Natural characteristic is such motor characteristic at which resistance in external armature circuit, rated voltage values and motor magnetic flux are absent

Stiffness of natural characteristic depends on internal resistance of armature circuit Ra, so the speed difference for natural characteristic would be:

,

.

The speed can be expressed through relative units, and at this the main is an angular open circuit speed. Taking into account previous condition, equation (3.14) can be written as:

,

from where the speed in relative units would be:

, or

. (3.15)

Formula of speed difference (3.15) by exterior view is identically with formula of sliding S for AC electric machines:

,

where - is a synchronous speed;

but physical essence of and S is different.

If to armature circuit an additional resistor (rheostat) will be connected, obtained characteristics would be artificial and called as rheostat characteristics (characteristics 2-5 of previous figure). Rheostat characteristics are intersecting at one point , the same as linear and natural, but have a big slope angle, i. e. less stiffness, and the greater connected in armature circuit additional resistance, the steeper these characteristics (lesser stiffness).

Дата добавления: 2014-01-05