- Dynamics of Electrical Drives Definition: The dynamics of electrical drives explain how motors and loads interact, especially when their speeds differ.
- Key Components: Important components include the polar moment of inertia (J), angular velocity (Wm), motor torque (T), and load torque (T1).
- Fundamental Torque Equation: This equation shows that motor torque balances load torque and dynamic torque, which is crucial during changes in motion.
- Dynamic Torque: Dynamic torque, J(dωm/dt), appears only during transient operations like starting or stopping, indicating acceleration or deceleration.
- Impact on Motion: By analyzing dynamic torque, we can determine if the motor is accelerating or decelerating, which is essential for efficient drive operation.
An electric motor accelerates a coupled load when its developed torque exceeds the opposing load torque. A gearbox can make motor and load speeds differ, so load torque and inertia must first be referred to the motor shaft.
The following one-shaft electrical drive model uses the angular-momentum balance.
For the model shown above:
J is the total equivalent moment of inertia referred to the motor shaft.
ωm is instantaneous motor angular velocity in radians per second.
T is instantaneous developed motor torque.
TL is instantaneous opposing load torque referred to the motor shaft.
Using one stated positive direction, the fundamental torque equation is:
For a drive with constant equivalent inertia,
The difference T – TL is the dynamic torque J(dωm/dt). Positive dynamic torque accelerates the shaft under this sign convention; negative dynamic torque decelerates it. Dynamic torque occurs whenever speed changes, including starting, stopping, reversing and responding to a new speed or load command. If inertia changes with time, the full derivative also contains the term ω(dJ/dt).
At constant speed and constant inertia, developed torque balances the referred load torque and dynamic torque is zero. During acceleration, the motor must supply load torque plus the torque needed to change speed. During controlled deceleration, motor torque can oppose the direction of motion. The drive may dissipate the recovered mechanical energy or return it to the electrical supply, depending on the braking method and converter.
