Estimating Generator amperage output

[k0balt]

Sorry if this is in the wrong category.... Working on a PM based wind generator, and I am trying to figure some things out.

I have an Idea how to calculate the voltage of a given winding in a PM based alternator, based on Gauss, distance, rate of change of the field, and number of turns in the winding, but how can one have an idea of the amps that a coil can source (into an Ideal load 1:1 Ohms - source : load,  or even to a short circuit (0 ohm) load)?

Obviously, the motive force is a factor, but lets set this as ideal, as in always sufficient to give the stated rate of field change regardless of loading.

Also, of course, the resistance of the coil must be known, but there must be other factors as well?!! What (magnetoinductive) design factors influence this parameter (other than voltage, coil resistance, heat capacity, etc), and how can the ampere sourcing capability of a coil - magnet interaction be estimated?

Any Ideas?

[Flux]

You don't give sufficient details to give more than a vague answer.

The emf is the basic starting point and you seem happy that you can calculate that.

From now on so much depends on the type of alternator and how you load it. There has been information on this board on predicting output from an air gap type alternator when loaded into a rectifier and battery. I presume this information is still here somewhere but I don't know if anything was lost in the change of board software.

Air gap alternators are more or less completely dominated by the winding resistance but you will find that the effective internal resistance is not exactly the same as the winding resistance which you measure at the terminals.

For any alternator involving iron cores and particularly those with windings in slots you will find that there is an inductive component to the impedance from leakage reactance and you will also most likely see effects from the armature reaction affecting the permanent magnet field.

Such things no doubt can now be calculated by the modern finite element analysis but in the past you had to carry out various tests to find the effective internal impedance. You could get fairly directly at the synchronous impedance and for what you need that may be enough but you would need a lot more tests to separate the effects of leakage reactance and armature rreaction.

If you want to look at all this then most of the text book details of wound field alternators apply to permanent magnet machines but you will not to be able to carry out many of the tests in the same way as you can't change the field.

I don't know whether you are looking to load the alternator with an ac load or whether you are rectifying to dc but as you mention resistive loads I can only assume you are not charging batteries and any information you fine here will be mainly directed to battery charging use.

Have a look through this board and see if it is the sort of information you need and come back with a bit more detail and I may be able to help a bit more.

Flux

[k0balt]

Thank you, That was very helpful, and helped to confirm my suspicions that empirical analysis or computer modeling might be necessary, and design factors play a large and not easily calculated part.

can you elaborate at all on the air core windings? How can the effective internal resistance be calculated or estimated? Or can you give empirical data for say, the 24v 10' turpine design on the otherpower website (most up to date one)? (in this case, the resistance would be for three coils in series, right?)


Thanks!

[Flux]

If you stick to air cored machines things are much simpler, you can forget the effect of leakage reactance and with neo magnets the effect of armature reaction is negligible.

As you are referring to the Otherpower machine I can only assume you are thinking of a battery charging situation. Assuming this is the case then you can regard the load as a constant voltage sink of negligible internal resistance ( don't treat it as a resistor), it is the same as a big zener diode.

If you take the ac rms volts per phase open circuit and multiply by 1.4 that gives the mean dc on open circuit and the speed at which this comes to 24v is your cut in speed.

Now at twice this speed the emf will be 48v, you now have 48 - 24 volts forcing current through the circuit when connected to the battery. If you know the total circuit resistance you can find the current. You can do this for any speed above cut in by scaling the emf.

Neglect battery resistance ( it's milliohms) you then have to factor the alternator effective internal resistance and any resistance in the line to the battery.

The resistance of the 3 alternator coils is the phase resistance, two phases in series carry the current at any instant in a star connected machine so you have a dc resistance equal to that of 6 coils in series. Ideally you measure this resistance between two terminals by some accurate method, ( bridge or volt drop method, multimeters are not good enough at such low values).

Alternatively you can get a good estimate from the length of wire in the coil or from the weight of copper, there are tables for this.

In real life this gives a somewhat optimistic figure for the current out, for some reason the machine behaves as though the internal resistance is higher than the measured value. for the size of machine you are looking at you will find that if you use 1.3 times the measured dc resistance you will be in the right order. Don't ask me to explain it, just call it Flux's constant or something.

You should find that this gives fairly good predictions. If you have a resistive load instead of a battery you may have to do things differently. You will then have voltage dividing itself between the alternator emf and the load in the ratio of the two resistances. I have never done any measurements this way so I have no idea of the factor for the effective resistance and i6t will certainly be different with dc load and 3 phase balanced load.

Hope this helps
Flux

[hysteresis]

This 'Flux Factor' of 1.3 is a differential constant from 1.414 -8% which is 1.414 - 0.11312=1.30088 which is aka efficiency loss from 100% inherent in the design structure of the alternator itself.

Normally it would be at or above 10% in loosely coupled magnetic and RF circuits. In laminated iron cores it gets down to under 5%, and with ferrite cores it can get to about 3%, which is usually in an oscillator driver of a regulated power supply.

With such low resistances and moderate currents in the otherpower system, the variables of capacitance are to low to play into this design in any way where it's a major factor. The long run of wire from the generator to the load is the biggest R that plays into the overall transmission. The capacitance of this run should be calculable and considered in real long situations.

The lack of iron cores and line transformers is what keeps this entire system concept simple for design and implementation.

[k0balt]

Thanks to all for the very informative and well elucidated explanations, I have what I need now to move into phase II  of my diabolical plan.... If it yields something useful I will be sure to share it with the fieldlines board - thanks for those who set up and maintain it, as well as the knowledgeable members.

For others stumbling on this thread, this spreadsheetl based calculator might prove interesting....

http://fieldlines.com/board/index.php/topic,143174.msg962234.html#msg962234