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Induction motors present a lagging (inductive) power factor to the
power line. The power factor in large fully loaded high speed motors can
be as favorable as 90% for large high speed motors. At 3/4 full load the
largest high speed motor power factor can be 92%. The power factor for
small low speed motors can be as low as 50%. At starting, the power
factor can be in the range of 10% to 25%, rising as the rotor achieves
speed.
Power factor (PF) varies considerably with the motor mechanical load
(Figure below). An unloaded motor is analogous to a transformer with no
resistive load on the secondary. Little resistance is reflected from the
secondary (rotor) to the primary (stator). Thus the power line sees a
reactive load, as low as 10% PF. As the rotor is loaded an increasing
resistive component is reflected from rotor to stator, increasing the
power factor.
Induction motor power factor and efficiency.
Large three phase motors are more efficient than smaller 3-phase
motors, and most all single phase motors. Large induction motor
efficiency can be as high as 95% at full load, though 90% is more
common. Efficiency for a lightly load or no-loaded induction motor is
poor because most of the current is involved with maintaining
magnetizing flux. As the torque load is increased, more current is
consumed in generating torque, while current associated with magnetizing
remains fixed. Efficiency at 75% FLT can be slightly higher than that at
100% FLT. Efficiency is decreased a few percent at 50% FLT, and
decreased a few more percent at 25% FLT. Efficiency only becomes poor
below 25% FLT. The variation of efficiency with loading is shown in
Figure above
Induction motors are typically oversized to guarantee that their
mechanical load can be started and driven under all operating
conditions. If a polyphase motor is loaded at less than 75% of rated
torque where efficiency peaks, efficiency suffers only slightly down to
25% FLT.
Frank Nola of NASA proposed a power factor corrector (PFC) as an
energy saving device for single phase induction motors in the late
1970's. It is based on the premise that a less than fully loaded
induction motor is less efficient and has a lower power factor than a
fully loaded motor. Thus, there is energy to be saved in partially
loaded motors, 1-φ motors in particular. The energy consumed in
maintaining the stator magnetic field is relatively fixed with respect
to load changes. While there is nothing to be saved in a fully loaded
motor, the voltage to a partially loaded motor may be reduced to
decrease the energy required to maintain the magnetic field. This will
increase power factor and efficiency. This was a good concept for the
notoriously inefficient single phase motors for which it was intended.
This concept is not very applicable to large 3-phase motors. Because
of their high efficiency (90%+), there is not much energy to be saved.
Moreover, a 95% efficient motor is still 94% efficient at 50% full load
torque (FLT) and 90% efficient at 25% FLT. The potential energy savings
in going from 100% FLT to 25% FLT is the difference in efficiency 95% -
90% = 5%. This is not 5% of the full load wattage but 5% of the wattage
at the reduced load. The Nola power factor corrector might be applicable
to a 3-phase motor which idles most of the time (below 25% FLT), like a
punch press. The pay-back period for the expensive electronic controller
has been estimated to be unattractive for most applications. Though, it
might be economical as part of an electronic motor starter or speed
Control.
An induction motor may function as an alternator if it is driven by a
torque at greater than 100% of the synchronous speed. (Figure below)
This corresponds to a few % of “negative” slip, say -1% slip. This means
that as we are rotating the motor faster than the synchronous speed, the
rotor is advancing 1% faster than the stator rotating magnetic field. It
normally lags by 1% in a motor. Since the rotor is cutting the stator
magnetic field in the opposite direction (leading), the rotor induces a
voltage into the stator feeding electrical energy back into the power
line.
Negative torque makes induction motor into
generator.
Such an induction generator must be excited by a “live” source
of 50 or 60 Hz power. No power can be generated in the event of a power
company power failure. This type of alternator appears to be unsuited as
a standby power source. As an auxiliary power wind turbine generator, it
has the advantage of not requiring an automatic power failure disconnect
switch to protect repair crews. It is fail-safe.
Small remote (from the power grid) installations may be make
self-exciting by placing capacitors in parallel with the stator phases.
If the load is removed residual magnetism may generate a small amount of
current flow. This current is allowed to flow by the capacitors without
dissipating power. As the generator is brought up to full speed, the
current flow increases to supply a magnetizing current to the stator.
The load may be applied at this point. Voltage regulation is poor. An
induction motor may be converted to a self-excited generator by the
addition of capacitors.
Start up procedure is to bring the wind turbine up to speed in motor
mode by application of normal power line voltage to the stator. Any wind
induced turbine speed in excess of synchronous speed will develop
negative torque, feeding power back into the power line, reversing the
normal direction of the electric kilowatt-hour meter. Whereas an
induction motor presents a lagging power factor to the power line, an
induction alternator presents a leading power factor. Induction
generators are not widely used in conventional power plants. The speed
of the steam turbine drive is steady and controllable as required by
synchronous alternators. Synchronous alternators are also more
efficient.
The speed of a wind turbine is difficult to control, and subject to
wind speed variation by gusts. An induction alternator is better able to
cope with these variations due to the inherent slip. This stresses the
gear train and mechanical components less than a synchronous generator.
However, this allowable speed variation only amounts to about 1%. Thus,
a direct line connected induction generator is considered to be
fixed-speed in a wind turbine. See
Doubly-fed induction generator for a true variable speed alternator.
Multiple generators or multiple windings on a common shaft may be
switched to provide a high and low speed to accommodate variable wind
conditions.
Some induction motors can draw over 1000% of full load current during
starting; though, a few hundred percent is more common. Small motors of
a few kilowatts or smaller can be started by direct connection to the
power line. Starting larger motors can cause line voltage sag, affecting
other loads. Motor-start rated circuit breakers (analogous to slow blow
fuses) should replace standard circuit breakers for starting motors of a
few kilowatts. This breaker accepts high over-current for the duration
of starting.
Autotransformer induction motor starter.
Motors over 50 kW use motor starters to reduce line current from
several hundred to a few hundred percent of full load current. An
intermittent duty autotransformer may reduce the stator voltage for a
fraction of a minute during the start interval, followed by application
of full line voltage as in Figure above. Closure of the S contacts
applies reduced voltage during the start interval. The S contacts open
and the R contacts close after starting. This reduces starting current
to, say, 200% of full load current. Since the autotransformer is only
used for the short start interval, it may be sized considerably smaller
than a continuous duty unit.
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