounts I've read are accurate, a motor like this one can be driven smoothly by applying a voltage sine wave on the first winding, and simultaneously applying a second voltage sine wave (offset 90 degrees) on the second winding. What is the best way to do that? I have found circuit diagrams for op-amp based sine generators which have a synchronized cosine output. Is the task as simple as using a pair of op-amps to amplify the generated signals and then using the amplified signals to drive the motor, or am I missing something that would prevent that idea from working? Is this whole scheme crazy for a reason I don't yet understand?

Thanks for any help you can provide.

Answer 1

You could do this with linear amplifiers such as high power op-amps, but that's not what would commonly be done today - it's very challenging (/expensive/inefficient) to build high power linear amplifiers.

Instead, it is more common to use full-on/full-off switching devices such as MOSFETs (or at very high voltages, IGBTs) in motor drivers. These are switched at a rate many, many times the commutation frequency, with a varying duty cycle for each pulse. The duty cycle varation - called pulse width modulation - effectively approximates the desired sine wave. Since the actual switching frequency is faster than the system can mechanically respond, it ends up averaging it out*

That said, a stepper would be an odd choice for a spindle motor. More common would be to use a brushless AC servo motor. Much as with a stepper this would be driven by several distinct phases of PWM-approximated sine waves. A main difference is that a servo motor typically has fewer poles - in effect fewer "steps" (electrical cycles) per mechanical revolution - and so needs a lower commutation frequency for a given rotational speed.

*though some brushless motor controllers intentionally modulate the applied voltage at audio frequency and make the motor itself emit audible beeps while you work through their options menu.