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Basis of automatic control of DC ED

Closed systems of automatic control

In opened control systems at the modest stiffness of static characteristics it is impossible to obtain the high accuracy regulation, because at such systems output value is determined by motor parameters and shaft load, and its changing is not compensated at different disturbances. Motor parameters and characteristics of executive mechanisms can be changed due to changing of winding resistance, operation mode of mechanism and so on. Because of all these reasons obtaining of high accuracy controlling is impossible.

In closed control systems automatic compensation of existent external disturbances is realized, and these systems, as opened ones, can be attended to provide automatic starting modes, braking, reversing, coordinates regulation, hence such systems allow to make called control modes by desirable way and to connect it more accurately with technology requirements.

In closed control systems, control, that acts on power part, is changed with deviation of true values for output parameters from set values, that is reached by introduction of feedbacks from output to the inputs (of all system or its specified elements). If feedback connects output and input of not all system, but its specified elements, then such feedback is called internal.

Feedbacks are divided into rigid and flexible.

Rigid – act in transient and steady state regimes.

Flexible – act only in transient regimes.

There are positive and negative feedbacks.

With increasing of regulated value a positive feedback increases it, and negative one decreases it.

Feedbacks can transmit signals proportional to the value (or differential of this value, seldom its integral) of voltage, current, moment and so on. In this case they are called feedbacks in respect to voltage, current, rotational angle and so on.

With this to realize an automatic control one should:

- to measure a feedback signal;

- this result of measurement in a form of voltage to compare with set signal represented in the form of voltage with value of regulated magnitude (to make an algebraic summing up);

- to direct the result of regulated object comparison;

- to amplify a signal, because there is not enough energy of measuring element for acting on regulated element.

Considered elements (measuring element, amplifier and regulating element) are included into a regulator, which realizes the regulation process.

For example let’s consider automatic control system of DCM IE angular speed with two different feedbacks:

а) with rigid negative feedback in respect to voltage (figure 7.15, fragment а);

b) with rigid positive feedback in respect to armature current of DCM (figure 7.15, fragment b).

System of equations in steady state regime for both functional schemes will be:

- voltage on amplifier input , (for scheme fragment а);

- voltage on amplifier input , (for scheme fragment b).

Parameters of figure 7.15 elements are such:

- e.m.f. of convertor;

- e.m.f. of motor;

- voltage of motor (negative feedback);

- voltage of motor (positive feedback);

- motor moment;

- amplification coefficient of feedback in respect to voltage;

- coefficient of feedback in respect to current;

- amplification coefficient of amplifier А;

- amplification coefficient of convertor;

- setting voltage;

- feedback voltage;

- electric machine constant of motor;

- angular speed of motor;

- resistance of convertor;

- resistance of motor;

- current of motor armature circuit;

- control voltage (on input of convertor );

- resistance of shunting resistor (shunt).

In fragment а with the help of rigid negative feedback in respect to voltage the voltage drop on internal resistance of convertor and oscillations of supply voltage ~are compensated (i.e. output voltage of convertor – voltage of motor powering is stabilized).

In fragment b with the help of rigid positive feedback in respect to current the compensation of motor angular speed decreasing with increasing of load by automatic increasing of e.m.f. of convertoris realized.

 

а – feedback in respect to voltage;

б – feedback in respect to current.

Figure 7.15 – Automatic control system of DCM IE angular speed.

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DCM starting control in time function | Basis of automatic control of AC ED
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