generators and commutators

[Sarahisme]

hey if i have a simple ac generator (i.e. a coil of wire roating in between two magnets), to get power out of it i need to connect the wire to a thing called a "commutator", is that right? i can't quite figure out exactly how this all works...

 

-Sarah

[mezarashi]

I think "commutator" is generally used when you are generating DC-current from a turbine (dynamo). Although I understand that it would probably work. In practical AC generators, electrical engineers use either synchronous or induction generators. In your case of a wire rotating between two electromagnets, this would be most similar to an induction generator. Induction motors/generators however dont use simply a wire. They use a squirrel-cage rotor. Its like two rings connected by some 12 long bars. Each pair of opposite bars acts like 1 loop. The cool thing about this is that it doesn't require any brushes at all !

[Sarahisme]

thanks that sounds great, would you be able to point me to some more info on these induction generators which don't require brushes?

[5614]

Commutators reverse the flow of current within a motor, or at least that is the context I've seen them in.

 

If you imagine a basic motor, with permament magnets on either side and a electromagnet in the middle, if you had a DC voltage through the electromagnet think about it, intially one side (lets call it the right side) of the electromagnet would repel the right hand permament magnet, once it has moved around to the other side it would be attracted, at this point the motor would stop turning... this is not what we want.

 

A commutator it attached to the axis of the motor and is also the link between the power and the electromagnet, hence every time the electromagnet rotates half way, the commutator has also turned half way, at which point the two parts of it have flipped and so the current is now flowing in the opposite direction, your motor now works.

 

See here: http://electronics.howstuffworks.com/motor5.htm

[mezarashi]

Sorry for the slow follow-up. How induction motors work is not an easy mechanism to explain. Usually induction motors are introduced at the very end of an Electric Power and Magnetism course in college that covers 3-phase power, although I feel it is still possible to understand nonetheless. It'll be some interesting reading indeed as these motors are the ones used in almost all industrial/commercial applications. Some of your house hold fans as well.

 

I think the best possible link I've found on this is probably:

http://www.windpower.org/en/tour/wtrb/async.htm

 

 

A couple of other things you may need to know to read this. It is easier to start with how an induction motor works. An induction generator is simply an induction motor that has its rotor turning slightly faster than the "synchronous speed" rather than slightly slower as in an induction motor. One second a motor can turn into a generator. Wind Mills which use induction generators, need to switch their induction generators on and off depending on the turbine speed, because once the rotor fails to turn at a certain speed, it becomes a motor and consumes energy from the power grid. On the other hand, you see here that an induction generator needs a constant "excitation source". They don't work off the bat. A properly configured DC motor running off gas should usually be able to jump start one.

 

Induction motors generally run on 3-phase power (3-wires). Your household plug is 1-phase (2 wires). 3-phase means that the sinusodial wave-form of the 3 lines are 120 degrees off from one another. 3-phase power can be created locally through use of capacitors and inductors which create an increased lead or lag in the current.

 

There are two main components to an induction motor, the stator and the rotor. The stator is where all the electrical connections are made. The stator is completely stationary and does not move at all. The use of three phase power enables the stator windings to imitate a turning magnetic field. These magnetic field lines cut through the rotor as they rotate.

 

Notice how the rotor is connected to nothing. The squirrel-cage rotor is not connected electrically. The existence of the cage bars is enough for the induction to occur and the rotor to start moving. This rotor is of course connected to a mechanical shaft of sorts where the energy is being delivered or transmitted.

 

There are other noteworthy but slightly more challenging topics on the induction motor, like operating speed, load considerations, properly starting up an induction motor (I remember a friend blowing up a few in my electric power labs), torque produced by induction motors (varies with speed), power factor, and voltage regulation.

[Sarahisme]

if i am trying to charge a battery (or store energy in a battery i suppose) using an ac generator would i need a commutator and/or slip ring things?

[Sarahisme]

oh and i am talking about the most basic possible ac generator here

[YT2095]

if it`s a moving magnet type you`ll need a diode rectifier, if it`s a simple moving coil type you`ll get DC anyway and won`t need it.

 

you only need the comutator or slip rings if the coil`s moving as the way to "pick-up" the current.

 

the easiest way would be to have a stationary coil and spin the magnet inside it at right angles

[Sarahisme]

well i have the coil spining inside the magnetic field, sorry bout that, its too late to change it now (dammit! ) so do i still need the slip rings? and/or the diode rectifier??

[YT2095]

is it a single coil winding?

[Sarahisme]

no , i estimated about 50 coils

[Sarahisme]

oh single coil winding? do you mean is there only one coil? because there is say a square with a wire wrapped 50 times around the squares circumfrence..

[YT2095]

no no, I mean is it a single wire, wound as a coil, does it have only 2 ends?

 

basicly if you use the rings and brushes you`ll need to rectify IIRC, but using a comutator then it`ll reverse polarity ever 180 degrees anyway and you`ll not need one

[Sarahisme]

yeah it has only 2 ends

 

doesnt a comutator need to be put somewhere within the generator or something like that , and so makes the design much more compicated, whereas a rectifier changes it to dc current after it has been generated as ac current in the generator??

[YT2095]

the armature needs to be inside the mag field as intimately close as possible, the actual Pickup points from the 2 ends of the loop don`t have to be though.

 

edit: I just found this, it may help: http://electronics.howstuffworks.com/motor5.htm

[mezarashi]

Saraisme, well first off you have to understand that currently our technology only permits us to generate electricity through some sort of AC generator. Well there are things like solar power and stuff, but those aren't the main power sources we use. AC generator means having some kind of electrical induction going on between a coil(s) and a magnetic field that are moving relative to one another.

 

A DC generator is actually just an AC generator with brushes and slip rings at its electrical connection to reverse polarity every 180 degrees. While the model that you are proposing for generation is not the type commonly used in real industry/commercial applications, it is a much easier model to understand.

 

If you want to charge a battery for example, you will definitely need DC current. Conventional batteries don't work on AC. They have a clear + and - sign on them if you've observed carefully. Creating DC current from an AC generator is not easy with the use of just mechanical parts. The use of the suggested commutator will result in your waveform being a series of bumps. However, since the advent of power electronics and highspeed switches, this task has become trivial. A more simple method would be the aforementioned rectifier circuits. Try the full-wave rectifier with capacitor filter. This configuration works with a direct AC input

[Sarahisme]

ok thanks guys