Guru
Joined: 05/10/2019
Location: United KingdomPosts: 8940
| Posted: 08:20am 09 Jan 2023 |
These little motors are pretty crude. A permanent magnet armature surrounded by two coils in a steel cage. Bent over "teeth" on those cages are the stator pole pieces. You can feel the 24 steps as you turn them by hand. However, mount them resiliently and couple them to a relatively big, heavy flywheel with a drive belt and you are firmly in "good enough" territory.
If you fiddle with the phase shift between the windings you can reduce the vibration, but even different samples of the same motor will often need different values. 90 degrees is a compromise. Some will be better at 92 degrees, others at 85. Also note that there are some of these motors which look identical that don't self-start no matter what you do with the phase shift - not suitable for turntables usually.
Once again, the value of the phase shift capacitor seems to be pretty none-critical in practice, providing that the windings stay within their voltage ratings. Generally anything from 0.15uF to 0.22uF can be considered normal. This can change the vibration characteristics a bit too.
Note - there is always some vibration from these no matter how you derive the 90 degree phase shift. You can never get rid of it entirely and while the additional complexity of producing the phase shift at low level then using a second power amp (and transformer in my case) usually helps it comes at a cost. That's because the voltages on the coils don't have to be the same - it's the phase shift that matters. You can always equalise the voltages a bit with a series resistor in the other coil circuit anyway.
All this falls to bits unless you drive from a reasonably clean sine wave. Anything less results in something more like stepper motor behavior (not surprising as that's more or less what these are). This is where this sort of speed control wins - the sine wave is much better than you'll get from the AC mains, especially at peak load times, the frequency is more accurate and has much better short term stability, which is what you hear when listening.