Generator heads

[tljones]

Been lurking here for weeks, reading voraciously, but have not been able to address this question anywhere.

I was a full time farmer for 8 years before I switched lifestyles. On the farm we always kept PTO Driven generators around to power the feed machinery and shop (along with the house.... wives, geez....). These generators were generally driven at a higher speed with gears. I presume these are the 1800 rpm generator head sets that I have seen in several of the threads I searched into.

OK, so now my naivete comes shining out. I understand the idea of cut in speeds and how it relates to axial flow turbines, but how do these gen head sets respond in aa wind situation? I understand that you cant drive a large rotor that fast, but do  these generator types produce anything before they reach 1800 rpm? I fuly understand the loss of efficiency in gear sets or adding pulleys and belts, but I also understand how you can gear something if you have the torque. (We had a roller mill for grinding corn that ran off a 1000 rpm pto that spun the 14 inch rolls at over 22000 rpm..... think a wind turbine is loud?) Also obviously load comes into play. I remember running a welder off these gen sets and being able to pull a tractor down a bit like that, so obviously we are talking about a large rotor turning at low rpm with high torque. What happens these in an overspeed situation?

I quess in general the many hours of research I have done here and other places lead me to believe that the axial flux designs produce power over a much wider range of rpm's, but has a real size limitation when you get above 1.5 kwts becoause of heating issues. The generator type (induction???) has less range at the bottom, but are more efficient as power outputs climb???

If this is a Newbie type post please move it, but I would appreciate input from the experts. I am a moderator at several other sites in the hunting vein and know that newbie type forums and posts get old for the veterans.

Could I suggest a reading list be posted for the Newbie forum. In our hunting forum we have made what they call a "sticky" post in what would be your wind forum.... Then when someone has a post that simply must be read by anyone who has an interest in that topic it can be stuck there at the top of the forum for everyone to find easily.

Just a suggestion. This is far and away the best site of its kind that I have found. I appreciate your time.

From the frozen plains of the southern Dakota...

Tom

Resectioned to Newbies because it likely will generate information that newbies could use. A good question, tho. -T

[SparWeb]

Welcome to the board, Tom!

I'm not the most experienced, but I didn't want your post to go unanswered.

The point of re-winding induction motors, or scratch-building the axial flux generators, is to create an electrical generator whose speed range matches the speed range of the wind mill.

1800 RPM, as you pointed out, is totally unsuitable for a windmill of any reasonable size - the blade tips would literally be spinning at Mach 1.  

Induction motors only work as generators in a small speed range.  A wider speed range is necessary.  A typical range could be 180 RPM to 400 RPM.  In that case, the generator produces enough voltage to give the battery system a charge at 180 RPM.  By 400 RPM, the current is heating up the wire, to the epoxy's limit.

The consensus among other members here is that the Axial Flux type (the flat disks with magnets type) is more sensitive to overheating - the disks warp and scuff.  A converted motor is much hardier, but also much heavier, of course.

Have you seen Dan's posting from Monday the 5th?  Your tractors will still be handy!

"From the frozen plains of southern Alberta..."

[maker of toys]

the biggest difference between the home-build style Permanent Magnet Alternators and the engine-driven commercial units is the source of excitation.

Permanent Magnet units are always of the 'self-exited' variety. . . says so right here on the label <G>. these are great for RE applications where you have a big battery bank and a small charging source; the bank holds the system voltage more-or-less constant, so the PM alternator's inherant lack of regulation isn't a problem. the UP side is, because there is no need for excitation current, every last milliamp goes to your loads. The worst thing is there is no way to shut them off without stopping thier rotation. (google 'dumpload' for some insight into the problems and opportunities this creates)

Automotive alternators can produce power over a very wide range, because they use an 'externally excited wound field' and depend on the battery supplying current through slip-rings to the field. the 'voltage regulator' varies the current through this field according to output voltage (usually referenced to the battery voltage; A very few use a different, internally referenced regulator) and hence controls the output current so that it balances what is being used. the problem with them is that they can use 50 or more watts of your hard-earned power just to keep their fields up; and they use more field current at lower rotational speeds. . . which in a constant-pitch, direct-drive machine, means that the field demand can be more than the total output of the alternator at low wind speeds. (to say NOTHING of the NET output!)

Some automotive alternators can be coerced into boot-strapping themselves with residual fields but it's not a phenomenon I'd be banking on. (I have one such in one of my cars; it would usually 'catch' about 3500 engine rpm, but once running, would regulate down to 500 engine RPM. Some days (usually cold, rainy nights) it refused to catch even at 6000 rpm, and I had to fiddle around with a test lead to get the thing operating.  after one particularly miserable evening, I got fed up and rigged a relay to kick it in the slats every time.)

the engine-driven sort of stand-by generator ALSO needs some sort of power source to build up its field. Ditto the big giga-watt baseload machines. sometimes they have a separate 'excitation' winding or section (often a residual field bootstraps these) and are 'self-exciting; others use the starter battery and/or the engine's alternator to provide external excitation to the field. the 'idle-down' sort of consumer-grade  portable genset, polyphase standby sets and diesel-electric locomotives tend to fit somewhere in this last category.  Baseload plants often use the grid itself for excitation.

It seems to me that an old engine-driven or PTO gen head might be a good candidate for an induction conversion; particularly the 1800 rpm units.  I would not count on them working well unmodified in a wind turbine setup, though.

-Dan

[maker of toys]

ggrrph.  cell modems don't get along with scoop.

[maker of toys]

geez, I'm brain dead tonight. the whole point of my little digression about my car was:

the problem with 'self excited' units that bootstrap on residual fields instead of permanent magnets is that they need to be turning pretty fast for that weak residual field to produce significant current in the field windings. . . once they do, they're fine, but they're not suited to low, speed, intermittant powersources.  Which leaves the externally exited and the permanently exited units as the viable options.  External excitation, by its very nature, is not suited to a low-energy-density prime mover, as the ratio of power used by the field windings to the total power available from the energy source is usually bad.

Hopefully that clears THAT up.  If not, ask qustions and I'll see if I'm any more coherent tomorrow.

I'm going to bed.

-Dan

[Flux]

Your pto driven alternator was intended to run at one speed because it was intended to supply power at a fixed frequency.

All synchronous alternators can work over a wide range of speeds if you are not committed to a fixed voltage and frequency. The type of alternator you were dealing with would be restricted at the lower speeds by the need to excite its field.

If you use permanent magnets to supply the field, the thing will produce power at any speed and the output voltage will be directly proportional to the speed.

With a wind turbine, for charging batteries the voltage produced must be above the battery voltage at cut in speed.

The ideal charging arrangement would let the alternator voltage rise with speed so that it worked at high efficiency over the range.

For simplicity the things are dragged down to constant voltage ( battery volts) and so they should be constrained to work at virtually fixed speed. This results in stalling the turbine blades and to gain a good overall efficiency the alternator efficiency is compromised to keep the turbine efficiency reasonable.

When an alternator is run in an overloaded state( low efficiency) much of the output goes into heating the windings. The converted induction motors ( permanent magnet iron cored alternators) have a fairly direct way to transfer the heat from the windings to the iron core and the core is cooled by the wind.

The air gap designs can only loose heat by convection and radiation and are fairly sheltered from the wind so they are more restricted in the heat that they can dissipate.

There are other factors that influence things as well, many of the motor conversions operate with higher cut in speeds and the speed range can be widened by a phenomena associated with the higher reactance of windings wound on iron cores.

I can think of no other application of an alternator where it is intentionally forced to work at low efficiency, but it is cheap simple and reasonably effective for modest sized machines.

The size and cost of any alternator for a given power out and efficiency is related to its operating speed and any very low speed alternator will be big and expensive.

Few other applications let you trade efficiency for size and cost. The solution to the heating problem is to make the alternator efficient and modify the loading scheme. The simplest way to do this is to add resistance in the line so that the heat is generated outside the alternator but it raises the cost and weight of the alternator. If you don't want the losses to occur as heat then you can't use the simple direct connection and have to do something clever but the efficient alternator stays big and expensive. When dealing with a free power source, absolute efficiency comes second to the initial cost and weight to some considerable extent.

I am not entirely sure what your question was, if I have answered the wrong thing, ask again.

Flux

[tljones]

I am almost getting it....

What really kicked this question off in my head was the post on the 11 kw system on the wind page.... In the shcematic, body transmission wise that looks just like one of our tractor mounted 10 or 12 kw generators (pto driven).

So am I correct stating that in order to use a horizontal axis generator like that you would need to convert it into a PMA style generator, (I still am not sure what all is involved in this conversion, I understand mounting magnets, but when we start talking about rewinding I get a headache...)? You need to do this because otherwise the generator will not produce usable quantities of power until you reached 1800 rpm through the gear mechanism.

Can someone point me to a good thread regarding this type of conversion? I think I could find one of these out here that has been damaged or something for relatively good money....

Tom

[ghurd]

You may want to seach here for 'conversion'.  Pick the right motor and it won't need rewound.

G-

[Flux]

That machine you are talking about uses a gearbox to raise the speed to somewhere near 1500 rpm average and it is still a wound field machine, it doesn't have permanent magnets.

Once you get above about 5kW things tend to change somewhat. The gearbox is expensive and often troublesome and is never worth considering for small machines.

With bigger machines it will be easier and cheaper to use a gearbox.

The wound field machine is much easier to control and has significant advantages, its down side is that it reduces the output in low winds. By the time that you get to a 10kW machine you may not be so worried at wasting several hundred watts in low winds,

The unfortunate thing is that the wound field machines are very large and not cost effective for direct drive at low speed.

Everything with wind is a compromise, the various schemes all have good points and bad points. Wind resource and initial cost will dictate the best choice for a given machine. One thing is absolutely certain the best choice will always be the most expensive in the short term. Over a period the economics may change.

Flux