# Electric motors, generators

1. Oct 30, 2012

### 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 ?

Last edited: Oct 30, 2012
2. Oct 30, 2012

### 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 eachother and creates torque spinning rotor faster. It can be seen form left hand rule.

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).

Last edited: Oct 30, 2012
3. Oct 30, 2012

### 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?

4. Oct 30, 2012

### gerbi

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