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Forum Index : Windmills : Alternative Rotors
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Megawatt Man Senior Member  Joined: 03/05/2006 Location: AustraliaPosts: 119 |
I have noticed under various topics that there is an interest in alternative sources of rotors for F&P units. The Suzuki Sierra uses brake drums that measure 261 mm internal and have enough thickness to provide a good magnetic circuit. Some machining would be needed on a hub that matched the wheel stud holes, unless the original hub was used. Megawatt Man |
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
Thats good info. The F&P stator has a diameter of 250mm. That gives us a 5.5mm gap, so 5mm thick magnets would fit perfectly and end up with a air gap of 0.5mm. Any brake repair shop could machine the drum out to 263mm, meaning you could use 6mm thick magnets. One of the bigest problems with swapping the magnets to neo's is the plastic F&P hub starts to buckle and twist, so a Suzuki brake drum could be the solution. Thanks Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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PeterD Newbie  Joined: 02/05/2006 Location: AustraliaPosts: 5 |
That sound like good info, they might be a bit hard to find tho. My question would be: wouldn't the steel drum actually short (divert) the magnetic field and decrease efficiency drasticaly? PeterD Let the wind blow high Let the wind blow low Through the streets in my kilt I go |
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| Gizmo Admin Group Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
The magnets go on the inside of the drum, so the backs of the magents are against the cast iron drum. The fronts of the magents are passing over the stator poles. By having this steel behind the magnet you are actually increasing the magnet flux in front of the magnet. If you ever get the chance to cut up a standard plastic F&P hub, you will see there is actually steel laminations embedded in the plastic behind the magnets. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| Gizmo Admin Group Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
Warwick Nichols posted this message, it was sent to my email but I think he intended it to go here..... Hello PeterD, Yes, Gizmo is right. Consider every rotating electrical machine you have met. They are all steel or iron cylinders in which the electro-magnetic poles are located. The working magnetic field is between adjacent pole pieces via the encasing magnetic material of the cylinder. The effect is to present as much of the magnetic field as possible to the rotating magnetic poles of the rotor, so that maximum possible magnetic forces are produced. Only a tiny air gap can greatly reduce the field intensity, because air is millions of times less effective as a carrier of magnetic fields than iron, say. So the desire in rotating machines is to have the smallest possible gap between rotor and stator. But this brings its own problems of course, only a little wear causing contact between parts for example. In the case in point, the closer the magnets to the f&p stator, the greater will be the attractive force that results in cogging. So, like many things in life, this is another area where compromise must act. I would be very interested to know just by how much electric power delivery is reduced by grinding the pole pieces down to reduce cogging. I can understand though why it happens, because if the gen can start in lighter wind, at least you generate some power. Another compromise! The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| RossW Guru  Joined: 25/02/2006 Location: AustraliaPosts: 495 |
I've been (half) thinking on this for a while and wondering if it's possible, or practical, to have enough magnets and poles, but with the number such that they do NOT all line up at the same time. In a really crude example, say you have 4 magnets and 4 poles. At one point, all 4 magnets will align with all 4 poles, while 45 degrees later all 4 magnets will be exactly between all 4 poles. If we re-arranged things so we had (say) 5 equally spaced magnets, there would only ever be one "meshed" at a time, substantially reducing the cogging effect. I'm sure it should be possible to arrange enough poles and magnets so the "effective" attraction at each point of rotation is identical, resulting in zero cogging. (I'm expecting somthing in the order of 36 poles and 35 magnets). I realize there are issues with where the coils are, and how they're connected... and perhaps it's even worth looking at a LOT more than 3-phase machines? |
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| Megawatt Man Senior Member Joined: 03/05/2006 Location: AustraliaPosts: 119 |
Using a lesser number of poles. I have not given this the depth of thought a thorough answer needs, but here is my in itial reaction. First, an even number of poles is needed so that we achieve an alternating sequence of North and South poles. An adjacent pair of the same polarity would be non-productive at least. I believe that the suggestion would result in less cogging, but at the expense of a lower output voltage. Megawatt Man |
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| PeterD Newbie Joined: 02/05/2006 Location: AustraliaPosts: 5 |
My motor theory is not great, specially when it comes to giant stepper motors(as is the f&p) but if you look at the pole-magnet arrangement as in the fp each magnet has ...end pole....centre pole....end pole. yes seemingly a 3 pole magnet. When you align the coils you get 3.5 coils (approx) per magnet providing an offset so that the motor steps. This is probably not the best arrangement for generating electricity but it works for us and does it quite well and probably lessens the cogging effect. Not sure if this thinking is correct but it doesnt stop me postulating (at night). PeterD Let the wind blow high Let the wind blow low Through the streets in my kilt I go |
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| Gizmo Admin Group Joined: 05/06/2004 Location: AustraliaPosts: 5078 |
Those large magnets you see are actually 4 magnets in one. More info here http://www.thebackshed.com/Windmill/WhatFP.asp. There are 56 magnets in a hub, and 42 poles in a stator. This gives us a 4:3 ratio ( four magnets per 3 poles ), meaning that at any one time, only 13 ( 42/3 ) poles are lined up with magnets. Its a true 3 phase machine, with spacings of 120 degrees. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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| Megawatt Man Senior Member Joined: 03/05/2006 Location: AustraliaPosts: 119 |
Postualte away, Mate! Leonardo postulated! Have a look at the info on the following site. www.st.com/stonline/books/ascii/docs/1679.htm It gives a good explanation of stepper motor drives. Back off after the first few pages, it gets a bit heavier than we need. I really don't know what system F&P use with their motors, BUT consider that a fixed direct current is flowing in the stator coils. There will then be a set of N & S poles around the stator and these will react with the rotor magnets and a state of equilibrium will result. There will be no continuous movement, in fact the rotor and stator will be locked together magnetically. Now reverse the current in the stator coils and the polarity of all poles changes. The rotor will then step to the new position of equilibrium and remain stationary again. Now continually reverse the stator coils polarities and you will have created a rotating magnetic field that will push and pull the rotor around with it. THat's the DC working p[rinciple. F&P motors are arranged as 3 phase windings, so if you apply a continuous 3 phase supply to the stator, the alternations in current polarity (current flow direction changes)will produce a smoothly rotating magnetic field that will cause rotation at a constant speed, directly related to the frequency of the supply. F&P could drive the motor from a frequency changer to produce several different speeds, or with a sophisticated frequency change arrangement, that could provide evbery possible speed from stopped tp the max, determined by the upper frequency available from the controller. I guess somebody would know what method they use and maybe they could tell us? Megawatt Man |
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| dwyer Guru Joined: 19/09/2005 Location: AustraliaPosts: 574 |
hi Megawatt man and Gimzo About the Suzuki Sierra's brake drum to be machine down at brake shop be aware they might refuse to machine down outside factory specification as they will not listen your stories and if something went wrong they can be sue if someone fit into the car So Just be aware of Dwyer the bushman |
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| PeterD Newbie Joined: 02/05/2006 Location: AustraliaPosts: 5 |
Thanks Glenn & Mega for pointing me at the various sources. I am familiar to some extent of rotating fields and the like and stepper motors are a variation on this principle. The f&p motors with their outer rotating magnets and inner coils is most suitable to our purposes. The four poles on the magnets are apparent when using another magnet to find them. The NZ site, as you point out Glenn is well worth a visit. Peter Let the wind blow high Let the wind blow low Through the streets in my kilt I go |
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