I. Соедините предложения, употребив слова в скобках.
1. Something goes wrong in commutation. It may be accompanied by excessive sparking (when, which). 2. There is excessive sparking at the commutator. The commutator and brush assembly must be checked (if, for). 3. There is little sparking. The correct commutation is taking place (when, where). 4. The brushes are made to bear on the commutator by a spring. It produces a pressure of one-and-a-half to two pounds per square inch of brush surface (which, when).
II. Предложите заголовок к каждому абзацу так, чтобы они составили развернутый план текста.
III. Письменно изложите краткое содержание текста на русском языке (не более 15 предложений).
Text 11
Insulation Breakdown
Under normal conditions of operation, the field and armature windings of generators and motors are completely insulated from the frame of the machine, which is bolted to the bench or floor. A resistance measurement from the frame to the armature or to the field should read infinity or several million ohms.
Sometimes, however, by reason of excessive heat generated by overloading the machine, because of high moisture content in the air, the resistance of the insulation decreases and some of the current leaks through to the frame. This leakage current adds to the break-down of the insulation; and if the leakage is not found in time, the breakdown will be complete and the winding will be shorted to the frame. This will cause it to overheat or burn out.
The armature and the field windings should therefore be checked at regular intervals to detect "leaks" and shorts to earth before they cause serious damage.
An ordinary ohmmeter cannot be used for insulation testing in large practical machines, since, the leakage will often show itself only when a high voltage is applied to it, and an ohmmeter cannot apply a high enough voltage. An instrument called a "megger" is used instead. This supplies the necessary high voltage and is calibrated to read very high resistance values.
N o t e s
| some of the current leaks | происходит небольшая утечка (потеря) тока |
| show itself | проявиться |
E x e r c i s e s
I. Одним предложением выразите мысль первого, второго, четвертого абзацев текста.
II. Письменно изложите краткое содержание текста на английском языке.
Text 12
The Megger
The megger is an instrument which is used for measuring insulation resistance, such as the resistance between windings and the frame of electrical machinery, and that of cables, insulators and bushings. The megger consists of two parts – (1) a hand-cranked d.c. generator (magneto), or a high voltage battery, to supply the voltage for making the measurement; and (2) a special type of meter movement.
Before using the megger, check that there is no voltage source connected to the circuit being measured. Then connect both meter leads to earth to ensure a good earth connection and good meter operation. Next, connect the megger across the circuit to be tested and turn the hand-crank so as to generate a high voltage across the megger terminals. As a result, current will flow through the circuit or insulation being tested.
This current flow is measured by the meter movement as it is an ohmmeter, but the megger is calibrated to measure megohms instead of ohms. The normal resistance reading for a circuit insulated from earth is several hundreds megohms. If the megger reads low, a short to earth exists, and the shorted circuit should be replaced.
An earth is a reference point for voltage and resistance measurements in electrical circuits. All the large metal objects (such as motor housings, switch-boxes, and transformer cases) which are associated with electrical equipment are directly connected to earth. A megger determines if any of the wires inside a motor or transformer has come into contact with the metal housing, or is in danger of doing so.
Sometimes moisture in the insulation will cause the insulation resistance to be as low as one megohm. Moisture can be eliminated by drying the insulation with heater lamps, or a hot air blower.
To test a d.c. machine for insulation leakage and shorts to earth, the leads of the megger are connected between the frame and the external terminals. The crank of the megger must be turned at a steady moderate speed. If the megger reads several megohms or more, the insulation is secure. If the megger reads less than one megohm, some of the insulation is defective and the fault must be insolated. Disconnect the field leads from the armature, and test both separately.
To test the field, the megger is connected between one side of the field and the frame. To test the armature, the megger is connected between the shaft and the commutator segments. If the megger reads several megohms, the insulation has its normal allowable leakage resistance. However, if the megger reads lower than this, say less than two megohms, the leakage is excessive and the insulation will eventually break down. Of course, if the megger reads zero, the insulation is broken and the coil is shorted to the frame of the machine.
Note, that it is possible to get a false impression of the condition of the insulation of a machine if the voltage generated by the megger chosen is very much greater than the voltages normal to the circuit. When selecting a megger for insulation tests, choose one of the appropriate voltages. Do not, for instance, test the insulation of 12 v d.c. motor with a 500-v megger. Choose one which generates, say, 100-v instead.
N o t e s
| a hand-cranked d.c. generator | генератор постоянного тока с ручным приводом |
E x e r c i s e s
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