Electric motors, generators

[Crazymechanic]

Hi , maybe a pretty simple question and about things that we have long used and know well but is there a electric motor or generator that is truly DC? As much as I think there isn't because either to create , induce electricity in windings (generator) u got to have a changing magnetic field which in turn creates changing alternating current AC right?

And in dc motors like the BLDC (brushless dc) again the moving of the magnetic rotor is made by changing the phase and PWM, sorry if I make some mistakes.

The brushed dc motor operates from true DC I guess.
Tell me how much am I right and is there something to add?

P.S. Also doesn't in BLDC the rotor bearings experience very little friction as the rotor is held precisely in middle all the time when it operates by the magnetic field ?

 

[gerbi]

Answer is as simple as asking uncle google - there are truly DC motors and generators (brushed).
How DC field can make rotor moving ? Current is switched by commutator as rotor moves to keep rotor and stator DC fields static to each other.
Two fields (from stator and rotor) interact witch each other and creates torque spinning rotor faster. It can be seen form left hand rule.
Please read more about it here:

http://en.wikipedia.org/wiki/Dc_motor
http://en.wikipedia.org/wiki/Commutator_(electric)
http://en.wikipedia.org/wiki/Fleming's_left-hand_rule_for_motors

About BLDC motor bearings.. so you say rotor levitates in the middle of stator ? Nonsense.. resulting force from magnetic field (on rotor) is zero - if rotor is perfectly in axis of stator (in other cases there is a "unbalanced magnetic pull" acting on rotor).

 

[Crazymechanic]

Yes I understand that you need to reverse the polarity to keep the rotor going otherwise it would just move like 180 degrees at best and stop, but isn't reversing polarity what AC is all about, so from this kind of perspective I was asking about is there a true dc motor?
As it seems to me that true dc without any polarity change done by commutator both mechanical or electric(transistor) one is impossible as static magnetic field cannot create motion. Am I correct by saying this?

 

[gerbi]

You do not reverse polarity of supplying DC current (it remains almost constant). This should be taken from this pont of view.

 

[alphysicist]

Hi there,

Thank you for your question. I can confirm that there are indeed electric motors and generators that operate on DC (direct current) rather than AC (alternating current). In fact, many modern electric cars and hybrid vehicles use DC motors for their propulsion systems.

DC motors operate by creating a constant magnetic field and using commutators to switch the direction of the current, which in turn causes the rotor to rotate. This is different from AC motors, which use alternating current to create a changing magnetic field.

As for BLDC motors, you are correct that they use a combination of changing phases and PWM (pulse width modulation) to control the speed and direction of the motor. However, the rotor bearings do experience some friction due to the rotation of the rotor, but this can be minimized through proper design and lubrication.

I hope this helps answer your question. Please let me know if you have any further inquiries. Thank you.