- •Electrical engineering unit 14 Direct-Current Generators
- •Exercises
- •VII. Supplementary reading.
- •Text 8
- •A.C. Generators principles
- •Of operation
- •Unit 15 Alternating-Current Generators
- •Exercises
- •Text 9 powerhouse auxiliary motors
- •Transformers
- •Exercises
- •VI. Answer the following questions:
- •VII. Describe the structure of a transformer.
- •VIII. Supplementary reading. Text 10
- •Voltage transformers
- •Unit 17 Single-Phase Motors
- •Exercises
- •VII. Be ready to answer the following questions:
- •VIII. Choose one of the topics below and prepare to talk on it:
- •IX. Write a summary of the text.
- •Text 11 direct-current motors
- •Unit 18 Polyphase Induction Motors
- •Exercises
- •Text 12 direct-current motors
- •Unit 19 Electrical Measuring Instruments
- •Exercises
- •VI. Retell the text. Unit 20 Electrical Measuring Instruments
- •Exercises
- •Text 13 electrical measurements
- •Instruments and meters
- •Unit 21 Ammeters and Voltmeters.
- •Types of Ammeters and Voltmeters
- •Unit 22 Wattmeters
- •Exercises
- •Text 15 ammeters and voltmeters Hot Wire
- •Unit 23 Resistance Measurement
- •Exercises
- •VII. Supplementary reading. Text 16 ammeters and voltmeters
- •Unit 24 Low and Medium Resistance Measurements
- •Medium Resistance Measurement.
- •Exercises
Unit 24 Low and Medium Resistance Measurements
The D.С potentiometer can be used for measuring resistances of very low value, such as instrument shunts. This is really a laboratory or test room method, and is based on the comparison of one resistance against another by an indirect method. It employs the potentiometer for the accurate measurement of the drop in voltage across both the shunt under test and another shunt of known resistance, when both are carrying the same current.
Medium Resistance Measurement.
This is perhaps the most frequently used workshop method of measuring resistance, using an indicating voltmeter and ammeter. The resistance being measured can be left connected in its normal working circuit if desired, with the ammeter and voltmeter added.
If the current I flowing through the resistance is measured in amperes when a voltage V is applied across the resistance, then, by Ohm's law, the resistance R is given by:
R =V:I ohms
The required range of voltmeter and ammeter to be used and the voltage of the supply necessary, if this be different from the resistor's normal working circuit, will depend on the size and rating of the resistance being tested. A high-value resistance will tend to require a high-voltage source, a high-range voltmeter, and a low-current range ammeter, whereas a low-value resistance will require in most cases a low-voltage, high-current source, a low-range voltmeter and high-range ammeter. The exact requirements will, of course, depend also on the rating of the resistance, as well as the instruments available.
Exercises
I. Find in the text synonyms for:
to demand, dimension, accurate, while, quantity, probably, to wish.
II. Give English equivalents for:
испытываемый, по желанию, точно так же, как; имеющийся в наличии.
III. Find in the text:
1) the Present Participles Active and Passive, the Past Participles and state their functions; 2) the Infinitive used Attributively.
IV. Translate and memorize such technical terms:
high-value resistance, a high-range voltmeter, high-current source, high-range ammeter, a low-range voltmeter, low-current range ammeter.
V. Put questions to the text.
VI. Translate the following questions and answer them:
1. Когда используется потенциометр постоянного тока? 2. На чем основан лабораторный метод измерения сопротивления? 3. Для какой цели используется потенциометр? 4. Как можно определить сопротивление? 5. От чего зависят изменения сопротивления?
VII. Retell the text.