Technology / Technical guide
Rotor Braking: Net Torque, Deceleration & Energy
A brake’s resisting torque is only one contribution to rotor motion. Deceleration depends on net torque acting on the defined inertia, while the energy to be handled also depends on continuing input. The brake’s name does not provide a stopping time or heat load.
Deceleration follows net torque, not the brake label alone.
Use motion and energy together to understand an ideal stop, then keep changing wind and equipment constraints outside that simplified result.
01 / Torque balance
Driving and resisting contributions act together.
For a rigid assembly about a fixed axis with constant inertia I, angular acceleration follows T_net = Iα.
For a rigid assembly about a fixed axis with constant inertia I, angular acceleration follows T_net = Iα. The chosen positive direction defines the signs. Aerodynamic torque, generator action, friction and a mechanical brake can contribute to that net quantity.
A resisting brake torque does not alone establish negative net torque if another contribution drives rotation. The inertia guide defines the rotating assembly; different included parts or axes describe a different physical model.
02 / Constant model
A linear speed decrease needs constant net torque.
Hypothetical model: let I = 5 kg·m², initial speed ω0 = 8 rad/s and net torque = −2 N·m.
Hypothetical model: let I = 5 kg·m², initial speed ω0 = 8 rad/s and net torque = −2 N·m. With no other torque, constant I and the stated constant resisting torque, α = −0.4 rad/s² and speed reaches zero after 20 s.
This model applies until zero speed; extending it predicts reversal, not a maintained stop. These invented inputs are not turbine or brake ratings.
03 / Energy change
Net work equals the change in rotational energy.
Rotational kinetic energy is 0.5Iω² in the fixed-axis model.
Rotational kinetic energy is 0.5Iω² in the fixed-axis model. The example begins with 160 J and ends at zero. Under its assumptions, the resisting action removes that 160 J from rotation. OpenStax connects net rotational work to this energy change.
Angular travel is 80 rad, giving net work −2 N·m × 80 rad = −160 J. This agrees with deceleration but supplies no actual brake heat distribution or temperature limit.
04 / Power variation
A constant torque does not give constant braking power.
For the resisting action’s magnitude, absorbed mechanical power is torque magnitude multiplied by angular speed.
For the resisting action’s magnitude, absorbed mechanical power is torque magnitude multiplied by angular speed. In the example it starts at 16 W and falls to zero with speed, giving 8 W mean over the ideal 20 s stop.
A brief peak and a total absorbed energy describe different thermal demands. An actual brake’s capacity depends on its design and conditions, not solely on average power. Continuing aerodynamic input can also add energy beyond the assembly’s initial rotational energy.
05 / Model limits
Several changing conditions can alter the stop.
Real torque contributions can vary with speed, wind, actuator state and electrical loading.
Real torque contributions can vary with speed, wind, actuator state and electrical loading. A complete rotor can also have mechanical behaviour beyond the rigid fixed-inertia model. A constant-net-torque calculation is therefore not a universal stopping prediction.
Achieving zero speed and maintaining a required state are different questions. The failure-response guide explains why no electrical output or a lost supply does not establish a particular brake action.
06 / Product scope
Brake construction and measured stopping performance differ.
FD14-50KW has a confirmed hydraulic disc-brake arrangement; that construction does not supply net stopping torque, stopping duration or an absorbed-energy rating.
FD14-50KW has a confirmed hydraulic disc-brake arrangement; that construction does not supply net stopping torque, stopping duration or an absorbed-energy rating. This information is not extended to the other four models.
The braking-function guide covers a different boundary. No brake operation, pressure setting, live stop test or protection sequence is provided here; the numerical stop is an ideal mathematical example only.
Continue the review with Fail-Safe & Fail-Operational: Wind Control Concepts.
Continue the review with Control Saturation, Integral Windup & Anti-Windup.
Continue the review with Wind Control State Machines: State, Trigger & Completion.
General background: OpenStax: Rotational work, kinetic-energy change and power. These references explain industry concepts; they are separate from the applicable MARSTON product and project documents.
Key concepts and conditions.
Read each quantity with its reference, units and stated conditions. A general relation does not establish a model-specific value.
| Concept / quantity | Meaning / relation | Conditions / limits |
|---|---|---|
| Net torque | Sum of signed contributions in N·m | Driving torque can offset resisting torque. |
| Inertia | I in kg·m² about the stated axis | Fixed assembly and constant-I model. |
| Deceleration | α = T_net/I in rad/s² | Constant torque gives the stated linear speed change. |
| Energy | 0.5Iω² in joules | Net work must account for continuing input. |
| Absorbed power | Resisting torque magnitude × speed | Constant torque does not mean constant power. |
Engineering questions.
Does brake torque alone determine stopping time?
No. The inertia and all net torque contributions matter.
Is the 20 s stop a MARSTON result?
No. It belongs to an invented constant-inertia, constant-torque model.
Does constant torque imply constant braking power?
No. Power changes with angular speed.
Can continuing wind add energy during deceleration?
Yes. Initial stored rotation need not be the entire energy handled.
Does reaching zero prove the brake maintains it?
No. Sustaining a required state is a separate question.
Does this page instruct a stopping test?
No. It explains physics without operating or testing procedures.
Technical enquiry
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