Matching the alternator to the blades

[DanB]

This is sort of an interesting chart we made up yesterday - based upon the tests we did last week.  Interesting to think about matching the alternators to the blades.

Alternator 1 uses the 1" x 2" x 1/2" blocks (N40 grade) and the coils are wound with  140 turns of #17 gage wire.  The cutin speed as tested was 140 rpm.  The resistance of this stator is around 3Ohms, so you can figure how much power is consumed overall - the curve only reflects what we get out - the power into the shaft is always more, especially at higher outputs.  This one is about 50% efficient at 700 Watts.

Alternator 2 uses teh 2" diameter x 1/2" discs (N40 grade) and the coils are wound with 110 turns of #15 gage wire.  The cutin speed of this one was around 130 rpm.  Resistance in this one is about half (1.5 ohms or so) of alternator 1 - so at any given output there's about half the heat in the stator.

'Betz' (the blue line) is based on a 10' diameter blade running at TSR 6.  So that is the power curve of a 'perfect' 10' diameter blade.  To get wind speed you can divide rpm by 16.8.  

Of course our blades are not nearly as good as they could be, so the 'betz' curve there doesn't reflect reality.  The TSR of the blades is also bound to change at different wind speeds because it's impossible to perfectly match the power curve of the blades with the alternator (maybe you could with an electronic MPPT controller.  In order to maintain the right TSR all the time, the power produced by the alternator (Both useful power and wasted power) would have to exactly match the power curve of the blades.  

The power curve of the alternator is linear.  It slumps off at the higher end on the graph, that's because of the wasted power (heat in the stator) is not shown there. The power curve of the blades is related to the cube of rpm and windspeed.  So a good match is tricky and we shoot to line things up best we can between about 6 and 25 mph or so.  (an intelligent MPPT controller should be able to match things up nearly perfectly I suppose)

Any time the power produced by the alternator (again, we have to consider power wasted in the stator and the line) is less than the power produced by the blades, then the blades will run at a higher TSR, and if the power produced by the alternator is less than that of the blades, then the blades will run at a lower TSR  - if it's too low, the blades will stall.

When we consider the power wasted in the stator, then Alternator 1 is a pretty good match to the blades - assuming our blades are somewhat less efficient than the betz limit, and so long as we furl at 600 - 700 Watts output or so.  This graph seems to agree with the sort of performance we've seen from those machines.

Alternator 2 would stall the blades and as it is - this would happen in very low winds because the cutin speed is too low.  It should be a pretty good match though if we open the airgap and add about 1 ohm to the line I figure.  Some don't like the idea of a wide airgap, but I sort of like the wide mechanical clearance - it's a bit wasteful of magnets but adds a safety factor I think.  The other safety factor in this alternator is greatly reduced heat in the stator.  Instead, we'll have that heat  in a resistor, between the rectifiers and the battery.

This is a 1 Ohm 1KW resistor which should be a good start in matching alternator #2 to a 48V battery.  Of course, if it were a 24 V machine we'd use a .25 ohm resistor and at 12 Volts it'd be 1/4 of that (about .06 ohm).  For a lower voltage machine one might want to build the heavier alternator just to make up for line loss - use the line as the resistor.

This is all sort of simple stuff but I thought I'd post it because I think the heavier alternator is a bit safer for 10' machines.  It might also be useful to some people building new machines to understand why you cant stick a big blade on a tiny alternator.  Getting the alternator matched to the blades is the trick with all this stuff.  

[Titantornado]

Thanks Dan. Matching is the one thing I still don't understand.  At this point, I am strictly guessing at my prop/alternator match, as there just doesn't seem to be definitive "this goes with that" numbers and a lot of it comes right down to trial and error.  Hopefully, I'll be fine.

On a side note, my project is currently stalled.  Too many necessary expenses are consuming any normally disposable income right now, along with an unstable job after a recent company buyout.  Hopefully, I can get the steel needed soon, so I can weld this thing up and finally see some electric come out of it.

[outback]

I was just wondering were those resitors came from or out of?

[DanB]

I dont really know - they're new (never used).  I got a bunch off ebay.

[willib]

i had a similar question

but i was wondering how big they were.

what size bolts and nuts are those ,that may help me give it a size relationship.

[DanB]

They're about 10" long, 2" diameter - made by 'ohmite'.  They mount nicely on 1/2" hardware.  

Ebay is a really good place to find bargains on big power resistors.

[Nando]

DAN:

Your idea of doing that chart is very good.

Check Betz curve that defines the power harvesting of a 10' (feet) machine and compare the root initial power and the discrepancy of the harvested power by the two mills.

The error maybe due to different setups without correlation to the 10 feet mill and the Betz of the two mills that may have a different power curve.

To have a GOOD correlation, I think, that instead of watts a initial cut in voltage and current is needed, then with the stator resistance a total power can be calculated to assign power lost due to the stator and power harvested by the load, though the stator-rotor gap voltage is not available. ( not measured I presume) a close total efficiency can be defined.

Determining the cut in voltage and current plus the mill size, one can calculate the Betz for such wind mill, assuming constant TSR, then data ( current and voltage) taken of the generated power with the stator resistance one can get two curves,

. 1) curve for harvested power into the load and

. 2) the power lost due to the stator resistance

which adding both, will show a total power that is below the Betz Curve, taking the difference from the Betz, One will find the stator-gap power loss ( also absorbed by the generator)

This way of taking data is too general and error prone due to lack of accurate Betz power definitions, which really requires a torque measuring device plus an accurate RPM reader attached to the mill shaft, but for this experimental basic conditions, I presume it is good enough to give a basic generalized power curves.

MPPT indeed will show a more accurate harvesting power curve with reduced stator losses, though the stator-rotor gap losses still be the same with equal currents levels.

Nando

[DanB]

Hi Nando -

Im not clear on what you mean by 'stator-rotor gap losses'.

I also think you may misunderstand the chart...

I put the betz limit on there just to show the cubic power curve of the wind at the sort of power the blades might make if they're perfect.

The alternators were not tested as wind turbines - those numbers come from testing alternators on the ground with the tractor driving them.  So there is not very much error there.  

[Nando]

DAN:

I would like to add about the RESISTORS.

I have a power resistor that is 0.6 ohms good for at least 4 KW, weight at least 12 pounds of so, the unit has two elements, spot welded, in parallel and if needed one can be cut and remove from the ceramic body.

The units looks like yours, but it is oval instead of rounded.

Size : (Inches) 5 * 2.5 * 20 and including the ceramic ends 25 inches

I believe that there are more than 100 available and if there is interest, I will photograph the resistor plus weight it to determine the shipping costs.

The power resistor is good for dump controllers, and of course your idea of series resistor for the mill.

Nando

[Flux]

Dan, that has me lost as well . I have a feeling that it is something that gets lost in translation, many technical terms translate very strangely.

I think Nando has had more experience with iron cored alternators and I think this gap term has something to do with synchronous impedance. Iron cored alternators have drops due to leakage reactance and armature reaction in addition to the resistive losses.

The whole effect is generally lumped together as synchronous impedance and it is quite an elaborate procedure if you want to separate armature reaction from leakage reactance.

With our air gap construction these factors are small enough to ignore and resistance alone is good enough to determine the fall in voltage with load.

flux

[Nando]

Dan:

Your testing and the Betz got me confused !!!.

The Stator-rotor gap is an existing fact that represents a voltage drop (in essence it is a reduction of the magnetic field) that can be represented as a resistance ( which causes power losses).

Nando

[DanB]

Yes, sorry to be confusing.

the 'betz' curve is based on a perfect 10' blade running at a constant TSR6 - so it's sort of the 'ideal' power curve.  The alternators are just data based on tests we did with the tractor into a 50V battery.

Im still confused about the stator rotor gap...

[Flux]

From Nando's comment , he does mean armature reaction.

With a conventional alternator with iron circuits and particularly with iron poles and wound fields, the magnetic field from current in the stator reacts on the field on the rotor and modifies the field.

With a purely resistive load the armature reaction just distorts the main field, but if any part of the iron is saturated it effectively results in field weakening.

Inductive loads result in field weakening, that is one of the reasons that small alternators are not capable of starting reasonable sized motors.

Capacitive loads cause field strengthening and a rise of volts.

Neo more or less acts as a large air gap ( saturated material) so pma's even with iron cores are much less affected by this effect ( magnet steels could be demagnetised)

The air gap alternators with neo show negligible change in gap flux with load so we can ignore it.

flux

[Drives]

Dan:

In the Drives industry this is a common resistor.  You find this value resistor typically used on DC drive applications to allow fast stopping of the load.  For example a crane's bridge or hoist motor drive, a printing press, ski lift, etc.  

We know that DC motors can act as generators.  As a generator, we can send power from the armature into a large resistor to actually turn the DC motor into a brake.

This feature is used to stop a load by taking the load's motion (inertia) and turning it into generated power creating braking torque.

This function to connect the armature to the resistor is performed by use of a "dynamic" brake contactor.  Once stopped, the load is held with a mechanical disc or drum brake.

AC drive dynamic brakes are the same wattage per HP size, but are higher resistance values, and probably not of much use for windmill applications.

Dean

[commanda]

I think another way of interpreting that graph, is to say that alternator 2 requires slightly bigger blades, to get the Betz curve above the alternator output at any given rpm.

Amanda