The power factor at which ac machines
operate is an economically important feature because of the cost of
reactive kilovoltamperes. Low power factor adversely affects system
operation in three principal ways.
(1) Generators, transformers, and
transmission equipment are rated in terms of kVA rather than Kw
because their losses and heating are very nearly determined by
voltage and current regardless of power factor.
The physical size and cost of ac
apparatus are roughly proportional to kVA rating. The investment in
generators, transformers, and transmission equipment for supplying a
given useful amount of active power therefore is roughly inversely
proportional to the power factor.
(2) Low power factor means more current
and greater 12 R losses in the generating and transmitting equipment.
(3) A further disadvantage is poor
voltage regulation.
Factors influencing reactive-kVA
requirements in motors can be visualized readily in terms of the
relationship of these requirements to the establishment of magnetic
flux. As in any electromagnetic device, the resultant flux necessary
for motor operation must be established by a magnetizing component of
current.
It makes no difference either in the
magnetic circuit or in the fundamental energy conversion process
whether this magnetizing current be carried by the rotor or stator
winding, just as it makes no basic difference in a transformer which
winding carries the exciting current. In some cases, part of it is
supplied from each winding.
If all or part of the magnetizing
current is supplied by an ac winding, the input to that winding must
include lagging reactive kVA, because magnetizing current lags
voltage drop by 90 °. In effect, the lagging reactive kVA set up
flux in the motor.
The only possible source of excitation
in an induction motor is the stator input. The induction motor
therefore must operate at a lagging power factor. This power factor
is very low at no load and increases to about 85 to 90 percent at
full load, the improvement being caused by the increased real-power
requirements with increasing load.
With a synchronous motor, there are two
possible sources of excitation: alternating current in the armature
or direct current in the field winding. If the field current is just
sufficient to supply the necessary mmf, no magnetizing-current
component or reactive kVA are needed in the armature and the motor
operates at unity power factor.
If the field current is less, i.e., the
motor is underexcited, the deficit in mmf must be made up by the
armature and the motor operates at a lagging power factor. If the
field current is greater, i.e., the motor is overexcited, the excess
mmf must be counterbalanced in the armature and a leading component
of current is present; the motor then operates at a leading power
factor.
Because magnetizing current must be
supplied to inductive loads such as transformers and induction
motors, the ability of overexcited synchronous motors to supply
lagging current is a highly desirable feature which may have
considerable economic importance. In effect, overexcited synchronous
motors act as generators of lagging reactive kilovoltamperes and
thereby relieve the power source of the necessity for supplying this
component.
They thus may perform the same function
as a local capacitor installation. Sometimes unloaded synchronous
machines are installed in power systems solely for power-factor
correction or for control of reactive-kVA flow. Such machines, called
synchronous condensers, may be more economical in the larger sizes
than static capacitors.
Both synchronous and induction machines
may become self-excited when a sufficiently heavy capacitive load is
present in their stator circuits. The capacitive current then
furnishes the excitation and may cause serious overvoltage or
excessive transient torques.
Because of the inherent capacitance of
transmission lines, the problem may arise when synchronous generators
are energizing long unloaded or lightly loaded lines. The use of
shunt reactors at the sending end of the line to compensate the
capacitive current is sometimes necessary.
For induction motors, it is normal
practice to avoid self-excitation by limiting the size of any
parallel capacitor when the motor and capacitor are switched as a
unit.
No comments:
Post a Comment