4-Engine choice

3.4. Study of dynamics

DefinitionFundamental equation:

and

• : Engine couple;

• : Accelerator torque;

• : Resistant torque opposed by the load;

• : Moment of inertia

DefinitionStarting conditions

The machine can only start if the starting torque of the machine is greater than the load torque of the load

=>

Example

The acceleration is higher as: is bigger than and is small.

Definition3.4.3. Running at OPERATING POINT):

In steady state the speed is constant. So the acceleration torque no longer exists.

If => =>

Definition3.4.4. Stable operation of the machine

  • The stable operating point of the machine is the point where the motor and resistive torque are equal.

  • Note:

    The motor is generally chosen so that the operating point A is as close as possible to the operation in nominal mode

Definition3.4.5. Natural slowdown of the machine

  • The natural slowdown of the machine is obtained by stopping the power supply of the engine at time t0.

  • Note :

    • Stopping the machine is shorter as the moment of inertia is low.

      from => =>

    • The acceleration is negative therefore slowing down the machine.

Definition3.4.6. Braking the engine

• To achieve a braking it is added at time , a braking torque .

from => =>

The braking torque can be produced by:

  • A mechanical element;

  • An external electrical system (powder brake, eddy current brake);

  • By the engine itself:

  • By DC injection;

  • Generator operation.

In case of mains failure, only the mechanical brake ensures the immobilization of the load.

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