Basis of automatic control of AC ED

For example let’s consider automatic regulation of asynchronous ED angular speed with the help of thyristor voltage regulators (TVR).

As known, in opened control system asynchronous motor can’t operate on unstable interval of static mechanical characteristic.

In closed control systems asynchronous motor can stably operate providing the smooth speed regulation in rather wide regulation range.

Functional scheme of ED with angular speed regulation with the help of thyristor voltage regulator based on AM with phase rotor have a view represented on figure 7.16.

Figure 7.16 – Functional scheme of ED with angular speed regulation with the help of TVR.

Such markings are accepted on scheme:

~- supply voltage of TVR;

- thyristor voltage regulator (TVR);

- control system of TVR;

- power thyristors;

- angle of thyristor control;

- angle of thyristor opening at absence of control signal ;

- control signal on input of (convertor control system);

- control signal on output of ;

- amplifier;

- voltage on input of amplifier ;

- setting voltage;

- tachogenerator (speed giver);

- signal (voltage) of feedback;

- transmission coefficient of tachogenerator ;

- additional resistance in rotor circuit of AM.

Voltage regulation on AM stator is realized by TVR, control angle of thyristors of which is changed by control system of convertor.

On input of the control signal is supplied from amplifier. This signal depends on difference of setting voltage and voltage of negative feedback with respect to speed which is taken from the terminals of tachogenerator

.

Mechanical characteristics of AM can be obtained in limits of minimal and maximal values of thyristors control angle.

Minimal thyristor control angle equals to phase angle of load (which is defined by value of sliding, active and inductive resistances of AM and character of load).

Maximal thyristor control angle for considered schemes is approximately equals to . At such angle developing moment of AM as a rule is less than static moment of motor idling rate .

In such a way at operation of AM in closed system with increasing of load control angle has to decrease from maximal to minimal. Thus thyristors control system with the absence of control signal must create impulses with control angle

,

and with increasing of control signal at increasing of AM load and with decreasing of its angular speed the angle must be decreased. (With this maximal control angle corresponds to minimal value of motor supply voltage, minimal control angle corresponds to nominal value of stator supply voltage):

(at ),

(at ),

because, .

Such process in convertor control system is represented on figure 7.16 by comparison unit on the output of. Correspondently to it one can write

,

,

where - amplification coefficient (transmission coefficient) of control system ;

- amplification coefficient (transmission coefficient) of amplifier.

Using the last equation, necessary values of and , that will provide necessary values of angular speed and rigidity of mechanical characteristics, can be calculated.

Дата добавления: 2014-01-05