Torque during braking 2

[twylks]

There's a turbine spinning at 30RPM, with 18,200Nm torque. This is normal for this turbine. There's a gearbox leading to a generator. The input from the turbine into the gearbox is where the brake is positioned. It takes 4 seconds for the brake to stop the turbine. My questions is, what is happening to that 18,200Nm of torque whilst the system is braking? Is there just a linear reduction in torque as the brake applies force?

Thanks for your help.

 

[btrueblood]

The torque may actually slightly increase, or stay roughly constant, or decrease... it would depend on the blade aerodynamics of the turbine, and whether the flow into the turbine is throttled/stopped (or, if a wind turbine, if the blades are feathered).

 

[twylks]

Thanks very much for your reply! That's exactly what I was looking for. The question is a hypothetical about a screw (hydro) so no variable pitch etc.

 

[rb1957]

isn't it a question about work ? the turbine is happily spinning away at 30 rpm producing a bunch of torque, doing a bunch of work. the brake has to do the opposite amount of work to stop the shaft; the brake will be (i guess) applying a tangential force, maybe a couple (braking on opposite sides of the shaft sounds reasonable), slows the shaft from 30 rpm to 0 in 4 sec.

Quando Omni Flunkus Moritati

 

[MintJulep]

Work is important in terms of thermal capacity, but not for torque.

The turbine produces torque, that may be a function of RPM and fluid properties.

The brake produces an opposing torque.

Typically turbines are connected to generators, and the generator produces an opposing torque.

If the turbine torque is greater than or equal to the sum of the opposing torques the turbine keeps spinning.

When the sum of the opposing torque exceeds the turbine torque the turbine slows down.

 

[tbuelna]

Turbine (wind or hydro) braking systems are sized to be able to safely slow/stop a turbine when when there is a generator failure. This means the drivetrain load produced by the generator must be ignored. The friction bake system is usually power limited, since it cannot absorb friction energy at a greater rate than it is capable of rejecting by heat transfer to the ambient air while maintaining safe temperature limits within the brake components.

The turbine brake must be able to provide sufficient absorption capability to overcome the combination of shaft torque produced by air/water flow over the blades plus the inertia of the rotating masses (blades, hub, bearings, gears, shafts, etc) within a specified period of time. Once the turbine drivetrain has come to a stop the brake system must only provide sufficient force to prevent any further shaft rotation. Wind turbine drivetrain brakes are normally located between the gearbox output and the generator input, which is a high speed, low torque shaft.

Turbine drivetrain brakes are usually designed to be operationally fail-safe and with very conservative margins, since a runaway turbine can result in a catastrophic failure.

 

[LittleInch]

I'm with btrueblood on this one. Torque can vary with speed, but so long as the motive force (water, wind, electricity etc)is still present, then the torque will still be there, but the power or work (torque X rotional sspeed ^2) will gradually reduce to zero.

what the torque does could stay the same or change depending on the energy going into the device to create the toque in the first place. If you remove that energy, then torque could fall to zero.

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