General characteristic

TRANSIENT PROCESSES IN ED

Transient (dynamic) regimes of ED are called operation regimes at passing from one steady state of ED to another, that occur during the starting, braking, reversing, abrupt application of load to the shaft. These regimes are specific by changing of e.m.f., angular or linear speed, moment and current.

Transient regimes are caused by two main reasons.

Firstly, technological factors. It is load changing caused by technological process, act on ED at control of such processes as starting, braking, reversing, speed regulation.

Secondly, unexpected factors, such as emergency, violation of normal electric supply conditions (decreasing, partial or complete disappearing of supply voltage, changing of network frequency, appearing of load asymmetry in symmetrical systems and so on).

Studying of ED transient regimes has essential practical importance for correct choice of electrical equipment (motor power, commutation and others apparatuses) for calculation of control systems, energy waste on starting, braking and reversing, for defining of ED operation influence on efficiency and operation quality of manufacturing mechanisms (for example, time reduction for passing of transient processes continues the time of ED operation in steady-state regimes, that define the efficiency of work).

However, construction of manufacturing mechanism and especially technological requirements and occupational safety requirements partially limit the load magnitude, speed, acceleration, temperature, noises and so on. From the side of technics and technology these limits are caused by desire to avoid failure of mechanisms, breakdown of equipment, faulty production. ED has to provide not only properly passing of transient processes, but also their character. Properties and characteristics of ED (avoiding of motor heating, its stopping, or failure of synchronization, deterioration of DCM commutation, failure of positioning accuracy and so on).

Such factors influence on character of transient process passing:

- properties of execution mechanism;

- type of electric motor;

- features of mechanical transmission (kinematics);

- principle of operation and control apparatuses properties;

- regime of operation.

Theoretical consideration of transient processes taking into account their main factors in many cases is very difficult. They can be described by differential equations of rather high order, often with non-linear relationships, the solution of which is connected with significant mathematical difficulties.

Hitherto such problems were not solved rather exactly:

- changing of winding inductances with changing of magnetic field intensity;

- resistance magnitude of transient contact collector-brush;

- accurate taking into account of magnetic material saturation.

This often make to use approximated calculations, experimental data (curves or coefficients), graphical constructions and graphical integration.

However not at all cases of practice detailed and accurate taking into account of all effects is necessary. So that, solving the certain problems transient processes can be considered with some approximations and simplifications.

In any transient process of ED simultaneously and interrelatively transient mechanical, electromagnetic and heating processes act. At high-speed processes changing of ED thermal state, that flows more or less slowly, in the most cases do not essentially effect on other processes, so at studying of such transient processes in ED the change of motor thermal state is not taken into account.

If thermal inertia is not taken into account, then only mechanical and electromagnetic inertias are considered. In some cases also electromagnetic inertia is neglected, so then only mechanical processes are considered.

In such a way, in ED transient process in certain quantitative ratio such inertias act:

- mechanical inertia of execution mechanisms, intermediate transmissions, moving parts of motors and apparatuses, so as rotating or translational motion;

- electromagnetic inertia, caused by inductance and active resistance of drive electric elements, mainly, they are windings and contacts;

- thermal inertia of electric machines and some electrical apparatuses.

All these factors slow down the flowing of transient processes. Degree of inertias influence of one or another type is defined by inertial constants.

Mechanical inertia of ED is characterized by electromechanical time constant Т_М, (more detailed consideration in paragraph 4.3). It doesn’t depend on mechanical parameters (rotating masses and static resistance moment), as well as on electromechanical properties, that in turn depend on resistance of motor electric circuits.

Electromagnetic inertia is characterized by electromagnetic time constant Т_Е. It is defined by ratio between inductance, and active resistance , :

, .

Thermal inertia is characterized by time heating constant . It is characterized by ratio between heat capacity, and heat conductance,:

, .

Calculation of ED transient regimes mainly is reduced to obtaining or construction of relationships

; ; .

Дата добавления: 2014-01-05