Lowest electrical noise type of motor for low speed applications - Page 1
Forum section: electronics / projects

author: ricko_uk
author_rank: Super Contributor
timestamp: May 23, 2020, 05:53:46 pm
content: Hi,what type of DC motor (up to 12V) has the lowest inherent electrical noise (both transmission and emission)? I mean by design (i.e. without considering any shielding or filtering I might add later).Ideally I am looking for a direct drive at a speed from 1 RPM to 200 RPM which I can control via a micro or a pot. If that's too low then I can use a gearbox. Load/torque is very minimal and not a concern and the same for acceleration/deceleration. I just need to rotate a light plastic plate.Thank you

author: Benta
author_rank: Super Contributor
timestamp: May 23, 2020, 06:19:11 pm
content: If you have AC available, I'd say a shaded pole motor.

author: nfmax
author_rank: Super Contributor
timestamp: May 23, 2020, 06:27:13 pm
content: Would a clockwork motor be acceptable? They don't generally have much in the way of electrical noise

author: Benta
author_rank: Super Contributor
timestamp: May 23, 2020, 06:37:55 pm
content: Quote from: nfmax on May 23, 2020, 06:27:13 pmThose are normally shaded pole motors

author: nfmax
author_rank: Super Contributor
timestamp: May 23, 2020, 06:43:30 pm
content: Quote from: Benta on May 23, 2020, 06:37:55 pmI mean a mechanical, spring-driven, wind-up motor. These are 'clockwork' in British English. A shaded-pole, or synchronous, AC motor would be OK, but you still have the possibility of mains-borne interference.

author: Benta
author_rank: Super Contributor
timestamp: May 23, 2020, 06:49:26 pm
content: Quote from: nfmax on May 23, 2020, 06:43:30 pmAh. This being an electronics forum, that aspect never occurred to me. Then let's include weights and strings, connected vessels etc. as well.AC noise is easily filtered, BTW.

author: ricko_uk
author_rank: Super Contributor
timestamp: May 23, 2020, 06:55:56 pm
content: Sorry, forgot to specify that the PCB has only DC (can be controlled/varied up to 12V).

author: nfmax
author_rank: Super Contributor
timestamp: May 23, 2020, 06:56:53 pm
content: Quote from: Benta on May 23, 2020, 06:49:26 pmThe application sounds like a turntable for some sort of EMC or antenna measurement system, so the OP may need really low noise. For a bigger turntable load (car, truck, tank...), you could also use a hydraulic motor!

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 24, 2020, 01:00:26 am
content: How about a stepper motor with slew rate controlled drivers?

author: ricko_uk
author_rank: Super Contributor
timestamp: May 24, 2020, 01:19:34 am
content: Thank you Mike,it sounds interesting. I did a search but couldn't find any devices. Can you suggest part numbers or keywords?Many thanks

author: Someone
author_rank: Super Contributor
timestamp: May 24, 2020, 01:34:13 am
content: Quote from: ricko_uk on May 24, 2020, 01:19:34 amStepper motors are a "special" case of more general brushless DC motors. You can drive them with purely analog signals so the only electrical noise is at the drive frequency (Hz to hundreds of Hz typically).You're way too vague on what is actually needed to suggest anything. Motion systems have too many dimensions/parameters to say any specific example is "low noise".

author: wizard69
author_rank: Super Contributor
timestamp: May 24, 2020, 01:38:25 am
content: Quote from: ricko_uk on May 24, 2020, 01:19:34 amDoes it have to be an electric motor? There are a variety of air driven motors, hydraulic motors and in a pinch you can find Steam and water driven motors. It really comes down to what you are doing, for example an air driven motor may provide rotation buy not real holding ability. If you need to position and hold like a stepper then you couldn't go that route. examples:https://www.atlascopco.com/en-us/itba/products/air-motors,https://www.mcmaster.com/air-motorsSome of those motors listed in the McMaster-Carr catalog show up in many Amish and Mennonite shops and farms. Their concerns are not electrical noise but an air line can offer a lot of isolation between whatever is your concern and the controls.Just out of the box back on the farm thinking there.Beyond that I really don't think modern stepper controller are advisable. You may need to take a step backwards to avoid the current limit switching of modern drivers. For example a driver with resistor current limiting. It really depends upon Where your electrical noise concerns are, is it audible noise, greater than 1MHz or something else.

author: ricko_uk
author_rank: Super Contributor
timestamp: May 24, 2020, 01:44:18 am
content: Thank you wizard69 and Someone,don't know much about motors apart from the basics. All I need to understand/find-out is whattypeof motor is best suited (brushless DC, stepper, etc).The only requirements are:- must be electrical- max voltage available on the board: 12V- as low noise as possible in terms of amplitude as well as frequency spectrum (this is as specific as I can get)- drive speed 1 RPM to 200 RPM that I can control through a micro- load/torque next minimal as I only have to rotate a 200 grams plastic disk. It can take few seconds to accelerate, it doesn't matter.That's all that is required. And all I need is to understand what is the type of motor with the lowest electrical noise (again, in terms of amplitude as well as frequency spectrum). Would it be a stepper with some slew rate device (after some Googling still no idea what they are) like NiHaoMike suggested or some other motor types?Thank you

author: Someone
author_rank: Super Contributor
timestamp: May 24, 2020, 05:17:11 am
content: Quote from: ricko_uk on May 24, 2020, 01:44:18 amThe bolded part is going to constantly prevent any sensible suggestions or analysis. There are extremely low noise (electrically) methods that would either put large magnetic fields, or concentrate all the "noise" to a very narrow frequency band. You have left too many dimensions unconstrained without anything to weigh/trade against. Hence the suggestions above for clockwork or hydraulic drives. A hydraulic servo keeps all the electrical noise away from the drive location, but may have more noise overall.

author: beanflying
author_rank: Super Contributor
timestamp: May 24, 2020, 05:27:28 am
content: Quote from: ricko_uk on May 24, 2020, 01:44:18 amYou will not get that sort of turndown ratio without a two stage (at least) mechanical reduction.EDITWhat you might get will be in the order of 20-200 RPM or maybe a bit better in particular if you kick it higher then come back down in speed.Steppers will be electrically noisy to hell along with their drivers so I wouldn't consider them either.For Quality motors the likes ofhttps://www.maxongroup.co.uk/maxon/view/catalogare great but by no means cheap. Give them your application and see what they offer.

author: Ian.M
author_rank: Super Contributor
timestamp: May 24, 2020, 07:17:31 am
content: 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!

author: filssavi
author_rank: Frequent Contributor
timestamp: May 24, 2020, 07:21:02 am
content: 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 boxIf 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 EMINow 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)

author: nfmax
author_rank: Super Contributor
timestamp: May 24, 2020, 07:44:51 am
content: 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'?

author: beanflying
author_rank: Super Contributor
timestamp: May 24, 2020, 07:55:06 am
content: Quote from: nfmax on May 24, 2020, 07:44:51 amHe wants a turndown rate from 1-200 RPM which is not feasible with a single reduction.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 24, 2020, 08:07:51 am
content: 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.

author: Someone
author_rank: Super Contributor
timestamp: May 24, 2020, 09:52:38 am
content: Quote from: beanflying on May 24, 2020, 07:55:06 amWhen you open up design concepts (and your mind) a little, its not hard at all to meet those speed requirements with direct drive.... coming from experience putting precision motion right on top of low noise electronic sensing.

author: beanflying
author_rank: Super Contributor
timestamp: May 24, 2020, 10:28:50 am
content: Quote from: Someone on May 24, 2020, 09:52:38 amWhen you consider the 'low noise' part of it initially requested it limits the options. If you ignore it then a stepper/gearbox makes a lot of sense but they are a long way from low noise.So rather than 'open your mind' then offer ZERO how about you make a suggestion?

author: Someone
author_rank: Super Contributor
timestamp: May 24, 2020, 12:30:52 pm
content: Quote from: beanflying on May 24, 2020, 10:28:50 amBecause there are too many variables/parameters to make any serious suggestions. What constitutes "noise"? A single pole rotor (permanent magnet) could be driven to the specifications required with electronics similar to an audio amplifier, almost zero EMI but likely huge magnetic fringing fields. An ultrasonic motor would do it with no audible noise. Both without any gearing as direct drive. If every concept of "noise" is sensitive then the suggestions already provided of hydraulics are valid."noise" is even less clear than "low", together they're just marketing speak.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 24, 2020, 12:37:34 pm
content: Float the turntable on water (or other hydraulic bearing), embed a bar magnet in it, and drive two coils in quadrature to position it. Drive can be via DAC into linear power amp, so the digital MCU control stuff can be at some distance (use isolation?) from the amplifier and compass\\\\motor.You know, like they did it two centuries ago.Tim

author: filssavi
author_rank: Frequent Contributor
timestamp: May 24, 2020, 04:07:37 pm
content: BLDQuote from: T3sl4co1l on May 24, 2020, 12:37:34 pmWhat you are suggesting is a poor man BLDC, then he might just as well use a proper one...The problem with BLDCs is that, similarly to steppers they are usually driven By fully energising a phase at a time, that has its advantages (mainly its cheap) But will generate quite a lot of noise. The least noisy way to drive a machine is with sinusoids (ok it is not that simple I know but let’s not go too much into details) so PMSM (also knows as BLAC) or induction machinesAlso the placement for the MCU is completely irrelevant, as the machine drive and machine themselves will spew out much much more EMILast but not least I would strongly Advice against rolling your own drive, as if you don’t have experience in the field it is pretty much guaranteed you will fail even the most basic compliance test, let alone do better than what you find on the marketAlso provided you are willing to spend I am sure you can find off the shelf solutions to the problem as your problem is common in many T&M specialities

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 24, 2020, 04:12:32 pm
content: Quote from: filssavi on May 24, 2020, 04:07:37 pmExactly. Stick to the classics -- AC motors (BLDC being just another type) are hard to beat, with good reason.QuoteWell, once you're driving the phases with a linear amp and sine wave, the MCU noise will be quite relevant!Also since mechanical constraints or available space is undefined, why not use a bog standard gear head motor or servo, and an insulated shaft of sufficient length?Tim

author: filssavi
author_rank: Frequent Contributor
timestamp: May 24, 2020, 04:33:44 pm
content: Pretty much spot onI did not consider linear driving because is is usually too inefficient (basically is becomes uncoolable really fast, however reading the OP specs more closely it is likely a very small setup (less than 10W) and in that case you are rightI would usually vote against long shafts as they are quite nasty to drive (lots of low frequency resonance modes, transient issues due to inertia and compliance etc) however if the load is basically almost zero and a those low speeds it could workHowever I would still use a belt (even without speed reduction) as it is much easier to design, certainly much cheaper and much more forgiving, it also insulates from drive side vibrations which can be usefull

author: wizard69
author_rank: Super Contributor
timestamp: May 24, 2020, 06:59:35 pm
content: Quote from: beanflying on May 24, 2020, 10:28:50 amStepper noise depends upon the driver. The motor itself isn't electrically noisy.As for the application here I'm not sure if a stepper fits the need as there is too much missing information. If not there are a number of Brushless designs on the market that would do the trick but you still have the issue of the driver creating most of the electrical noise.

author: wizard69
author_rank: Super Contributor
timestamp: May 24, 2020, 07:23:34 pm
content: Quote from: ricko_uk on May 24, 2020, 01:44:18 amThat is where the problem is, it is hard to say for sure from the information available.QuoteThat right there can be an issue, I really like to see motor supplies separate fro the supplies going to the controls.QuoteThis really isn't useful. First of in a stepper or even the brushless solutions a vast amount of electrical noise comes from the driver. Beyond that you leave open a range of possibilities from below one Hz to many MHz.Consider this in many applications audible noise is a big factor to be controlled. A common approach is to implement drivers with very high switching frequencies. That however can lead to radiated high frequency noise.QuoteOpen loop or closed loop. How tight on the measured RPMs? I ask these questions because various methods are available and frankly you may have to experiment to find the best fit. I don't really thing that anybody can say for certain that one solution is the best here.For example back a few decades ago I worked on machinery that need spindle control to one hundredth of an RPM. In that case we used an AC driver (3 phase) to an synchronous AC motor. The drive was rather huge compared to what you can buy today for a 1/3 HP motor. But was probably the best solution at the time, using a ring counter driving hilariously large switching modules.Would that be the right choice today? probably not. In fact I believe there was an upgrade to a DC drive that had a really good velocity control loop, a couple of decades later.QuoteThis really can't be answered as there are too many depends that must be addressed.

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 24, 2020, 07:50:19 pm
content: Are there times when bursts of noise can be tolerated? Perhaps the solution could be as simple as a flywheel with a DC motor pulsed on to keep it turning.