It consist of a stationary ferromagnetic core and a moving ferromagnetic part (armature), which is separated from the core by an air-gap . A winding w is also provided, serving as a source of power to energize the magnetic circuit. After the switch S is closed, the current I will create magnetic flux. When moving towards the core (-x direction), the armature will reduce the air-gap , thus converting part of the magnetic field energy, stored in that system, into mechanical energy. When the switch S is opened, the spring force Fc, which is counter-acting, will bring the armature back to its initial position.
It encompasses the energy delivered by the electrical power source, the energy stored in the magnetic circuit and the mechanical energy utilized for the armature motion. As a result, a second (general) approach for the force Fd determination has been introduced. It states:
This equation assumes firstly that the reluctance of the ferromagnetic part of the circuit is zero, and secondly, that no fringing exists in the air-gap. Thus SM is regarded as the core surface area in front of the armature, as depicted in figure:
If the winding mmf remains constant (Fw=Iw=const), the force Fd will increase rapidly when the air-gap decreases. The curve Fd = f() is depicted in figure:
together with the spring characteristic curve Fs = f(). The curve Fd = f() is known in engineering as static electromechanical (or load) characteristic of the electromagnet. Actually this is a family of curves when current I delivered by the source voltage U varies. The lower the current I value, the lower is the position of the curve in the diagram.
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Electromagnetic Module. Modelling, Analysis and Design.
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