Contactless Traveling Switches

Contactless traveling switches (the converters of a way working without mechanical influence from a moving emphasis) are used in schemes of control of electric drives of machines, mechanisms and cars. Contactless switches are intended for switching of chains of management by means of electromagnetic relays or contactless logical elements which is carried out under the influence of the operating element.

Classification of contactless traveling switches

Contactless traveling switches can be classified on: 1- to a way of impact on a sensitive element, 2- physical principle of operation of the converter, 3 - design,

4 - class of accuracy, 5 - extent of protection.

On a way of impact on a sensitive element contactless traveling switches can be divided into switches of mechanical and parametrical action.

In switches of the first form the operating element influences directly mechanically to the primary drive of the contactless traveling switch which interacts with a sensitive element without contacts. In switches of the second form depending on the position of the operating element which is mechanically not connected with the contactless traveling switch any physical parameter of the converter changes. At a certain value of this parameter the condition of a relay element changes.

Classification of contactless traveling switches by the physical principle of operation of the converter includes the following types:

1. The inductive switches constructed on change of induction, mutual inductance, and also induction switches.

Now the majority in lots of the contactless traveling switches which are producing the industry are inductive devices. In turn converters of inductive contactless traveling switches can be constructed according to the following schemes: resonant, auto generator, differential, bridge, direct transformation.

2. Magnet inductive switches which are constructed on the following principles: effect of the Hall, magnet resistor, magnet diode, magnet thyristor.

3. Capacitor switches: with the changing area of plates, with the changing gap between plates, with the changing dielectric permeability of a gap between plates.

4. Photoelectronic switches with elements: photodiode, phototransistor, photoresistor, photothyristor.

Photo-electric switches and adjoining to them beam switches in which along with rays of visible light beams of other physical nature can be used beams, for example radioactive radiation.

According to the design contactless traveling switches are subdivided on:1- slot-hole, 2-ring (semi-ring), 3-plane, 4-face, 5-switches with the mechanical drive, 6- multi element switches.

On an accuracy class (size of the main error) contactless traveling switches share on switches low (about ±0,5 mm and more), average [about ± (0,05 - 0,5) mm], raised [about ± (0,005 - 0,05) mm] and high (about ±0,005 mm and less) accuracy.

Contactless traveling switches can possess various extent of protection against hit of foreign solid bodies and penetration of water in the device.

Technical characteristics of contactless traveling switches:

1-Precision (metrological) characteristics, 2- speed, 3- electric characteristics,4- the overall and adjusting dimensions and weight, 5- nominal and admissible working conditions, 6- reliability indicators, 7- cost and so on.

One of the main characteristics of contactless traveling switches which is directly influencing its design and some other technical characteristics is defined by a geometrical arrangement of the operating element of rather sensitive surface in operating time. For plane contactless traveling switches as the main characteristic the working gap is the distance between a sensitive surface of the switch and the operating element at which there is an operation of the switch is accepted. The main characteristic of the face switch is the maximum distance of influence, i.e. the maximum distance between a sensitive surface of the switch and the operating element at which change of its switching state is possible. The main characteristic of a slot ring switches is width of a crack and internal diameter of a ring of these switches respectively.

Speed (reaction time) of the contactless traveling switch this is time between the moment of establishment of coordinate of operation and the moment of achievement of the established value of tension at the exit of the contactless traveling switch. Knowing the size of speed of the contactless traveling switch, it is possible to define dynamic errors of operation of contactless traveling switches at changing of speed of the managing director of an element.

Electric characteristics of contactless traveling switches include the demanded parameters of the power supply (power line) and load characteristics.

It is necessary to carry also overall and adjusting dimensions of contactless traveling switches to the major parameters.

Contactless switches of the slot-hole BVK-24 type

Contactless switches of slot-hole type with the transistor amplifiers working in the generating mode were widely adopted. In fig. 1, and the general view of the BVK-24 switch is shown. Its magnetic conductor placed in the case 4 consists of two ferrite cores 1 and 2 with an air gap 5-6 mm wide between them. In the core 1 primary winding of wk and a winding of positive feedback, in the core 2 – a winding of negative feedback is placed. Such magnetic conductor excludes influence of external magnetic fields. Coils of feedback are switched on consistently – opposite. As the switching element the aluminum petal (plate) 3 up to 3 mm thick which can move to cracks (in an air gap) magnetic system of the sensor is used.

Contactless traveling BVK-24 switch: 1- general view; 2 - basic electric circuit

1 2

Scheme of turning on of the BVK contactless switch

Scheme of consecutive turning on of two BVK contactless switches

Scheme of parallel turning on of two BVK contactless switches

KVD contactless switches

Contactless KVD limit switches are intended for switching of electric chains of management and the alarm system at automation of various systems. The electric schematic diagram includes the generator and the trigger on transistors. At introduction to a working gap of a metal plate there is a reduction of coefficient of feedback causing generation failure, the trigger overturns, and normally closed output transistor opens that causes operation of the relay or a logical element. Supply voltage - 12 or 24 V.

Вправа 3. Дайте відповіді на питання.

1. Where are contactless traveling switches used?

2. For what contactless switches are intended?

3. How can be classified contactless traveling switches?

4. What are the differences between mechanical and parametrical switches?

5. What do you know about the inductive switches?

6. What are the other types of contactless switches on operating in physical principle?

7. How are contactless traveling switches subdivided on according to the design?

8. What are the technical characteristics of contactless switches?

9. How do contactless switches of slot-hole type work?

10. For what are contactless KVD limit switches intended?

Вправа 4. Знайдіть еквіваленти з колонки А до слів з колонки В.

A	B
mechanical influence	ланцюги управління
moving emphasis	механічний вплив
chains of management	рухомий упор
оn a way of impact	під час роботи
change of induction	габаритні
the majority	напівкільцеві
semi-ring	багатоелементні вимикачі
multi element switches	зміна індуктивності
overall	за способом дії
in operating time	більшість

Вправа 5. Доповніть речення.

1. The electric schematic diagram includes the generator and … … ….

2. Contactless switches of slot-hole type with the transistor amplifiers working in the generating mode … … ….

3. Electric characteristics of contactless traveling switches include the demanded parameters … … ….

4. It is necessary to carry also overall and adjusting dimensions of contactless traveling switches … … ….

5. For plane contactless traveling switches as the main characteristic the working gap is the distance between a sensitive surface of the switch and the operating element at which there is … … ….

Граматичний матеріал: Питання до різних членів речення. Спеціальні питання

Питання до окремих членів речення називаються спеціальними.

Питання до окремих членів речення називаються спеціальними. Вони починаються словами who, what, where, why, how тощо. Питання можуть ставитися до різних членів речення.

1. Питання до різних членів речення, окрім підмета

Питальне слово	Допоміжне дієслово	Підмет	Смислове дієслово	Доповнення
Where	do	you	live?
How	does	he	study	English?
What	is	he	doing	now?

2. Питання до підмета

У спеціальних питаннях, які відносяться до підмета, порядок слів такий же, як і в розповідному. Якщо питальне слово є підметом, то після нього, як і в розповідному реченні, ставиться присудок:

Питальне слово	Присудок	Підмет	Додаток
Who	works	-	here?
Who	likes	-	oranges?
What	is	-	happening?
Who	is teaching	-	this class?

Якщо питальне слово є визначенням підмета, то після нього одразу ставиться підмет, а потім присудок.

3. Спеціальні питання зі словом what. Вживаються наступні поєднання зі словом “what” What color is your car? What colors are his eyes? Wha t time is it? Wha t size is your shirt? What kind of job What sort of job What type of job is it? Wha t make is your TV set? Wha t day is it today?

What може вживатися без іменника: What is your name? What is his address? What is her favorite color?

4. Спеціальні питання зі словом how How tall are you? How big is the house? How old is your mother? How far is it from here to the airport?

Виконання вправ на закріплення граматичної теми

Вправа 6. Складіть питання до кожного члена речення.

1. Contactless switches are intended for switching of chains of management by means of electromagnetic relays or contactless logical elements which is carried out under the influence of the operating element.

2. On a way of impact on a sensitive element contactless traveling switches can be divided into switches of mechanical and parametrical action.

3. Electric characteristics of contactless traveling switches include the demanded parameters of the power supply (power line) and load characteristics.

4. The electric schematic diagram includes the generator and the trigger on transistors.

5. The main characteristic of the face switch is the maximum distance of influence, i.e. the maximum distance between a sensitive surface of the switch and the operating element at which change of its switching state is possible.

Заняття 52

Тема: Апаратура керування та захисту електроприводів: кнопкові пости, рубильники, контроери, запобіжники

Граматичний матеріал: Порядок слів у спонукальних та окличних реченнях

Вправа 1. Прочитайте таперекладіть слова за допомогою словника.

motionless parts [ˈməʊʃənlɪs pɑːts]

three-arms (shoulder) lever [θriː-ɑːmz (ˈʃəʊldə) ˈliːvə ]

axis [ˈæksɪs]

contact-maker [ˈkɒntækt-ˈmeɪkə] з

brass contact plate [brɑːs ˈkɒntækt pleɪt]

spiral spring [ˈspaɪərəl sprɪŋ ]

screw [skruː]

wire [ˈwaɪə]

tension [ˈtɛnʃən]

strengthened [ˈstrɛŋθənd]

nuts [ nʌts]

at short circuit [æt ʃɔːt ˈsɜːkɪt]

cross over knife switch [krɒs ˈəʊvə naɪf swɪʧ ]

rotation sense [rəʊˈteɪʃən sɛns]

handhold [ˈhændhəʊld ]

fuses [ˈfjuːzɪz ]

admissible value [ədˈmɪsəbl ˈvæljuː]

safety plugs [ˈseɪfti plʌgz ]

melting insertion [ˈmɛltɪŋ ɪnˈsɜːʃən]

cut-outs [kʌt-aʊts]

Вправа 2. Прочитайте та перекладіть текст українською.

Push-Button Switch, Knife Switches, Controllers, Fuses

PUSH-BUTTON SWITCH

The rotary switch (Fig.1) serves for switch on and switch off the receivers of electric energy up to 220 V.

Fig.1 The rotary switch

1- foundation; 2, 4 - stationary spring contacts; 3 – drum; 5 – contacts for joining of wires; 6 - clamping screw; 7 - contact plate.

The simplest switch of a low voltage calculated on low power is the push-button switch (Fig. 2). It consists of mobile and motionless parts.

Fig.2 The push-button switch

a) rotary part; б) the stationary part

The mobile part (above) has two buttons 1, three-arms lever 3 fixed on axis 2, and a contact-maker 4 with a brass contact plate 5. In the case of a contact-maker a spiral spring 6 and a steel ball 7 are located.

The motionless part (below) consists of two contacts with contact springs 8 which join by means of screws 10 of a wire 9 from an electric circuit.

When pressing the button 1 located at the left the three-arms lever 3.1 falls down. His right shoulder together with the second button rises up. At the same time average third shoulder of the lever works through a ball 7 and a spiral spring in on a contact-maker 4, moving it to the right. The last the plate 5 connects contact springs of the 8th motionless part of the switch and thus closes a chain of wires 9.

When pressing the button located on the right the contact-maker moves to the left and disconnects a chain.

KNIFE SWITCHES

For switch on and switch off any electrical units with tension to 500 V at the currents exceeding 6 A apply knife switches.

There are unipolar, bipolar and triple-pole knife switches which are mounted on boards.

The main parts of a triple-pole knife switch (Fig. 3a) are copper knives 1 which can turn on the axes 5 strengthened in motionless contacts 4.

Fig.3 Knife switches

a) a triple-pole knife switch; б) crossover switch

Wires from the receiver of electric energy join bolts of contacts by means of nuts. At short circuit of a chain copper knives 1 enter intervals between the springing contacts 3 to which clips wires 2 from an electric circuit are connected.

Along with usual knife switches apply knife switches which are called also cross over (Fig. 3б).

Switches, except the top springing contacts, have the same number of the lower contacts. Knives of a cross over knife switch can be connected both with top and to the lower contacts.

By safety regulations knife switches and switches are closed by protection covers.

CONTROLLERS

For making several switches at the same time in electrical circuits in case of start-up of electrical machines, regulation of speed, change of a rotation sense of electro motors it is used controllers.

The controller (Fig.4) consists of copper segments 1 (segments of the rings) strengthened on a shaft 2 of insulating material. The segments which are mobile contacts move in case of turn of the handhold 3 together with shaft.

Except mobile contacts, the controller has the fixed springing contacts 4 isolated among themselves.

Number of segments of the controller equally to number of its springing contacts which wires are going from the electrical machines, rheostats and other devices controlled by the controller join.

In case of turn of a shaft one segments of the controller are shorted with fixed contacts, and others are disconnected. By such way difficult switches in electrical circuits are carried out.

FUSES

In case of short circuit, and also in case of an overload circuit the electric current proceeding on the wires, electrical machines and instruments switched on in a circuit increases and exceeds admissible value. Wires of a circuit overheat, and their insulation can light up, and electrical machines and instruments can fail.

For protection of electrical circuits against short-circuit currents of the long overloads sequentially with receivers of electrical energy safety plugs turn on. Their operation is based on use of thermal action of current.

Each fuse shall withstand unlimited time the current intensity specified on it. In case of relocation of this current intensity the melting insertion melts the quicker, than more overload.

If in an electrical circuit in case of a uniform load (without current intensity changes) labor of current I=10 A, it is impossible to turn on the fuse expected smaller current intensity in such circuit.

In case of a fuse choice for an electrical circuit in which will turn on electro motors, it is necessary to consider the short-time force of a starting current exceeding labor of current approximately by 5-7 times. For this reason that the fuse doesn't interrupt a circuit in case of each start-up of the engine, it is necessary to set the fuse on the rated current intensity making about 40% of a starting current of the engine. It must be kept in mind that the fuse selected in such way protects the engine only from short-circuit currents, but doesn't protect it from the long overload. For such protection of the engine time cut-outs and the relay serve.

In Fig.5 Fuses of different construction are shown

Вправа 3. Дайте відповіді на питання.

1. What is the simplest on/off switch?

2. What does it consist of?

3. When do we use knife switches?

4. When do we use cross over knife switch?

5. What must we do by safety regulations?

6. When do we use controllers?

7. What does the controller consist of?

8. How are difficult switches in electrical circuits carried out?

9. When does the electrical current exceed admissible value?

10. Why do we use fuses?

Вправа 4. Вставте замість крапок необхідне за значенням слово або висловлювання.

1. It consists of … and motionless parts.

2. His … … together with the second button rises up.

3. When … … … located on the right the contact-maker moves to the left and disconnects a chain.

4. For switch on and switch off any … … with tension to 500 V at the currents exceeding 6 A apply knife switches.

5. Wires from the receiver of … … join bolts of contacts by means of nuts.

6. Knives of a cross over knife switch … … … both with top and to the lower contacts.

7. Except mobile contacts, the controller has the fixed … … isolated among themselves.

8. In case of turn of a shaft … … of the controller are shorted with fixed contacts, and others are disconnected.

9. Wires of a circuit overheat, and their insulation can light up, and … … and instruments can fail.

10. For such protection of the engine time … … … … serve.

(one segments, cut-outs and the relay, electrical machines, mobile, electric energy, pressing the button, right shoulder, electrical units, can be connected, springing contacts).

Вправа 5. Знайдіть еквівалентні пари.

А. поворотний вимикач, кнопковий вимикач, рухома частина, контактні пружини, електрична мережа, замикати ланцюг, розмикати ланцюг, змонтовані на щитах, мідні ножі, електрична енергія, захисний кожух, для декількох переключень, при пуску електричних машин, регулювання швидкості, мідні сегменти, ізолюючий матеріал, вал, перенавантаження мережі, прибори, для захисту електричних ланцюгів, плавка вставка.

B. insulating material, melting insertion, an overload circuit, devices, for protection of electrical circuits, shaft, copper segments, several switches, regulation of speed, start-up of electrical machines, electrical energy, protection covers, copper knives,

are mounted on boards, rotary switch, push-button switch, motion part, contact springs, electrical circuit, to close a chain, to disconnect a chain.

Граматичний матеріал: Порядок слів у спонукальних та окличних реченнях

Наказові речення в англійській мові (або, як їх ще називають, спонукальні речення) висловлюють спонукання до певної дії, прохання, наказ, а в негативній формі – заборону.

Наказове речення в англійській мові має звичайний, прямий порядок слів, але починається з дієслова, а іноді складається з нього одного:

Give me that! – Дай мені це!

Run! – Біжи!

Для пом’якшення наказу і перетворення його в прохання в кінці речення можна вжити will you (won’t you), would you (wouldn’t you):

Do as much as you can, will you? – Зроби так багато, як зможеш, добре?

Також для цього можна використовувати конструкцію let’s + інфінітив англійського дієслова без частки to:

Let’s make a pizza! – Давайте зробимо піцу!

Окличним може стати будь-який тип речення, якщо він буде виголошений з емоційним забарвленням:

I’m thirsty! – Мене мучить спрага!

Be quiet, Jack!

What’s is that?! – Що це?!

Для додаткового підсилення емоції в окличних реченнях можуть використовуватися слова what, that та how:

What a garden you have! – Який в тебе сад!

That‘s right! – Це вірно!

How it was?! – Як пройшло?

That‘s pity! – От жаль!

What‘s going on?! – Що відбувається?!

Але окличні речення можуть включати в себе лише 1 слово або словосполучення:

Awesome! – Класно!

Wonderful! – Чудово!

Wow! – Супер!

Well done! – Добре зроблено!

Виконання вправ на закріплення граматичної теми

Вправа 6. Перекладіть речення.

- Давай зберемо електричний ланцюг.

- Зробіть заземлення.

- Використовуйте пакетні вимикачі.

- Давайте використаємо автоматні вимикачі.

- Не треба робити цього!

- Перевірте роботу шляхових перемикачів.

Заняття 53

Тема: Апаратура керування та захисту електроприводів: автоматичні вимикачі, електромагнітні пускачі, контактори, теплове реле

Граматичний матеріал: Реферативний переклад

Вправа 1. Прочитайте та перекладіть слова за допомогою словника.

switches machine guns [ˈswɪʧɪz məˈʃiːn gʌnz]

reach [riːʧ ]

anchor-latch [ˈæŋkə-læʧ]

core [kɔː]

adjust [əˈʤʌst]

shutdown [ˈʃʌtdaʊn]

on a certain current [ɒn ə ˈsɜːtn ˈkʌrənt]

widespread [ˈwaɪdsprɛd]

contactor [ˈkɒntæktə]

excite [ɪkˈsaɪt]

operational electric current ˌ[ɒpəˈreɪʃənl ɪˈlɛktrɪk ˈkʌrənt]

usual type [ˈjuːʒʊəl taɪp]

anchor [ˈæŋkə ]

clips[klɪps]

winding [ˈwɪndɪŋ ]

pivot-center [ˈpɪvət-ˈsɛntə ]

flexible wire [ˈflɛksəbl ˈwaɪə]

become demagnetized [bɪˈkʌm ˌdiːˈmægnɪtaɪzd]

rectilinearly [ˌrɛktɪˈlɪnɪəli]

movable frame [ˈmuːvəbl freɪm]

crossbeam [ˈkrɒsbiːm]

densely [ˈdɛnsli]

magnetic actuator [mægˈnɛtɪk ˈæktjʊeɪtə]

top basis [tɒp ˈbeɪsɪs]

three contact crossing points plates [θriː ˈkɒntækt ˈkrɒsɪŋ pɔɪnts pleɪts]

thermal relay [ˈθɜːməl riːˈleɪ]

thermo switch [ˈθɜːməʊ swɪʧ]

extension of metal[ ɪksˈtɛnʃən ɒv ˈmɛtl]

linear expansion [ˈlɪnɪər ɪksˈpænʃən]

brass[ brɑːs]

sequentially [sɪˈkwɛnʃəli ]

be bend [biː bɛnd]

intermittently [ɪntə(ː)ˈmɪtəntli]

curve [kɜːv ]

pressure gages [ˈprɛʃə ˈgeɪʤɪz]

Вправа 2. Прочитайте та перекладіть текст українською.

Switches Machine Guns, Electromagnetic Actuators,

Contactors, Thermal Relay

SWITCHES MACHINE GUNS

For automatic switching off and protection to electric circuits switches machine guns use at violation of normal operating conditions. They are two types: maximum and minimum.

The machine gun which is disconnecting a circuit when current in it reaches size, bigger than admissible, it is called maximum. The main parts of such machine gun (Fig.1 a) are: electromagnet, anchor-latch and switch.

Fig. 1a) Fig. 1б)

Arrangement of switch machine of:

а) maximum; б) minimum

When current in a circuit of the machine gun exceeds the admissible size, an electromagnet 1 attracts an anchor latch 2. The knife of the switch 3 is released and under the influence of a spring 4 automatically disconnects a circuit in which it is switched on the machine gun.

Repeated turning on of the machine gun is made manually.

The machine gun which is disconnecting a circuit when tension in it becomes less admissible is called minimum (Fig. 1б).

When passes current in a circuit of the machine gun less admissible, the core of an electromagnet 1 isn't able to hold vertically an anchor latch shoulder. As a result of it the spring 5 delays an anchor from the electromagnet core, and a latch 2, rising up, releases a knife of the switch 3 which under the influence of a spring 4 disconnects the circuit which is protected by the machine gun.

Machine guns can be adjusted on a certain current at which there is a shutdown. The biggest advantage of machine guns is the accuracy of their installation on a certain current, considerably bigger than at protection of circuit by fuses.

CONTACTORS

The most widespread device for remote short circuit and disconnection of electric chains is the contactor.

Unlike devices in which switching on and switching off of electric chains make manually (knife switches), in contactors these operations are happen automatically under the influence of the magnetic field excited at switching on of operational electric current.

Two main types of contactors differ: rotary and usual type.

The rotary contactor (Fig. 2a) consists of an electromagnet 1, the anchor 7 located on axis 6, the main contacts 2 and clips 3 and 4 for accession of wires of the power supply network. All parts of the contactor are strengthened on the panel 8.

Arrangement of contactors:

Fig. 2 a) rotary; 2 б) usual type

When operational current passes on a winding of an electromagnet of the contactor, his core is magnetized and attracts an anchor. An anchor, turning round its pivot-center, closes the main contacts.

Current in the circuit created by the contactor passes from a clip 3 through the main contacts 2 and a flexible wire 5 to a clip 4, and from here to the electrical machine which is operated by the contactor.

At disconnection of a chain of an electromagnet its core become demagnetized and the anchor disconnects the main contacts.

In the usual type contactor (Fig. 2б) the mobile part of magnetic system (anchor) doesn't turn on an axis as in the contactor of rotary type, and moves rectilinearly.

The contactor consists of the Sh-shaped core 10, an electromagnet 11, an anchor 9, the movable frame 12, a crossbeam 13 of insulating material, double contacts of 14 and motionless contacts 15 which wires join of an electric chain. When on a winding of an electromagnet of the usual type contactor electric current proceeds, the anchor is attracted to the motionless core and rises up, entraining the movable frame and connect them to the double contacts come on a crossbeam which densely touch two motionless contacts. There is a short circuit of the chain operated by the contactor.

Contactors for chains of a direct current are unipolar, and contactors of three-phase alternating current is triple-pole.

Contactors widely apply to control of electrical units at plants and factories.

MAGNETIC ACTUATOR

Remote control of electric motors of three-phase alternating current is exercised by means of magnetic actuators — more difficult devices in which triple-pole contactors enter.

The magnetic actuator of usual type (Fig. 3a) has an electromagnet 1 with the steel core attached to the top basis 2. The anchor 5 is located below on which three contact crossing points plates isolated one from another 4 are strengthened. The basis 2 actuators is supplied with contacts 3 which the wires of L1, L2, L3 from a circuit of three-phase alternating current and a wire going from the electric motor join.

Magnetic actuator

Fig. 3 a) general appearance; 3 б) the scheme of turn on

When passing operational electric current through a winding of an electromagnet the magnetic field and an anchor is excited attracted to the core. Contact plates crossing points of an anchor connect among themselves contacts to which wires from a circuit of three-phase alternating current are connected and from the electric motor. At switching off of current the anchor under the influence of a body weight falls and contact plates crossing points disconnect the engine from a circuit.

Start-up of the engine by means of the magnetic actuator can be carried out, having included it according to the scheme provided on Fig.3. Contacts of the magnetic actuator 1, 2, 3 are normally opened. The EM actuator electromagnet is attached by one end to contact of the Start-up button, and another - to a network L2 wire. Contacts of the Feet button are normally closed, and contacts of the Start-up button are opened. Block contacts of BQ are connected parallel to contacts of the Start-up and Stop buttons.

Using two triple-pole contactors, it is possible to turn on the asynchronous engine so that with their help it is possible to make an engine reversal.

The push-button station of the reversal magnetic actuator consists of three buttons: "Forward", "Back" and "Stop".

By pressing the Forward button the anchor of an electromagnet of one contactor will be attracted to the core and will close the main contacts of the actuator on which current will come to the electric motor.

By pressing the Stop button the chain of this electromagnet owing to what the anchor disconnects the main contacts and windings of the engine from circuit wires is disconnected.

For change of the direction of rotation of the engine press the Back button. Thus the electromagnet of the second contactor joins. Current from a circuit comes to engine windings through the main contacts of the second actuator.

THERMAL RELAY

Thermal call the relay reacting to change of temperature (thermo switch). Action of a thermo switch is based on extension of metal in case of its heating up.

Bimetallic thermal relays were widely adopted. The working part of such relay represents the bimetal plate consisting of two metals with different temperature coefficients of the linear expansion. Materials for plates select so that they had perhaps big difference of expansion ratios, for example copper - steel, steel - nickel, invar - brass.

In case of the current overloads the thermal maximum relay use (Fig. 4) to protect of electric motors. The electric heater of 1 thermal relay is the perceiving part. The bimetallic plate 2 is used as the intermediate part, and as an execution part contacts 7 serve. The heater joins sequentially in an engine circuit, and contacts - in a circuit of an electromagnet of the actuator making start-up of the engine.

In case of a normal load the bimetallic plate is bent. Thus the lever 3 rests the upper shoulder against a plate 2, and his lower shoulder shorts contacts of 7 relays.

When current in a heater exceeds admissible value, the bimetallic plate 2 is bent towards metal with smaller coefficient of thermal extension, i.e. up. Then the upper lever arm 3 under the influence of a spring 5 is turned on axis 4 to the left, the lower shoulder - to the right, and contacts of the relay disconnect a circuit in which they are included.