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I am designing a braking system for a direct drive (no gear box) wind turbine. I am not sure how to calculate the braking time and number of brakes required. I made this in two methods, which gave different values. What is the exact method for calculating the number of brakes and braking time?

Here is my problem

Generator capacity = 20kW

Rotational speed = 100 rpm

Method-1 -finding number of brakes (Equate torque by rotor and braking system)

Total torque at rotor Tt = Aerodynamic torque Ta + Rotor torque Tr

Aerodynamic torque (P=2.pi.N.Ta/60) Ta = 1909.8 N-m

Rotor torque = Tr = I.α

Mass MI of rotor I = 650 kg-m^2

Time to stop the rotor t = 0.5 sec

Alpha = ω2-ω1/t

ω1 = 2.pi.100/60 = 10.47 rad/sec

ω2 = 0 rad/sec (stopped condition)

Alpha = 10.47-0 / 0.5 = 20.9 rad/s^2

Rotor torque = Tr = 6500*20.9 =13585 N-m

Tt=1909.8+13585 = 15494.9 N-m

Braking force = 10600N

Brake pad distance from centre = 0.3 m

Braking torque = 3180 N-m

No of brakes required = 15494.9 / 3180 = 4.87 (5 brakes)

I need five brakes to stop the rotor in 0.5 sec

Method-2 - finding time required to stop by fixing number of brakes

Kinetic energy of rotating body = ½.I.omega^2

=0.5*650*10.47^2 = 35626 N-m

Adding aerodynamic torque = 35626+1909.8=37536 N-m (Is adding torque right?)

For bringing the rotating body to rest work done by the braking system should be equal

Wd = F.D

D = 37536/(10600*5)=0.708 (assume 5 brakes)

0.708 = pi.(0.3*2).N -> pi.dia.Number of revolutions

N=0.376 revolutions

Revolutions in 1 sec = 100/60=1.66 rev

braking time = 0.375/1.66 = 0.23 sec

Thanks

Raj

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# Braking torque question

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