Electromachine Converter (G-M system)

Scheme of such system called the generator-motor system is shown below.

Here armature of motor is directly (metallic) connected to the armature of generator (without starting, regulating and commutating apparatuses). Excitation circuit of machines is feeding from the separate source (rectifier of exciting machine).

Generator G together with its driving motor (asynchronous motor AM) make up an electromachine converter П – controlled converter of unregulated alternating supply-line voltage U₁ (that feeds AM) into regulated constant voltage U_м (that feeds the driving motor M) – controlled rectifier.

Voltage regulation U_м for feeding the motor armature realized due to change of generator excitation current by means of resistor R_3G. Voltage U_м may be obtained quite small (at minimal generator excitation) and may be increased to the maximal value (to the allowed limit of generator magnetic system saturation). But it isn’t the limit of motor speed rise M.

The upper bound of speed regulation will be tolerated measure of motor field weakening due to decreasing from nominal down off its excitation current by resistor R_3M. in such a way we will have two-zone regulation: down off from primary – due to supply voltage regulation, and upper from primary – due to motor field weakening (7, 8 characteristics on the previous graph). Motor reverse is realized by changing the polarity of feed current of generator excitation winding. Motor starting is realized due to gradual voltage increase U_M from the zero-value.

Static characteristics of speed regulation for the case when feeding the motor from electromachine converter (G-M system) are shown on the previous graph.

Main advantages of G-M system

Wide range and smoothness of regulation.

High inflexibility of static characteristics and their linearity.

Capability of recept of all energetic modes of operation of DC motor of independent excitation, including regenerative braking.

Simplicity of speed regulation and realization of all operating duties (starting, reverse, braking, etc), since all regulating, starting and commutating apparatuses are removed from armature circuit (when the primary current is large) into excitation circuit (where control current is large).

It is economically profitable as there are small energy losses and low costs of active material (copper for contacts) in the commutating apparatuses.

G-M system practically doesn’t garble the main harmonisity and doesn’t make radio noise.

Disadvantages of G-M system

Low quick response, that is the most considerable disadvantage when using the modern deepregulated and accurate electric drives.

High control signal power (excitation current), that doesn’t allow the usage when controlling directly from modular control elements on the level of integrated circuits.

High preseted power (triple).

Low efficiency coefficient (about 70-80% in the most powerful).

Unstable operation of motor at low speeds (value of U_M is commensurable with the value of voltage drop on armature).

High operating costs (restoration, service), high overall dimensions indexes (basements, etc).

Significant vibrations and noises.

Static converter – motor system (SC-M)

The basic type of controlled converters in modern controlled DC electric drive is semi-conducting static thyristor or transistor converters.

They are constitutes the controlled reversible or irreversible rectifiers assembled by zero- or bridge single-phase scheme of three-phase scheme.

Consider the principle of operation, features and characteristics of the thyristor converter – motor system by the scheme where the three-phase irreversible controlled thyristor rectifier assembled by the bridge scheme is chosen as converter.

Converter has matching transformer TH (see the figure), six thyristors VS1-VS6, system of impulse-phase control SIPC (СИФУ) and smoothing reactor L.

Converter provides regulation of voltage that feeds the motor armature circuit due to change of thyristor control angle α. Angle α is delay angle of thyristors opening relatively to the moment of their natural opening (see the figure).

When α=0 thyristors receives the control impulses U₁ from SIPC in the moment of their natural opening, converter realizes the full-wave rectification and the full voltage applies to the DC motor. If with the help of SIPC the impulse feeding will occurred not in the moment of thyristor natural opening, but with the shift by angle , than EMF of converter will decrease and the lower voltage will be applied to the DC motor.

The pulsating character of converter EMF leads to the current pulsations in the DC motor armature circuit; it is the reason of bad influence on the DC motor operation, worsening of commutation, i.e. worsening of commutator operation, increases energy and heat losses.

For decreasing of current pulsation the smoothing choke is connected into the armature circuit. Besides, the increasing of number of rectifier phases leads to decreasing of pulsation.

Static characteristics o electric drive have the next view.

Дата добавления: 2014-01-05