Why, on a given turbine, a smaller generator would generate less output?

[daniTW]

Hello everyone,

I am a mechanical engineer with litle knowledge about electrical so I need advice from people knowing about electrical system.

Having a wind turbine that I am testing at the moment, I first placed a big generator (PMG 500kW 690V @ 300RPM - 18000Nm) behind it and it could produced a 100kW @ 260 RPM.
From this I deducted that my turbine only produce 3500Nm so it was obvious this generator is oversized.
Luckily someone gave me a smaller PMG (300kW 520V @ 300RPM - 9000Nm).
After swaping them, the test result left me dumbfounded, expecting a better output, it only produced 70kW @ 260 RPM.

Coud you explain me why or what should I checkout?
Or do you need other informations before you could answer?

Looking forward to your help

 

[telectrix]

this is above my pay grade. others ( Darkwood, Rob, and a couple of others more expert in this field may be able to help).

 

[Wilko]

Hi - any specs on the blades or the generator? Also, how are you measuring the 70 and 100kW and rpm. Is there a controller? Pics would be nice to see

 

[FatAlan]

Again, way out of my sphere of knowledge but why would you expect a smaller generator to produce more power than a larger one? Just Logic like!

 

[Wilko]

Perhaps confirming the mechanical aspects are identical? That is, the wind turbine may have pitch control so that with a range of wind speeds a more regulated blade rpm is maintained ... if it has this, then when you got 100kW with 260rpm the wind was stronger, for example.

 

[marconi]

You need to consider the turbine and generator and load as a complete system which converts wind kinetic energy into electrical energy.

So, you need to know the power versus rpm curves for the turbine design for a range of steady state wind speeds. With this information you can determine at which combination of power and rpm (for a given wind speed) the turbine develops peak power.

Next you need to know the power versus rpm curves for the generator and superimpose these on the earlier power v rpm for the turbine. The aim is for the generator curve to pass through the peaks of the turbine curves.

Finally you need a load which can be adjusted in resistance to draw a current from the generator at that rpm's voltage output which can absorb the peak generator power output - look up 'maximum power transfer theorem -

http://ebooks.bharathuniv.ac.in/gdl...Maximum Power Transfer Theorem - kathleen.pdf

But you also have to take into account the rating of the generator so that it does not overheat through Ohmic heating.

Alas, the most energy efficient operating point (electrical power out/wind power in) will not be the same as the theoretical maximum power output point because of other real-world losses such as, variation in wind speed, inertia, turbulence, friction, Ohmic heating, viscosity of oils and greases, bearing losses, cooling requirements -some of which are non-linear.

Here are a few references I have turned up which elaborate further for you:

Wind turbine power curves. - http://www.wind-power-program.com/turbine_characteristics.htm

http://www.windpowerengineering.com/construction/calculate-wind-power-output/

http://www.rpc.com.au/pdf/wind4.pdf

Generator Design - http://www.calebengineering.com/generator-design.html

Hope this is of some help.

 

[telectrix]

good post, even if most of it was way above my limited knowledge, which consists of press the button and watch it work.

 

[Wilko]

I'd pay real money for that quality advice Mr Marconi !

 

[marconi]

In #6 I wrote 'With this information you can determine at which combination of power and rpm (for a given wind speed) the turbine develops peak power.'

To do this new need to study 'maximum power point tracking algorithms/methods' as applied to wind turbine driven generators. Similar techniques are used in photo-voltaic panel electricity generation.

To start you off look at:

Maximum power point tracking - Wikipedia - https://en.wikipedia.org/wiki/Maximum_power_point_tracking

A Novel Maximum Power Point Tracking Control for Permanent Magnet Direct Drive Wind Energy Conversion Systems - http://www.mdpi.com/1996-1073/5/5/1398/htm