' 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?
B:...
A: That’s clear. But what about compound motors? I’m afraid I don’t see any difference between compound & series motors.
B:...
A: I’ve got it now. One point I’d like still to clear up. I’ve found out that compound motors are a compromise between series & shunt motors. Will you tell me what it means?
B:...
A: Thanks a lot. You were of great help to me. Now I have clear & complete comprehension of motors.
B:...
41. /5 мин./ Прочтите следующие тексты и укажите номера предложений, которые дополняют тему “Типы двигателей постоянного тока”.
А 1. The magnetic system of DC machines comprises two>; poles. 2. These poles are stationary ki space. 3. They create a magnetic flux of constant magnitude. 4. Between these poles an armature is inserted. 5: To rectify the current in the external circuit, the machine is furnished with a special device called the commutator.
В 1. There are three types of dc motors: the series motors, the shunt motors, &' the compound motors. 2. These types are alike externally. 3. They differ in construction of the field coils.4. They differ in the connection between the fjjfcjd & the armature.
C 1. The dc shunt motor contains a field composed of many turns of wire. 2. Its field is connected in parallel with the armature.
3. The motor has constant-speed characteristic. 4. It is used on applications that require constant speed.
42. /40 мин./ Прочтите следующий текст, стараясь понять его содержание. Напишите по несколько вопросов к каждому абзацу. Будьте готовы обсудить основные моменты текста, используя эти вопросы.
Main Structural Elements of DC Machines
The dc machine consists of two main parts: the stationary part, designed mainly for producing the magnetic flux, & the rotating part. These parts are separated fron each other by the air gap. The stationary part of a d-c machine comprises: a/ the main poles; b/ the commutating poles; с/ the frame.
A. The main pole. It consists, of laminated core. At the end directed towards the armature, the pole core.widens into pole shoe. Mounted onto the core is field coil. The coil is wound on frame. For better cooling the field may be subdivided along thp height into two or more parts with ventilation channels between them. The poles are bolted to yoke with the help of special bolts.
B. The commutating pole. It is similar to the. main pole. The commutating poles are arranged strictly midway between the main ' poles & are bolted to the yoke. Commutating poles are usually made of solid steel, but for machines operating on abruptly varying load they are made of sheet steel.
C. The frame is the name given to the stationary part of the machine to which ате fixed the main & commutating poles & by means, of which the machine is bolted to its bedplate. The ring-shaped portion is called the yoke. The frame of modern machine is made of cast iron or steel. The rotating part is called the armature. The armature is a cylindrical body, rotating in the space between the magnetic poles & comprising: d/ a slotted armature core; e/ a winding inserted in the armature slot; f/ the commutator & g/ the brush gear.
D. The armature core is assembled of steel sheet laminations.
To reduce eddy current losses the sheets are insulated with a thin layer of varnish. The armature core is pressed from both sides by clamping device either specially fixed onto the shaft or tightened by bolts. To improve the cooling of machines they have a special ventilator fan mounted on the shaft.
E. Armature winding. All modern dc machines employ the drum-wound armature. The winding is laid entirely on the outside of the armature core periphery.-The winding is built up in sections wound on special moulds & then laid in the armature core slots.
