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Permanent Magnet alternator Synchronous reactance

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TheMerovingian

Wed Feb 23 2011, 07:59PM

Registered Member #14 Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383

I have converted a car alternator into a PMG in order to use it for a wind system. I have read that the main parameter determining the performance of a synchronous alternator is the synchronous reactance. I have some questions that i cannot find around:
1) this "impedance" is frequency dependant, or only the "leakage" part of it (Xsynch = Xind + Xleakage)?
2) In a textbook i found that this reactance has a value that is 10-30 times the ohmic phase resistance.

Steve Conner

Thu Feb 24 2011, 09:00AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Hi Jonathan,

The reactance of an alternator is made of two parts, synchronous and subtransient.

The subtransient reactance is exactly like leakage inductance in a transformer, and depends on the construction. It determines how the alternator reacts to very sudden changes in load. Compulsators are just alternators designed for lowest possible subtransient reactance.

The synchronous reactance is a measure of how easily the field flux can be distorted by stator current. Reactive current in the stator produces a flux that fights the field winding and tries to demagnetize it. Since the rotor spins at the same speed as the stator flux, the demagnetizing field looks like DC, so the synchronous reactance doesn't depend on frequency, as far as I can see.

Both factors are under the machine designer's control. That's you since you started messing with it. :) Using a permanent magnet for the field should decrease the synchronous reactance. Ceramic and Nd both have permeabilities the same as air, so the demagnetizing flux won't be able to pass through them.

Small machines always have more impedance than you'd like, big ones less. Power station sized alternators have their reactance deliberately increased to prevent expensive and messy compulsation in the case of a short circuit on the generator busbars.

Marko

Thu Feb 24 2011, 07:19PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

Hi,

Both factors are under the machine designer's control. That's you since you started messing with it. :) Using a permanent magnet for the field should decrease the synchronous reactance. Ceramic and Nd both have permeabilities the same as air, so the demagnetizing flux won't be able to pass through them.

This is most surprising for me - so you're implying that if I demagnetized a ND magnet some way, it wouldn't be attracted to other magnets at all? It's even more puzzling for ceramic magnets, aren't those basically just high-hysteresis ferrite? And wouldn't a low permeability material just refuse to get magnetized externally after all? (no dipole-turning force and hence no energy transfer)

I don't recall too well, but I think that the typical way of preventing the "synchronous reactance" problem is installing a sort of shading rings on the rotor coils.
But maybe I'm wrong since this would imply that the demagnetizing flux is AC, not DC.

Marko

Steve Conner

Thu Feb 24 2011, 07:38PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The shading ring resists rapid changes in flux, so it reduces the subtransient part of the reactance. The synchronous reactance is more about slow changes. Actually that is an oversimplification, the thing will have several LR time constants due to eddy currents in various parts of it. But before computer simulation, two reactances was all engineers could cope with.

Also, yes. Try putting a neo magnet in a coil and see how it affects the inductance. Watch out for eddy current losses in the nickel plating though. One way of seeing it is that the hysteresis is so high, the dipoles refuse to budge until a very high coercive force is applied, then bam, they all flip at once and the magnet is magnetised. It has a large permeability for large signals, but an incremental permeability of 1. Alnico and magnetic steels have permeability for small signals too.

TheMerovingian

Thu Feb 24 2011, 08:14PM

Registered Member #14 Joined: Thu Feb 02 2006, 01:04PM
Location: Prato/italy
Posts: 383

I gathered the following data about the converted car alternator:

Open Circuit voltage: 1.25Vrms (x phase) 2.2Vrms (star) / 100 RPM

Short Circuit current:

Issscc (x phase) (steady state short circuit current)
230 RPM 4.15 A
280 RPM 5.10 A
460 RPM 8.35 A

With those data the syncronous reactance must be in 0.67 Ohms range (70mOhms is the ohmic resistance, 0.69 total impedance) with the "leakage" part (subtransient) being negligible at those frequencies. This value is quite high, compared to my air cored permanent magnet generator (piggott-type) but of course the field generated by the stator coils will oppose also to permament magnets lowering their field by sovrapposition of fields.

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