There are a few drive systems on the market, and one project here, which seek to drive a turntable's AC motor better than if it was just plugged into the mains. There are some differences in how they do this.
However, it seems to me that they all create a sinusoidal wave (or multiple sinusoidal waves).
What's the ultimate, best-est waveform for a turntable's AC motor? Is it possible that a square or triangular, or a wave of complex shape will be better than a sinusoidal wave?
If so, can one produce such a wave on a PC- or Mac-based signal generation program, have it do an endless repeat, send the wave to the headphone output jack, amplify that signal to the required wattage and then send it into the motor?
However, it seems to me that they all create a sinusoidal wave (or multiple sinusoidal waves).
What's the ultimate, best-est waveform for a turntable's AC motor? Is it possible that a square or triangular, or a wave of complex shape will be better than a sinusoidal wave?
If so, can one produce such a wave on a PC- or Mac-based signal generation program, have it do an endless repeat, send the wave to the headphone output jack, amplify that signal to the required wattage and then send it into the motor?
Assuming a perfectly wound motor with symmetrical stator cutouts and lots of other perfect assumptions. Probably a slightly skewed sine might be better.
Perhaps if you have detailed design information about the motor and know what you are doing you might be able to pre-distort the waveform to compensate for these imperfections, but in almost all cases you don't and you can't. IME with the application of pulse width modulated variable frequency drives on motors as large as 5500 horsepower in industrial environments, the motors operate cooler and with lowest vibration when the VFD output is closest to a pure sinewave.
Ray K
Define "best-est". Are you looking to maximize torque, minimize vibration, maintain constant impedance, constant phase angle between windings, reduce power consumption, maximize efficiency?
I don't think we'd mind if it took a longer time to reach the desired speed if there are other advantages to be had. And perhaps reducing power consumption and max efficiency are very low down on priorities. Reducing vibration might be a consideration though this may already be addressed by other things on the turntable.
I suppose the high-level question is: what sort of waveform could make the music sound better? We're not, for example, talking about what's required for a train or a cooling fan. So perhaps controlling things like.... cogging?
Cogging creates vibration and it is a function of the drive frequency, the strength of the magnet and the power being delivered to the motor. It can be reduced by proper alignment of the phase between the coils in AC synch motors and reducing power (at the expense of available torque), but is never completely eliminated. As far as waveform, a pure sinewave will give you the lowest noise.
AC synch motors are not the optimal drive solution for turntables IMHO. 3 phase motors produce more torque, are more efficient and have little to no cogging, but require 3 phase sinewave drive to achieve the best results.
AC synch motors are not the optimal drive solution for turntables IMHO. 3 phase motors produce more torque, are more efficient and have little to no cogging, but require 3 phase sinewave drive to achieve the best results.
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