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Description / Abstract:
This Guide presents geneal recommendations for determining, by
approximate methods from name plate data, the short-circuit
characteristics of direct-current motors and generators. The method
is applicable to general-purpose general industrial mill motors and
special-purpose direct-current machines. This includes
constant-speed and adjustable-speed motors and machines with and
without compound windings, and with and without compensating
windings.
This Guide is intended to help those concerned with the
transient characteristics of direct-current machines and
direct-current systems and the effects of these transients on their
operation. The calculated values of rate of rise and peak armature
current will facilitate the proper selection of system components
which will safety withstand the mechanical and thermal stresses
during fault. When using nameplate data, the approximate methods
presented will yield results sufficiently accuate for estimates.
Values computed by this method will be high, in general, thus on
the safe side. Where no flashover exists, a range of accuracy of
-20 to +30 present can be attained.
An exact and rigorous analysis is made difficult, if not
impossible, by the variability and nonlinearity of the parameters
involved in short-circuit calculations, and is beyond the scope of
this Guide. However, those wishing to pursue a more exact approach
are referred to [6], [9], [20], and [21] for methods of computation
utilizing design and test data. A range of accuracy of -5 to +10
percent can be attained where no flashover exists.
PURPOSE
The purpose of this Guide is
(a) to review the factors that affect the short-circuit
characteristics of direct-current machinery.
(b) to recommend methods for approximating the ate of rise and
peak value of armature current from name plate data.
(c) to discuss the causes of flashover and its effect on
short-circuit current and effective armature circuit
resistance.