Lowest Emissions Electric Motor

[ricko_uk]

Hi,
I don't know much about the various types of electric motors so just wondering if someone could give me some pointers...

A client asked me to design a simple stirrer (just a motor with a shaft) rotating with adjustable speeds between 50 RPM and 200 RPM. The load is just a tiny steel shaft mixing 500ml of liquids. It can take quite a bit o get up to speed too, that is not a problem.

Efficiency and power dissipation are also not a problem, so linear drive can also be used (I assume that would be less nosy than a PWM drive).

But they need it to be very low emissions (electric, magnetic and electromagnetic) because they have all sorts of sensors around the container which are extremely sensitive to all those emissions.

What type of (low-power) electric motor has the lowest electric, magnetic and electromagnetic emission?

Or, if when decreasing one, another increase (for example by decreasing the magnetic field the electric field increases) then which solution is easier to shield? I think perhaps higher voltage lower current because the electric and electromagnetic fields are easier to shield than the magnetic ones, perhaps?

Any clarifications and pointers are welcome.

Thank you

[uer166]

The lowest noise might be something that can run directly off of mains such as a shaded pole induction motor (or any kind of mains induction motor). Unfortunately this is not compatible with the variable speed requirement, any chance you can vary speed mechanically via belt or gearbox? If not, then some kind of brushless motor driven by a linear power-amplifier instead of PWM as mentioned. A good start would be quantifying the acceptable amount of magnetic/electric field leakage..

[langwadt]

who not a hydraulic or pneumatic motor? could be made of plastic for zero emmisions

[CatalinaWOW]

First question I would ask is what the sensitivities are.  Is the concern strong magnetic fields which might saturate or over bias a sensor.  Are there frequencies that must be avoided and are there any that can be allowed?

Depending on those answers you might look at a homopolar motor, which has no varying emissions in steady state operation.  The magnetic fields can be fairly well contained in the motor.  Going the other direction, a motor with very high pole count could drive the emissions up and out of the operating band of the sensors. 

If the problem is extreme, look into a hydraulic motor or an air motor.  By making the piping/hoses long enough you can get all the pumps and controllers isolated.  You could use fiber optics to couple a shaft position sensor to the remote location for speed control.

[coppercone2]

run it on a pulley from a distance


the lowest emissions would be hand powered from a pully, perhaps with a wet rope

[moffy]

Just use a brushed DC motor and feed it with a variable DC supply, speed control, for suppression put a capacitor across the brushes.

[David Hess]

A DC brushless motor will be electrically quieter than a brushed DC motor because of the lack of brushes and commutator.  Speed can be varied by changing the voltage.

[moffy]

A DC brushless motor will be electrically quieter than a brushed DC motor because of the lack of brushes and commutator.  Speed can be varied by changing the voltage.

Not sure you can generalise that, probably specifics dependant. Where I worked we got a 'dunny cart' for the australian army through its stringent emissions testing, it had a brushed DC motor for the pump, we just had to fit a capacitor across the brushes. The BLDC motor has a drive built in which could be noisy, and good commutation doesn't have to spark especially at low voltages.

[robzy]

What type of (low-power) electric motor has the lowest electric, magnetic and electromagnetic emission?

The type which is placed far away from the sensors and uses linkage to transfer the stirring force.

A hydraulic or pneumatic solution might be the cheapest and easiest way to go.

Also you need more specific requirements than "very low emissions". Exactly how low do you need?

[Andy Chee]

But they need it to be very low emissions (electric, magnetic and electromagnetic) because they have all sorts of sensors around the container which are extremely sensitive to all those emissions.

Can you tell us what those sensors are?

How about just using DSP to get rid of any noise?

[David Hess]

A DC brushless motor will be electrically quieter than a brushed DC motor because of the lack of brushes and commutator.  Speed can be varied by changing the voltage.

Not sure you can generalise that, probably specifics dependant. Where I worked we got a 'dunny cart' for the australian army through its stringent emissions testing, it had a brushed DC motor for the pump, we just had to fit a capacitor across the brushes. The BLDC motor has a drive built in which could be noisy, and good commutation doesn't have to spark especially at low voltages.

Low power brushless DC motors can use electronic commutation without PWM drive to control speed.  Speed is controlled by the back EMF just like with a brushed DC motor.  The circuit can be incredibly simple, as shown below with a temperature controlled fan.  Modern devices either use one hall effect sensor or sense through the motor windings.

[Siwastaja]

But they need it to be very low emissions (electric, magnetic and electromagnetic) because they have all sorts of sensors around the container which are extremely sensitive to all those emissions.

What kind of sensor? This sounds like a classic X-Y problem, with assumed problem which very likely does not exist at all. If the sensors are available in the market, and the motor+driver is available in the market, they must comply to both EM emissions and immunity requirements, and will work fine. This is required by law; sensor manufacturer has tested that sensors operate within specifications under radiated and conducted emissions, and motor manufacturer has tested that the motor does not exceed these emission limits. ... in theory, of course.

Add to mix some specialty prototypes, Ebay crap etc., and then it might matter. Still, I would not stress about it too much in advance, chances are very high there is no problem with any motor type.

Avoid low quality brushed motors. High quality brushed from decent manufacturers are OK; brushless types with short motor wiring are quite safe.

[moffy]

A DC brushless motor will be electrically quieter than a brushed DC motor because of the lack of brushes and commutator.  Speed can be varied by changing the voltage.

Not sure you can generalise that, probably specifics dependant. Where I worked we got a 'dunny cart' for the australian army through its stringent emissions testing, it had a brushed DC motor for the pump, we just had to fit a capacitor across the brushes. The BLDC motor has a drive built in which could be noisy, and good commutation doesn't have to spark especially at low voltages.

Low power brushless DC motors can use electronic commutation without PWM drive to control speed.  Speed is controlled by the back EMF just like with a brushed DC motor.  The circuit can be incredibly simple, as shown below with a temperature controlled fan.  Modern devices either use one hall effect sensor or sense through the motor windings.

Thanks David, I am moderately familiar with the BLDC topology and you present the operation so concisely. I was just pointing out that the drive electronics perhaps better described as the switching/commutation electronics is a source of noise. A good commutator (round and seasoned), with bedded in brushes, especially at low voltages doesn't have to be noisy, but it would have to be a quality DC motor.

[coppice]

Nobody seems to have mentioned ultrasonic motors. They have essentially nothing magnetic going on, which is why they get used for things like powering the bed in an MRI machine, although the piezo devices may produce an electric field which can be shielded. They are not great for high speed rotation, but you are only looking for 200RPM.

[BrianHG]

For DC Brushed motors, not only do you need to place a cap directly across the motor terminals, but you also need to solder a cap on each motor terminal directly to the motor chassis.  No wire lead lengths allowed.  This is the trick really old, like 1975, RC cars used to use to stop motor commutator noise from affecting the radio receiver which at the time were ~25mhz am radios, sensitive to any noise at all in the system.  The caps directly to the motor chassis turned the motor casing itself into a Faraday cage.

The cap values would typically be 0.1uf or 0.01uf, ceramic.

[BrianHG]

See here: https://www.eevblog.com/forum/projects/brushed-motor-emi-reduction-with-x2y-caps/

[Haenk]

Maybe hack something together for a test - use a low noise PC fan motor (brushless) and a metal can to contain the power supply and a PWM controller. So that's a less than 20 bucks setup to check if this is "good enough".
A used electrical drill head from a dentist might be a good choice as well, some of them also go down to low 3-digit rpm (brushless motor, too).

[Martinn]

If you can place a VFD (which generates a lot of noise) away from the test setup, you could use a sine output filter, removing most of the harmonics. These are expensive, but at this power level (few watts?) maybe acceptable. Small induction motor then. Sounds overkill though.

More exotic: Use a small BLDC motor and drive it with three sine waveforms. Use three (linear) audio amplifiers to create the phase voltages. Drive the BLDC open loop (for a stirrer this should do) in stepper motor fashion (enough current to have plenty of torque, with slow enough acceleration profile).

[wilhe_jo]

A client asked me to design a simple stirrer (just a motor with a shaft) rotating with adjustable speeds between 50 RPM and 200 RPM. The load is just a tiny steel shaft mixing 500ml of liquids. It can take quite a bit o get up to speed too, that is not a problem.

Well, I'm currently building a linear stepper motor driver to be used in an EMC-Chamber... lowest emission is essential.
My first prototype does run, but there'll be some time to go since this is one of my commercial filler-projects that may get eventually sold.

If your problem is not time-critical, this could work.

I'd stay away from speed drives (VFDs, BLCD-drivers,...).
Depending on your mechanical setup, filtering may get a non-trivial task...

73

[Siwastaja]

When I built a film scanner with a linear CCD and 14-bit ADC, I did build a linear stepper motor controller (basically DAC + class AB amplifier stage) for it, "just for safety" against noise. In retrospect, I don't know if an even semi-decently designed normal PWM stepper motor controller would have caused any issue. Probably not. The motor is a huge inductor which limits dI/dt. That leaves capacitive coupling, which attenuates pretty fast.

[Jeroen3]

I've been dealing with a project with similar constraints (military) of silencing a brushed generator. Impossible job, even the flexcoupling emits emi.
Brushes are out, they generate soo much noise. You won't be able to keep this contained. So no DC motors.
If static field is a problem, then you can't use brushless dc's motor with magnets. So shaded pole/induction is the only option then.

You could use a vfd/motor controller with reactor filter (inductors). If it's very low power you can maybe use a low voltage one through a transformer, as that would completely remove the common mode. Starting torque may be limited.

However, this all focusess on the electric options. What about mechanical options?
- increase the distance with a long shaft or flex axle (like those dremels have)
- mechanical gearing
- hydrolics?

[David Hess]

If static field is a problem, then you can't use brushless dc's motor with magnets. So shaded pole/induction is the only option then.

I suspect Tektronix had the opposite conclusion.  They originally used shaded pole motors, but had to add extra magnetic shielding in some cases.  Later they made their own DC brushless motor which required no external shielding, but I assume they took steps in its design to control leaking magnetic flux.

[BrianHG]

You can use a mechanical wind up motor, with nothing more than a solenoid to enable/disable it.
This used to be used in old battery powered radios for the auto-radio station seeking feature.  No motor meant no interference in the rf signal level radio station detector.

Though, you will need to wind up the thing every once in a while...

[wilhe_jo]

In retrospect, I don't know if an even semi-decently designed normal PWM stepper motor controller would have caused any issue. Probably not.

Well, the common-mode noise introduced is pretty high if you need to ground the motor.

Adding filters makes the current control of the drivers I tested pretty unhappy (works in principle, but the motor screams as hell).

YMMV, but I go for linear drives inside the EMI chamber

73

[mikeselectricstuff]

Clockwork?