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Lowest electrical noise type of motor for low speed applications
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Ian.M:
If you don't give a flying **** about efficiency and are willing to have a big heatsink for the drive output stage,  consider a RC hobby sensorless BLDC gimbal motor, driven open loop as a PM synchronous motor with smooth AC currents from three linear class B amplifiers, each with a current feedback loop so it acts as a power voltage controlled current source/sink.  Generate the control voltage signals using three 12 bit DACs, fed with data from an approximate sinewave* lookup table in a MCU using software DDS techniques to generate three phases at 120 deg separation, at frequencies from DC up to 3.333 * N_poles Hz for direct drive.  Control frequency ramp-up/ramp-down to limit the acceleration so the motor doesn't slip out of synchronous operation as there is no provision to resynchronise.

There will be minimal electrical noise from the motor and drive amplifiers, and most of that will be at the drive frequency and first few harmonics, so as long as you can screen the magnetic field from the motor and minimise the wiring loop area, provided the digital side is very well screened and its supply and the DAC outputs are well filtered, EMI shouldn't be a problem for all except the most demanding applications.

* Due to the winding pattern of a BLDC motor, the lookup table probably wont be an ideal sinewave, but may need to be pre-distorted to minimise cogging, to get a more linear relationship between LUT pointer and rotor physical position.   Drive the motor shaft at  a  fairly high speed so it generates, connect a heavy resistive load and scope the coil waveforms to get an idea of the required waveform shape!
filssavi:
For the lowest EMC I would use a fixed speed three phase induction machine directly connected to the grid (No VFD), and then use a CVT transmission to vary the speed, I would also push the machine as far away from your load as humanely possible with a belt or chain To connect it to the transmission box

If you NEED an all electric solution (which is the worst in term of noise) there is no avoiding a VFD, and in that case it’s all degrees of bad, steppers and DC machines are god awful piles of junk spewing EMI from DC to light (sometimes literally)
 BLDCs are less awful but still bad, the best are PMSM and Induction machines (avoid slip rings though, as they might arc with vibration and wear). Now the problem is the VFD, first and foremost buy the best one you can afford (bonus points if it is a CSU inverter, if you manage to find it small enough) put it as far away from anything sensitive as possible (realistically at least in another room, better still if in another building) use the best, most expensive shielded cables you can find and afford, you will still need to filter the crap out of the VFD output, I would say allocate a sizeable chunk of the budget to it (let’s say 25 to 50% of the overall budget, skimp on the machine instead if you really need to) You will want a very large series inductor for low pass, tens to hundreds of millihenry at least, it will need to be custom designed to minimise parasitic capacitance, otherwise high frequencies will pass straight through, so it will be physically large, heavy and very expensive, this will also smooth out the pwm to sinusoids, then you follow it up with as many stages of traditional common and differential mode filtering as needed to get the required performance. The efficiency will be atrocious, however there is not much you can do about that without compromising on EMI


Now when it’s all said and you will have a system that is worse off in pretty much any way imaginable to a hydraulic one
- more expensive
- bigger
- heavier
- not much more efficient
- noisier(in emi terms)
nfmax:
If the diameter of the turntable is large enough, you can use a flexible belt drive, as your torque and acceleration requirements are low. A speed reduction of 100:1 should be feasible. This is how they did it in record player turntables back in the day, though the primary concern was mechanical noise rather than electrical. Both shaded pole induction motors and BLDC motors were used.

With a suitably large moment of inertia, would it possible to simply turn the motor off briefly while you make your measurements, and let the turntable 'coast'?
beanflying:

--- Quote from: nfmax on May 24, 2020, 07:44:51 am ---If the diameter of the turntable is large enough, you can use a flexible belt drive, as your torque and acceleration requirements are low. A speed reduction of 100:1 should be feasible. This is how they did it in record player turntables back in the day, though the primary concern was mechanical noise rather than electrical. Both shaded pole induction motors and BLDC motors were used.

With a suitably large moment of inertia, would it possible to simply turn the motor off briefly while you make your measurements, and let the turntable 'coast'?

--- End quote ---

He wants a turndown rate from 1-200 RPM which is not feasible with a single reduction.
Siwastaja:
Obviously, brushless, called BLDC or PMAC. Stepper motor is brushless, as well.

Designing the inverter (or motor controller, or driver, many names for the same thing; applies for steppers, as well) yourself allows sacrificing a tiny bit of efficiency to slow down switching, reducing EMI. Keep the layout tight and place the inverter as close as the motor as possible, it could be a small PCB right at the motor terminals.

For such low-torque, low-speed application requiring precision speed, stepper motor is the most obvious choice. If higher range of speeds, and/or high torque is needed, then BLDC+gearbox.

I have designed a class AB sine-wave microstepping stepper driver once for the absolute lowest EMI (overengineering, no actual specification nor actual EMI problem, just wanted to do it), but usually you wouldn't do that.
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