Low-voltage motor without permanent magnets?

[l_donini]

Hi all,

For a research project I'm looking for a small motor without permanent magnets (because I need there to be no significant magnetic fields when the motor isn't running), but am a bit confused as most DC motors seem to have magnets or not specify whether they do. Ideally should be <USD100, torque doesn't need to be more than a few N cm. To be operated at low voltage (<40V) and e.g. controlled via Arduino (plus whatever chips are necessary). I'll probably use an encoder so it doesn't matter how precise the motion is. Would anyone be able to point me in the right direction / suggest options?

Thanks!

[Psi]

Look up series wound and shunt wound motors, as well as universal motors
They have electromagnets instead of permanent magnets.
However they are usually not 'small' and often run on higher DC voltages,  though i guess 'small' is relative.

Another option might be a sonic vibration/ultrasonic motor.
Cameras use them for auto-foucs and you can buy them for $30 or so as spare parts.
https://www.aliexpress.com/item/1005003578865128.html
I would assume that one has no magnet inside, but i dont know for sure. It could be a normal motor as replacement for OEM ultrasonic motor. But it appears to be a true ultrasonic motor from the pics.
You can also buy ultrasonic motor rings, but i don't know enough about how they work.
https://www.aliexpress.com/item/1005002546531733.html

Some research might be needed before purchase.

[andy3055]

If you are looking for a really small motor, search for an AC operated model train motor from a Marklin locomotive. Those are quite old. Works on 12 to 18 volts AC.

[james_s]

The universal motors used in things like blenders, mixers, rotary tools, vacuum cleaners and other small AC powered appliances will run on much lower than their rated voltage. IIRC a blender I was playing with a while back would start on less than 10V and keep spinning all the way down to around 3V. Most should run pretty well on 24V, obviously not anywhere near rated power but quite possibly adequate for these needs. As the name implies, they are universal, ie will run on either AC or DC.

[Doctorandus_P]

I've got similar experiences with universal motors, a motor made for 230Vac runs on 12Vdc, but at a much lower rpm and not with much torque. You can run them at an even lower voltage by putting the field windings in parallel with the rotor instead of in series.

I have also got one variable reluctance stepper motor (about size Nema23) and this also does not have a permanent magnet inside. But it's an 50 year old motor, I don't know if these are still being made.

I don't know how sensitive your application is to magnetic fields but the "normal" hybrid stepper motors have a relatively small permanent magnet in their rotor, which is surrounded by a lot of iron of the stator. Maybe magnetic leakage is low enough for your application.

[Siwastaja]

"Series wound" DC motor or "Sepex" DC motor.

Might not be easy to find a small/cheap motor for such low voltage.

[Kleinstein]

It is not so much low voltage, but small size that makes it attractive to use permanent magnets.  At larger size a electromagent is working well, but it does not scale down as well to smaller size (more heat).
Universal motors can also run with DC, usually with a lower voltage than there rated AC motor. The universal motors would still have some residual stray field from remanence in the iron, though one could reduce it with a demagentizing phase.

The already mentioned Märklin model train motor would be a possible source for a small universal motor, about as small as they get.

If it needs to be really small/low power and nonmagnetic there would be ultrasonic motors, so using piezo drive and no magentics. They are used in better camera lenses, but it could be hard to source one separately as a more normal motor form factor.

p.s. The usual starter motors for cars are also series wound.

[WattsThat]

You left out a critical piece of information, the load characteristics. What are the torque requirements for starting, low speed and high speed? One way to classify is constant torque or variable torque.

A constant torque load can require full torque at zero or very low speeds. A common type of variable torque load is a centrifugal pump or fan where the required torque goes up with the speed squared.

An overly broad characterization:

A constant torque load will normally require a compound wound dc motor (if the field is not separately excited/controlled). A variable torque application can usually be done with a universal type motor.

[TimFox]

The useful suggestions above do not contain permanent magnets, but are built with ferromagnetic components ("cores").
Is that a problem for your application?
When I was working in MRI, a colleague built a custom DC motor without any iron (air-core coils), relying on the DC magnetic field from the main electromagnet far from the imaging area, for tuning vacuum capacitors (where efficiency was the least of his worries).
He also used piezoelectric motors ("ultrasonic") for the same purpose.

[Infraviolet]

"because I need there to be no significant magnetic fields when the motor isn't running"
I'd be interested to know why you have that requirement. Because if this is something to do with having a magnetometer compass near to a magnetised motor then you can compensate out the motors effects from the magnetometer readings in software and still have a good compass reading however strong the motor magnets' field is*. In a brushed DC motor the magnets are on the stator not the rotor, so the distortion you'd need to compensate for would not vary with the present angular position of the motor.

*as long as it is not so huge as to go above the magnetometer's maximum possible output reading

[Zero999]

There will be residual fields in any ferromagnetic core. The only way to avoid this is to use an aircore design, but it's much less efficient and generally needs to run at a higher speed, as the back-EMF is lower.

How about better shielding or a hydraulic/pneumatic transmission, so the load is far enough away from the motor to avoid the field?

[Psi]

Perhaps a ratcheting motor using a piezo transducer to push each step in the ratchet.