1. Decipher and translate the following abbreviations:

AC generator, DC, elec, eng, neg, ft, 12in, 1.5yd, pos, 3,000 r.p.m, 5kW DC motor, e.m.f, h.v. power supply, 220v 50 Hz mains

2. Answer the following question:

1. What type of the current is there in your cell phone?

1. Does a car accumulator is a source of AC, isn’t?

2. What types of DC do you know?

3. What type of the power supply may be used for portable calculator?

4. What transformers are designed for?

5. Can you distinguish any difference between terms converter and inverter?

6. Is it possible to use DC installations in AC power supply lines?

7. Do you know electrical motor which can run on either DC or AC electric power?

1. Translate the following text into Russian:

Universal Motors

If you replace the permanent magnets of a DC motor with electromagnets and con­nect these electromagnets in the same circuit as the commutator and rotor, you will have a universal motor. This motor will spin properly when powered by either direct or alternating current.

When you connect DC power to a universal motor, the stationary electromagnets will behave as if they were permanent magnets and the universal motor will operate just like a DC motor. The only difference is that the universal motor will not reverse direc­tions when you reverse the current passing through it. It will continue turning in the same direction because reversing the current through the rotor also reverses the current through the electromagnets. Since the universal motor contains no permanent magnets, every pole in the entire motor changes from north to south or from south to north. Be­cause all the poles change, the motor’s behavior does not. It keeps turning in the same direction. If you really want to reverse the motor’s rotational direction, you must rewire the stationary electromagnets to reverse their poles.

Since the universal motor always turns in the same direction, regardless of which way current flows through it. it works just fine with AC electric power. Universal mo­tors are commonly used in kitchen mixers, blenders, and vacuum cleaners. While these motors are cheap and reliable, their graphite brushes eventually wear out and must be replaced. To repair a motor with a worn brush, you simply remove what’s left of the old brush and replace it with a new one from the nearest hardware store.

2. Translate the following material into English:

Трансформатором называется статический электромагнитный аппарат, пред­назначенный для преобразования переменного тока одного типа (первичного) в переменный ток с другими характеристиками, такими как другое напряжение и сила тока. Трансформатор состоит из: 1) сердечника, выполненного из трансфор­маторной стали, и 2) двух или нескольких обмоток, связанных между собой элект­ромагнитным полем. Трансформатор с двумя обмотками называется двухобмоточ­ный; а с несколькими обмотками - многообмоточными. По роду тока различают трансформаторы однофазные, трехфазные и многофазные.

Обмотка трансформатора, к которой подводится переменный ток, называется первичной обмоткой, а обмотка на выходе - вторичной. Если напряжение на вы­ходе трансформатора выше первичного, то трансформатор называется повышаю­щим, а если ниже - понижающим.

ADDITIONAL MATERIAL:

Arc welding refers to a group of welding processes that use a welding power supply to create an electric arc between an electrode and the base material to melt the metals at the welding point. They can use either direct (DC) or alternating (AC) current, and con­sumable or non-consumable electrodes. The welding region is sometimes protected by some type of inert or semi-inert gas, known as a shielding gas, and/or filler material.

Gas metal arc welding Power supplies

Power supplies

A constant current welding power supply capable of AC and DC To supply the electrical energy necessary for arc welding processes, a number of different power supplies can be used. The most common classification is constant cur­rent power supplies and constant voltage power supplies. In arc welding, the voltage is directly related to the length of the arc, and the current is related to the amount of heat input. Constant current power supplies are most often used for manual welding processes such as gas tungsten arc welding and shielded metal arc welding, because they maintain a relatively constant current even as the voltage varies. This is important because in manual welding, it can be difficult to hold the electrode perfectly steady, and as a result, the arc length and thus voltage tend to fluctuate. Constant voltage power sup­plies hold the voltage constant and vary the current, and as a result, are most often used for automated welding processes such as gas metal arc welding, flux cored arc welding, and submerged arc welding. In these processes, arc length is kept constant, since any

fluctuation in the distance between the wire and the base material is quickly rectified by a large change in current. For example, if the wire and the base material get too close, the current will rapidly increase, which in turn causes the heat to increase and the tip of the wire to melt, returning it to its original separation distance.

The direction of current used in arc welding also plays an important role in welding. Consumable electrode processes such as shielded metal arc welding and gas metal arc welding generally use direct current, but the electrode can be charged either positively or negatively. In welding, the positively charged anode will have a greater heat concentra­tion and, as a result, changing the polarity of the electrode has an impact on weld proper­ties. If the electrode is positively charged, it will melt more quickly, increasing weld pen­etration and welding speed. Alternatively, a negatively charged electrode results in more shallow welds. Non-consumable electrode processes, such as gas tungsten arc welding, can use either type of direct current, as well as alternating current. With direct current however, because the electrode only creates the arc and does not provide filler material, a positively charged electrode causes shallow welds, while a negatively charged electrode makes deeper welds. Alternating current rapidly moves between these two, resulting in medium-penetration welds. One disadvantage of AC, the fact that the arc must be re-ig- nited after every zero crossing, has been addressed with the invention of special power units that produce a square wave pattern instead of the normal sine wave, eliminating low-voltage time after the zero crossings and minimizing the effects of the problem.

LESSON # 3 MEASURING INSTRUMENTS

A multimeter set to measure voltage. Instruments for measuring potential differ-, ences include the voltmeter, the potentiometer (measurement device), and the oscilloscope. The voltmeter works by measuring the current through a fixed resistor, which, according to Ohm’s Law, is proportional to the potential difference across it. The potentiometer works by balancing the un­known voltage against a known voltage in a bridge circuit. The cathode-ray oscilloscope works by amplifying the potential difference and using it to deflect an electron beam from a straight path, so that the deflection of the beam is proportional to the potential difference.

A multimeter is an electronic measuring in­strument that combines several functions in one unit. The most basic instruments in­clude an ammeter, voltmeter, and ohmmeter.

A multimeter can be a handheld device useful for basic fault finding and field ser­vice work or a bench instrument which can measure to seven or eight and a half digits of accuracy. Such an instrument will commonly be found in a calibration lab and can be used to characterise resistance and voltage standards or adjust and verify the perfor­mance of multi-function calibrators.

Current, voltage, and resistance measurements are considered standard features for multimeter. AVO multimeters, a manufacturer of early multimeters, derived their name from amperes, volts, and ohms, the units used for the measurement of current, voltage, and resistance.

A multimeter may be implemented with an analog meter deflected by an electro­magnet, as a classic galvanometer; or with a digital display such as an LCD or Vacuum fluorescent display.

Analog multimeters are not hard to find in the used market, but are not very accu­rate because of errors introduced in zeroing and reading the analog meter face.

Analog meters may be implemented with vacuum tubes to precondition and am­plify the input signal. Such meters are known as vacuum tube volt meters (VTVM) or vacuum tube multimeters (VTMM).

Modem multimeters are exclusively digital, and identified by the term DMM or digital multimeter. In such an instrument, the signal under test is converted to a digital voltage and an amplifier with an electronically controlled gain preconditions the signal. Since the digital display directly indicates a quantity as a number, there is no risk of parallax causing an error when viewing a reading.

Similarly, better circuitry and electronics have improved meter accuracy. Older an­alog meters might have basic accuracies of five to ten percent. Modem portable DMMs may have accuracies as good as ±0.025%, and bench-top instmments have accuracies in the single-digit parts per million figures.

Digital meters often feature circuitry or software to accurately measure AC voltag­es at any frequency. These meters integrate the input signal using the root mean square method, and will correctly read the true voltage of an input signal even if it isn’t a per­fect sine wave.

Modem meters may be interfaced with a personal computer by IrDA links, RS-232 connections, or an instmment bus such as IEEE-488. The interface allows the computer to record measurements as they are made or for the instrument to upload a series of results to the computer.

As modem appliances and systems become more complicated, the multimeter is becoming less common in the technician’s toolkit. More complicated and specialized equipment replaces it. Where a service man might have used an ohmmeter to measure resistance while testing an antenna, a modem technician may use a hand-held analyzer to test several parameters in order to determine the integrity of a network cable.

Read this useful information:

C ommon voltages 1.5 volt С-cell batteries

Nominal voltages of familiar sources:

Nerve cell action potential: 40 millivolts Single-cell, rechargeable alkaline battery: 1.2 volts

Single-cell, non-rechargeable battery (e.g. AAA, AA, С and D cells): 1.5 volts Lithium polymer rechargeable battery: 3.7 volts Automobile electrical system: 12 volts

Household mains electricity: 120 volts North America, 230 volts Europe and 220volts, 50 Hz Russia.

Rapid transit third rail: 600 to 700 volts

Trams and trolleybus power supply in Russia: 600 volts

High voltage electric power transmission lines: 110 kilovolts and up (1150 kV is the record as of 2005)

Lightning: 100 megavolts

WORDLIST:

Measuring instrument - электроизмерительные приборы

multimeter - универсальный измерительный

прибор; многофункциональный измерительный прибор; мультиметр bridge circuit - мостик

cathode ray oscilloscope - элетронно-лучевой осциллограф bench instrument - стендовый измерительный прибор

calibration - калибровка, проверка, тарирование

vacuum fluorescent display - вакуум-люминесценный дисплей

(индикатор)

vacuum tube voltmeter - электровакуумный вольтметр

controlled gain - управляемое, регулируемое усилие

parallax - смещение

single-digit - одноразрядный

circuitry - компоновка схемы

root mean square method - метод среднеквадратных значений

sine wave - гармоничная волна; синусоидальное

колебание

с®

IrDA (infrared data association) - инфракрасный стандарт instrument bus - шина для подключения приборов

interface - устройство сопряжения

analyzer - прибор для проверки; анализатор

rapid transit third rail - напряжение скоростного поезда

(третий контактный рельс)

EXERCISES: