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Forum Index : Electronics : General Generator Questions
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| jcbabb Newbie  Joined: 24/02/2009 Location: Posts: 2 |
Greetings: Is there any advantage to using multiple generators on a single windmill? For instance, can you have a shaft from the windmill turn three generator shafts, and get power from one, two, or all three generators at the same time? Along those lines, what effect will different loads on each of the generators have? In other words, can the first be loaded, and the other two be open circuited? I believe that shorting a generator will stop the shaft, but how is rotational resistance affected by varying load on the generators (or is it affected at all?). I've actually had lots of electrical engineering coursework, but I have forgotten so much due to lack of regular use. Any refreshers or pointers you can provide would be great, thanks! |
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Gizmo Admin Group  Joined: 05/06/2004 Location: AustraliaPosts: 5030 |
One advantage of using multiple generators/alternators is the ability to match them to the tubine speed and power. If you have a turbine that runs at 300 RPM, but your single alternator only makes 10 volts at 300 RPM, then 2 alternators on the one shaft, conneced in series, will give 20 volts, suitable for charging a 12v battery. You can also switch from series to parallel connections as the RPM increases. But, friction and resistance losses do increase with multible alternators. A typical bearing will suck up a few watts. As a rule, a single alternator is the better aproach. Glenn The best time to plant a tree was twenty years ago, the second best time is right now. JAQ |
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Bolty Regular Member  Joined: 03/04/2008 Location: AustraliaPosts: 81 |
Is there any advantage to using multiple generators on a single windmill? For instance, can you have a shaft from the windmill turn three generator shafts, and get power from one, two, or all three generators at the same time? Yes you could have three generators off the one shaft. However it would be difficult for me to find an advantage to this configuration. There would be many disadvantages, including extra friction in the bearings, and the difficulty of physically arranging the connections. You could individually tap of energy from any of them as you desired. Along those lines, what effect will different loads on each of the generators have? In other words, can the first be loaded, and the other two be open circuited? Yes! But why would you want to do that? I believe that shorting a generator will stop the shaft, but how is rotational resistance affected by varying load on the generators (or is it affected at all?). Depending on the rotational energy of the turbine and blades and the internal resistance of the windings of the generator, shorting CAN stop the shaft. However if the blades are heavy, spinning fast, and the internal resistance of the windings is high, in strong winds the turbine can spin to destruction, even though the generator is shorted. This is why furling is needed to prevent excessive rotor speeds. I am assuming that by load you mean an electrical load placed on the windings. I also assume that by rotational resistance you mean a braking torque on the generator. A fundamental understanding of Physics can be considered as "You cannot get something for nothing" Hence if you are loading a generator, this is causing a current flow. Current flow means energy is used to move electrons. Hence loading a generator takes work(energy) from the system. This energy comes from the kinetic energy of the wind and the rotor and blades. In a steady wind, loading a turbine will result in a reduction of rotational velocity of the blades. The greater the load, the greater the retarding effect. This is fundamentally an illustration of Lenz's Law. ie The direction of an induced EMF is such as to oppose the force producing the EMF. Hope this helps! |
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| jcbabb Newbie Joined: 24/02/2009 Location: Posts: 2 |
Thanks, guys. That makes sense. So is the relationship between the electrical load on the generator and the rotational resistance pretty much linear(neglecting mechanical resistance such as bearings, etc)? Assuming a DC generator: How does the rotational resistance of the generator with no load (open circuit) compare to a generator with a large resistance connected? Will it be like "free-wheeling" with an open circuit(neglecting the friction in the bearings)? If there actually was a windmill with infinite speed and torque potential, could I theoretically apply more load to the generator(s) to keep the rotation of the rotor under control? Granted, at some point, the force from the wind could overcome the emf in the generators, but up until that point, would it work? I realize I'm sort of coming at this from left field, but I'm trying to better understand the principles involved. Maybe multiple generators are not the answer, but here's my thought: Most generators are most effective at a given rpm, and require a certain amount of torque in order to turn at that rpm. Because the largest percentage of torque applied by the generator and against the rotation of the rotor is from the emf, is it possible to begin with no emf (possibly by open circuiting the gen), then gradually apply emf (possibly by loading one or more gens w/ electrical loads), and possibly maxing out the emf (by loading all gens)? Again, thanks for your input! |
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| Bolty Regular Member Joined: 03/04/2008 Location: AustraliaPosts: 81 |
Hi again jcbabb. You should have a good look at the windmills section on capacitors. Even though it is a very long topic, it should give you a whole lot of information about what you want to know. You are right in your desire to properly match the load with a given (varying) speed. This is why windmills are much more complex with load matching than say more constant speed water turbines. The voltage doubler, tripler or quadrupler proposals suggested in the capacitors topic effectively gives easily achievable, and seamless load matching over varying speeds. It is also far less complex amd more reliable than maximizers, due to passive components. Arguably, the more active components you have in your matching circuits, the less reliable. |
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oztules Guru  Joined: 26/07/2007 Location: AustraliaPosts: 1686 |
[quote] So is the relationship between the electrical load on the generator and the rotational resistance pretty much linear(neglecting mechanical resistance such as bearings, etc)? [/quote] Ignoring mechanical resistance and all loses in the alternator... power in = power out... so in that sense linear. In the real world, it is about here that the wheels fall off. If we ignore all mechanical losses and concentrate on an air cored machine (axial flux type), we are faced with an alternator that has internal impedance = internal resistance of the windings only (mostly anyway). This is an easier place to start. We can generate emf for no cost in torque, as no power is generated .. only emf. W=ExI.. so at no current w=0 no output so no input power (torque x rpm). If you place a high resistance on the output, then current will flow proportional to the combined resistance of the alternator and the load in series.... I=E/R where R= alt resistance + load resistance. The current must flow through both of these in series... so power will be used to turn it, to provide the output power (small as that may be at high resistance) Terminal volts will be a function of the alternator's internal resistance and the current flowing through the coils for any given load. If your alternator had 1ohm resistance, and you were driving a 10 amp load, you will lose E=IxR so 10 volts will be lost inside the alternator and so terminal volts will be EMF-10v... and ExI (10) watts lost inside the alternator heating up the stator. So by knowing the resistance of the alternator, we know what our losses are inside. What our terminal volts will be at given current etc etc. It is the load that will mess this up for us with battery charging. The batteries will present as a 0r load, so all the EMF generated must be lost in the distribution system, the alternator, and the battery terminal voltage (according to state of charge at the time) and the diodes. Before the alternators EMF reaches battery voltage, no torque is required to achieve cut-in (ignoring mechanical). Once cut in voltage (ignore diode voltage drop as well) is reached, we can see the power. W=(EMF-Battery voltage)^2/alternator resistance= power lost in the stator itself. This is the power that needs to driven by the shaft PLUS the power to charge the batteries... which is terminal volts x current. It does not matter too much how you achieve this, 1 or 2 or 3 alternators. If any are open circuit, they will generate EMF but use no power doing it, so you can switch them in and out as your needs require. Once you bring the wind into the equation, all bets are off. You need to match the load to the available power in the wind and this is in nightmare territory to calculate accurately. When you use F&P style alternators, there is a raft of other factors to take into account to do the power figures, as internal impedance changes with current and frequency... not simple resistance as in the air core version. There is lots to learn about these things, electrical theory is only a small almost insignificant part compared to matching the wind to your genny... what ever that may be.... as any alternator can be driven to good effect (how simply is another question), it is matching the wind to your load requirements (voltage and impedance) thats tricky. Too make matters worse, as you change 1 small thing, lots of other things may change as well that you didn't figure on. For starters, just build a proven design and then tinker with it.... and it will come naturally. Don't try and re-invent the wheel until you have some hands on experience, as that will give you the best insight as to which brainstorm is the one to persue... if any. Above all, have fun, and if you have specific questions regarding any aspect, then this is the place to glean answers. .........oztules Village idiot...or... just another hack out of his depth |
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