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.

5. 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?

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

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

7. 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.

stationary – нерухомий

rotate – обертатись

stator – статор

rotor – ротор

circuit – ланцюг, коло

insulate – ізолювати

carry – проводити

steel – сталь

laminate – розщеплювати 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

flange – фланець

mount – монтувати

magnetic path – магнітопривід

set – набір, комплект

slot – проріз, паз

reduce – зменшувати

eddy current – вихровий струм

losses – втрати.

2. Memorize the words and word combinations and their equivalents

eddy currents losses – втрати від вихрового струму

heating – нагрівання

winding – обмотка

cross-sectional – поперечний 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

bar – шина

install – установлювати

die-cast – литий

rugged – жорсткий

flow – протікати, текти

support – основа, опора, підтримувати

transmit – передавати

torque – обертаючий момент

cooling fan – охолоджуючий вентилятор

forced – примусовий

terminal – затискач, клема, вивід

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

3. 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

4. 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.

5. 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?

6. 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.

7. 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.

supply – джерело постачання

power supply – джерело 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

perimeter – периметр

cycle – цикл

supply voltage – напруга живлення

revolution – оберт

per – за, на

pole – полюс

pole-pair – пара полюсів

permanent – постійний

magnet – магніт

interact – взаємодіяти

induce – наводити, індукувати, збуджувати, викликати

induced current – індуктивний струм

air-gap – повітряний зазор.

2. 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

establish – створювати, установлювати

iron path – магнітопровід

penetrate – проникати

cut – перетинати

rate – швидкість

short circuited – коротко замкнутий, закорочений

rotational force – обертаюча сила

reduce – зменшувати

direction – напрямок

accelerate – прискорювати

starting – запуск, пуск

decrease – зменшувати(сь), спадати, знижувати(сь)

proportionately – пропорційно

settle – установлюватись

overcome – подолати

friction – тертя

windage – опір повітря

draw – живитись

actual speed – дійсна швидкість

slip – ковзання

slip speed – швидкість ковзання.

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

amount – величина, ступінь, міра

load torque – обертаючий 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

sequence – послідовність, порядок слідування

phase sequence – послідовність фаз

peak – пік, максимум

IEC (International Electrotechnical Commission) – Міжнародна електротехнічна комісія

field winding – обмотка збудження

rated speed – номінальна швидкість

inertia – інерція

exceed – перевищувати

stall – зупинятись, перекидатись

starting torque – пусковий момент

output torque – вихідний обертовий момент

breakaway torque – гальмовий момент

match – узгоджувати

requirement – необхідна умова, вимога

pull away – відриватись, розганятись.

4. 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: n_o = 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 n_o = 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.

5. 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