Controlling 7.5 deg Stepper Motor Speed for Turntable

Okay so I'm working on my senior project which is creating my own turntable. I'm going to use a 7.5 deg stepper motor to actually turn the platter.

I'm a complete novice, I know absolutely nothing about controlling motors. What can I use to control the motor? How do I supply power to the motor? I want to be able to switch the motor from 33 rpm to 45 rpm. If that's too complex, how can I make it run just at 33 rpm?

I have no experience with programming, that's why this is a stretch for me. Please just point me in the right direction, maybe articles, or youtube videos. Maybe even to a website that sells motor controls.

A direct drive turntable, in days of yore, was a very specialized
motor. Your stepper 7.5 degrees is probably so "notchy" that
you will have audible flutter ("wow" and "flutter" being specs
of interest to audiophiles, as much as raw frequency response
of the pickup).

A belt drive setup was always superior even after decades
of motor engineering, soaking up small torque ripples.

I would therefore look for controllers with micro-stepping
features. I'd look for an integrated motor control chip
that drives power FETs or bridge drivers. Although you
could probably do this in software on a uC platform (and
there may be hybrid products comprising a uC and a
stepper motor control engine) I'd expect a lot of learning
to be required (which may be the point of all this, but
learning as little as necessary expedites things). You
might even find an eval kit which matches your chosen
motor nicely, and relieves a lot of the hardware design
/ build / debug / cursing.

Dick is absolutely right, if you use the motor directly it will try to rotate the platter into one of just 48 angles! You might look at the way disc drive spindle motors work or even the motor in electronic fans (like in a PC) where an oscillator is used to drive several coils in sequence. They have the same requirements as your platter but you can use variable frequency drive to adjust the speed. They are synchronous so the platter speed would follow the frequency of your drive waveform.

If you use a stepper motor and gear it down so the platter angles are smaller, you will likely run into problems with the high stepping speed you will need. It's technically possible but would you want to listen to a record with a whining motor running all the time?

Brian.

If you drive the "stepper" motor with sine waves it will produce smooth rotation.
It works just like a three phase ac motor, but having four phases.

This technique is known as "microstepping". An internet search on microstepping should answer all your questions.

Although one full step might be 7.5 degrees, you can position the motor anywhere in between steps, by suitably proportioning the currents in the windings.

By driving the motor with sine and cosine waveforms derived from a lookup table, it will be no problem to smoothly and directly drive your turntable at any speed within reason.