Energy balance in an electromechanical device

Introduction

This section approaches the energy balance/power balance in an electromechanical device.

Example: electromechanical conversion

Let us consider a device which, supplied by an electric energy source, generates a force or a torque. It is an electromechanical conversion system that transforms the electrical energy into mechanical energy.

In order to compute the efficiency of this device, an energy balance can be carried out on the basis of the ‘conservation of energy's principle (nothing is lost, nothing is yielded). The energy balance can be written as follows:

The electrical energy provided by the source is not integrally transformed into useful mechanical energy that is delivered to the driven system. Part of this energy is stored as magnetic energy in the volume of magnetic circuits and part is transformed into thermal energy (heat). The heat can be accumulated inside the device and/or transferred to the cooling system.

To determine the useful mechanical energy, it is therefore necessary to compute:

• on the one hand the stored magnetic energy
• on the other hand the dissipated thermal energy (thermal losses)

Generalities

In a more general manner, in order to carry out the energy balance of a complex system, it is necessary to take into consideration:

• the stored potential energies (electrical/magnetic/mechanical)
• the dissipated energies (losses) (electrical/magnetic/mechanical)

This is presented in the figure below.