# FIXED and VARIABLE wind turbines, help pls!

Hello all!

First of all i want to say that i am happy to be part of this big community

I have a question: What is difference between FIXED and VARIABLE wind turbines?

I have read a lot about them but couldnt figure out difference :\ when i look at power curve, both have the same.

where:

Fixed-speed fixed-pitch (FS-FP)
Variable-speed fixed-pitch (VS-FP)

Please if somebody can, explain me the difference and how they work. As i understand from the name FIXED speed means that turbine works at some fixed speed (e.g. 7 m/s) and if speed is less, it doesnt work and if speed is higher it tries to lower the RPM to match the RPM for 7 m/s. Whereas variable wind turbines can work at the range of cut in and cut out speed. I am really confused in those two terms, and it affects my advances in studying wind turbines.

thank you!

A fixed-speed wind turbine always spins at the same generator/rotor speed during operation, regardless of the wind speed. Thus, the tip-speed ratio (TSR) would change with wind speed and the rotor aerodynamic performance would only be optimal at a given wind speed. The generator torque of a fixed-speed wind is dictated solely by the induction generator (only small speed deviations are expected as a result of the slip of the induction generator).

A variable-speed wind turbine allows the generator/rotor speed to vary proportional to wind speed between cut-in and rated speed, thus maintaining a constant TSR and optimal aerodynamic performance. Above rated speed, the generator/rotor speed is then held constant. The torque must be actively controlled.

I hope that helps.

Best regards,

Dear Jason,

Thank you for your reply. What i can not understand is, if â€śA fixed-speed wind turbine always spins at the same generator/rotor speed during operation, regardless of the wind speedâ€ť how power generation is increasing with the wind speed? If rotor spins at constant speed regardless of the wind speed, it should have same output always, no? i mean on the graph its clearly shown that with wind speed the power output is increased.

best regards,

In an induction machine, the slope of the torque-speed is steep (the slip is small), so, the torque (and hence power) can vary considerably for small changes in speed.

Best regards,

Dear Jason,

Thank you for you reply! Could you please tell me if this definition if physically correct?

â€śWind turbines are devices which convert kinetic energy in the wind to mechanical ENERGY and then into electricityâ€ť

Thank you!

Sounds reasonable.

Dear Jason,

So do you mean the power output variation of fixed-speed wind turbine is caused by small changes in turbine rotating speed? However, according to graph below, even though rotating speed is really fixed at one point, if wind speed is different, power output is still different. Do I misunderstand your meaning or meaning of graph below? Besides, could you please tell me if the fixed speed is just determined by optimal torque of induction machine? Kindly let me know your comments.

Best,
Yanda

Dear Yanda,

Yes, small changes in rotor speed together with a steep torque-speed curve will result in large variations in torque and power. The fixed rotor speed will be selected to maximize aerodynamic efficiency at a given wind speed together with structural design decisions of the wind turbine.

Best regards,

Hi Jason,

Thx so much for your prompt reply. So the label of y axis of the graph above is not electrical power output of generator, but only mechanical power of turbine, right? Since if we suppose it is electrical power output, the x=Wm5 will intersect with several curves of different wind speed, which means their electrical power output is different caused by their different wind speeds, nothing else. Is that correct? Waiting for your reply. Many thx in advance.

Best,
Yanda

Dear Yanda,

The point is that the fixed-speed turbine actually has a small variation in generator speed, so that large variations in wind speed result in small variations in generator speed and large variations in torque / power i.e. the torque-speed curve is steep.

Best regards,

Dear Jason,

Thx so much. I got it. But why did forerunners make the rotating speed constant? Just because it is simple or easy to control? Since it seems, using this way, a lot of energy will be wasted due to force applied on shaft to control speed.

Best,
Yanda

Dear Yanda,

The power grid operates at a â€śfixed frequencyâ€ť, so, coupling a fixed-speed turbine to the grid is straightforward. Coupling a variable-speed turbine to the grid requires a fancy power electronics i.e. AC-to-DC-to-AC conversion.

Best regards,

Dear Jason,

Thx very much for your help. I appreciate it.

Best,
Yanda