重新分配任务的英语作业第一课Word下载.docx

1、1. Electrical Measuring InstrumentsElectrical personnel use many different types of measuring instruments. Some jobs require very accurate measurements while other jobs need only rough estimates. Some instruments are used solely to determine whether or not a circuit is complete. The most common meas

2、uring and testing instruments are voltage testers, voltmeters, ammeters, ohmmeters, continuity testers（连续性测试仪）, megohmmeters, wattmeters（功率计）, and watt-hour meters（电度表，电表）.All meters used for measuring electrical values are basically current meters. They measure or compare the values of current flow

3、ing through them. The meters are calibrated and the scale is designed to read the value of the desired unit.【1】1.1 SAFETY PRECAUTIONCorrect meter connections are very important for the safety precaution of the user and for proper maintenance of the meters. A basic knowledge of the construction and o

4、peration of meters will aid the user in making proper connections and maintaining them in safe working order.【2】Many instruments are designed to be used on DC or AC only, while others can be used interchangeably. Note: It is very important to use each meter only with the type of current for which th

5、e meter is designed. Using a meter with an incorrect type of current can result in damage to the meter and may cause injury to the user.Some meters are constructed to measure very low values. Other meters can measure extremely high values.CAUTION: Never allow a meter to exceed its rated maximum limi

6、t. The importance of never allowing the actual value to exceed the maximum value indicated on the meter can not be overemphasized. Exceeding maximum values can damage the indicating needle, interfere with proper calibration, and in some instances may cause the meter to explode, resulting in injury t

7、o the user. Some meters are equipped with over correct protection. However, a current many times greater than the instruments design limit may still be hazardous.【3】1.2 BASIC METER CONSTRUCTION AND OPERATION Many meters operate on the principle of electromagnetic interaction. This interaction is cau

8、sed by an electric current flowing through a conductor which is placed between the poles of a permanent magnet. This type of meter is especially suitable for direct current.Whenever an electric current flows through a conductor a magnetic force is developed around the conductor. The magnetic force c

9、aused by the electric current reacts with the force of the permanent magnet. This causes the indicating needle to move. The larger the amount of current, the farther the needle will move.【4】 以上716完成The conductor is formed into coil, which is placed on a pivot between the poles of the permanent magne

10、t. The coil is connected to the terminals of the instrument through two spiral springs. These springs supply a reacting force proportional to the deflection. When no current is flowing, the springs cause the needle to return to zero.【5】The meter scale is designed to indicate the amount of current be

11、ing measured. The movement of the coil （and thus the movement of the indicating needle） is proportional to the amount of current flowing in the coil. If it is necessary to measure larger currents than the coil can safely carry, a bypass circuit, or shunt, is included. The shunt many be contained wit

12、hin the meter housing or connected externally.【6】Example 1A meter is constructed to measure 10 A on a maximum scale. The coil can safely carry 0.001 A. The shunt must be designed to carry 9.999 A. The meter is designed to indicate 10 A when 0.001 A flows in the coil.Fig.1.1（a） illustrates a permanen

13、t-magnet meter. Fig.1.1（b） show an external shunt connected across the meter terminals. The permanent-magnet meter can be used as an ammeter or a voltmeter. When the scale is designed to indicate current and the internal resistance is kept to a minimum, the meter functions as an ammeter. When the sc

14、ale is designed to indicate voltage, the internal resistance will be relatively high, depending upon the value of voltage for which the meter is designed. Note: Regardless of the design, the distance the needle moves is determined by the amount of current flowing in the coil.【7】以上717完成A slight chang

15、e must be made in the design in order to use this type of meter on AC. A rectifier is a device which changes AC to DC. It must be incorporated into the meter and the scale must be drawn to indicate the correct value of AC voltage. Rectifier-type AC meters cannot be used on DC and are generally desig

16、ned as voltage meters.【8】The electrodynamometer, Fig.1.2 is another design for both ammeters and voltmeters that can be used on alternating current. This instrument consists of two stationary coils and one movable coil. The three coils are connected in series with each other through two spiral springs. The spring also support the movable coil. When current flows through the coils the movable coil moves in a clockwise directio