# In an Alternator, is the armature an electromagnet?

Tags:
1. Mar 19, 2016

### Metals

Hey,

I'm lightly studying 'Generating' as a topic in my Physics curriculum. I have just been introduced to an AC Generator, an Alternator, and I have a simple question: Is the armature in an alternator an electromagnet?

From my understanding, there is a U-Shaped length of wire(s) which rotates, generating a current (in a generator). This then produces an AC current in the stator coils surrounding. I believe the U-shaped rotating piece is the armature. I have been confused by the multiple interpretations of an AC generator, and do not have access to one myself. Could someone please explain this to me. Some say an iron core (nail) in a solenoid, whilst others show the familiar U-Shape I have seen in online motor demonstrations.

Also, do the electrons of an AC in the stator coils literally move back and forth? Is yes, does that mean that when they move back, there is a very short moment when no current powering the appliance?

2. Mar 19, 2016

### Staff: Mentor

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

If you ask that, then you don't understand even the basics of AC electric power. You should study that before studying generators.

3. Mar 19, 2016

### Tom.G

Yes, the rotor is an electromagnet and the current thru it is controlled by a regulator to control the alternator output.

Most alternators, those in cars, motorcycles, and power plants are 3-phase devices, i.e. there are three separate stator windings arranged so that when one winding has zero output, the other two windings still have some output to supply the load.

The different shaped cores for electromagnets just depends on the desired end usage. The main requirement is that they are magnetic; that is, they are attracted to a magnet.

4. Mar 20, 2016

### Metals

Thank you for the response.

So the armature in an alternator is an electromagnet? This rotating electromagnet is a rotor? And the electromagnet is surrounded by a radial field from the two poles of a permanent magnet?

In regard to the shape, does that mean that the u-shaped coil (shown below) would be a solid iron piece wrapped in copper wires to make it an electromagnet?

5. Mar 20, 2016

### jim hardy

There's some finesse in the definition of "armature"
Webster:

Your picture in post 4 is not an alternator , it's a DC generator.
Note the commutator rectified the voltage that got induced into the moving armature wires. Most DC machines are built that way, with a segmented commutator.

Oops, Lavioiser would chide me - Have we yet defined commutator?
Webster, again
and just to be thorough, Commutate :

Yes the u-shaped coil(s) wrap around an iron core.
But not to make it an electromagnet.
The iron core allows the field, which is in that picture the permanent magnets, to push more magnetic flux through the region where armature conductors are spinning.
That way the armature conductors have induced in them more voltage because e=BLv and flux is B.

No. The armature in any rotating machine is the winding where the magnetic flux from the field induces voltage.
That's where the electro-mechanical energy conversion occurs.
That can be either the rotating part or the stationary part.
Doesn't apply to the picture you posted. It's not an alternator.
Here is an alternator, from http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/motorac.html:

Note that the slip rings are not segmented so you get AC out not DC. That's the difference between a simple alternator and a DC machine, the commutator.
In this pink & gray picture just above the alternator's armature rotates just as in a commutated machine.
Observe the rotating part is not an electromagnet.

Most alternators are built the other way, armature stationary and field rotating for a practical reason.

The field rotates and the armature , being in the stator, is stationary.
Observe the rotating part IS an electromagnet. But it's the field.
With this design your slip rings need only be big enough to handle a few amps of field current, not the hundred amps from armature.
And they needn't be segmented which saves a lot of machining. Time is money....
Take apart a junk car alternator and study it. It is a three phase synchronous alternator. Usually all they need to restore them to operation is brushes and a front bearing.

Here's a master's thesis on car alternators for wind turbines , i thought it might be interesting to you:
repository.tudelft.nl/assets/uuid:60a3ca0e-25f0-4892-ae52-300dcb4443ab/MSc_Thesis_Application_of_automotive_alternators_in_small_wind_turbines.pdf

it's where i got that fig 2.5

have fun,
and keep your thinking simple - define all terms....

old jim

#### Attached Files:

File size:
9.1 KB
Views:
152
Last edited: Mar 20, 2016
6. Mar 21, 2016

### Tom.G

7. Mar 23, 2016

### Metals

So in the examples seen, the armature would be the multiple copper coils joined together?

What and where would the 'iron core' be? Also a u-shaped loop, but one that's covered in copper wiring? Also, is an iron core used in alternators but not dynamos or motors?

Aah, I kind of understand why an AC current is produced now after you explained the slip ring having no segments. Even when not reversing the current and only as one piece, can the slip ring be referred to as a commutator?

So the alternator diagram examples we see are not considered 'practical' generators, does that imply they are not used in actual operational devices? What would these be referred to, basic or demo alternators?

Thank you for the response.

8. Mar 24, 2016

### sophiecentaur

There are thousands of low power alternators that use a permanent electromagnet as a rotor. Regulation is often carried out in the output.
Of course they are not 'practical' generators', any more than all the other circuits and mechanisms that are used to teach Physics are 'practical'. They are simplified to give students a chance of seeing the main principles involved. I assume that you have done a google search for 'Images" of alternators and you will have seen the sort of design that's used for real alternators. The actual shapes that are used for coils and cores are very hard to see because they are fitted inside the stator windings to get the spacing as small as possible for good magnetic coupling.

But, as has already been recommended, I suggest you start at the beginning of Electricity and don't try to jump in half way through.

9. Mar 24, 2016

### jim hardy

Here is a picture of a medium sized real alternator.
You can see the u-shaped armature turns in the stator.
They run through slots in the core.

The field (rotor) fits in the round hole at center. It carries u-shaped turns that magnetize it.
Here's a picture of one, probably not from this exact machine though.

So the "Core" really is the whole magnetic path consisting of both the rotor and stator iron.
We sometimes mis-use the term to mean just the stator, though.

This would be a cross section of the rotor
u-shaped turns lie in the machined slots , well braced braced so centrifugal force doesn't sling them out.
Use your right hand rule to see that magnetic flux is indeed up as shown in the sketch.

each slot contains several turns , on purpose not all the same so as to produce sine wave voltage at generator terminals.

That "Flux Probe" on left is used to monitor flux at each rotor slot as the rotor spins by

a slot with a shorted turn in it will show less than its fair share of magnetic flux.
Shorted rotor turns are difficult to find because they usually disappear when the machine is stopped and the conductors are no longer pushed together by centrifugal force. You have to find them when machine is at speed. They cause vibration that changes with vars(see Anorlunda's insights) because of magnetic unbalance.
This little company built a business providing that measurement.
http://www.generatortech.com/index.html

So yes, what you saw is simplified teaching models.

I really recommend you spend some time at this slideshow
http://www.slideshare.net/waleedmahmoud1881975/generator-fundamentals-2 [Broken]

Lastly

slip rings are slip rings and commutators are commutators. Remember "Commutate" has a mathematical meaning.

Here's a commutator for a DC generator where armature rotates

see the segments? The ends of of your u-shaped turns connect here .

Here's a fair sized slip ring

but no segments - it doesn't commutate.
Picture came from that slideshow linked above

Have fun..

old jim

Last edited by a moderator: May 7, 2017