Types of generators used in wind turbines

[mull982]

Does anyone know what type of generators are typically used in wind turbines? Are they Synchronous, Induction, DC, etc...? I think I read somewhere that the majority of them are induction generators?

Is there any rhyme or reason why one generator type is chose over another in a particular appliation?

 

[kingpb]

Asynchronous permanent magnet generators.

Based on weight and speed control.

That's my best and final answer.

 

[gar]

101130-1421 EST

Brush type DC generators are avoided because of brush maintenance reasons.

Permanent magnet AC alternators are the likely type. The magnets would most likely be on the rotor to avoid slip-ring maintenance. This is basically an AC synchronous motor or generator in construction as is a so called brushless DC motor. The DC brushless motor has other stuff added to it for control purposes.

AC induction motors are not an effective generator for energy purposes but are commonly used in rotary phase converters for cost reasons.

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[mull982]

On the topic of generators I'm trying to determine what type of generator our 750kW standby desiel generator is here at our facility. It is a CAT C27 geneator with a battery and AVR which I'm assuming is used for excitation. The generator has SCR's mounted on the shaft which I'm assuming is to provide the DC rotor field current and therfore this generator is a brushless synchronous motor. Does this sound right?

What type of generators are typically in smaller portable generators used at campsites or construction sites etc..?

So for a PM synchronous do the permenant magnets mounted on the rotor create the rotor field or DC field that would typically be created by the SCR's in a normal synchronous generator?

What is the main difference between a PM asynchronous generator and a PM synchronous generator?

 

[Jraef]

There is no "one-size-fits-all" answer to this. Every supplier has their own cost / benefit analysis that goes into that decision. But there are a few generalities.

Induction generators (actually alternators if the output is AC) can only be used for "on grid" systems, where they are connected to the utility line which is supplying the excitation for the induction machines. Without that, they are just big wind sculptures. They are very common for "wind farms" because of low maintenance.

Brushless Synchronous alternators are used for larger stand-alone or "island mode" generators, such as you would find on remote site power systems where AC is required.

PMDC generators are sometimes used on smaller systems because of the cost (the magnets are expensive) and where DC can be used or stored in batteries, but need an inverter if AC is necessary.

 

[gar]

101130-1530 EST

If permanent magnet excitation is used then there is no energy lost in a current excited field. However, there is no easy direct ability to adjust field excitation. Thus, the output has to be converted to DC or chopped into small pieces for voltage regulation.

A coil type field allows for adjustment of the magnetic field intensity. Here the problem is how to eliminate slip-rings. A rotating transformer can be used.

In a three phase machine there is a rotating magnetic vector in physical space of approximately constant magnitude.

If this is a motor, then the vector is generated by the three phase coils.

If the motor is a synchronous type, then there is a constant DC field from the rotor that is dragged around by the rotating field in exact frequency synchronization, but with some phase lag resulting from the torque on the rotor. Excessive torque load will cause loss of synchronization.

If the motor is an induction type, then there has to be slip to induce current in the rotor. The amount of slip is a function of torque until a breakdown point.

If a synchronous motor shaft is mechanically driven, then the motor becomes a generator with the rotating magnetic field being created by the rotor rotation. This induces the the three phase voltages in the stator coils. Exact sync exists between the shaft and the output voltages.

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[dkarst]

Many of the generators in the utility-class are doubly-fed wound rotor machines which are more flexible for reactive power control and better at handling varying wind speed. The downside is the maintenance of the slip-rings and brushes with the upside being only a fraction of the power (20 - 30%) has to be handled by the converter.

The PM generator seems to be the way of the future, as the power handling capability of semiconductors continues to increase they can handle the full power required of the conversion and you get rid of the slip ring maintenance issues.

I have no affiliation with Clipper Windpower but saw a presentation and they have an interesting concept where they use 4 smaller ~ 600kW PM alternators with rectifiers at the back of a planetary gearbox, then feed DC down the tower. This enables the generators to be smaller and more easily replaceable in the event of failure.

 

[mull982]

Many of the generators in the utility-class are doubly-fed wound rotor machines which are more flexible for reactive power control and better at handling varying wind speed. The downside is the maintenance of the slip-rings and brushes with the upside being only a fraction of the power (20 - 30%) has to be handled by the converter.

The PM generator seems to be the way of the future, as the power handling capability of semiconductors continues to increase they can handle the full power required of the conversion and you get rid of the slip ring maintenance issues.

Can you explain what a doubly-fed machine is? With a PM rotor there would be no need for any electrical excitation so where does the "doubly fed" terminology come into play?

With PM's on the rotor why is there a need for semiconductors as you mentioned?

How is reactive power control accomplished with PM's on the rotor if the rotor field can not be varied since they are permenant magnets instead of coils?

 

[dkarst]

I tried to separate my two trains of thought (doubly fed wound rotor vs. permanent magnet - they are very different) but must not have succeeded

For more details consult any good textbook (I prefer Theodore Wildi - Electrical Machines, Drives and Power Systems) but in short a wound rotor induction machine has windings (duh, right?) on the rotor in contrast to a squirrel cage machine. A wound rotor motor is sometime employed where you need speed control although now you would likely use a VFD.

Many wind turbines in the 1 - 3MW range use a doubly-fed wound rotor generator where the stator is tied to the grid and the rotor is tied to the grid but though a converter that allows you to vary the voltage/frequency of the rotor. This allows the machine to operate in either sub-synchronous or super-synchronous mode. This converter doesn't have to handle the full rated output of the generator.

In a permanent magnet generator (really alternator to be precise), the output frequency will vary with rotor rpm so you would typically rectify and then invert to lock to the grid. In this case, the converter has to be rated at the full nameplate power of the turbine, e.g. 3MW.