in DC machines, how do commutators convert induced AC voltage into DC ? 2

[atifmwali]

Hello guys
I have been reading about DC machines and I read that commutators are responsible for converting AC to DC voltage
my question is:
how do commutators convert induced AC voltage into DC exactly ?

 

[panter]

Atif,
bad book.. Commutator just use DC voltage for making rotating field and operation of DC motors.

 

[BrianPetersen]

In an old school DC motor, the field windings are fixed (DC) and there is indeed AC in the rotor.

The commutator is simply a fancy multi-pole switch. When the AC is in let's call it the "positive" phase, that fancy switch connects that phase on the rotor to the "positive" DC brush and the opposite phase somewhere else on the rotor to the "negative" brush. When the AC is in the "negative" phase, that fancy switch connects the winding to the terminals the other way around.

http://electronics.howstuffworks.com/motor1.htm

 

[Clyde38]

Are you thinking of a Universal motor?

https://en.wikipedia.org/wiki/Universal_motor

 

[Skogsgurra]

Brian is correct. And he has obviously also seen a DC motor IRL.

The commutator is indeed a "fancy switch". Another way of saying the same thing is that the commutator and its bars work as a mechanical rectifier that connects the part of the winding that has the highest induced EMF to the brushes. Since there is a set of positive brushes and a set of negative ones, the resulting voltage across the brushes is a rater smooth DC voltage. There is a slight "commutator tone", but the more bars there are the smoother the voltage.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[flexoprinting]

Hello Gentleman

What!!, How does it know when it's in the positive cycle or negative cycle and than deliver the current to the correct brushes? Are the rotor bars and or commutator
just set up so they are lined up no matter what position it's started in? Would a 50Hz motor need to be made different than 60Hz. Scratching my head.

Chuck

 

[davidbeach]

DC to the brushes. The commutator keeps supplying the positive DC to exactly right place relative to the DC in the field. That means that any specific winding in the armature sees all voltages from max positive to max negative, looks kinda like AC. There's no motor that actually has DC on both the armature and the stator. If the armature could actually work with DC, it would have slip rings rather than a commutator. The rotor of an AC synchronous machine has a DC field and uses slip rings; the rotor (armature) of a DC machine uses AC to work against the DC field.

 

[Skogsgurra]

It is really very simple, Flexo. The information is in the brush position versus the excitation field. The brush bridge can be adjusted so the commutation is at maximum voltage, which is the normal way of doing it. It can also be adjusted perpendicular to that position, which produces no counter-EMF at all. And, if you adjust it further, the positive brush gets negative and vice versa.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[flexoprinting]

Of course! Jeeze I never looked at it that way and I certainly will not explain how I originally thought it worked.
Thank you for the perspective David and Gunnar, enlightened and embarrassed.

Chuck

 

[Skogsgurra]

Sorry, Flexo. No embarrassment meant.


Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

Another way to visualize this is to consider only two coils and two commutator segments.
Look at one coil rotating clockwise in a horizontal magnetic field.
As the one coil moves down across the field to the right, from 0 degrees to 180 degrees, a voltage will be induced in the coil.
As the other coil moves up across the field to the left, a similar voltage will be induced in the coil.
These voltages appear in parallel at the brushes.
As the coils continue fro 180 degrees to 360 degrees, a similar voltage but of opposite polarity is induced.
However as the coils pass the zero degree and 180 degree position the brushes transfer to the opposite commutator segments and reverse the coil connections.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[flexoprinting]

Hi Bill

Yep that is a visualization I can rap my head around with additional coils and segments performing the timing and all
part of the "Fancy switch" as described, Two coils would of coarse be very choppy but as Gunnar described with more coils
and segments added the voltage will smooth out.

Thank's Sir!

 

[waross]

When I was a kid in high school, our school shop had a small demonstration motor with two poles. The first motor that I built had two poles. If they stopped in the wrong position they would not start.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[crshears]

So, atifmwali, was any of this of use to you? If so, please let us know; same applies if you're still confused...

It is considered bad form to post once and then never respond or request clarification, or express thanks and appreciation for the assistance of complete strangers who owe you nothing.

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[robert122]

To understand this you have to know the working of DC machines. Actually the generated voltage is always AC. In simple the commutators just conduct in only one way making the current output is always one way.