4. Applying this rule to conductor it may be seen that an emf induced there changes its direction twice for each revolution of armature. The time T, during which one complete variation of emf takes place, is called the emf cycle. The number of cycles per one second iscalled the frequency & is measured in cycles per second. If we consider the general case, when a machine has p pairs of poles, the frequency of the induced emf increases in proportion to p, i.e., f=pn/60 where n is the rotational speed measured in revolutions per minute /rpm/.
5. To rectify the current in the external circuit, the machine is furnished with a special device called the commutator which functions as follows: The ends of the turn are connected to two copper segments - bars of the commutator, insulated from each other & from the shaft. It is necessary to arrange the brushes so that the emf induced in the turn is zero at the instant when the brush passes over from one bar to another. In this case the current in the external circuit will flow in one direction. The induced alternating emf in the turn & the corresponding ac are rectified into a pulsating emf & a pulsating current at the brushes & in the external part of the circuit. The emf pulsations may be smoothed out if a-multi-conductorwinding specially arranged on the armature & conne'cted to the commutator is used.
Задания на расширение лексико-грамматического материала основного текста
39. / 20 мин./ Парно-групповая работа. Обсудите тему “Электродвигатели постоянного тока”. Осветите, в частности, их типы, характеристики, конструктивные особенности и применение.
40. / 20 мин./ а) В следующем диалоге между наставником и практикантом даны только реплики практиканта /А/. Мысленно: обращаясь к изученному материалу, воспроизведите реплики наставника /В/ и запишите их в тетрадь, б) Используя записи реп-? лик наставника, воспроизведите реплики практиканта, в) Работая в парах, воспроизведите полностью диалог по памяти.
A: I’m sorry, sir, but would you like to help me?
B: Well, I’m listening to you.
A: You see, I’ve some difficulties in distinguishing all these types of dc motors. Can you tell me what types of dc motors; depend on?
' B:...
A: Oh, now I see what differs them. In this case shunt motors have the: field coils in parallel with the armature circuit & series, motors have the field coils connected in series with the armature, circuit. & would you mind telling me where shunt motors are used & where series motors are used?
