Consideration of machine losses is important for three
reasons:
(1) Losses determine the efficiency of the machine and
appreciably influence its operating cost
(2) Losses determine the heating of the machine and hence
the rating or power output that can be obtained without undue deterioration of
the insulation
(3) Voltage drops or current components associated with
supplying the losses must be properly accounted for in a machine
representation.
Machine efficiency, like that of transformers or any
energy-transforming device, is given by:
Efficiency = output/ input
Rotating machines in general operate efficiently except at
light loads. For example, the full-load efficiency of average motors ranges
from 80 to 90 percent for motors on the order of 1 to 10 kW, 90 to 95 percent
for motors up to a few hundred kW, and up to a few percent higher for larger
motors.
Efficiencies determined from loss measurements can be used
in comparing competing machines if exactly the same methods of measurement and
computation are used in each case.
For this reason, the various losses and the conditions for
their measurement are precisely defined by the American National Standards
Institute (ANSI), the Institute of Electrical and Electronics Engineers (IEEE),
and the National Electrical Manufacturers Association (NEMA). The following
discussion summarizes some of the various commonly considered loss mechanisms.
Ohmic Losses
Ohmic, or I2R losses, are found in all windings of a
machine. By convention, these losses are computed on the basis of the dc
resistances of the winding at 75°C.
Actually the I2R loss depends on the effective resistance of
the winding under the operating frequency and flux conditions. The increment in
loss represented by the difference between dc and effective resistances is
included with stray load losses, discussed below.
In the field windings of synchronous and dc machines, only
the losses in the field winding are charged against the machine; the losses in
external sources supplying the excitation are charged against the plant of
which the machine is a part.
Closely associated with I2R loss is the brush-contact loss
at slip rings and commutators. By convention, this loss is normally neglected
for induction and synchronous machines. For industrial type dc machines the
voltage drop at the brushes is regarded as constant at 2 V total when carbon
and graphite brushes with shunts (pigtails) are used.
Mechanical Losses
Mechanical losses consist of brush and bearing friction,
windage, and the power required to circulate air through the machine and
ventilating system, if one is provided, whether by self contained or external
fans (except for the power required to force air through long or restricted
ducts external to the machine).
Friction and windage losses can be measured by determining
the input to the machine
running at the proper speed but unloaded and unexcited.
Frequently they are lumped
with core loss and determined at the same time.
Open-Circuit, or No-Load, Core Loss
Open-circuit core loss consists of the hysteresis and
eddy-current losses arising from changing flux densities in the iron of the
machine with only the main exciting winding energized. In dc and synchronous
machines, these losses are confined largely to the armature iron, although the
flux variations arising from slot openings will cause losses in the field iron
as well, particularly in the pole shoes or surfaces of the field iron.
In induction machines the losses are confined largely to the
stator iron. Open-circuit core loss can be found by measuring the input to the
machine when it is operating unloaded at rated speed or frequency and under the
appropriate flux or voltage conditions, and then deducting the friction and
windage loss and, if the machine is self-driven during the test, the no-load
armature I2R loss (no-load stator I2R loss for an induction motor).
Stray Load Loss
Stray load loss consists of the losses arising from
nonuniform current distribution in the copper and the additional core losses
produced in the iron by distortion of the magnetic flux by the load current.
It is a difficult loss to determine accurately. By
convention it is taken as 1.0 percent of the output for dc machines. For
synchronous and induction machines it can be found by test.
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