Ac Motor used as a generator on wind turbine 1

[Feg]

Hi

I don't have any real experience in this area but i have been looking at a job where a 37Kw 4pl IEC Ac Motor, rated voltage range 380-420D/660-725Y 50Hz is being proposed to run as a generator using soft starts for starting. The motors will be driven by the wind turbine generating 28Kw. My question is this, is a std Ac motor with std windings ok for this job. The proposed motor is a Siemens 37Kw 4pl B3 cast iron asynchronous induction motor with std windings. Any thoughts would be welcome.

 

[waross]

The soft starts puzzle me. Can you regenerate through a soft start, and can you limit regenerative current? There should be a speed sensor to connect the motor to the line when it is above syncronous speed, and disconnect it when it drops to syncronous speed.
As for motors, I have set up demonstrations a couple of times.
Once the results were excelent. Another time and place with different equipment, the results were still positive, but nowhere near as good as the first time.
I hope someone can tell us what properties of an induction motor are beneficial to use as an induction generator.
yours

 

[jraef]

This is done all the time, but I would assume you have the motor connected to a grid (network), because an asynchronous motor cannot self-excite without some additional trickery. If it is in island mode, i.e. the sole source of power in a remote site, you need an excitation system to get generation started and keep it generating. There was a thread devoted to that issue in this forum a few years ago, you might want to try a Keyword Search on "wind turbines" or "wind induction generators". A Google search on those terms would reveal other sites as well, but I would add "self exciting" for Google to narrow it down.

Waross, the reason for the soft starter is that typically the turbine cannot produce enough torque to start itself from a standstill. In order to avoid over speeding, the turbine blades are designed to shear off excess torque producing wind, and of course, it's that "excess" torque that is needed as accelerating torque. So they use the soft starter to get it up to motor slip speed, then bypass it with a contactor and let the turbine take it the rest of the way to super-synchronous. You are correct in that you can't have reverse current flow through a normal soft starter. There are however specially designed units specifically for the wind generator industry (Google Enerpro). Most users don't want to pay the extra money for them however, because a bypass contactor works fine and is cheap, and that way they can use an industry standard soft starter that they can quickly get a replaced or serviced.

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[dpc]

Any induction motor can run as an induction generator. You need to rotate it faster than the motor's synchronous speed. Speed control of the wind turbine will be fairly critical.

But as an induction generator, it must be connected to the grid to generate power - it cannot operate in an isolated configuration, at least not very easily.

Also, as a generator, its power factor will be lousy (low). If this is a system that is going to actually run for long periods of time to produce power, you may want to consider adding some switched capacitors to help provide some of the excitation.

I'm guessing the motor will be the least of your problems. I agree with waross re the soft start - I'm not seeing the purpose for that, but I haven't thought about too hard - maybe I'm missing something. I don't think you'll be able to backfeed power through the softstarter, unless it has a bypass contactor.

 

[jraef]

Here is a paper on self-excitation of induction generators in gory detail. As I said, it is possible, just not pretty.

http://www.nrel.gov/docs/fy05osti/33138.pdf

As I said though, if you are connected to a grid, you don't need to worry about this.

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[Warpspeed]

Feg, as soon as you specify an induction motor, you pin yourself down to constant Rpm (synchronous) operation. Constant operating speed is a rather tricky thing to achieve when there is rapidly varying wind speed. Power will be flowing into and then out of the turbine. While variable pitch blades are feasible, I can quickly see the whole thing becoming a bit of a drama to control properly.

Personally I would prefer a dc machine with a synchronous inverter to feed the grid. Turbine voltage and Rpm can fluctuate, but the inverter will respond very quickly.

 

[itsmoked]

I agree with Warpspeed but would include using the induction generator with rectified output to make the DC.

 

[davidbeach]

DC would be a very bad choice - brushes. The much better solution for this application would be a synchronous generator with permanent magnet field, running wild, feeding a rectifier and an inverter. No brushes, and no worries about speed (within reason).

 

[jraef]

Induction generators are very common, wind speed is not as big an issue as you may think. As I said earlier, modern turbine designs are such that excess wind is spilled off of the blades to maintain a relatively constant torque on the shaft, then wind brakes (small blades deployed to inhibit speed) are used beyond that, up to a point. When the wind gets too strong, they just shut down rather that risk burning up the generator. That part about wind generation is down to a science now so speed isn't the problem it used to be. The the bigger problem is in exciting the induction motor if not connected to a grid. That's why generally you only see induction generators used in wind farms that are supplying power to utilities.

From a cost standpoint, AC regenerative inverters are typically only used in the biggest generators because they require a lot more capital, protection and maintenance, so you only see them in 400kW and up (last I checked). They do that because they want to maximize the revenue by taking advantage of low wind days where the turbine has trouble getting super-synchronous. On small induction generators it isn't worth the cost and headaches for the small amount of additional power you would get.

That's why small island mode wind turbines tend to be DC. They don't ned the additional complexity to make an AC induction generator work, even though brushes are a pain. You still need an inverter if your loads are AC, but it's a much simpler off-the-shelf inverter, really more like a chopper drive because it is just doing the DC to AC part of the job.

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[LionelHutz]

Are you looking at just supplying a motor or are you looking at the whole system?

The blades will want to spin faster to produce more power if the wind increases. So, when you limit them to a fixed speed they tend to somewhat "stall" as the wind increases. But, they will still produce more power with a stronger wind so you will have to have some control means for them.

Using permanent magnet generators running wild seems to be the standard these days. Every turbine 10kW and under that I've looked at uses PM generation. Above that, it's a mix of technology but even the largest machines are using PM generation.

 

[aolalde]

”37Kw 4pl IEC Ac Motor, rated voltage range 380-420D/660-725Y 50Hz is being proposed to run as a generator using soft starts for starting”.

My personal opinion is that it will not work. First it needs the greed voltage for excitation, the power factor is low consuming high reactive power from the greed and a super sophisticated fast mechanical control is required to connect it only when the speed is above synchronous, considering the typical wind performance.

The induction generators used in the windmill farms are not simple induction motors. Those are wound rotor induction machines which have a controller connected to the rotor winding to make the generating window broader. A detail of the operation is complicated to me for explanation here. May be you will find some information at:

http://www.abb.com/global/abbzh/abb...17ec2&e=us&c=B8C8356383489150C1256CE0004E0BD0

 

[Warpspeed]

One thing that still baffles me about this, why does a wind turbine need "starting up" with externally applied power anyway ?

 

[jraef]

I answered that on Feb. 10th for Waross.

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[LionelHutz]

The blades are basically shaped like an airplane wing. The "lift" created by the blades is the main rotational force. No rotation = no lift which means the blades can't start the typical gearbox supplied on the large turbines. Direct drive turbines can start if they have no load load other than your basic bearing drag. If you want to find blade design info look up a guy named Hugh Piggot on the internet and you'll find some documentation he's written on blade design and the forces causing rotation.

aolalde is right, the wound rotor control seems to be the other main type of generator used. I forgot about that one.

 

[jraef]

I beg to differ on the point about wind generators being wound rotor. I live next to the Altamont Pass, the 2nd largest wind farm in the US. They are currently undergoing an upgrade of 2500 wind generators here, and not one of them is a wound rotor machine, they are all standard squirrel cage induction generators. These are large units and there are minor differences in bearing design etc. from motors, but electrically they are no different that any squirrel cage induction motor. My contacts up there say they have heard of using wound rotor machines, but point out that having slip rings would be a maintenance problem for them to consider. They also do not use inverters except on the largest MV units because of cost / benefit ratio as I mentioned earlier. For the most part they use soft starters or autotransformer starters to motor the turbine up to slip speed as I mentioned earlier, then connect to the grid using a simple contactor.

Here is a very informative article on the whys and wherefores of using induction generators on grid connected systems. Danish wind power technology link

Despite what the ABB drives division would like you to believe about the prevalence of inverters in induction generators, here is what ABB sells the most of world wide.
ABB induction generators link

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[dpc]

Vestas has a lot of wound rotor wind turbines in the US.