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Old 7th July 2011, 10:25 PM   #1
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Default Turntable AC motor - optimizing the circuit

I've been toying with my TD-160's two phase motor a bit, trying to get it to run with as little vibration as possible. This is an extraordinarily simple circuit, with a coil ~ 4 kohms on its own, then a coil ~ 6 kohms in series with the phase-shift cap. Obviously these coils (each fed 120V) see very different currents and voltage drops. I've gone ahead and added ~ 3 kohms of resistance in series with the lone coil, which cut down on the noise in the table's top panel ("measured" with a stethoscope). This brought both the current and voltage drop closer to that of the other coil.

I'm curious though, if someone knows whether I should be aiming to match either of these quantities between the two coils, or perhaps the power dissipated, instead? I could imagine benefits from any of these methods, but am wondering if there is theory I'm ignorant of.

Thanks in advance!
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Old 8th July 2011, 01:53 AM   #2
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My measurements indicate that phase angle is the most important criterion, followed by voltage balance. It is quite common to find the "sweet spot" doesn't correspond to 90 degrees and 1:1.

Depending on the design of the motor (specifically the variation in magnetic reluctance with rotor position) the distortion components in the drive waveform can also be important. With better quality motors such as your Thorens it is possible to remove a lot of the cogging by tuning the third harmonic. For lower quality motors such as the Hurst used by VPI, this is usually not possible.
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Old 8th July 2011, 02:12 AM   #3
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You can measure the phase shift by measuring the voltage across the two coils and between the them and doing some maths. The voltage across the coil with the series capacitor is usually HIGHER than the lone one.
If your mains supply is 110V or more, add a series resistor with the entire circuit. These motors work smoothly with 90~100V ac

Harmonics are probably less of a problem in the USA. The simple dropper resistor used in 230V countries boosts harmonics to the motor windings as it looks like a high pass filter with the coil inductance
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