Wind Turbine electrical braking system

Discussion in 'The Projects Forum' started by TomRij, Jan 16, 2012.

  1. TomRij

    Thread Starter New Member

    Jan 16, 2012
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    Hi all, I am currently in my last year of study at University and I am working on my final year project for a braking system for a wind turbine generator.

    I am not actually building a working turbine as the wind power and rotational speed will be simulated in a lab. I will be using a 24v DC motor as the generator (I have others at my disposal) and will be designing and building my own circuit to control the rotary speed.

    I am looking for advice on whether to go for regenerative braking or an H bridge design or a DC injection system for the DC motor brake (mainly looking at the latter two). The aim of the braking system is to cap the speed of the wind turbine and not to brake the motor to a full stop, what feed back system is best for this? Also, is it feasible to design a control system that is capable of running off the power generated by the turbine?

    You advice would be much appreciated, thanks in advance.
     
  2. jimkeith

    Active Member

    Oct 26, 2011
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    The current rating of the generator (motor) is the torque limiting factor, regardless of the source of the power, whether external via DC injection or self-generated.

    I am assuming that you have no ability to feather the turbine--correct?

    I believe that a self-powered system is not only feasible, but the best way to go--external power may not be reliable during a wind storm--guaranteed failure mode!

    Voltage feedback is appropriate as it is directly proportional to rotational velocity--and it can be easily regulated via a simple PI control system.

    Establishing limits: Current must be measured and limited in order to keep it below the demagnetizing current of the PM motor. Since wind often blows in gusts, overcurrent should be allowed to this current level--beyond that it must be allowed to overspeed to prevent motor demagnetization. This current level is perhaps 5x nameplate value, so a current limit of 200 to 250% should be safe.

    Since motor current will not be regulated to nameplate levels, temperature must be monitored--note that wind velocity will work wonders at reducing motor temperature and increasing current rating. However, when the motor temp reaches the insulation limit, overspeed must again be allowed to prevent motor thermal failure. Note that during a wind storm, the ambient temp will likely be far below rated motor ambient.

    I envision an input capacitor and basic buck regulator that powers a resistive load--the load also cooling enhanced via the high wind velocity.

    Note that while I am knowledgeable about controls, I have no experience or training on windmills--just self think tanking...
     
    Last edited: Jan 16, 2012
  3. wayneh

    Expert

    Sep 9, 2010
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    But isn't that the whole point, to avoid overspeed on the blades? When the available wind power exceeds the maximum load current that the generator can produce, the energy has to go to heat generation somewhere else. If you could just let it run free to high rpm, the solution would be easy, but you can't. You need brake pads or something.

    IMHO, regenerative braking doesn't make much sense. That's just another small generator following the big one. Might as well put the money into the big one to begin with.

    Old-fashioned windmills turn the blades out of the wind when it gets to be too much. I think that's a very logical first step - take the wind out of the sails.
     
  4. jimkeith

    Active Member

    Oct 26, 2011
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    Good point, a mechanical default solution may be in order, but a variable speed friction brake may be difficult to implement--stalling is simple, but difficult to recover from without external power.
     
  5. TomRij

    Thread Starter New Member

    Jan 16, 2012
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    At the moment I am struggling to find any achievable method using DC injection to a generatior. It is ideal for stopping a free running motor using a mains supply but I can't think of any way to implement it to brake a generator (motor) running on wind power alone, also I think it is impossible to apply DC injection on the generator using its own source of power.

    For this project I will not be using pitch control on the blades to reduce rotation, it will be electrical braking only. My most favored design thus far is the H bridge with the use of MOSFET transistors acting as switches. My aim is to not stall the motor as I want to be able to continually produce an electrical current whilst controlling the motor speed.

    Using the power produced from the generator, how do I use that to control the MOSFETs to keep the motor at a certain speed?
     
  6. wayneh

    Expert

    Sep 9, 2010
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    I can't help think you're chasing your tail. If the wind power is 4X the amount the generator can capture, how are you going to use the 1X captured by the generator to resist the other 3X?
     
  7. jimkeith

    Active Member

    Oct 26, 2011
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    DC injection is generally used to stop an induction motor--I do not think I have ever seen this done to a DC motor, nor do I think it will work--DC motors always use dynamic braking--just throw a resistor across the terminals...
     
  8. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    I assume the power the blades generate is proportional to wind speed and RPM product. So to keep the generator from overheating, you should aim for lower than maximal RPM during high wind, right? Anyway, you would simply shunt the DC into a resistor to get some desired output voltage. Also some protection to stall the motor permanently should the RPM get too high (for example should the regulator fail) could be a nice touch.
     
  9. strantor

    AAC Fanatic!

    Oct 3, 2010
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    (assuming you are using a permanent magnet motor) if you shunt the terminals together with a resistor as mentioned, you get braking action. If you short them together, you get shaft locking action. I suspect (not based on any research or testing) that you might be able to PWM the motor terminals together (shorted) to achieve variable torque braking. If that works, then you could divert the power (from) the load (to) shorted together, and back to the load, at high frequency with big filter caps so that you get braking and usable power at the same time.
     
  10. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    The power in the wind is the CUBE of the windspeed, and generator output electrical power can follow this pretty closely.

    So electrical braking is considered a really poor way of doing this. If the turbine stator makes its max power of 100W at 20 knots, then it is running at the limit.

    Then if wind speed increases to 40 knots the stator would be required to generate anywhere up to the 2 cube ie 800W. Which requires a much bigger stator and increases the cost of the turbine, ie; if you paid for an 800W stator why not make the max usable output power go up to 800W now?

    You may be able to limit shaft power by loading the generator in a way that really reduces the overall RPM and efficiency of the whole system so you don't need a huge stator or huge waste energy disspator, but that would involve most of the work going into very specific turbine mechanical and electrics design, (a good subject for a final year). But not just "try to load a generator" to safe limit the whole system.

    Small commercial turbines use mechanical feathering (changing the angle of the whole turbine) to limit power, larger expensive turbines use variable blade pitch to do the same thing. Nobody that works with wind power (that I know of) tries to load a standard generator as a solution to peak winds!
     
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