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Document # 15
ROTATING MACHINES
1 Whenever a conductor is moved in a magnetic field, an electromotive force (e.m.f.)
is induced ( or generated ) in the conductor and the direction of the e.m.f.
depends on the direction of both the magnetic field and the conductor.
2 In a single-loop generator, the conductor, shaped as a rectangular loop, is
rotated at constant speed between the two poles of a permanent magnet. An e.m.f.
of similar magnitude and direction is then generated and the current is
collected by two carbon brushes pressing on slip-rings attached to each end of
the loop. However, this effect is reversible and instead of converting motion
into electricity, it is possible to use a current to produce motion ( counter
e.m.f.).
3 This, of course is the simplest form of rotating machine. Indeed, instead of a
single loop, an actual DC machine mainly consists of an armature, a field
circuit, and a brush gear.
The armature carries coils wound around acore: this is the moving part which is
mounted on a shaft and which transmits mechanical power from a motor or to a
generator. The whole piece rotates inside field magnet poles separated from it
by an air gap.
Only relatively few rotating machines use permanent magnets: in most cases, a
field circuit is generated by poles and field coils wound around each of them.
The brush gear is an essential part of a DC machine. In addition to collecting
(or feeding) current through carbon brushes, the brushgear consists of a
commutator whose job is to rectify the current of the different coils for the
e.m.f. to be unidirectional.
4 Indeed, consider the single-loop generator: when the conductor is parallel to
the magnetic field, no e.m.f. is induced in the loop. Then, as it turns through
a quarter of a circle, the magnitude of the e.m.f. increases to a maximum when
the loop is at right-angle to the field, then the e.m.f. decreases to zero
during the end of the first half-revolution and during the second
half-revolution the direction of the e.m.f. is reversed while the magnitude
first increases then decreases to zero. The shape of the curve representing the
current is sinusoidal, each repetition is called a cycle, the time taken
corresponds to the period and the number of cycles per second is the frequency.
5 Each revolution of the coil of the single-loop generator corresponds to one
cycle. Thus to obtain a 50 Hz frequency, the coil must rotate at 3,000 r.p.m.,
and such high speed being often undesirable, multipolar generators are used so
that f(Hz)= r * p (where p is the number of pole pairs and r the number of
revolutions per second). It should be noted that European countries use 50 Hz
power supplies while in the US, for instance, the standard frequency is 6O hz.
As a result, a motor designed to run on 50Hz current and operating on a 60 Hz
supply would be faster but its power would be decreased.
UNDERSTANDING THE TEXT
1 Pick up passive verbs in paragraphs 1,2,3,5.
2 Pick up the ING forms in the whole text and analyse them. You can have :
a present participle equivalent to a relative clause.
a present participle used as an adjective.
a present participle expressing cause.
a gerund required after prepositions
Careful, in § 4, an –ING form is neither of these above.
3 Pick up the modal verbs in para 5. What do they mean ?
4 - Find in the text the word / phrases which mean ... (mind the preposition)
§1 - dépendre de §2 - tourner … une vitesse (de X tours) - fixer sur - §3 -
consister en (careful, do not mix with 'consist in')- §4 - parallèle … tourner
de (X tours) - angle droit - tendre vers zéro.
5 (§1,2) Answer the questions
When can an e.m.f. be generated ?
What is the influence of direction ?
What are the main physical features of the generator mentioned in the text ?
What are the tow carbon brushes for ?
What is counter e.m.f. ?
Does the generator described here actually exist ? Why or why not ?
What diffrences are there between this generator and an actual one ? In
particular, explain the double part played by the brishgear of a DC machine.
6 (§3,4) Use sentences of the text to explain what happens when the coil is in
position A1-B1 ; A3-B3 ; A4-B4 ; 45-B5 ; A6-B6.(See Fig.2)
7 (§5) Answer the questions.
How can a 50Hz frequency be obtained with a single-loop generator ?
From a practical point of view, how is such frequency generated ?
What difference is there between US and european power supplies ?
What are the consequences ?
LANGUAGE STUDY
8 – Both / the two
a. Study the difference
both: two independent persons or objects.
the two : two objects or persons considered as a whole / together.
e.g.
Both letters I and O are vowels = I is a vowel in itself, just as O.
In I/O, the two letters mean 'input' and 'output' = each letter takes this
meaning when it is used with other letter.
b. Use BOTH or THE TWO :
-6 is an odd number. So is 8.
-8 and 6 form the number 86.
-Motors and generators are based on the same principle.
-A magnetic field is generated between the poles of a magnet.
-Direct current generates a magnetic field. It's the same with A.C.
-The positive pole must be connected. Same thing for the negative one.
-A battery is a type of generator. An alternator is also a type of generator.
-The rotor and the stator carry coils.
9 - How to express aim.
a. Study the examples
A commutator rectifies the current for the e.m.f. to be unidirectional.
A commutator is required to rectify the current
(i) - Note: a/ Sentence 1 is concerned with two notions 1/ a commutator
rectifies the current and 2/ the e.m.f. is unidirectional;
b/ Sentence 2 is concerned with a single notion: a commutator is required, and
the (same) commutator rectifies the current
(ii) - Translate these sentences. You will notice that
-TO is the equivalent of ... in French
-FOR...TO is the equivalent of ... in French
b. Transform with TO or FOR ...TO.
-If you want to stop a shaft in any position, you must use a stepping-motor.
-You have to use multipolar generators if you don't want the coil to rotate too
fast.
-You should use a special induction motor when you want to position or clamp
elements on machine tools.
-You can move a conductor through an magnetic field, so it produces an e.m.f.
-You must use a magnet or field coils if you want to produce a magnetic field.
-You don't have to rectify the current if you want to produce A.C.
10 - Turn into the passive.
-If somebody moves a coil in a magnetic field, this induces a current which you
can pick up by brushes. Conversely, when you mount a coil on spindle and when
you place it between the two poles of a magnet, it will start rotating when you
supply it with electricity.
-In brushless D.C. motors, electronic sensors replace the commutator. You can
vary the speed with a potentiometer. You find these motors in record turntables,
computer drives and high-speed printers.
11 - Turn into the passive the sentences you have made up in exercise 9.
12 - Fill in with THE / A / AN or Ø
... ammeters and voltmeters consist of ... following parts:... deflecting
device, ... controlling device, and ... damping device. ... former instrument is
based on ... moving coil located between ... poles of ... magnet: ... passage of
... current through ... coil sets up ... torque. ... control spring resists...
turning effort, which brings ... coil to ... rest position. ... magnitude of ...
current flowing through ... coil being proportional to ... turning effort, a
simple hand will show ... desired magnitude. ... range of ... ammeter or ...
voltmeter can be extended by connecting ... resistances to ... instrument.
13 – How to add a new idea. Transform with IN ADDITION; IN ADDITION TO + ING /
noun; MOREOVER / FURTHERMORE.
-Alkaline cells are robust both mechanically and electrically . They have
practically unlimited rates of charge and discharge.
-Brusless D.C. motors deliver maximum torque on starting. This ensures minimum
maintenance.
-Stepping motors have a high output torque. They have infinitely variable speed.
14 - Translate into French so as to make clear the meaning of AS.
-It is possible to use a generator as a motor.
-According to Ohm's law, tension increases as voltage increases when the
resistance remains the same.
-Copper segments separated by insulating sheets of mica are used as commuatators.
-As a cycle corresponds to a revolution, a single-loop generator should rotate
very fast.
-As in a D.C. generator, the magnetic field poles of an A.C. generator are
alternatively + and -.
15
a. Compare the two examples
As iron is extremely magnetic, it is used to manufacture magnets.
Iron being extremely magnetic, it is used to manufacture magnets.
b. Give the other form:
-As current magnetizes some materials, powerful magnets can be made.
-The frequency depending on the alternator rotation speed, it must be very
regular.
-As D.C. servomotors deliver a high torque, they are suitable for heavy machine
tools.
-Having no brushes, brushless D.C. motors require little maintenance.
-As alternators have no commutators, the D.C. for the magnetic field must be
obtained from a separate source.
16 - Write each of the following sentences as one (avoid BUT and AND)
-The dry Leclanche cell is extremely useful. It's compact and portable.
Consequence: it's the most commonly used.
-This type of cell has an e.m.f. of 1.5V. It quickly falls to 1.4V and remains
constant. It is the type we use everyday.
-A carbon rod is surrounded by paste. The carbon rod acts as the positive
element. The paste contains carbon powder and manganese dioxyde.
-This substance is a depolarizer. A depolarizer prevents hydrogen from being
formed. Consequence: it avoids reduction in potential difference.
-The container of the cell is made of zinc. It acts as a negative electrode.
-The electrolyte is between the zinc case and the carbon/ manganese mixture.
This electrolyte is a paste.
-The electrolyte is not a liquid. The cell is said to be dry.
-The electrolyte is composed of ammonium chloride. It also contains zinc
chloride.
-The top of the cell is carefully sealed. This prevents the paste from drying.
17 - Explain the principle of operation of this electric bell.
You may use phrases such as :
-be composed of
-to flow through the transformer
-voltage induced in the secondary windings
-magnetic field / to set up / core
-armature / attract
-attach / clapper to armature
-make an break contact / to open as a result
-armature pulled back
-make and break contact / to close
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