III. Mind your Grammar:
Put the verbs in brackets in the correct form - Present Simple Ac tive or Present Simple Passive.
Fig 1 ... (illustrate) the diagram of a shunt-wound motor.
This type of D.C. motor . . . (exhibit) the following properties.
The speed of rotation of a shunt-wound motor . . . (keep) coti stant.
The field current and hence the flux . . . (remain) unchange able.
Stepless speed regulation within wide limits . . . (accomplish by putting a regulating rheostat in the field circuit and an addition resistor in the armature circuit.
The motor torque characteristic . . . (make) a straight line.
While running idle a series motor . . . (tend) to overspeed.
The motor . . . only . . . (start) under load.
The speed regulation of a series motor . . . (perform) by shunting the field winding.
A series motor . . . (use) where a large starting torque . . . (require).
A differential compound motor . . . (feature) a small starting torque.
This type of D.C. motor . . . (sub classify) into series- compounded and shunt-compounded.
Series-compound motors . . . (provide) with a predominant series field winding and an auxiliary shunt field winding.
Series-compound motors . . . (not tend) to overspeed while running idle.
Shunt motors . . . (find) application in the electric drives of fans, pumps, compressors etc.
Unit 5. Transformers
1. Purpose, construction, types
A transformer is an electromagnetic device converting an alternating current of one voltage into an alternating current of another voltage but of the same frequency. Although there are many types of transformers and a great variety of different applications, the operation principle is the same in each case.
A transformer (fig.6) is composed of a laminated iron core and two separate windings. The core is made of electrical sheet steel. The winding, to which the voltage U1 is led, is called the primary winding W1. the one from which the voltage U2 is collected, is called the secondary winding.
The operating principle of a
transformer is based upon the phenomenon of mutual induction. If the
alternating current with the voltage
U1
is
led to the primary winding, the current I1
passing through it will produce the alternating magnetic flux Ф
in the core which
cutting the turn of the primary windings will induce the emf of
self-induction E1
in iе,
but in the turns of the secondary winding the emf of mutual
induction E2
will be induced. Since the magnetic flux is the same, in each turn
of both
the primary winding and the secondary winding there will be induced
one and the same in its magnitude emf -
e. Therefore, the emf
с
the primary
winding
the
one of the secondary winding E2=ew2.
If we disregard the voltage drop in the windings, emf may be equate
with the voltages:
Dividing on equation by another, we obtain:
U1/U2 = E1/E2 = w1/w2 = k;
where k is a transformer ratio, i.e. the ratio of the connected voltage to the obtained one or the ratio of the number of turns in the primary winding to that of the secondary winding.
The currents in the transformer windings are inversely proportional to their voltages.
In construction, transformers are divided into two types - a bar- type transformer and an ironclad transformer. Their cooling may be either airy or oily.
On board ships, in accordance with the work safety rules, only air- cooled transformers are used.
In the number of phases, transformers are one-phased, two-phased, or multiphased; in the number of windings they are either two-circuit or multicircuit, there are also transformers which have windings with special tappings.
In application, transformers are classified as power transformers, special purpose transformers and autotransformers.