Homopolar, Brushless and Brushed DC Motors

[Samson4]

I've seen examples of each of these types of dc motors. What I haven't found is a dc motor that doesn't use pwm, commutators/brushes, or some sort of slip contact. Is there a such thing? If not, would there be a demand for such a motor?

 

[jim hardy]

There exist "Brushless DC motors" that are really AC motors with a DC to AC inverter inside the case. Take apart an old VCR or computer disk drive to see one.
https://en.wikipedia.org/wiki/Brushless_DC_electric_motor

There also exist "Linear motors" , but they don't rotate just give some amount of linear motion.
https://www.machinedesign.com/motion-control/what-voice-coil-actuator
PC disk drives use one to position the heads.
Take one apart - the actuator magnet works fantastic for holding a calendar to your fridge.

Bottom line is you can't make a coil in a field rotate more than a half turn without reversing current, which we call commutation.
You've already named the three ways to achieve that.
from http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/motdc.html

 

[Merlin3189]

... a dc motor that doesn't use pwm, commutators...

I'm not sure what you mean here? No brushes makes sense, but what have you got against commutation? Is electrical commutation acceptable?
If so, how do you define PWM, since electrical commutation requires switching current to coils very similar to PWM?

Since you've seen brushless DC motors, what is it about them that doesn't meet your requirements?

 

[Baluncore]

What I haven't found is a dc motor that doesn't use pwm, commutators/brushes, or some sort of slip contact.

You are ignoring linear electric motors such as solenoids that need no ancillaries.
The need for commutation and brushes comes from the continuous rotation requirement.

Even if you had a magical DC rotating motor, you would efficiently control the speed and direction through PWM of the DC supply to the motor.

DC is a real problem. Insulation and switches cost more because DC arcs do not self extinguish.
Three phase AC is significantly more adaptable to industry as the voltage can be easily transformed.

 

[neilparker62]

If you are interested in homopolar motors, you might find it interesting to wade through following - albeit a little dated.

http://www.dtic.mil/dtic/tr/fulltext/u2/a436704.pdf

Most searches using "homopolar motor" just bring up various toy motors. There's also a company called Guina Energy Research supposedly developing new forms of hp motors. But having watched them for a while I'm really not sure whether the technology is going anywhere or not!

 

[Svein]

Brushless motors are usually driven by an electronic circuit:

(copied from https://www.digikey.com/en/articles/techzone/2013/mar/an-introduction-to-brushless-dc-motor-control).