Self excited induction motor/generator [Archive]

Tony Savage

29th March 2007, 21:22

Hi Mark,

Will post a picture of motor name plate soon. I picked up a used transformer today. It's a 10kva unit with multiple input taps (570-630 volts) and 240/120 volt output. I'll post pictures of it too. Now that I'm able to load up the motor better, my poor old lathe can't keep up. It blows the thermal overload. Now I am using a 5.5 HP honda gas engine to spin it up. Because I had to split up the capacitors into the series parallel arrangement, I don't have the capacity (pun intended) that I had before. I cleaned princess auto out of their supply and will have to see if I can get more. I was running 135/270 micro farads before. Now am using 90/180. I did check voltage across capacitors with no load at full voltage output and it is 330 volts. Cutting it close with 370 volt rated capacitors. They do kind of resemble sticks of dynamite lol. Haven't had much of a chance for testing but here's the first run. Engine running full throttle. Motor turning at 1200 rpm. 410 volts at transformer primary and 6.6 amps. Works out to 2700 watts. I need more power to turn up the motor faster. I have a 454 chevy sitting in the garage, I wonder... Will try to post pictures and more results tomorrow.

Tony Savage

31st March 2007, 19:10

After some experimentation today I realized that the capacitors play a big role in dertermining the start-up speed, the voltage and current available and the load in order to turn the shaft. The amount of power that it takes to turn the shaft, even with no load, is substantial with a larger capacitor bank. One thing that I have to investigate further is the effect that the transformer has on everything. As it stands right now with the 90/180 capacitors, the Honda engine can rev up fairly well (about 3000 rpm). If I increase capacitance to 135/270 then engine will hardly rev at all. This is with no load. Adding load at this point drags down engine slightly but not very much. The conclusion that I have arrived at is that it may be advantageous to start with a smaller capacitor bank to keep resistance of turning the shaft lower while maintaining a higher voltage and frequency. Then, when voltage reaches a predetermined point, cut in extra capacitors for added output and resistance to turning the shaft. One nice thing with this feature is that the blades will have a large resistance to turning even with no load on the motor/generator, especially with a large capacitor bank. I really do need something with more power in order to more proper testing. That 454 chevy might not be so far fetched after all.

Tony Savage

31st March 2007, 19:32

I'm not sure if I should have started new thread for this next part but it is part of my setup so I'll post it here for now.

We just received (at the marina where I work) a new Xantrex XC5012 battery charger. I was able to borrow it for the weekend for testing purposes. It is a 50 amp, 12 volt charger with an automatic universal AC input of 100-260 volts, 47-63 hertz.

Attaching the unit to my transformer directly resulted in not enough load incurred to prevent generator from self exciting and producing power. Once voltage reached a certain point, the charger turned itself on and started to charge my battery bank. It's difficult to pinpoint to exact turn on voltage because the voltage drops when it does turn on and it has quite a delay to turn on when the voltage is low. This thing is pretty impressive I'd have to say. Once it is on and charging, the voltage can drop down to 28 volts (charger AC input) and it still hangs in charging at 10 amps. It takes 48 volts to charge at 30 amps and 59 volts for 40 amps. It peaks to 42 amps until input voltage reaches 110 volts where it then kicks into max charge rate of 52 amps. There are, of course, a couple of drawbacks. One, it has a hefty price tag of $512.00 can. my cost. Two, only 12 volt output. They do make a 24 volt unit but only 25 amp output.

Laurie Forbes

31st March 2007, 19:46

Tony, I am uneasy about the way your gen is behaving ie. the large input required to turn the thing with no load applied. This implies to me one of two things; there is something amiss in your setup (maybe a shorted cap, wire or winding) or, the gen is internally consuming a lot of power thru the self-excitation process. The first is correctable but the second, if the case, is going to eat a lot of your output and, make the turbine hard to start.

I'm not expert for sure but I've never heard of anyone using a gen of this nature for wind power (maybe the losses are just too high). I guess you will get a better handle on the efficiency once you get a more powerful driver but if the efficiency turns out low, I would definitely consider a PM rotor solution.

I don't recall if you are on-grid but if you are, another route is a grid excited motor/gen which I understand work quite well but with the disadvantage of achieving no useful output until the gen is revved to above synchronous frequency. Advantages are automatic frequency and voltage control plus no mods needed to the motor.

Not trying to rain on your parade...........

Tony Savage

31st March 2007, 20:01

Yes, I was quite uneasy too at first thinking the same things. But after switching out the capacitors and changing the values, I'm pretty sure that it is just the nature of the beast. By adding capacitors as wind speed picks up, I can go from a 1000 watt generator to a 5000 watt one. The load that occurs when I start turning the shaft with the larger capacitor bank, seems to be just "recirculating" power in the motor windings. The power is there, it is just not being used. Capacitors remain stone cold to touch and the motor is barely noticeably warm after hours of testing. I think that this thing could really crank out the power if I could turn it fast enough. The other problem is that the Honda engine does make any power until it is reved up to 3600 rpm. I can't get past 1200 rpm with the bigger capacitors. Will chuck motor back in my lathe tomorrow and compare previous results to now with the transformer attached to see if anything has changed.

Mark Parsons

31st March 2007, 20:26

Hi Tony,

Great info. The capacitors present a reactive power load on the motor. I suspect this is why most people only use the minimum capacitance required to ensure self excitation. Turbine blade stall could also be a problem with large starting torque requirements.

You make a very good point though about using more capacitance to get your cut-in at a lower RPM. This method may be sacrificing some electrical efficiency. Further data and experimenting will determine. Switching in capacitors above sepoint voltages is a simple matter with voltage monitoring relays - http://www.crouzet-usa.com/catalog/_voltagecontrol.shtml

I use a Xantrex C40 Charge Controller at my cottage and am happy with it. The LCD display doesn't like the extreme cold in my garage (fades out) but other than that no problems. It only handles up to 55VDC input so not an option with your concept.

A little googling brings up this Dutch made universal charger (90-265VAC in) http://www.victronenergy.com/upload/documents/CCH012020000-D-cEN.pdf It is a marine duty unit that has 3 outputs that allows it to simultaneously charge 3 battery banks each at full rated ampacity. At 24V*100A = 2,400W * 3 circuits is 7,200W for the largest of the family. Very close to your 10HP (7,460W). Price tag is 1,500 euros.

Mark

Tony Savage

1st April 2007, 08:18

Hi Mark, thanks for the links.

The Xantrex unit has three bank capacity too and is able to supply full output to any bank but not all three simultaneously. I think you'll find the same thing with the Victron unit as well. Other than that it looks like a great unit, certainly a lot more powerful than the Xantrex. Will have to enquire about pricing.

I did post photos of the motor plate etc. It was my first post yesterday but it did show up. Do posts with pictures have to be screened by the administrator before they are posted? If not I will have to re-post.

Rob Beckers

1st April 2007, 14:14

Hi Tony,

There's no approval/censoring of text or images. If you post images they should show immediately. There are two ways to include one or more images in a post: The first is to include a URL to the image in the text, the easiest to do this is to hit the little 'picture' icon at the top of the editor (it'll then ask you for a URL). The second way is to include it as an attachment. This is done by clicking the small 'paperclip' icon at the top of the editor. This will open the attachment manager and let you browse for the image on your computer, then upload it.

Hope this helps!

-Rob-

Mark Parsons

1st April 2007, 17:05

Hi Tony,

I just re-read that Victron Energy Centaur battery charger spec sheet, and it states:
Three outputs that each can supply the full output current
Three isolated outputs to simultaneously charge 3 battery banks
Each output is capable to supply the full rated current.

Might be semantics or advertising spin but sounds to me like 3 x rated current.

1 Euro = CAD $1.54 at today's rate.

Regards,
Mark

Tony Savage

3rd April 2007, 17:45

It would be nice if it did output all three batteries full power simultaneously but it sounds like the typical carefully worded sales pitch that we hear all the time. Hopefully I'm wrong because that would be real kick a$$ unit. It is still pretty good even at 100amps @ 24 volts on one output. A little pricey but two would still do me fine as long as the negative output post is isolated from the chassis ground. I could run into problems hooking up to a 48 volt bank otherwise. A 48 volt/100 amp unit would be the cat's meow.

Mark Parsons

3rd April 2007, 20:58

Hi Tony,

Here is a Canadian company offering a 1000W universal input charger for 12 -72 volt batteries.
http://www.delta-q.com/products/quiq.shtml

You may be able to parrallel them up for more power. Best to ask the manufacturer.

Mark

Tony Savage

4th April 2007, 19:18

Hi Mark,

Tried to upload pictures of nameplates on motor and on transformer again. I apologize for the crappy quality. Interesting link for the Delta charger. You continue to amaze me with sites that you find, one of the reasons why I like this forum so much. Keep up the good work.

I received eight more capacitors today so hopefully will be able to do some real testing this easter weekend. I think I've figured out how to apply the necessary torque to use the larger capacitor bank. Time will tell.

Tony Savage

8th April 2007, 10:36

Well, I learned something about capacitors today. I have my Kohler generator engine driving the 10HP motor and it handles the load with no problems. It should, it's a 4cyl. engine after all. I secured the motor in my lathe so that I was able to measure the torque being applied. With transformer wired into the circuit and my capacitors wired in a series/parallel configuration, I proceeded to do extensive load/rpm/capacitor comparisons. I started to realize that my outputs did not seem to concur with previous experiments. I was beginning to think that I had some dud capacitors or something. After checking on the internet I discovered that my capacitor bank is only half of what I thought it was. My error was made by assuming that two 30 micro farad caps in series doubles the voltage (which is true) and maintains 30 micro farad capacitance (false). Actual capacitance is only 15 mF! I have 24 capacitors right now. It's going to cost me another $250 to double that. Oh man, the price you pay to run at 600 volts. Anyway, the testing still looks promising. The transformer doesn't seem to affect efficiency in any appreciable amount. For example, at 830 rpm which is about 30 Hz, I get 741 watts of output for 1030 watts of input. That efficiency gets better the more load that is applied, I can't give exact numbers because my clamp on AC ammeter isn't reading correctly and I can't use the inline one with the big 4800 watt heater. It reaches approximately 85 % at 1000rpm with around a 4000 watt output.

Rob Beckers

8th April 2007, 11:38

Tony, could you snap a picture of the whole test setup. I'd love to see how you hook up the motor to your engine and lathe. How do you measure torque? Strain gauges?

-Rob-

P.S. You (and others) probably know this, but just in case: To attach a picture there's no need to resize it before uploading. Just take the entire (large) camera .JPG file and upload it to the attachment manager. The system will automagically resize the resolution and size to fit within the forum limits.

Mario De Castris

8th April 2007, 13:56

My thought exactly, I need to find out some way to measure torque for when I start my test, I'm sure there is some mathematical way unfortunately I must have slept through that math lesson.

Mario

Mark Parsons

8th April 2007, 20:47

Hi Tony,

More great info.

Sorry, I didn't warn you about the resultant capacitance of series connected capacitors. Series capacitors provide a net capacitance similar to parrallel resistors. In a series connected chain of capacitors or parrallel connected resistors add up the inverse of the values and invert this total. i.e. a series string of 4 capacitors 2 - 10uF and 2 - 30uF = 1/10uF + 1/10uF + 1/30uF + 1/30uF = 166,666. inverted = 6uF net.

Just to double check your efficiency data. You are calculating your input power from the measured torque and RPM? Torque is your pull (fish?) scale weight reading muliplied by a tape measured (machinists scale?) radius to the weigh scale connection point on the motor housing? Your output power is from measured Volts AC * Amps AC from inline ammeter? This measured output power is on the secondary side of the transformer feeding a purely resistive (4800W) load? You mention your amp clamp is unable to read properly - due to lower than 60Hz frequency? A true RMS amp clamp should resolve this inaccuracy.

Your data indicates 830RPM is 72% and 1000RPM is 85% efficient. The 830RPM is likely about 60Hz*830/1800*95%(slip) = 26 Hz. Good to see the transformer not presenting much impedance at this lower frequency.

Is the system much good below the 830RPM point? A 20' diameter 3 blade wind turbine will have a power harvesting cut-in speed of about 50RPM.

Mark

Mark Parsons

8th April 2007, 21:39

Hi Tony,

Just tried to find some further data on your 10HP Lincoln motor. The picture of the rating plate answered a few questions but raised more.

I couldn't read the frame number from the photo - is it a 215T? I also couldn't read the model number. I see that Lincoln was bought by Leeson motors so am sure some product rationalization has occurred.

A concern on the plate is the statement "Drip proof". This is normally only referenced for open frame motors. Is it a TEFC (Totally Enclosed Fan Cooled) motor? I wouldn't use anything environmentally rated lower than a TEFC motor for a wind turbine application. A TENV (Totally Enclosed Non-Ventilated) motor might be slightly better due to no spinning fan working against the wind and efficiency. These are typically heavier and more expensive. Easiest to just remove the fan from a TEFC motor. The wind provides lots of cooling. Wind blown rain and snow will quickly degrade an open frame motor rotor and bearings - even if designated "drip proof".

It looks like a 1.15 Service Factor rating. A 1.15 service factor means the motor can provide rated power without meltdown at 115% of spec conditions - i.e. ambient temperature, dust / grime limiting cooling ability. Most heavy duty TEFC motors will be rated with a 1.25 service factor.

Here is a Leeson catalog link showing Lincoln TEFC info (no 600V) along with MSRP:
http://www.leeson.com/cgi-bin/nicefetchpdf.cgi/literature/bulletins/pdf/1050/lincoln_ac_3phase_tefc_rigid_cface.pdf

Thanks,
Mark

Tony Savage

10th April 2007, 18:30

Okay, where to start.

Rob, I will take a picture and upload ASAP. I use a digital fish scale to measure the input torque. Initially I was measuring off of the frame and doing the calculation to convert to foot lbs. I now have an arm attached to the studs holding the motor together. It has a hole at exactly 12 in. from the centre of the motor shaft. Thanks for instructions for JPEGs.

Mario, other than having a lathe to hold the motor shaft, I don't know how else you could easily measure input torque. When I post a photo you'll see how I do it.

Mark, I am not near the motor right now but will confirm frame number when I upload photos. It is an open frame motor with cooling fan. The entire thing will be totally enclosed so I'm not too concerned about the bearings etc. The fan doesn't seem to be much of a load. I may try a test with and without to see if there is any measurable difference now that you have mentioned it. The loads that I am using are purely resistive and I am measuring the output side of the transformer with an inline ammeter where possible. I'm sure my clamp on ammeter is a true RMS one, I just can't get it to work properly, even using line voltage. I had this problem before but can't remember how to resolve it, it'll come to me. Right now, the cut in rpm is 800 with 90/180 mF capacitors. I need more capacitors to try at the higher rpm/voltages. At slower speeds with the capacitors hooked up individually at 135/270 mF, cut in rpm was much lower, 500 rpm, I think. With a 10 to 1 gear reduction/increase that translates to your 50 rpm blade speed which puts me in the ballpark at least.

Tony Savage

10th April 2007, 20:52

Mark, frame number on motor is what you thought it was, 215T. What do you guys think of my $100.00 Kohler generator? It came out of an old salty dog of a boat and didn't look quite that pretty when I got it. I had to add the radiator and muffler. There is an alternator as well but had to remove it for this testing. You can see some of the capacitors on the floor.

Ralph Day

11th April 2007, 06:10

Tony,

Please excuse if this is a total insult to your intelligence post...When using your clamp meter are you sure to be only clamping around one conductor? I didn't know that when i first got a clamp meter. True RMS is a must for ac current measure as you probably know. Just separate the conductors and use one for your measurement.

Nice genset, nice price! What kind of output, what kind of HP on drive unit? Diesel/gasoline/majic smoke?

ralph

Tony Savage

12th April 2007, 18:22

Hi Ralph,

No insult taken. I do only measure one conductor, it's just something quirky with this meter. The generator is 7.5 KW and has a four cylinder gas engine that runs at 1800 rpm. Not sure how much horse power, but it seems to have lots for what it does. It's what we call a "flat head" engine and is super smooth and quiet. It appears to be economical too. I ran it for an hour with about 2000 watts of load and it used less than 1.5 liters of fuel.

Ralph Day

13th April 2007, 06:04

Hi Tony

your gennie sounds like a sweet unit! A 4 cyl gas would put you at about 20hp i'd guess. When running the 1800 rpm is a great improvement over 3600. When you run some gens the din is unbearable.

Fuel consumption is good too. My diesel 10kw will use about 2-2.5 liters per hour under 70% load (or under no load too, friction losses i guess). Can you remote start or does it require your presence...do you need to remote start ie are you grid tied? I wuould assume so by the look of your shop equipment!

I went to a buddy's to weld some solar H2O racking this week. I've not run my genset->welder for more than a year. I had some genset engine control module issues when i'd weld, intermittent shutoffs were very inconvenient. The welder was the only load that would make the genset's governor "operate" (lug, respond) meaning the genset was at or beyond operating limits while striking and holding an arc (stick ac welder). Mig is more fun anyway:)

Ralph

Tony Savage

13th April 2007, 19:49

Hi Ralph,

Yes, welders can be quite demanding. I have a TIG/Stick welder and it has tripped the 60 amp breaker. Max current draw is 90 amps!! Definitely grid use only unless you happen to own a 24 Kw generator or better.

Ralph Day

14th April 2007, 16:21

I think it's time to take my welder to the neighbour's and swap hardware for watts (when i need to weld something). I thought it was a bit of overload for my gennie.

ralph

Mark Parsons

15th April 2007, 22:38

I have been thinking further about applying more off-the-shelf technology to extract the best 'bang for the buck' in a DIY wind turbine. Tony's data creates a compelling argument to consider an induction solution.

Photo below shows a cut away for a Nord double reduction helical gearmotor. These are very robust units that utilize hi-efficiency TEFC induction motors. Efficiency of the gearbox is rated over 95% and for outdoor use with the proper synthetic oil. With a 95% gearbox coupled to a 92% motor, blade to wire efficiency could be up to 87% efficient. We integrate a lot of these gearmotors over the course of a year. Prices start in the C$600 range for 1HP size and I would guess about C$1,500 for a 7.5HP unit.

With a variable pitch blade system (Laurie is perfecting) coupled to one of these gearmotors with a 4:1 - 8:1 ratio depending on blade diameter and TSR, add a few excitation capacitors and an over voltage relay and you're making power very efficiently and relatively economically.

Prairie Turbines uses a 15:1 ratio. This seems too high for a double reduction ratio and is likely into the triple reduction range, reducing efficiency. They need the high ratio to get the induction motor up to synchronous speed for direct grid tie. If direct grid tie is not required, and running the induction motor at 50% speed gets self-excitation operational, then a lower ratio, more efficient, lower starting torque gearbox is preferable.

Mark

Laurie Forbes

16th April 2007, 12:52

I have been thinking further about applying more off-the-shelf technology to extract the best 'bang for the buck' in a DIY wind turbine. Tony's data creates a compelling argument to consider an induction solution.

Photo below shows a cut away for a Nord double reduction helical gearmotor. These are very robust units that utilize hi-efficiency TEFC induction motors. Efficiency of the gearbox is rated over 95% and for outdoor use with the proper synthetic oil. With a 95% gearbox coupled to a 92% motor, blade to wire efficiency could be up to 87% efficient. We integrate a lot of these gearmotors over the course of a year. Prices start in the C$600 range for 1HP size and I would guess about C$1,500 for a 7.5HP unit.
Mark, how does these gearboxes react to being driven backwards (any derating required)? An efficiency of 95% is certainly attractive - I'm surprised it's that high considering seal friction etc.

And, what is the nominal motor speed?

Another possible advantage is that they would not require an enclosure(?).

I'm still kind of partial to conversion to a PM gen however - I believe that lower speed efficiency (which is where most of the KWHs are generated) would be considerably better than self-excitation.

Mark Parsons

16th April 2007, 13:29

Laurie Forbes

16th April 2007, 14:28

Hi Laurie,

Some of our applications involve overhauling (normally due to gravity). The gearbox reflects this overhauling back to the motor efficiently. Where overhauling is present we have to use a brake motor. The manufacturer (Nord) doesn't publish any specs on reverse power transmission. This also means warranty is likely void for the application. :(
I would guess void warranty is to be expected for any such conversions.

I'm thinking that one of these gearmotors with a condensate drain option and filled with appropriate oil should be adequate for direct outdoor exposure. Output shaft and coupling may need some protection.

Yes, and the main shaft/bearings, and the pitch mechanism - looks like running naked is probably not that great an idea (unless all the exposed moving bits were stainless but that could get expensive). Might be worth looking at though as making an enclosure is pretty tedious and of course it has to be removed or opened for maintenance or fiddling. I made a wood enclosure for my previous mill (the gen & sprockets/chains only) and it was a a pain to work with.

One would need a stainless main shaft and a pitch shaft & torsion spring for each blade, with two bearings for each shaft along with stainless (or bronze?) pitch sync gears.

Just looked in a MSC catalogue and found 1" stainless pillowblocks for $135 and 1 1/2" for $282 giving a total of $1,374 for a three blade turbine (ouch!).

They also however list nickel plated bearings ("excellent for outdoor use") at $42 & $67 respectively giving a total of $386 so maybe that would be worth considering. OTOH, maybe Princess Auto style bearings would stand up satisfactorily (even if they had to be replaced every year or so). Cost for that option is only about $61 (which is what I have on my mill). Maybe if they were greased up properly, they would be OK.

The problem with PM generators is brush maintenance or make your own axial flux. It looks like you are flying a brush unit on your Flying Goose machine.

Mine is actually an induction motor conversion - mount magnets in the rotor (and cut off the rotor end stuff that circulates the induced current) is all that's required. No brushes to maintain, low cut-in speed and high efficiency (apart from the gear-up requirement).

Tony Savage

17th April 2007, 18:00

Hi Mark,

Thanks for the vote of confidence on the self excited motor/generator. I'm not sure that I'm wholeheartedly there myself yet. Because I'm running at 600 volts, the expense of the capacitors required becomes a factor. 600 volt plus capacitors seem to be scarce and therefore expensive. Smaller caps need so many, they get expensive too. If I were to start over I think I would go with a 208 volt motor. On the other hand, it would be hard to find a 10 HP/208v motor. I like the bigger motor to better handle higher wind speeds. I like the gear drive motor too, nice to have everything together and pre-fabbed. I suspect the efficiency going in reverse would not be as good as their rating, although the 4 to 1 ratio couldn't be too bad. I wonder if that ratio is a little low though. I can't remember if Laurie mentioned what his final gear ratio is and how well it seems to work.

Mark Parsons

17th April 2007, 21:33

Hi Tony,

I had the Emerson-EPT (Browning) sales rep in today. They have a full line of helical units similar to the Nord and SEW. Outdoor service is no problem. Rated -25F to +125F. They claim stage reduction efficiencies of 98%. That puts a double reduction at 96%. They make double stage reductions of up to 50:1.

I quizzed him on using a helical gearmotor in reverse. His literature states can be backdriven. No mention of derating when backdriven. When I told him the specific application of wind turbine he deferred to request opinion from factory experts. I have a technical enquiry in with the factory on the application. Will advise this group of factory expert's opinion. I suspect a wind turbine application falls into AGMA - class M with frequent stop /starts for 24/7 operation requiring a service factor of 2.00 or higher for robust solution.

I know the motor run capacitor voltage rating problem. To find anything rated 600V requires power factor correction caps. $$$. I would go with a 230/460V motor. Wire it in 230V mode. Run the output into a universal charger or Windy Boy for grid-tie. No transformer required. Keep the output voltage inside the charger input range using variable pitch RPM control. 10HP is about max for the inexpensive 230/460V mass produced motors. Above 10HP is mostly 460V. I can buy a brand new 10HP, 230/460V 3 phase TEFC motor for under $1,000. Used or refurbished are way less.

You may be right about the 4:1 ratio. If the gearbox motor option is only a 2-pole (1800RPM) then 10:1 - 12:1 is more in line for larger blade diameters. Ideally, a 6 (1200RPM) or 8-pole (900RPM) motor would allow a lower ratio maybe into a single reduction stage. The Prairie Turbines design is backdriving a 15:1 Nord double reduction using 20' blade diameter. They have to spin the motor at about 1850RPM to sync to the grid and generate any power. The shock loading they put into the gearbox when throwing the motor across the line must be enormous. Self excitation removes the 105% motor nameplate RPM requirement, and the shock loading from line syncing, when power harvesting into charging or inverter circuits.

Mark

Laurie Forbes

17th April 2007, 22:29

The Prairie Turbines design is backdriving a 15:1 Nord double reduction using 20' blade diameter. They have to spin the motor at about 1850RPM to sync to the grid and generate any power. The shock loading they put into the gearbox when throwing the motor across the line must be enormous. Self excitation removes the 105% motor nameplate RPM requirement, and the shock loading from line syncing, when power harvesting into charging or inverter circuits.

Mark, I am curious as to why there should necessarily be shock loading when connecting to the grid (with direct connect, grid excited motor). I was under the impression that the connection is normally made at the synchronous frequency (1800 RPM in your example) and, if so, no current flows into or out of the motor at that point. Only when the RPM further increases would current begin to flow to the grid which would make for a v smooth transition (?).

Mark Parsons

18th April 2007, 06:52

Laurie Forbes

18th April 2007, 11:17

Hi Laurie,

I might be wrong about the method Prairie Turbines uses. They sell the idea, their book and of course, the electronic control box.

I am basing my opinion on this thread discussion with one of the owners on the Otherpower board.
http://www.fieldlines.com/story/2007/2/23/43328/2693

Sync'ing a generator to grid requires matching both RPM and phase. A machine spun up on its own is highly improbably 'phased'.

I believe they use solid state relays to switch the motor across the line when above synchronous RPM by measuring back EMF. One of the photos of their control panel shows a capacitor. Under low wind conditions it bleeds the inertia of the turbine into the grid and slows the motor below synchronous RPM. It is disconnected and allowed to spin up, then bled into grid, repeatedly, until windspeed rises to keep the machine above synchronous RPM.

Please correct me if I'm wrong.

I don't think you are wrong here Mark. What they are apparently doing, as you explain above, is waiting until the RPM rises to some degree above syn speed b/f cutting in to the grid, supposedly to make use of rotor inertia. Under that situation you may have to phase match to the grid to avoid a bump. This is also necessary with self excited gens such as used by power utilities. I'm not sure that Prairie Turbines gains much if anything by this strategy - you get a burst of power but then have to disconnect and wait for the RPM to build up again so it's a case of intermittant bursts vs a smaller steady amount of generation that you would get if connecting at 1800 and leaving it there. The Otherpower post you reference mentions their claim of 2.1KW at "kickin" and it would appear then that such is the power "burst" I mentioned above - I would suggest that is in fact deceptive as it confuses peak power with steady-state power (kind of like the exaggerated HP claims you often see for power equipment and Wattage for audio amplifiers). Maybe that is why they use that particular grid cut-in technique.

I have to say I'm also a bit skeptical about their claims for not needing any sort of active rotor control, such as pitch control, to limit input to the gen. They do have a gen brake and some sort of torque limiter but I still wonder what happens in sustained strong winds - the energy has to go someplace and one would think the brake or torque limiter would heat up eventually to self-destruction. Maybe the brake is strong enough to bring the prop to a halt if it trips(?). Those wood plank blades also sound pretty iffy - I'd like to see some actual energy production data. I also would not want to be close by if one broke off :eek:

Laurie Forbes

18th April 2007, 13:43

Hi Mark,
I can't remember if Laurie mentioned what his final gear ratio is and how well it seems to work.

My ratio is 4.5 which drives my gen to 120VAC (& 1 KW) at about 1350 RPM (corresponding to a prop speed of 300 RPM). It seems subjectively at least to be about right but I don't really know as I don't have a wind speed device. I don't think I would want to run over 300 RPM and start-up capability seems pretty good also (I have devised a simple adjustable triac circuit that gradually cuts in the load after the prop RPM picks up).

Mark Parsons

18th April 2007, 19:06

I'm not sure that Prairie Turbines gains much if anything by this strategy - you get a burst of power but then have to disconnect and wait for the RPM to build up again so it's a case of intermittant bursts vs a smaller steady amount of generation that you would get if connecting at 1800 and leaving it there. The Otherpower post you reference mentions their claim of 2.1KW at "kickin" and it would appear then that such is the power "burst" I mentioned above - .

Exactly my point about shock loading on the gearbox .....

My ratio is 4.5 which drives my gen to 120VAC (& 1 KW) at about 1350 RPM (corresponding to a prop speed of 300 RPM). It seems subjectively at least to be about right but I don't really know as I don't have a wind speed device. I don't think I would want to run over 300 RPM and start-up capability seems pretty good also (I have devised a simple adjustable triac circuit that gradually cuts in the load after the prop RPM picks up).

Are you back feeding the grid with your induction motor conversion?

Mark

Laurie Forbes

18th April 2007, 19:32

Are you back feeding the grid with your induction motor conversion?
Nope - it's allowed here but there is a lot of expense and hoops to jump through. I don't think it would make much sense with only a 1KW unit - maybe 5 or 10(?).

Tony Savage

18th April 2007, 21:37

Hi Mark,

When you talked about shock load when connecting to the grid, it reminded me of an incident that occurred when I was on co-op for my engineering course. It was at Denison Mines in Elliot Lake. They use a lot of steam to process the uranium ore. Some one decided to use the excess steam to turn a generator through a turbine. On the very first run, they had it up to speed and "threw the switch". It was out of phase and it locked up. It sheared a key that was 2 inches wide, one inch thick and 10 inches long! Hows that for shock load. They did eventually getting it working. Didn't save the company though.