[SOLVED] why permanent magnet is not used in alternator?

hello,
i have seen many motor has permanent magnet on stator and rotor(practically). but in my study, there is another coil in both(stator and rotor!)
and also why alternator(A.C. generator )also like this?
in alternator, the rotor windings needed to be supplied with external or exciter (at the end of shaft)! what happen if they use permanent magnet ?
don't you think this rotor windings make it complicated?opcorn:
i have seen permanent magnets in small and medium size motors! is that reason that in large motor-generator, large permanent magnet is not available?
what will if they use magnet that made(excited)by another magnet?

thanks for the reply.
vraj

i have seen many motor has permanent magnet on stator and rotor(practically). but in my study, there is another coil in both(stator and rotor!)

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The field strength of a coil is controlled by the current if you have windings acting as a source of magnetic flux. The magnetic flux in a coil as you know is the integral of the voltage applied to the coil, so high magnetic fluxes can be achieved using this approach.

Achieving big magnetic fluxes with permanent magnets it is not very practical because they need a lot of space.

The rotor field strength required in alternators can be well generated by permanent magnets. A simple and sufficient reason why PMSG isn't used for alternators is lack of the simple voltage regulation feature, it would need an additional high power switch mode converter.

I observed that an automotive plain alternator has the rotor halves, one magnetized and the other not. You can check/confirm with iron filings on a disassembled rotor; the left toothed core and the right toothed core.

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So yes, there is a faint? permanent magnetism in the rotor to provide initial/startup generation for the built-in regulator to sense/respond/start without field being energized. I suspect one is steel and the other iron and after factory test, one stays magnetized, if not magnetized before assembly.

The field rotor coil can be replaced by a speaker toroidal magnet and will generate no problem. But the voltage will depend on speed, with no regulation. That is why there is a coil, to provide constant generation under varied demand, its current governed by the regulator.

Vraj said:

i have seen permanent magnets in small and medium size motors! is that reason that in large motor-generator, large permanent magnet is not available?

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The only kind of motor on which I've seen permanent magnet it was within a small pump on which the magnet was apparently taking the place of the bearings, perhaps to reduce dynamic friction.

andre_teprom said:

The only kind of motor on which I've seen permanent magnet it was within a small pump on which the magnet was apparently taking the place of the bearings, perhaps to reduce dynamic friction.

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magnet as bearings? how they would provide flux?

Permanent magnet excited synchronous motors and generators are frequently used, but for the said reasons not well suited as alternators if direct voltage regulation is intended. Most brushless DC motors are e.g. of this machine type.

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Magnetic bearings or magnetic couplings are purely mechanical functions and don't have to do with motor operation.

FvM said:

direct voltage regulation

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means? .

means?

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Means you want to regulate the generator output to a specific voltage despite of varying load and speed. You should know if it's the case in your generator application.