Static mechanical characteristics of DC mixed excitation electric motor (DCM ME)

In DCM ME, the scheme of which is presented on the next figure, magnetic flux is developed by common mmf action of independent excitation winding LM1 and series excitation winding LM2.

Therefore mechanical characteristics of DCM ME take intermediate position between characteristic of DCM IE and DCM SE. Independent excitation winding, producing constant value of mmf, provides magnetic flux at absence of current in armature circuit, i. e. the ability of DCM ME operation at open circuit.

Mmf of winding with series excitation increases magnetic flux of the motor, which assists to increasing of starting moment, and, together with this, will cause decreasing of the speed at load increasing.

DCM ME has significant saturation of magnetic system and at these conditions its mechanical characteristics (as in DCM SE) can`t have précised analytic expression. At computations it is should use (as for DCM SE) universal characteristics, given in catalogues and reference books for corresponding types of DCM ME.

Mmf relation between independent excitation windings and series excitation ones can be different for different types of DCM ME – the view of mechanical characteristics depends on this relation. Generally more used is such relation, which at rated current gives the same mmf for both excitation windings (fifty-fifty).

Mechanical characteristics of DCM ME are intersected at one point, which corresponds to the speed of ideal open circuit ω0.

With load arising on the shaft the speed decreases abruptly (as in DCM SE) and then this decreasing is decelerated and realized almost on the straight line (as n DCM IE). It is explained by that fact, that at low load moments magnetic system is not saturated, the flux is significantly increases because of addition to constant mmf value of independent excitation winding alternative mmf value of series excitation winding.

At substantial load moments and at substantial mmf of series excitation winding saturation of motor magnetic system comes, the flux is the constant, and speed decreasing by such a way will stipulated mainly by voltage drop in armature circuit.

The more influence of series excitation winding, the softer mechanical characteristic. At increasing of resistance in armature circuit the motor speed decreases (see the previous figure) and characteristics become softer. At high resistances in armature circuit (for example, Raa4) the motor at the influence of potential static moment can slowly pass from motor mode into braking one.

Дата добавления: 2014-01-05