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Basis of eds automatic control




 

7.1 Introduction

 

To control electric drives means to run-up, to control the rate, to brake, to backspace and to keep up some conditions and operating modes of electric drive according to the requirements of manufacturing process.

Manual control is used when the simplest events occur (switches, communicators, start and control resistors, brackets etc)

But it isn`t the best way of controlling because it has additional time losses, power losses, fatigability and possibility of faults at controlling by human, low efficiency, impossibility of remote control and automation problems.

Manual control is complicated in powerful electric drives because of substantial efforts of human.

Shifting the control functions onto automaton occurs in the next areas:

- A person can but he or she shouldn`t realize control functions because it is useless (temperature control, pressure control, level monitoring);

- heavy-load conditions, that threaten person`s health (high temperature, heightened radiation, toxic environment etc);

- large amount of control objects, expedience of usage the control devices is stipulated for impossibility to solve control task by human (for example: execution of control operations when it is necessary to process a large amount of information per units of time - traffic flow control, power supply system control).

 

Functions which execute auto control devices:

 

-self-acting execution of common control operations (start, braking, speed reversal etc);

- automatic coordination and upkeep of destination of electric drives (speed, moment etc);

- monitoring of input signals;

-auto control separated mechanisms or set of mechanisms at the specified program;

-auto control separated mechanisms or set of mechanisms ith auto choose the most reasonable regimes;

-complex automation.

The basic elements of auto control systems are the following:

Power and slave micro motors.

Contact and non-contact control apparatuses

Analogue and digit transducers and relays

 

Electric auto micro machines (tacho-generators, rotary transformer, transmitters etc) which execute functions of transducers, converters, amplifiers etc

Correcting links (proportional, integrating, inertial, differential, aperiodic and so on), that are used in direct connections and in feedback connections to set the desirable dynamic properties of system.

Block control units (digit, analogue, analogue-digits); they consist of integrated circuit and other integrated components, they used to realize different functions (command devices, transducers, transformers, galvanic separation, logic elements etc).

Microprocessors and computers

Unstable and stable sources.

Auto control cen be realized with open or closed loop.

Open loop systems have a peculiarity that changingof external effects (load, for example) leads to changing of previous electric drive regime.

Specified electric drive regime is kept up in closed loop control systems irrespective of external loads

Modern auto devices have a great amount of elements which are connects electrically in common system, which is intended to realize specified operations in specified order.

In this case there is a lot of difficulties in scheme reading as well as in designing. Taking into account aforesaid scheme drawing should be done with the help of the system that facilitates its reading.

7.2 Representation and designation of electric scheme elements. Common rules of scheme execution.

Scheme is an engineering document that represents compounds of hardware and connections between them in reference designation form.

Schemes are used in the following stages: designing, manufacturing, operation.

Regulations to fulfillment and execution are stated in government standards. There is United system of engineering documents (USED) in the territory of Ukraine.

According to it, electric schemes have the next classification.

 

 

7.2.1 Electrical structure chart. Signification of E1 document

There are plotted the main functional parts of device and the connections between them in the chart.

Functional parts of scheme should be plotted in rectangular form with appropriate caption. It is acceptable to present some objects in the pictorial symbol form and to put the caption inside rectangular.

 

afore-cited diagram presents electrical structure chart of AC voltage transducer into DC voltage with rectifier VD and regulator in the AC circuit.

The source is a power transformer; AC voltage adjuster is AAV of transformer. Uncontrolled rectifier consists of diodes that work in bridge scheme or in one with a center tap. Filter is a smoothed choke; a load is represented by DC motor and its armature circuit.

 

7.2.2 Functional electrical structure chart. Signification of E2 document

 

This scheme is intended to make all device processes clear, thus there can be not just one but several functional structure charts. There are functional components of device and connections between them in the chart.

Functional elements and their connections are painted with help of the pictorial symbol. If there is no need to detail some components are plotted in rectangular form.

Note, that functional detailed parts are painted using electrical structure charts regulations; but detailed ones are painted using electrical based charts regulations.

Real position of devise elements can not be taken into account.

 

There is presented a simplified functional scheme of controlled thyristor rectifier in the diagram, where

Ui- input signal (control voltage);

PPCC- pulse-phase control circuit

a- control angle of thyristorМЫ

ed-EMF of rectifier;

L- smoothed choke;

Id- rectified current (load current);

ZL- load;

U1- supply voltage;

When electrical and non-electrical elements are used in the chart, then functional charts are painted in the next ways:

1st way. Several functional charts are plotted (electrical functional chart, optical functional chart, hydraulic circuit etc)

2nd way. Combined functional chart is plotted. There should be signed the electrical and non-electrical elements and connections.

7.2.3 Basic electrical scheme. Signification of E3 document

Basic electrical scheme is the most fulfilled electrical scheme of device. All electrical elements and devices are plotted on this diagram, that are used to realize and control specified electrical processes. Also there are plotted connections, switching elements of input and output circuits.

Electrical elements are presented in pictorial symbol form, the dimensions, view, literal captions of which are stated in the standards.

Partly used elements can be presented in the incomplete form, painted just those parts, that are used in the diagram.

Schemes are plotted in one- or multiline views.

a) Three-phased multiple line scheme with Q1 three-phased switch;

b) Three-phased multiple line scheme with Q1, Q2, Q3 single-phased switches;

c) Three-phased single line scheme with a Q1 three-phased switch;

d) Three-phased single-line scheme with three single switched:Q1;Q2; Q3.

Signification of element is given to the right or above its image. Electrical key diagram is supposed to have text document, so-called list of elements. This document has all information about the elements, that are in the diagram. When there is empty space on the diagram, it is appropriate to give these data in the diagram.

 

List of documents is given in the first sheet of diagram above the corner caption or it is given as an A4 document with the title block for text document.

Relations between pictorial symbol and list of elements are realized via positional signs, that are in the list of elements as well. List of elements is a table, that has the positional signs in the first column.

 

In the list columns there are the following data:

 

The column “Note” is signed technical data of element, which wasn`t mentioned before in column “Caption”.

List should be filled top-down. Elements should be grouped alphabetically in number ascending order.

Basic electrical scheme is used to study the principle of device operation, to control and to repair it.

 

7.2.4 Electrical scheme of connections. Signification of E4 document (in the corner block of document)

 

Electrical scheme of connections defines a way of device construction, its electrical connections between elements especially. All parts of device and connections between them are depicted in the scheme.

Devices are shown as rectangular or simplified external contour, and elements are shown in pictorial symbol form, rectangular form or simplified external contour.

Inside these graphical signs it is allowed to put pictorial symbols, or literal signs. Input and output elements are depicted with pictorial symbols, or literal-numerical signs.

Position of input and output devices or leads inside of pictorial symbols should fit their real position in the device.

Conductors, wires, cables, harnesses have their own serial number. They are numbered in the edges of device. Cables and wires should be numbered separately.

Wires which are in harnesses are numbers in the edges, thread of cables are numbered in the edges of them. Numbers of wires and cable threads are filled in near the both edges of their image. Numbers of cables are allowed to put in circles, which are signed in the gap of cable images. Numbers of harnesses are put in the axis of offset near the branching of cables, group numbers of wires are filled in near the axis of offset.

Grade, number of harnesses and cross-section of cables is depicted in the scheme. These data are put near the lines, that the conductors and cables are represented.

Input and output characteristics are recommended to give in the table, which is put near the graphical symbol of slots or clamps.

If there is no sign of wire or harnesses connection or there is a lot of such connections the table of connections should be made, which has all information about wires and cables.

Tables are put in the first sheet of scheme above the corner block.

 

7.2.5 Electrical scheme of commutation. Signification of E5 document (in the corner block of document)

 

Electrical scheme of commutation depicts external connections of device.

There should be depicted in the scheme:

-a device;

-Input and output elements of device (connectors, clamp etc);

-edges of cables and wires (which lead to input and output elements), which are part of external assembly.

Devices and their parts are depicted in rectangular form in the scheme, input and output elements (connectors) are depicted in pictorial symbol form. It is appropriate to depict an element, input and output lead as a simplified contour.

Input and output elements are situated inside pictorial symbol of device according to their real position in the device.

 

 

7.2.6 General electric scheme. Signification of E5 document (in the corner block of document)

 

General electric scheme is defined the parts of complex and connections between them in the operation. Scheme is used to get acquainted with the complex and wires, cables that connect them.

7.2.7 Arrangement electric scheme. Signification of E5 document

 

Arrangement electric scheme defines the comparative position of device parts and, if there is need, harnesses, cables, wires. If there is need, it also defines simplified construction, premises, surrounding, where the device is placed.

If there is a great amount of device components this scheme and list of elements is allowed to compose.

 

 

Arrangement electric scheme can be made in sections of construction, in sections or plans of premises or using axonometric (foresaid) diagram inside service room.

 

7.2.8 Schemes of digital and computing technology

 

Schemes of digital and computing technology have their own peculiarities. If there is need to look through them and foresaid information as well one can do it in USED

 

7.2.9 Symbolic notation in electrical schemes

 

Each element in the electrical scheme has its own notation. Such notions are stated by standards. Literal symbols of elements have their general sign in a letter, i.e.:

All element are divided by types in groups of elements, every type group of elements is signed by one letter. To precise type of element double-letter sign are used (look at the table)

First code letter Group of elements Examples Double-let. sign  
A Devices(common signification) Amplifiers, lasers -  
B Convertors of nonelectrical quantities selsyn receiver selsyn transmitter temperature sensor pressure sensor accelerometer velocity sensor photo cell BE BC BK BP BR BV BL  
C Capacities   -  
D integrated circuit Analogue digital DA DD  
E lighting facilities Lightning lamp EL  
F Fuses, safety device current cutoff, fuse unit current inertial protection FA FU FP  
H Attention and display device acoustic alarm indicatinglight HA HL  
K Contactors Starter Relay Contactor solenoid starter current relay indicating relay polarized relay hot-wire relay time relay potential relay KM KM KA KH KP KK KT KV  
P Instrument, measuring equipment ammeter frequency meter watch voltmeter wattmeter PA PF PT PV PW  
Q Breakers Disconnectors(powerful) Automatic cutout short circuitor disconnector QF QK QS  
R Resistors   Thermo resistors variable resistor instrument shunt varistor - RK RP RS RU  
S Commutation elements (not powerful) Switch Button switch Automatic switch Level switch Pressure switch Arrester Rotation switch Tempreture cutout SA SB SF SL SP SQ SR SK  
T Transformers autoconnected transformer Power transformer Current transformer Voltage transformer Magnetic stabilizer TH TA TV TS  
V Semi conductive elements Diode voltage reference diode, thyristor transistor VD VD VS VT  
X Connections Sled Skate finger contact slot demountable joint XA XA XP XS XT  
Y Mechanical devices with an electrical drive Electromagnet Brake with electromagnetic drive Clutch with electromagnetic drive electromagnetic chuck YA YB   YC   YH  

Schemes circuits are divided into two groups:

-circuit of main current;

- auxiliary circuits.

Power circuits of motors and generators are regarded to the circuits of main current.

Auxiliary circuits consist of control circuits (connected coils, relays, control contacts etc), protection circuits, alarm system, measuring system etc.

Commutation devices should be depicted in the initial position in the figures. As the rule, It is a position of currentless state and zero-position of controller.

7.3 Opened systems of automatic control

7.3.1 Automatic control principles in opened contactor-relay systems

As it is known there are no feedback in the open-loop systems, thus if the deviation of input takes place the control signal stays constant.

As an example is Motor, supplying by convertor. Convertor is connected to the network and the control input signal is given.

 

Motor drives operation device via mechanical part of ED – reducer.

As the rule the output value is velocity.

Disturbance acts the convertor, motor and device in the way of voltage supplying changing etc. This disturbance leads to the output deviation of input. Magnitude of this deviation specifies by convertor motor or operation mechansm parameters. Value of voltage, as there is no feedback, is constant that is why it has no influence on the output. The possibility of influence the output value isn`t excluded by control deferent coordinates of ED as armature current, voltage, frequency, excitation current etc.

Automatic control principle can be considered as the example of EM starting automation in the opened circuits.

The simplest way of AC motor automation of starting is to apply it to the motor of average or low power, because starting control is the type of control that uses direct connection AC motor to the network. If there is another case resistor starting is used frequently that decreases gradual starting resistance and controls starting current and runaway motor speed.

To design control starting schemes of DC motor and asynchronous phased-motor the explicit data of starting should be taken into account.

These data is convenient to give as the starting motor diagrams at every stage of starting resistor.(look the 5 section)

At the figure 7.7 the starting motor diagram is depicted with 3-staged starting resistor, built acoording to the conditions of current changing from upto.

Steady values of velocity and current are equal:

 

,

.

Starting of motor can be realized automatically or manually.

If the manual staring takes place, the operator sets up the resistor handle to the maximum value in the zero moment t=0, and watches the ammeter reading at the time moment t1, when current is equal I2 then the operator takes out the first stage of resistor, the second one is taken out when starting current is equal I2 again and so on.

Automatic control allows to execute all foresaid operation more precisely and to free a human from the annoying staring operations.

It is obvious there are several criteria that are realized at the automatic control. Taking out the resistor stages can be done:

-by angular velocity control, executing at the ω1, ω2, ω3velocities;

-by current value control and;

-by time control in 1,2,3 intervals

So it is obvious that the starting control can be realized by velocity, current and time functions. There is another indirect function that is connected with those three, the path function. Afore-cited diagram depicts starting process at the staged control with help of relay-contact equipment in the open-looped control systems.

 

7.3.2 DC motor control of starting in angular speed function

 

When control of angular speed function takes place this speed should be controlled and the signal should be given to the appropriate switching unit at the appropriate moment

There are relays that operate when angular velocity changing occurs, e.g. centrifugal relay. But they are not widely used. It can be explained by the structural complexity and low reliability. Using tachometer generator the angular speed can be measured in electrical way, but it complicates the scheme.

So the speed can be easier fixed in indirect way by measuring the secondary parameters referredto the angular velocity. The best variant of such parameter is a motor EMF for the case of DC motor or current frequency and armature EMF for the case of synchronous or phased asynchronous motors.

This way of control can be named the control in function EMF

 

The simplest scheme is shown above. Depending on the armature EMF voltage is applied to the contact coils, they operate when some value of voltage

occurs. The contacts switch resistor stages in strict succession. Every of switched on contacts KM1, KM2, KM3 is adjusted to the strict pull-in voltage. At the first moment voltage of contact coils is equal to the voltage drop in the armature circuit. As the angular velocity is growing up, armature EMF increases.

At the w1 angular velocity the contactor voltage KM1 reaches the value that KM1 can operate and close its contact in the R1 start resistor stage, i.e. the 1st resistor stage is shunted. KM2 and KM3 are adjusted to the larger value of pull-in voltage, that is why they don`t operate.

After the operation of the last 3rd stage of start resistor motor start is finished and operates in the natural characteristic.

Disadvantage of the scheme solution is different voltages of KM1, KM2, KM3, according to them these contacts should be chosen. It can be changed in the following scheme solution.

In this scheme only one output of every coil is connected to the armature winding and another one to the appropriate stages of start rheostat. Pull-in voltage is smoothed, but the contacts operate at differentvalues ofω1, ω2, ω3.

Thus after starting all contacts are operated to the same voltage the net one.

Disadvantages of the starting control:

- Blowing of starting resistor stages;

- Not precisely contact operation in the cold or hot state;

- Not precisely operation of contactors at the oscillations of net voltage.

These disadvantages make this way no suitable to the drives of large power. It is applied for metal-cutting equipment.

 

 




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