Features abstract of AM mechanical characteristics

Mentioned above analysis of AM mechanical characteristic gives an opportunity to formulate some features of it mechanical characteristic (have earlier already determined by us):

a) current value of AD moment depends on square of supply voltage, active and inductive resistance of rotor and stator windings at a given sliding and synchronous angular velocity (see the equation (3.33));

b) moment made by a motor is a complex sliding function. The curve has 2 extremums (one is in a motor mode, and other is in generator mode), slidings corresponding to these extremums are called critical and they are equal by module (equations (3.35) and (3.36));

c) critical sliding value depends on value of rotor active resistance (see the equation (3.33));

d) the value of AM critical moment doesn`t depend on on value of rotor active resistance (see the equation (3.36));

e) simplified Kloss`s formula is very simple from one hand, because only rated data is in this formula , , and from the other hand – with sufficient for practice accuracy it is represent the physics of a process (see the equation (3.39));

f) operation area of AM mechanical characteristic is straight linear, as well as DCM IE characteristic, that made consideration of processes significantly simpler;

g) AD mechanical characteristic in a view, describing some certain AM operation, provides its location in graphical representation in a one quadrant of rectangular coordinate system.

Considered and formulated features allow to determine some parameters and AM design peculiarities related to these features.

Value of rated sliding depends on value of active resistance of rotor circuit.

AM with cage rotor of normal design will have the less(at equal powers and number of pole pairs). In these motors under design peculiarities the rotor active resistance will be not significant (short-circuited cage) that causes the decreasing of and. Due to the same reasons with AM power increasing its become smaller (because the resistance decreases) and the stiffness of mechanical characteristic increases.

Figure 3.37 – AM rated sliding.

AM features (depends on , and doesn`t depend on ) applies in AM with phase rotor to increase the starting moment due to increasing of active resistance of rotor circuit.

When using additional resistance (characteristic 4) . Characteristic 1 is mechanical natural characteristic, and the characteristics 2, 3 and 4 are rheostat ones.

Figure 3.38 – Natural and rheostat characteristics of AM with phase rotor.

AM rotor winding is wounded from ordinary winding wire with sufficient number of turns. So, the rotor current doesn`t reach significant values, because the effect of current displacement doesn`t manifest its activity, that`s why its mechanical characteristic mostly corresponds to simplified Kloss`s formula. In the motor with cage rotor (high current of squirrel cage) the phenomena of current displacement is significant, that’s why the winding parameter and can not be constant, because the mechanical characteristics are not constant. Primarily it is concern to AM with a deep slot and two-cage AM.

Besides in AM with a cage rotor there is no possibility to connect the starting rheostat into rotor circuit at the start moment. That`s why its starting features is worse than in AD with phase rotor.

So the multiplicity of starting moment is , and the multiplicity of starting stator current is .

Absence of proportionality between and explains by significant decreasing of magnetic flux at starting AM and decreasing of .

Abnormal symmetrical modes are significantly effect on mechanical characteristics. Such as decreasing of voltage supply value of stator and increasing of active and inductive resistances in stator circuit.

Figure 3.39 – Natural and artificial characteristics of AM.

Since the motor current is proportional to the voltage in 1^st power, and the moment is proportional to the supply voltage in the 2^nd power, the characteristics when decreasing supply voltage will change both qualitative and quantitative and will have a view as in a graph. Critical velocity (critical sliding ) does not changed for every voltage values, that is doesn`t depend on voltage value. Moment value changes significantly with voltage change, so , because supply voltage is decreased in 2 times. In real conditions in all industrial mains the voltage can drop, that`s why it should always control the decreasing of maximal and starting ED moments when decreasing voltage and to control the satisfaction of technological process requirements.

In real conditions of ED operation with the purpose of starting current (of high-power motors) limiting and with the purpose of unallowable voltage drop eliminating at the start symmetrical active or inductive resistances are connected into circuit at starting period (that is equivalent to the stator supplying with decreased voltage). Sometimes the same measures are applied for light-load drives with purpose to limit starting moments, in order to cushion in mechanical part and provide the surgeless acceleration of actuating mechanism.

When applying active resistance is a little increase, and losses are increase, but when applying reactance the losses are decrease.

Дата добавления: 2014-01-05