Mass Imbalance

Good Morning everybody,

I’ve got a pretty straight-forward technical query. By using Bladed, I am trying to evaluate the effect of a purely mass imbalance on a 5MW wind turbine. I’ve got from the manufacturer a mass imbalance of 2000kg acting 1m from the hub point.

My data are listed below:

Blade Mass:22985 Kg
Rotor Diameter: 136.05m
Blade Center of Mass: 20.70 m ca.

Some guideline don’t really specify a limit for mass imbalance, saying to relate to the manufacturer data. Anyway it seems to me that 2000kg is a too high value (it the 8.7% of the blade’s mass).

In some scientific reports I noticed how the mass imbalance is often bounded to the 0.5% of the blade mass, or respectively, the 0.5% of the rotor radius.

Roughly speaking, I would like to ask if 2000kg is reasonable or not: if yes why, and if not why.

Kind Regards,
Francesco Perrone

This should be simple to work out.
The center of gravity of the rotor with the one overweight blade will be offset by half a meter from the axis of rotation!
It sounds terrifying to me. I hope they mean it as a “fail-safe design limit”. The turbine must be shut down by its accelerometers well before this point.

Before using Bladed, try “Excel” to work out the effect of imbalances. You only need statics and vector sums to work out this problem.

My guess is it will produce a 16 kN radial load when the rotor turns at 6 RPM.

Hi Steven,

I thank you for your reply. Anyway, after some talks, I noticed that in Bladed I set a 2000kg1m static imbalance, which is pretty close to the 0.5% of the blade static moment (S=blade massblade c.o.g), so I don’t really have a 2000kg mass imbalance. Furthermore, some Bladed versions manages mass imbalance as a mass moment about the hub point: that means no difference between 1000kg2m , 2000kg1m, 100kg*20m as load effect.

Anyway, I would like to know if is possible to estimate somehow the lever arm of a particular mass imbalance value: note that I don’t mean how much the rotor c.o.g shifting. Unfortunately it’s not easy to model properly mass imbalance.

If you have any suggestion, they will be welcome.

I’m a little concerned that you are not stepping away from the FEA to do the hand-calculations properly.
As I pointed out, it is a statics and vectors problem. You can feed the resulting conditions into the FEA when you know them, but a “reality check” is also needed.

I fear that I’ve misunderstood, but you seem to be speculating about what imbalances could occur. Your own design requirements dictate how mass imbalance can occur, and how much. Is moisture ingress possible? Assume it is, where will it enter? How far will it propagate? Ice accretion on the blades usually grows evenly, but when it breaks off the chunks can be large and uneven. Will the leading edges be heated to prevent ice formation or will the machine be shut down in those conditions? What is the maximum permissible variation in core density (assuming these blades have a cellular core)? How strictly controlled will be the fiber volume ratio and ply thicknesses? Quantify these things, and you won’t be speculating.

Many other factors will affect how the “flaw” in the mass distribution can occur. This was just off the top of my head. These are things I cannot answer from outside of the design team.