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  1. Answer the questions.


  1. How are DC motors commonly constructed?

  2. What types of field coils are traditionally used?

  3. What are the advantages of a permanent magnet type of DC motors?

  4. What type of rotors were used originally in all large industrial motors?

  5. What allowed the creation of high-intensity permanent magnets?


3-phase AC induction motor

Text A


  1. Listen to the words and word combinations from the text. Pay attention to their meaning.


AC (alternating current)змінний струм

motorдвигун

induction motorасинхронний двигун

phaseфаза

applicationзастосування

mainsмережа

adjustableрегульований

frequencyчастота

driveпривод

totalзагальний, повний

energy – енергія

convert – перетворювати

manufacturingвиробництво

processingобробка

pump – насос

fan – вентилятор, лопать

compressorкомпресор

mixerзмішувач

agitatorрозмішувач

millпрокатний стан

crusherдробильна установка

machine toolметалорізальний станок

squirrel cageбіляча клітка.


  1. Memorize the words and word combinations and their equivalents.


squirrel cage induction motors – коротко замкнутий асинхронний двигун, асинхронний двигун з білячою кліткою

variable – змінний, регульований

converter – перетворювач

variable voltage variable frequency (VVVF) converter – перетворювач з регульованою напругою і частотою

variable speed drive (VSD) – привід змінної швидкості

technique метод

designрозробляти, конструювати, проектувати

automated – автоматизований

frame – рама, станина, корпус, конструкція

performance characteristics – робочі (експлуатаційні) характеристики

reliability – надійність

DC motor – двигун постійного струму

bearing– підшипник

slipring – контактне кільце

brush – колекторна щітка

pre-lubricate – заздалегідь змазувати

single-phase – однофазний.


  1. Read and translate the text.


For industrial and mining applications, 3-phase AC induction motors are the prime movers for the vast majority of machines. These motors can be operated either directly from the mains or from adjustable frequency drives. In modern industrialized countries, more than half the total electrical energy used in those countries is converted to mechanical energy through AC induction motors. The applications for these motors cover almost every stage of manufacturing and processing. Applications also extend to commercial buildings and the domestic environment. They are used to drive pumps, fans, compressors, mixers, agitators, mills, conveyors, crushers, machine tools, cranes, etc. It is not surprising to find that this type of electric motor is so popular, when one considers its simplicity, reliability and low cost.

In the last decade, it has become increasingly common practice to use 3-phase squirrel cage AC induction motors with variable voltage variable frequency (VVVF) converters for variable speed drive (VSD) applications. To clearly understand how the VSD system works, it is necessary to understand the principles of operation of this type of motor.

Although the basic design of induction motors has not changed very much in the last 50 years, modern insulation materials, computer based design optimization techniques and automated manufacturing methods have resulted in motors of smaller physical size and lower cost per kW. International standardization of physical dimensions and frame sizes means that motors from most manufacturers are physically interchangeable and they have similar performance characteristics.

The reliability of squirrel cage AC induction motors, compared to DC motors, is high. The only parts of the squirrel cage motor that can wear are the bearings. Sliprings and brushes are not required for this type of construction. Improvements in modern pre-lubricated bearing design have extended the life of these motors.

Although single-phase AC induction motors are quite popular and common for low power applications up to approx 2.2 kW, these are seldom used in industrial and mining applications. Single-phase motors are more often used for domestic applications.

The information in this chapter applies mainly to 3-phase squirrel cage AC induction motors, which is the type most commonly used with VVVF converters.


  1. Match the words and word combinations (a-e) to the sentences (1-5)


  1. bearings

  2. frequency

  3. squirrel cage

  4. AC induction motors

  5. single-phase


        1. More than half the total electrical energy used in different branches of industry is converted to mechanical energy through …

        2. AC induction motors can be operated either directly from the mains or from adjustable … drives.

        3. AC induction motors, as compared to DC motors are much more reliable.

        4. Improved pre-lubricated … have extended the life of squirrel cage AC induction motors.

        5. motors are more often used for domestic applications.


  1. Answer the questions to the text


    1. What kind of AC induction motors has become commonly used for variable speed drive applications?

    2. Are sliprings and brushes required in the modern design of the squirrel cage motors?


  1. Say if the statement to the text is true or false


  1. 3-phase AC induction motors are common for low power applications.


  1. Translate the sentences paying attention to Indefinite Tenses in Active and Passive. Correct the mistakes in the sentences.


  1. AC induction motors is used to drive mills, machine tools, compressors, conveyors, pumps, etc.

  2. Their applications also extends to domestic environment.

  3. To understand how the variable speed drive system work it is necessary to understand the principles of operation of this type of motor.

  4. Single-phase AC induction motor are seldom used in industrial and mining applications.

  5. International standardization means that motors from manufacturers has similar performance characteristics.


Text B

Basic construction


  1. Listen to the words and word combinations from the text. Pay attention to their meaning.


stationaryнерухомий

rotate – обертатись

stator – статор

rotor – ротор

circuit – ланцюг, коло

insulate – ізолювати

carry – проводити

steel – сталь

laminateрозщеплювати на шари, пластини

laminated steel – пластинчата сталь (шихтована)

magnetic flux – магнітний потік

outer – зовнішній

welded – зварний

sheet – лист

cast iron alloy – легований (сплавний) чавун

aluminum alloy – легований алюміній

feet опора

flangeфланець

mount – монтувати

magnetic path – магнітопривід

set – набір, комплект

slot – проріз, паз

reduce – зменшувати

eddy current – вихровий струм

losses – втрати.


  1. Memorize the words and word combinations and their equivalents


eddy currents losses – втрати від вихрового струму

heating нагрівання

winding обмотка

cross-sectionalпоперечний розтин

power rating – номінальна потужність

wound rotor – фазний ротор

shaft – вал

external – зовнішній

slipring motor – асинхронний двигун з контактними кільцями

squirrel cage rotor – коротко замкнутий ротор, ротор з білячою кліткою

barшина

install установлювати

die-castлитий

rugged жорсткий

flow протікати, текти

support основа, опора, підтримувати

transmitпередавати

torqueобертаючий момент

cooling fan – охолоджуючий вентилятор

forced – примусовий

terminal – затискач, клема, вивід

terminal box – коробка зовнішніх з’єднань, розподільна коробка.


  1. Read and translate the text


The AC induction motor comprises 2 electromagnetic parts:

  • stationary part called the stator;

  • rotating part called the rotor, supported at each end on bearings.

The stator and the rotor are each made up of:

  • an electric circuit, usually made of insulated copper or aluminum, to carry current;

  • a magnetic circuit, usually made from laminated steel, to carry magnetic flux.

The stator. The stator is the outer stationary part of the motor, which consists of:

  • the outer cylindrical frame of the motor, which is made either of welded sheet steel, cast iron or cast aluminum alloy. This may include feet or a flange for mounting;

  • the magnetic path, which comprises a set of slotted steel laminations pressed into the cylindrical space inside the outer frame. The magnetic path is laminated to reduce eddy currents, lower losses and lower heating;

  • a set of insulated electrical windings, which are placed inside the slots of the laminated magnetic path. The cross-sectional area of these windings must be large enough for the power rating of the motor. For a 3-phase motor, 3 sets of windings are required, one for each phase.

Stator and rotor laminations


The rotor. This is the rotating part of the motor. As with the stator above, the rotor consists of a set of slotted steel laminations pressed together in the form of a cylindrical magnetic path and the electrical circuit. The electrical circuit of the rotor can be either:

  • wound rotor type, which comprises 3 sets of insulated windings with connections brought out to 3 sliprings mounted on the shaft. The external connections to the rotating part are made via brushes onto the sliprings. Consequently, this type of motor is often referred to as a slipring motor;

  • squirrel cage rotor type, which comprises a set of copper or aluminum bars installed into the slots, which are connected to an end-ring at each end of the rotor. The construction of these rotor windings resembles a ‘squirrel cage’. Aluminum rotor bars are usually die-cast into the rotor slots, which results in a very rugged construction. Even though the aluminum rotor bars are in direct contact with the steel laminations, practically all the rotor current flows through the aluminum bars and not in the laminations.

The other parts. The other parts, which are required to complete the induction motor are:

  • two end-flanges to support the two bearings, one at the drive-end (DE) and the other at the non drive-end (NDE);

  • two bearings to support the rotating shaft, at DE and NDE;

  • steel shaft for transmitting the torque to the load;

  • cooling fan located at the NDE to provide forced cooling for the stator and rotor;

  • terminal box on top or either side to receive the external electrical connections.



Assembly details of a typical AC induction motor


  1. Match the words and word combinations (a-e) to the sentences (1-5)



  1. windings

  2. magnetic path

  3. rotor

  4. stator

  5. frame


    1. The outer cylindrical … of the motor may include feet or a flange for mounting.

    2. The … is supported at each end on bearings.

    3. To have a high power rating of the motor the cross-sectional area of insulated electrical … must be large.

    4. The outer stationary part of the motor is called a … .

    5. To reduce eddy currents as well as to lower losses and heating the … is laminated.


  1. Answer the questions to the text


      1. What types of electrical circuit can have the rotor?

      2. What does the wound rotor comprise?

      3. What does the squirrel cage rotor comprise?

      4. The AC induction motor comprises two electromagnetic parts: stator and rotor. What other parts are required to complete it?

      5. Where does the steel shaft transmit the torque?

      6. What kind of cooling does the fan provide for the stator and rotor?


  1. Say if the statements to the text are true or false


    1. Two bearings support two end-flanges.

    2. Cooling fan is located at the drive-end.

    3. The rotating shaft is supported by two bearings.


  1. Translate the sentences paying attention to Indefinite Tenses in Active and Passive. Correct the mistakes in the sentences.


  1. Current flow through the copper or aluminum bars and not in the laminations.

  2. Slots is connected to an end-ring at each end of the rotor.

  3. Rotor bars is in direct contact with the steel laminations.

  4. Wound rotor type of motor are often referred to as a slipring motor.

  5. To receive the external electrical connections there are a terminal box on the top on either side of the frame.


Text C

Principles of operation


  1. Listen to the words and word combinations from the text. Pay attention to their meaning.


supplyджерело постачання

power supplyджерело живлення

terminal – вивід

set up – викликати, спричиняти

flux – потік

magnetic flux – магнітний потік

speed – швидкість

be in synchronism with – бути синхронним з ...

produce – створювати

synchronous speed – синхронна швидкість

perimeterпериметр

cycle – цикл

supply voltage – напруга живлення

revolutionоберт

perза, на

pole полюс

pole-pair пара полюсів

permanent постійний

magnet магніт

interact – взаємодіяти

induce – наводити, індукувати, збуджувати, викликати

induced currentіндуктивний струм

air-gap – повітряний зазор.


  1. Find the words and combinations of words in the text and translate the sentences containing them.


arrange – розміщати, розташовувати

fit установлювати, складати, збирати, монтувати, приганяти

suit задовольняти, відповідати

establishстворювати, установлювати

iron pathмагнітопровід

penetrateпроникати

cut – перетинати

rate – швидкість

short circuited – коротко замкнутий, закорочений

rotational forceобертаюча сила

reduce зменшувати

direction напрямок

accelerate – прискорювати

starting – запуск, пуск

decreaseзменшувати(сь), спадати, знижувати(сь)

proportionatelyпропорційно

settleустановлюватись

overcomeподолати

frictionтертя

windage – опір повітря

draw – живитись

actual speed – дійсна швидкість

slip – ковзання

slip speed – швидкість ковзання.


  1. Memorize the words and word combinations and their equivalents.


amountвеличина, ступінь, міра

load torqueобертаючий момент при навантаженні

no-load без навантаження

frictional losses – втрати від подолання тертя

windage losses – втрати від подолання опору повітря

increase – збільшуватись, зростати

full load – повне навантаження

sequenceпослідовність, порядок слідування

phase sequenceпослідовність фаз

peak – пік, максимум

IEC (International Electrotechnical Commission) – Міжнародна електротехнічна комісія

field windingобмотка збудження

rated speed – номінальна швидкість

inertia – інерція

exceed – перевищувати

stallзупинятись, перекидатись

starting torque – пусковий момент

output torque – вихідний обертовий момент

breakaway torqueгальмовий момент

match – узгоджувати

requirement – необхідна умова, вимога

pull away відриватись, розганятись.


  1. Read and translate the text


When a 3-phase AC power supply is connected to the stator terminals of an induction motor, 3-phase alternating currents flow in the stator windings. These currents set up a changing magnetic field (flux pattern), which rotates around the inside of the stator. The speed of rotation is in synchronism with the electric power frequency and is called the synchronous speed.

In the simplest type of 3-phase induction motor, the rotating field is produced by 3 fixed stator windings, spaced 120 apart around the perimeter of the stator. When the three stator windings are connected to the 3-phases power supply, the flux completes one rotation for every cycle of the supply voltage. On a 50 Hz power supply, the stator flux rotates at a speed of 50 revolutions per second, or 50 x 60 = 3000 rev per minute.

Basic (simplified) principle of a 2 pole motor


A motor with only one set of stator electrical windings per phase, as described above, is called a 2 pole motor (2p) because the rotating magnetic field comprises 2 rotating poles, one North-pole and one South-pole. In some countries, motors with 2 rotating poles are also sometimes called a 1 pole-pair motor.

If there was a permanent magnet inside the rotor, it would follow in synchronism with the rotating magnetic field. The rotor magnetic field interacts with the rotating stator flux to produce a rotational force. A permanent magnet is only being mentioned because the principle of operation is easy to understand. The magnetic field in a normal induction motor is induced across the rotor air-gap as described below.

If the three windings of the stator were re-arranged to fit into half of the stator slots, there would be space for another 3 windings in the other half of the stator. The resulting rotating magnetic field would then have 4 poles (two North and two South), called a 4 pole motor. Since the rotating field only passes 3 stator windings for each power supply cycle, it will rotate at half the speed of the above example, 1500 rev/min.

Consequently, induction motors can be designed and manufactured with the number of stator windings to suit the base speed required for different applications:


  • 2 pole motors, stator flux rotates at 3000 rev/min

  • 4 pole motors, stator flux rotates at 1500 rev/min

  • 6 pole motors, stator flux rotates at 1000 rev/min

  • 8 pole motors, stator flux rotates at 750 rev/min

  • etc

Flux distribution in a 4 pole machine at any one moment


The speed at which the stator flux rotates is called the synchronous speed and, as shown above, depends on the number of poles of the motor and the power supply frequency.

Where: no = Synchronous rotational speed in rev/min

f = Power supply frequency in Hz

p = Number of motor poles

To establish a current flow in the rotor, there must first be a voltage present across the rotor bars. This voltage is supplied by the magnetic field created by the stator current. The rotating stator magnetic flux, which rotates at synchronous speed, passes from the stator iron path, across the air-gap between the stator and rotor and penetrates the rotor iron path as shown in Figure 2.4. As the magnetic field rotates, the lines of flux cut across the rotor conductors. In accordance with Faraday’s Law, this induces a voltage in the rotor windings, which is dependent on the rate of change of flux.

Since the rotor bars are short circuited by the end-rings, current flows in these bars will set up its own magnetic field. This field interacts with the rotating stator flux to produce the rotational force. In accordance with Lenz’s Law, the direction of the force is that which tends to reduce the changes in flux field, which means that the rotor will accelerate to follow the direction of the rotating flux.

At starting, while the rotor is stationary, the magnetic flux cuts the rotor at synchronous speed and induces the highest rotor voltage and, consequently, the highest rotor current. Once the rotor starts to accelerate in the direction of the rotating field, the rate at which the magnetic flux cuts the rotor windings reduces and the induced rotor voltage decreases proportionately. The frequency of the rotor voltage and current also reduces.

When the speed of the rotor approaches synchronous speed at no load, both the magnitude and frequency of the rotor voltage becomes small. If the rotor reached synchronous speed, the rotor windings would be moving at the same speed as the rotating flux, and the induced voltage (and current) in the rotor would be zero. Without rotor current, there would be no rotor field and consequently no rotor torque. To produce torque, the rotor must rotate at a speed slower (or faster) than the synchronous speed.

Consequently, the rotor settles at a speed slightly less than the rotating flux, which provides enough torque to overcome bearing friction and windage. The actual speed of the rotor is called the slip speed and the difference in speed is called the slip. Consequently, induction motors are often referred to as asynchronous motors because the rotor speed is not quite in synchronism with the rotating stator flux. The amount of slip is determined by the load torque, which is the torque required to turn the rotor shaft.

For example, on a 4 pole motor, with the rotor running at 1490 r/min on no-load, the rotor frequency is 10/1500 of 50 Hz and the induced voltage is approximately 10/1500 of its value at starting. At no-load, the rotor torque associated with this voltage is required to overcome the frictional and windage losses of the motor.

As shaft load torque increases, the slip increases and more flux lines cut the rotor windings, which in turn increases rotor current, which increases the rotor magnetic field and consequently the rotor torque. Typically, the slip varies between about 1% of synchronous speed at no-load to about 6% of synchronous speed at full-load.

and actual rotational speed is


Where no = Synchronous rotational speed in rev/min

n = Actual rotational speed in rev/min

s = Slip in per-unit


The direction of the rotating stator flux depends on the phase sequence of the power supply connected to the stator windings. The phase sequence is the sequence in which the voltage in the 3-phases rises and reaches a peak. Usually the phase sequence is designated A-B-C, L1-L2-L3 or R-W-B (Red-White-Blue). In Europe this is often designated as U-V-W and many IEC style motors use this terminal designation. If two supply connections are changed, the phase sequence A-C-B would result in a reversal of the direction of the rotating stator flux and the direction of the rotor.


  1. Match the words and word combinations (a-f) to the sentences (1-6)



  1. induces

  2. slip

  3. friction

  4. accelerate

  5. air-gap

  6. interacts

    1. The magnetic field in a normal induction motor is induced across the rotor … .

    2. This magnetic field … with the rotating stator flux to produce the rotational force.

    3. When the rotor starts to … in the direction of the rotating field, the rate at which the magnetic field cuts the rotor windings reduces and the induced rotor voltage decreases proportionately.

    4. As the magnetic field rotates, the lines of flux cut across the rotor conductors and this … a voltage in the rotor windings.

    5. The amount of … is determined by the load torque required to turn the rotor shaft.

    6. The rotor settles at a speed slightly less than the rotating flux, which provides enough torque to overcome bearing … and windage.


  1. Answer the questions to the text


    1. What currents set up a changing magnetic field which rotates around the inside of the stator?

    2. What is a synchronous speed?

    3. What is the rotating field produced by in the simplest type of 3-phase induction motor?

    4. How many rotations for every cycle of the supply voltage does the flux perform when the three stator windings are connected to the 3-phase power supply?

    5. What is a 2 pole motor?

    6. Where is a magnetic field induced in a normal induction motor?

    7. What must be present across the rotor bars to establish a current flow in the rotor?

    8. What speed must the rotor rotate at to produce torque?

    9. What is a slip?

    10. What is a slip speed?

    11. What is the amount of slip determined by?

    12. What does the direction of the rotating stator flux depend on?

    13. What is a phase sequence?




  1. Define the functions of Participle I and Participle II in the following sentences


      1. When the induction motor is supplied from a power source of constant voltage and frequency, the current drawn by the motor depends primarily on the slip.

      2. At no-load, the motor will draw only no-load magnetizing current.

      3. When matching motors to mechanical loads, the two most important considerations are torque and speed.

      4. Induced voltage reappears in the rotor, but in the opposite direction.

      5. When set up a changing magnetic field rotates around the inside of the stator.


  1. Say, which of the sentences are in the Active and which are in the Passive Voice


      1. In contrast with a DC motor, the AC induction motor doesn’t have separate field windings.

      2. The performance of the 3-phase AC induction motor has been described for the speed range from zero up to its rated speed at 50 Hz.

      3. Typically, the slip varies between about 1% of synchronous speed at no-load to about 6% of synchronous speed at full-load.

      4. The induction motor will always run at a speed lower than synchronous speed because, even at no-load, a small slip is required to ensure that there is sufficient torque to overcome friction and windage losses.

      5. Inertia can be calculated using the formula.


  1. Translate the sentences paying attention to the Sequence of Tenses


  1. The motor accelerates if the motor torque always exceeds the load torque.

  2. At starting, the motor will not pull away unless the starting torque exceeds the load breakaway torque.

  3. If the load torque increases, the motor speed drops slightly, slip increases, stator current increases and the motor torque increases to match the load requirements.

  4. If the rotor speed was increased to the point that there was no slip, the induced voltage and current in the rotor fell to zero and torque output.

  5. If the rotor speed is increased about the mentioned one, the rotor will run faster than the rotating stator field and the rotor conductors again start to cut the lines of magnetic flux.

  6. When the motor torque is less than the load torque, the motor will stall.


  1. Translate the following Conditional sentences


  1. If there were a permanent magnet inside the rotor, it would follow in synchronism with the rotating magnetic field.

  2. If the three windings of the stator were re-arranged to fit into half of the stator slots, there would be space for another 3 windings in the other half of the stator.

  3. If the rotor reached synchronous speed, the rotor windings would be moving at the same speed as the rotating flux, and the induced voltage (and current) in the rotor would be zero.

  4. If two supply connections were changed, the phase sequence A-C-B would result in a reversal of the direction of the rotating stator flux and the direction of the rotor.

  5. If the load torque increased to a point beyond Tmax, the motor would stall.

  6. If the acceleration torque were constant over the acceleration period the formula of the total acceleration time would simplify.



Transformer

Text A


  1. Read and memorize words and word-combination


transformer – трансформатор

transfer – передавати

coupling – зв’язок, взаємодія

magnetic coupling – магнітна взаємодія

core – стержень, сердечник (осердя), стрижень

winding – обмотка

magnetic flux – магнітний потік

induce – індукувати

range - діапазон

coupling transformer – трансформатор зв’язку

microphone – мікрофон

unit – пристрій, апарат, установка

gigawatt - гігават (109)

interconnection - міжз’єднання

power grid – електрична мережа живлення

hypothetical – гіпотетичний

magneto motive force (MMF) – магніторушійна сила

link - звязувати

coil – виток, обмотка

primary winding первинна обмотка

secondary winding вторинна обмотка

mutual inductance взаємоіндукція

electro magnetic induction електромагнітна індукція

apply прикладати, подавати

turn виток

produce створювати

electro motive force (EMF) електрорушійна сила

drive – рухати

magnetic circuit – магнітне коло (ланцюг)

back electro motive force проти електрорушійна сила (проти е.р.с)

proportional пропорційний

rate швидкість.


  1. Make sure that you know these words and word combinations.


derivative похідна

derivative … with respect to …– похідна ... по ...

number кількість

substitute підставляти

solve вирішувати

respectively відповідно

ratio – відношення, співвідношення

turns ratio – коєфіціент трансформації

voltage ratio – коєфіціент трансформації по напрузі

alternatively інакше

inversely proportional обернено пропорціональний

cause заставляти

tend – намагатися

cancel – знищувати

reduce – зменшувати

increase – збільшувати

offset – зміщувати

feed подавати, живити

deliver – подавати, живити

flux density – густота потоку

steady – постійний

resistive load – активне навантаження

supply – подавати

quantity – величина, кількість

relation ship – співвідношення, залежність

equation рівняння

universal універсальний

root mean square(rms) – середньоквадратичний

frequency частота

hertz герц

cross-sectional area площа поперечного розтину

peak пік, максимум

tesla тесла.


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A transformer is an electrical device that transfers energy from one circuit to another by magnetic coupling, without requiring relative motion between its parts. A transformer comprises two or more coupled windings, and, in most cases, a magnetic core to concentrate magnetic flux. A changing voltage applied to one winding creates a time-varying magnetic flux in the core, which induces a voltage in the other windings.

The transformer is one of the simplest of electrical devices, yet transformer designs and materials continue to be improved.

Transformers come in a range of sizes from a thumbnail-sized coupling transformer hidden inside a stage microphone to huge gigawatt units used to interconnect large portions of national power grids. All operate with the same basic principles and with many similarities in their parts.

Michael Faraday built the first transformer in 1831, although he used it only to demonstrate the principle of electromagnetic induction and did not foresee its practical uses.

Coupling by mutual induction The principles of the transformer are illustrated by consideration of a hypothetical ideal transformer. In this case, the core requires negligible magnemotive force to sustain flux, and all flux linking the primary winding also links the secondary winding. The hypothetical ideal transformer has no resistance in its coils. A simple transformer consists of two electrical conductors called the primary winding and the secondary winding. Energy is coupled between the windings by the time varying magnetic flux that passes through (links) both primary and secondary windings. Whenever the amount of current in a coil changes, a voltage is induced in the neighboring coil. The effect, called mutual inductance, is an example of electromagnetic induction.