الفهرس 
INTRODUCTIONOnly 14 pages are availabe for public view
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Abstract
The direct current machine was manufactured before the
begining of this century when the available power supply was
direct current.Today, almost all electrical power-supply networks
are ac systems, and it is accepted practice in industry to employ
ac motors wherever they are inherently suitable or can be given
appropriate characteristics by means of power electronic devices.
Yet there remain important fields of application in which de
machines offer economic and technical advantages. Separately
excited de motor drives are still chosen in the industrial field
for their simplicity in control especially when the wide range
and accurate speed control are necessary.
On the other hand the de motor requires special measures for
starting, except for the smallest motor. The commutator of de
motor consists of large number of copper segments separated by
thin sheets of mica insulation . This increases the
construction cost of the de motor and reduces the power/weight
ratio. DC motor requires periodic maintenance, therefore it
cannot be used at all in certain environmental conditions e.g. in
submerged pump and in some chemical/gases plants.
The maximum voltage between the segments of a commutator is
about 20 volts for successful commutation. Therefore the total
armature voltage is limited to a maximum of about 1500 volts.
But the advantages of de motor are enough to make these disadvantages insignificant.
The tremendous progress in semiconductor technology have made
solid state de motor drives suitable for many practical
applications. Such drives has a lot of advantages. Among of
these are very low speed operation down to 1 r.p.m , fast
response, high efficiency and cheapness except for power ratings
below 10kw.