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Forum Index : Windmills : ACIM
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mel n rob Newbie  Joined: 02/01/2010 Location: AustraliaPosts: 4 |
hello folks would somebody be kind enough to tell me , do i need to energise the field of an ACIM to use it for a generator ? if this 5 kW 4 pole has its rating at 1450 RPM , then would i be getting approximately 2.5 kW at 700 RPM ? is it roughly linear ? (and 10 kW @ 2900 RPM if it was geared to get that fast) i know efficiency will drop off below a certain RPM threshold any experience with this setup ? |
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oztules Guru  Joined: 26/07/2007 Location: AustraliaPosts: 1686 |
Q: do i need to energise the field of an ACIM to use it for a generator A: Yes Q: then would i be getting approximately 2.5 kW at 700 RPM A: No Q: is it roughly linear A: No If you want to use mains to energise the field then look at this site: breezy5.5 If you want to use capacitors to excite it, you will need to select the caps for the resonant frequency it will run at... usually around the label RPM. (1450) Also, older motors will usually be better then newer cheaper motors. They used to worry a bit more about the laminations quality, and eddy currents were much lower. As this system uses the remnant magnetism in the rotor to start the sloshing in the tank circuit, eddy currents may hamper startup, as will some steels that hold little to no remnance. Beware of over exciting (too much cap), as the voltage can get out of hand very quickly. This system falls down if the load gets high or is inductive, as it kills off the resonance in the cap circuit... and it free wheels. This is a mill killer. ..........oztules Village idiot...or... just another hack out of his depth |
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| Dinges Senior Member  Joined: 04/01/2008 Location: AlbaniaPosts: 510 |
You don't specify how you intend to mechanically power the motor, but I'll assume for the moment it's with wind. If so, this site may interest you too. It's a windmill that's a tad more sophisticated than the Breezy: http://windmolensite.be/generator.html (see 'generator' for example) For the benefit of other readers, 'ACIM' apparently means Alternating Current Induction Motor. Never came across this acronym myself, and took me a while deciphering it. You can energise either by using the grid (when grid-connected) or by using capacitors (stand-alone). More information on what you intend to do and why would be helpful in properly answering your questions. The relationship is far from linear - when grid-connected, at 1450RPM it would motor...at 1499 RPM it would motor... at 1501 RPM it would generate.... at 1550 RPM it would generate even more. The term you're looking for is 'slip'. Googling that term will answer many of your questions. The link below also provides a wealth of information: http://www.allaboutcircuits.com/vol_2/chpt_13/7.html 
You normally operate at maximum -2...-3% slip (motor) or +2...+3% slip (generator), or roughly between 1440-1499 RPM as a motor and 1501-1550 RPM as an alternator. Peter. |
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| oztules Guru Joined: 26/07/2007 Location: AustraliaPosts: 1686 |
The other thing that complicates it a bit, is that the torque to slip is effected by the rotor bar resistance for a given power rated motor The higher the startup torque design, the less resistance will be in the rotor bars. So compressor type motors which need high start torque will have very low resistance rotor bars, and so the torque changes hugely for a small change in slip. Centrifugal pump motors which need very little start torque will have more resistance in their rotor bars, and seem more forgiving ... and so a little more likely to break away. What this means is that the motor you choose will have a bearing on how you drive and control it. .... and Dinges, have you eaten my half of the candy bar yet?..... happy new year. Village idiot...or... just another hack out of his depth |
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| mel n rob Newbie Joined: 02/01/2010 Location: AustraliaPosts: 4 |
wonderful help thanks the dutch guy has done some pretty nice work there , but 5 kW is not much from that blade diameter and infrastructure cost 
he gets generation from 1200 - 1500 RPM , but the friction losses through a 20:1 wont be attractive. i have seen guys using ACIM in electric vehicles which gives them brake regen charge back to batteries all the way down to about 150 RPM by the way .... so i wonder if a different controller might improve the output. australian electric vehicle association forum probably better to just go ax-fx now to find a reliable blade fabrication method. got to be low labour input , consistant in strength , low mass , resistant to fatigue and overspeed failure not much to ask ;) fibreglass / plywood sandwich has strength but is heavy and labour intensive carbon fibre is expensive even once you had moulds and vacuum forming techniques set up for repetition , and would require expensive computer design analysis. aluminium ? , extruded blades look safe enough up to 3 metres but over that you would have to be looking at larger section custom extrusions aluminium hollow fabricated spar ( like a plane wing ) is time consuming and risk exists from jointing integrity due to the large number of joint interfaces timber blades made with a CNC router have potential but australia has very poor supply of suitable timber , has anyone found a source for imported timber for blades ?? i wondered if a set of moulds could be made , filled with a few sheets of spaced carbon fibre and then injection filled with high strength epoxy with similar properties to PVC ( UV resistant , tough as boots , high impact strength ) thanks as someone here said ... a week in the lab is worth an hour in the library ....... ;) |
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KarlJ Guru Joined: 19/05/2008 Location: AustraliaPosts: 1178 |
Timber blades cut with a chainsaw seem to be up to the job! OZtues has a few posts on how to. Aluminium and PVC blades are available in lengths upto 3m giving a no-join max of 6m diameter. This is going to be OK as long as the rpm is kept in check. This is based on an educated guess and your blades having adequate internal supports. a 2m diameter set will be good for 900rpm a 3m diameter set will be good for 700rpm a 4m diameter set will be good for 500rpm a 5m diameter set will be good for 300rpm a 6m diameter set will be good for 125rpm All comes down to the forces at the hub, if they are kept within reason by keeping the rpm reasonable, then they'll be OK. Doing the maths on 3m diameter at 1000rpm was scary enough but Phills PVC blades have survived 750rpm with minimal damage (still running unrepaired I might add) There was a long post on the stresses (thanks Perry) and the conclusion was thick walled stainless / mild steel tube would do the 1000rpm in a pinch, that said it was agreed that a) this wasnt realistic as we're not operating at these rpms and no consideration was made for the aluminium profile (intentionally but which is very strong also). The (unfortunately) wisest choice at this size of blade may well be a chinese set of blades, available and cheap, reasonable profiles too. chinese blades these are about 6m diameter and good for 5KW mill (should think it would do that easily) Luck favours the well prepared |
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