For three-phase bridge irreversible rectifier

The presence of the segment of discontinuous current is its peculiarity (marked as a dashed-line curve and crosshatched region). At this segment it is noticeably the change of inflexibility of characteristics, in consequence of which the characteristics integrally will be nonlinear.

At final values of inductance of smoothing reactor and small loads the mode of discontinuous currents occurs, with this the abrupt increment of characteristics exists. Phenomenon of discontinuous currents stipulates the decreasing of amount of energy accumulated of the induction L by decreasing of load, and there are occurs moment when generated by inductance EMF of self-induction isn’t sufficient for maintenance of current that leads to the voltage and speed increasing at the idling rate. As a result of this the static characteristics will be non-linear. And as a result of single-zone conductivity of such scheme the static characteristics locates only on the I and IV quadrants. For obtaining of characteristics, located on every of four quadrants, reversible converter in which besides of the thyristor group for specified polarity (there are 6 thyristors in the scheme) the thyristor group for opposite polarity is needed, should be used. This groups are connected between each other by the crossed or inverse-parallel constructions.

In the given scheme the DC motor can operate at starting, braking modes, in the feedback with active static moment (at constant direction of armature current), and in the regenerating mode at α=90˚, i.e. converter EMF equals to zero and there are voltage on armature.

Advantages of SC-M system

- Smoothness and considerable range of speed regulation up to D=10:1 – 16:1

- High inflexibility of characteristics;

High efficiency factor of electric drive due to efficiency factor of power elements of converter (; );

- Quick response that is especially important for high-speed deep-regulated electric drives;

- Small control signal strength;

- Smaller overall dimensions factors, there is no need in basements, additional electric machines, etc;

- Noiseless in operation, simplicity in service.

Disadvantages of SC-M system

- Single-sided conductivity of semi-conducting elements needs two rectifier groups for reversing;

- Rectified current have pulsations that determines the worsening of DC motor operation;

- Converter garbles the form of supple-line voltage and current;

- Converter has low jamming protection and it is a source of radio noises;

- Small overload capability by currents and voltages;

- Discontinuous currents mode occurs, and it decreases the inflexibility, and increases the non-linearity of static characteristics.

4.4 Basic Methods of Speed Regulation of DC Motors with Series Excitation

Regulation of DC motor with series excitation coordinates may be realized in the same ways as in the DC motor with independent excitation:

- By means of change of resistance of additional resistor in armature circuit;

- By means of changing of magnetic flux;

- By means of change of motor supply voltage.

Regulated electric drive of DC motor of series excitation is mostly used in the lifting-transport mechanisms and as traction drive.

4.4.1 Speed Regulation of DC Motor with Series Excitation by Resistors in Armature Circuit

The scheme for realization of this method has the view.

At constant rheostat characteristics it is necessary to remember that ordinate (speed axis) is asymptote for all characteristics,

І_я®0; М®0; Ф®0; w®.

Basic arguments about speed regulation by means of this method remain the same as for the DC motor with independent excitation. Speed regulation realizes down off from primary one, inflexibility of characteristics decreases with increasing of the resistance value. Range of regulation D=(2-3):1 and depends on the load value (decreases with the decreasing of load). In order to use completely DCM SE on the various speeds one should regulate the speed at constant loading moment (that corresponds to nominal current). The method is uneconomic due to great energy losses on the additional resistor in the armature circuit. But this method is widely used in electric drives of lift-and-carry mechanisms because of realization simplicity from the one side, and short period of motor operation at low speeds from the other one. With this the rheostat for starting, braking and speed regulation is the same.

Дата добавления: 2014-01-05