The Field Energy
Based on the principle of conservation of energy: energy is neither created nor destroyed; it is merely changed in form.
Fig. 3.3(a): a magnetic-field-based electromechanical-energy-conversion device. A lossless magnetic-energy-storage system with two terminals
The electric terminal has two terminal variables: (voltage), (current).
The mechanical terminal has two terminal variables: (force), (position)
The loss mechanism is separated from the energy-storage mechanism.
– Electrical losses: ohmic losses.
– Mechanical losses: friction, windage.
A simple force-producing device with a single coil forming the electric terminal, and a movable plunger serving as the mechanical terminal.
The interaction between the electric and mechanical terminals, i.e. the electromechanical energy conversion, occurs through the medium of the magnetic stored energy. Equation (3.9) permits us to solve for the force simply as a function of the flux λ and the mechanical terminal position x. Equations (3.7) and (3.9) form the basis for the energy method.
Consider the electromechanical systems whose predominant energy-storage mechanism is in magnetic fields. For motor action, we can account for the energy transfer.
The ability to identify a lossless-energy-storage system is the essence of the energy method. This is done mathematically as part of the modeling process. For the lossless magnetic-energy-storage system of Fig. 3.3(a), rearranging (3.9) in form of (3.10) gives
Here E is the voltage induced in the electric terminals by the changing magnetic stored energy. It is through this reaction voltage that the external electric circuit supplies power to the coupling magnetic field and hence to the mechanical output terminals. The basic energy-conversion process is one involving the coupling field and its action and reaction on the electric and mechanical systems.