Alternators vs. generators

[NE2000]

OK I spent the whole semester thinking an alternator was where you rotate a magnet in a coil to induce current (as opposed to rotating a coil in a magnet). Then I read on the study design about AC alternators and DC generators. This was reinforced by a past paper where it talked about an AC alternator where the magnet was fixed.

So is it simply that alternators are for AC and generators for DC?

[naved_s9994]

Quote from TT Cheat Sheet;
Alternator: Alternating emf is induced by rotating a permanent magnet (or electromagnet) beside a coil. External circuit is connected directly to the terminals of the coil and no slip rings are required.

[NE2000]

Yeah that's what I thought an alternator was. Then this threw that definition into doubt:

http://www.vcaa.vic.edu.au/vce/studies/physics/pastexams/2007/2007physics2.pdf

p. 6 - Figure 5 shows an alternator consisting of a rectangular coil ... rotating in a uniform magnetic field

[krzysiek]

Yes, I don't see what's wrong with that?

The coil can rotate in a uniform magnetic field, because when it is perpendicular to the field, the flux through the coil is a maximum. When it rotates to be parallel with the field, as it is rotating, the flux is changing (it is becoming less) and thus an induced current is being produced, to oppose the change in magnetic flux.

That response is only if you think it cannot be in a uniform magnetic field - if that's not what you were asking, then i'm sorry

And to answer your initial question, I am pretty sure that rotating the magnet within a fixed coil will still produce an induced current - but I think the VCAA mainly uses scenarios where the magnet is fixed and coil turns.

[NE2000]

There's nothing wrong with it. I think my question probably wan't clear

My question is what is the definition of an alternator and what is a generator. Up till now I have believed that rotating a magnet within a fixed coil is what an alternator is, and TT's cheat sheet appears to suggest the same thing, but based on that VCAA exam example it appears that that is not the correct definition.

[krzysiek]

Hmm, in that case I am not too sure how much help I can be. However, a generator can be DC or AC, and from my knowledge, an alternator is just an AC generator.

[moekamo]

From Study Design:
"explain the production of DC voltage in DC generators and AC voltage in alternators, including
the use of commutators and slip rings respectively;"

so i think we only have to worry about generators making DC and alternators making AC

[sachinmachin]

yeh some company trial exams can get adventurous and test your knowledge in being able to decipher between an alternator and a generator. if that was on tmmrw's exam, i wud be really surprised

[krzysiek]

Now I'm getting a little confused of what you guys are saying. An Alternator IS a generator... just an AC one, and a generator is a generator, either DC or AC?

[sachinmachin]

an alternator is an "AC" electrical generator, so yes it can be called a generator. there are also dc generators. below gives a more detailed explanation of the differences.

'The key different between an alternator and a generator is what spins and what is fixed. On a generator windings of wire (the armature) spin inside a fixed magnetic field. On an alternator, a magnetic field is spun inside of windings of wire called a stator to generate the electricity. This allows the wires to be directly and easily connected to their outputs without the need for sliding contacts to carry the relatively high output current. The magnetic field is still generated via electro magnets mounted on a rotor, and the relatively small field current that powers them is supplied to the rotor by two small brushes that each ride on a separate and continuous slip rings. These smooth slip rings (unlike the comparatively rough contacts on a commutator in a generator) and the fact that the relatively heavy windings are fixed instead of rotating allows the alternator to be spun to much higher speeds.'

[avram_grant]

all you have to know that an alternator produces AC ONLY. A generator can produce AC or DC. i.e an alternator is a generator that produces AC, but if you add a commutator to a generator it will produced DC. So all u need to know that alternator is AC

[crappy]

Look, the simple fact is a generator is a rotating coil in a magnetic field. An Alternator is a rotating magnet inside a solenoid. There is a difference, and you must know this. NE2000, your definition of a magnet within a fixed coil is correct. I have verified this in a number of trial exams.