Variable speed control for turntable motor

[kirt1974]

I'm working on a project in which I am using an old 60Hz turntable motor to turn a small wooden wheel that's coated with rosin in order to rub against a guitar string to make sound (similar to the inner workings of a hurdy gurdy/wheel fiddle.) Does anybody here know what kind of potentiometer I could use to control the speed of the motor?
Thanks

[markrob]

Hi,

Cool project! Most of the cheapy turntable motors were shaded pole induction motors. These do not respond well to speed control by varying the voltage applied. To slow these down, you would need to vary the AC line frequency. Even then, they won't allow much of a speed range. You would be better off using a DC permanent magnet type motor and a variable power supply.

Mark

[kirt1974]

I just purchased a box of large turntable motors, the ones with the big magnets. They look very similar (almost identical) to the motors on my RekOkut Imperial Lathe and my Presto lathe. Are these the types of motors you're talking about? I sure hope so, because I got a great deal on these motors and they all seem to work very well. I'm just trying to figure out a way to control the speed without burning them up.

[kirt1974]

Do you think this would work:http://www.amazon.com/Elenco-Variable-Voltage-Power-Supply/dp/B0002D0ROO/ref=sr_1_7?s=electronics&ie=UTF8&qid=1408942328&sr=1-7&keywords=dc+power+supply+adjustable

[Fela Borbone]

Hello!
Your project are very interesting! Agree with Markrob.
AC motors are complicated for speed control.Maybe you want to use them because you already have them, but it will complicate things.
DC are more easy, I dont think you will need a monster for your project.Normal potentiometers will burn if you want to control speed like it was a volume control,too much current across them,even for a small motor.PWM (puse width modulation) is te best way, you can keep good torque along wide frequency range.Variable power supply is another way,if is more easy for you to find.
Good luck!

[markrob]

I just purchased a box of large turntable motors, the ones with the big magnets. They look very similar (almost identical) to the motors on my RekOkut Imperial Lathe and my Presto lathe. Are these the types of motors you're talking about? I sure hope so, because I got a great deal on these motors and they all seem to work very well. I'm just trying to figure out a way to control the speed without burning them up.

If they are AC motors, they will not have any magnets. The Presto and Rek-oKuts used either AC induction or AC synchronous motors. If the motor requires a capacitor, it is probably synchronous. Otherwise, they are standard AC induction motors. You can control both of these with a VFD (variable frequency drive). The synchronous type will not allow a large range of speed control due to the use of the capacitor. The AC induction types can be run easily over a 2:1 range if you use a VFD. An off the shelf drive can get expensive and hard to find for single phase motors. If you have some electronics skills, you can cobble one together using a automotive AC inverter (pure sine wave are the best for this application). You have to hack the inverter such that you can vary both the AC mains frequency and the voltage such that the ratio of voltage to frequency is constant. So, as you lower the frequency, you have to reduce the voltage to the motor by the same proportion or you will burn the motor up.

I still think you will better off using a DC permanent magnet motor for this application.

Mark