启动电动机时应注意什么？
参考答案：
（1）如果接通电源开关，电动机转不动，应立即拉闸，查明原因消除故障后重新启动。
（2）接通电源开关后，电动机发出异常响声，应立即拉闸，检查电动机的传动装置及熔断器等。
（3）接通电源开关后，应监视电动机的启动时间和电流表的变化，如启动时间过长或电流表迟迟不返回，应立即拉闸，进行检查。
（4）启动时发现电动机冒火或启动后振动过大，应立即拉闸，停机检查。
（5）在正常情况下，厂用电动机允许在冷状态下启动两次，每次间隔不得少于5分钟；在热状态下启动一次。只有在处理事故时，以及启动时间不超过2-3秒的电动机，可多启动一次。
（6）如果启动后发现运转方向反了，应立即拉闸，停电，调换三相电源任意两相接线后再次启动。
| Let us take a brief look at the planet on which we live. As Earth hurtles through space at a speed of 70,000 miles an hour, it spins, as we all know, on its axis, which causes it to be flattened at the Poles. Thus if you were to stand at sea level at the North or South Pole, you would be 13 miles nearer the centre of the earth than if you stood on the Equator. The earth is made up of three major layers — a central core, probably metallic, some 4000 miles across, a surrounding layer of compressed rock, and to top it all a very thin skin of softer rock, only about 20 to 40 miles thick — that’s about as thin as the skin of an apple, talking in relative terms. The pressure on the central core is unimaginable. It has been calculated that at the centre it is 60 million pounds to the square inch, and this at a temperature of perhaps 10,000 degrees Fahrenheit. The earth’s interior, therefore, would seem to be of liquid metal — and evidence for this is given by the behavior of earthquake. When an earthquake occurs, shock waves radiate from the centre just as waves radiate. outwards from the point where a stone drops into a pond. And these waves pulsate through the earth’s various layers. Some waves descend vertically and pass right through the earth, providing evidence for the existence of the core and an indication that it is fluid rather than solid. Thus, with their sensitive instruments, the scientists who study earthquakes, the seismologists, can in effect X-ray the earth. Iceland is one of the most active volcanic regions of the world. And it was to Iceland that Jules Verne sent the hero of his book A Journey to the Centre of the Earth. This intrepid explorer clambered down the opening of an extinct volcano and followed its windings until he reached the earth’s core. There he found great oceans, and continents with vegetation. This is conception of a hollow earth we now know to be false. In the 100 years since Jules Verne published his book, the science of vulcanology, as it is called, has made great strides. But even so the deepest man has yet penetrated is about 10,000 feet. This hole, the Robinson Deep mine in South Africa, barely scratches the surface; so great is the heat at 10,000 feet that were it not for an elaborate air-conditioning system, the miners working there would be roasted. Oil borings down to 20,000 feet have shown that the deeper they go, the hotter it becomes. The temperature of the earth at the centre is estimated to be anything between 3,000 and 11,000 degrees Fahrenheit. Some scientists believe that this tremendous heat is caused by the breaking-down of radio-active elements, which release large amounts of energy and compensate for the loss of heat from the earth’s surface. If this theory is correct, then we are all living on top of a natural atomic powerhouse. |
Since the publication of Jules Verne’s book, it has been proved that ______ .
A．( the centre of the earth is not hollow
B．( oil borings cannot go deeper than 20,000 feet
C．( there are active volcanic regions at the centre
D．( the earth is not in danger of exploding
