机长英语_11规则管理级轮机英语教材中关联题13篇.doc

The operation of paralleling two alternators requires the voltages to be equal and also in phase. The alternating current output of any machine is always changing, so for two machines to operate together their voltages must be changing at the same rate or frequency, and must be reaching their maximum (or any other value) together. They are then said to be in phase. Use is nowadays made of a synchroscope when paralleling two a.c. machines. The synchroscope has two windings which are connected one to each side of the paralleling switch. A pointer is free to rotate and is moved by the magnetic effect of the two windings. When the two voltage supplies are in phase the pointer is stationary in the 12 oclock position. If the pointer is rotating then a frequency difference exists and the dial is marked for clockwise rotation FAST and anti-clockwise rotation SLOW, the reference being to the incoming machine frequency. To parallel an incoming machine to a running machine therefore it is necessary to ensure firstly that both voltages are equal. Voltmeters are provided for this purpose. Secondly the frequencies must be brought into phase. In practice the synchroscope usually moves slowly in the FAST direction and the paralleling switch is closed as the pointer reached the 11 oclock position. This results in the incoming machine immediately accepting a small amount of load. A set of three lamps may be provided to enable synchronizing. The sequence method of lamp connection has a key lamp connected across one phase with two other lamps cross connected over the other phases. If the frequencies of the machines are different the lamps will brighten and darken in rotation, depending upon the incoming frequency being FAST or SLOW. The correct moment for synchronizing is when the key lamp is dark and the other two are equally bright. 87. What device is used to parallel two alternators nowadays? A. A pointer. B. Two windings C. A synchros