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Description  |
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FIELD OF INVENTION
The invention relates to the field of alternating current (a.c.) electric
induction motors, particularly of the larger sized fractional horsepower
and integral horsepower "squirrel cage" rotor variety, which commonly run
at a subsynchronous speed. Single phase motors of this type ordinarily
have a START winding and a RUN winding, while polyphase (e.g., 2 and
3-phase) motors of this type have an effectively separate RUN winding for
each phase. In a general sense, the field of the invention pertains to
load dependent controlled operation of a novel configuration of such
motors in order to obtain significant ENERGY SAVINGS.
BACKGROUND OF INVENTION
Electric induction motors are by far the most common, popularly used form
of a.c. motor. They find ubiquitous application in refrigerators, air
conditioners, major appliances, and a host of other machine applications.
When fully loaded, induction motors may be designed to exhibit
exceptionally good efficiency and quiet, long-term operation with
negligible maintenance. The art of induction motor manufacture is so
highly developed that a wide variety of motors are routinely made for all
sorts of applications with such motors providing predictable performance
characteristics and low unit cost.
While efficiency of induction motors may be readilly maintained at a high
level when driving a full load, they also have notorious inefficiency
problems when unloaded or lightly loaded. Ordinary induction motors
literally waste a large percentage of their electrical power consumption
as unecessary heat when they are delivering intermediate levels of output
member (drive shaft) torque. It is this area of ENERGY waste which occurs
under operating conditions that present less than a full load to the motor
which has been previously addressed by several of my earlier inventions
and remains the technical area which continues to be improved upon by my
instant invention.
Refrigerations and air conditioners are two of the most prodigious
producers of unecessary electrical ENERGY WASTE that, to a substantial
extent, is caused by induction motor losses. As is well known, common
hermetic compressors used in refrigerators utilize small induction motors
ordinarily rated between about 1/6 and 1/2 horsepower for operation. Motor
design is dictated to a large extent by engineering windings that develop
sufficient magnetic field strength to produce adequate running torque in
the motor under worst case conditions of high compressor loading, typical
of extreme climatic conditions of heat and humidity. Obviously a motor
carefully designed so as to be adequate for extreme climatic conditions
will be considerably over-rated under milder conditions.
Domestic refrigerators, as a categorical induction motor application, are
known to consume about 7% of the total amount of electric energy produced
in the United States. More significantly, these same domestic
refrigerators claim about 20% of the electrical consumption of the average
household. As a result of this, a mere 14% improvement in refrigerator
motor operating efficiency would unburden our nation's electric power grid
by about one percentage point. Said another way, one out of every one
hundred power plants could be "turned off" if this mere 14% efficiency
improvement were in place in every domestic refrigerator. Air conditioners
are even bigger energy hogs, particularly in summertime and in warmer
regions of the nation. Energy consumption by air conditioners may dwarf
all other uses, particularly when weather is severely hot and/or humid. It
also behooves the layperson, if not the practicing artisan, to realize
that the true mechanical load presented to the typical air conditioner
motor varies widely, depending upon ambient air temperature, humidity,
heat-load changes, and so forth. Again as with refrigerators and other
appliance applications, induction motors intended for air conditioner use
are intentionally over-rated (over designed) to accomodate "worst case"
scenarios, while in fact they may ordinarily operate under conditions
demanding much less motor running torque.
Operating an induction motor with magnetic field strengths nearing the
stator core material saturation level is a common operating mode in modern
motor design, where the central goal is to get the most torque for the
least unit cost. High flux densities are often obtained from windings
having a minimum of copper (e.g., reduced circular mil wire cross-section)
in order to cut cost and weight, particularly with the advent of modern
high temperature insulating materials. The net result is a motor field
which runs hot and with low efficiency under all but full load running
conditions. As is well known, when an induction motor is unloaded (e.g.,
the instant driven mechanical load ordinarily coupled with the output
member is reduced or decoupled) it produces a corresponding decrease in
power factor. While in theory this reduction in power factor with the
current lagging the voltage phase by perhaps 30 to as much as 70 degrees
or so can cut true power consumption (e.g., power actually drawn from the
source), it must be realized that the apparent power (the product of
voltage and current flowing through the motor winding) still remains high.
An artisan familiar with the ramifications of power factor changes in an
a.c. induction motor circuit knows that power draw from the a.c. line is
of course reduced as the load coupled with the motor is reduced, but that
the proportional reduction in a.c. line power wattage draw is not nearly
so strong as what ought to be obtained in view of the extent of mechanical
load reduction. What actually happens is that the "apparent" level of
circulating current through the field windings continue to set up magnetic
flux fields in the stator core which bring about almost as much eddy
current and hystersis loss as what is produced when the motor is fully
loaded. Additionally, this same current continues to produce resistive
losses in the windings, a so called "copper loss" which results in a
considerable level of heating sometimes approaching the heating that
occurs when the motor is fully loaded.
An example quickly makes this problem apparent. A General Electric model
35JN23X motor draws about 6.6 amperes from an 117 volt source while
providing 1/3 horsepower. Power factor is about 82%, and knowing that one
horsepower equals 746 watts, the motor performance appears as:
((117 v.times.6.6 a).times.82%)/100=633 watts
((746 w/3)/633 w).times.100=39% efficiency
((100-39%).times.633 w)/100=386 watts waste power
Under about 25% "partial load", this same motor continues to draw an
apparent current of about 5.1 amperes albeit the power factor appears to
drop to about 40%, resulting in about:
((117 v.times.5.1 a).times.40%)/100=238 watts
((746 w/3).times.25%)/238 w=26% efficiency
((100-26%)/100).times.238 w=176 watts waste power
It would be better if the apparent current draw (said as 5.1 amperes) were
considerably reduced when the load is light. This apparent level of
current flows through the windings of the motor field, inducing magnetic
fields in the structure. It is the energy returned by the inductance of
the motor which lowers the power factor and keeps power draw down.
However, this same level of apparent current flow produces copper losses
in the winding in the form of "IR" heating losses. Additionally, the flux
field induced in the field core produces considerable eddy current losses
in the iron (silicon steel) making up core structure. Indeed,
substantially reducing (such as "halving") the current draw under light or
no load conditions is known to bring about considerable savings in both of
these common areas of power loss. I have found that lower winding current
under reduced motor load can be readily obtained by increasing the winding
inductance. Such operation with increased winding inductance is
correlational with having otherwise reduced the applied motor voltage
coupled with an unmodified winding under conditions of reduced or no load.
As well known to artisans, the ampere-turn relationship of the winding is a
principal determinant of magnetic flux levels in a motor's field
structure. Therefore, a mere 10% increase in turns-count increases the
effective inductance about 21%, and results in about a 10% decrease in
overall ampere-turn excitation level. A BASIC computer routine may quickly
show this relationship;
__________________________________________________________________________
10 'COMPUTATION OF EFFECTIVE AMPERE/TURN EXCITATION LEVEL CHANGE
20 '(c) H. Weber 10/12/90 K1VTW MBASIC ATL-1.BAS
30 '- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
40 INPUT "Original Number of Turns ";NTA
50 INPUT "Additional Number of Turns ";NTB
60 NTX=NTB/NTA
70 LX=1/((1+NTX) 2)
80 AT=LX*(1+NTX)
90 PRINT:PRINT "AMPERE TURN level = ";AT;"% of ORIGINAL LEVEL"
100
'finis
__________________________________________________________________________
In refrigerator and air conditioner applications, any unecessary power
waste under reduced load compounds itself as an unecessary increase in
overall system loss. This comes about due to the configuration of the
motor, where it is hermetically sealed into a module integral with a
compressor. What occurs is that the heat load of the motor inefficiency is
contributed to the overall refrigerant system (e.g., the Freon gas loop)
where it must be disposed of through increased condensor size and
additional compressor effort. In practical effect, the overall size of any
given refrigeration system is substantially upscaled to accomodate waste
motor power.
In U.S. Pat. No. 4,266,177 for "Power Factor Control System for AC
Induction Motors", Frank Nola described an early effort in obtaining a
reduction of power input to a less than fully loaded motor. In this early
work, a.c. power to the motor's main RUN winding was simply ON or OFF.
Operation was akin to that obtained with a phase angle delayed electric
lamp dimmer, aside from the control signal being derived from current lag
changes (e.g., relative with power factor). In this NASA invention, the
sudden inrush of triac turn-ON power introduced harmonic distortion of the
a.c. power flow. This distortion was found to be objectionable by others
attempting to practice the invention, and considerable parasitic loss
(so-called third-harmonic distortion, in particular) was introduced into
the utility power grid which was thought to offset any gains which might
otherwise be produced by this invention. In any event, in a build-up of
Nola's configuration as essentially taught in this earlier patent I found
negligible power savings between having the controller ON or OFF (e.g.,
bypassed) when operating an ordinary 1/2 horsepower split-phase motor at
partial load and with actual power consumption having been measured on a
Westinghouse type CS watthour meter (as ordinarily used by utility company
customers). I also found the motor operation erratic and noisy, producing
buzz which was probably caused by triac turn-ON.
In yet another U.S. Pat. No. 4,533,857 for "Electrical Energy Savings
Device for Motors", Ten-Ho Chang et al said that savings could be obtained
by measuring motor current and providing phase-angle delay of the motor
power turn-ON during each half-cycle. Unfortunately, common induction
motors typically maintain relatively high levels of apparent current flow
even when unloaded and certainly when partially loaded. Mainly, the phase
lag of the current changes. In Chang's device, only apparent current is
measured and thus the scheme is inapplicable to an awful lot of ordinary
cheap induction motors. Additionally, Chang's device suffers the aforesaid
shortfalls which afflict Nola: that being the losses introduced by partial
cycle power flow caused by phase-angle delay of power turn-ON.
I have already taught advantages which reducing motor excitation levels
under all but full load provide in terms of power savings and ENERGY
CONSERVATION. In U.S. Pat. No. 4,806,838 "A.C. Induction Motor Energy
Conserving Power Control Method and Apparatus" issued Feb. 21, 1989 I
described a motor having two sets of parallel RUN windings. A main RUN
winding set is fully energized by direct connection with the a.c. power
source. Through engineered design, this main RUN winding set is ordinarily
sized to produce just sufficient field flux to alone operate the motor
under minimum load conditions. As motor load increases, additional power
is introduced into a secondary RUN winding which is wound so as to
contribute to the field flux produced by the main RUN winding and result
in a stronger field. The power increase in the secondary RUN winding is
related to motor loading, and full a.c. power is fed to the secondary RUN
winding when the motor is fully loaded. I sampled current flow through the
main RUN winding with effective motor loading being determined by instant
phase lag of this current flow. In other words, increased loading produces
a decrease in current phase lag. Although a special motor, having multiple
RUN windings, is needed to implement my earlier invention, its benefits in
power savings are substantial due to reduced eddy current losses and
lessened winding losses under any running conditions less than that of
full load. Unlike Nola and Chang, my invention maintains substantial power
flow over the full 180 degrees of every a.c. power half-cycle even when
less than fully loaded. The result is at least minimal, and usually nearly
negligible levels of loss caused by a.c. power distortion.
In another U.S. Pat. No. 4,823,067 issued Apr. 18, 1989 for "Energy
Conserving Electric Induction Motor Control Method and Apparatus" I have
again taught the use of more than one parallel RUN winding acting in
concert to modulate field flux in relation to instant motor loading. In
its usual embodiment, my earlier invention employs two separate RUN
windings wound effectively in parallel to produce additive flux
contribution to the motor's magnetic field strength. In this arrangement
of my earlier invention the first RUN winding is fully excited from the
a.c. power source, with the ampere/turn design of the first RUN winding
engineered to alone produce sufficient field flux to ordinarily operate
the motor near full subsynchronous speed under minimum load. As motor load
increases, subsynchrous speed decreases introducing an increase in motor
speed slip. It is this increase in speed slip that is sensed and used to
determine an increase in a.c. power which may flow to the secondary RUN
winding. As before, when the motor is fully loaded immediate circuit
operation is established to bring about full a.c. power coupling with both
RUN windings thereby producing a maximal level of field excitation and a
resulting full-torque operation of the motor's rotor coupled output
member.
In yet another U.S. Pat. No. 5,013,990 issued May 7, 1991 for "Energy
Conserving Electric Motor Control Method and Apparatus" I further teach a
reactor coupled in series between an induction motor's usual RUN winding
and a.c. power source. The reactor is sized to introduce some voltage
drop, typically about 10-20%, and maintain sufficient magnetic flux level
in the motor's main RUN winding to keep the motor running properly under
reduced load. When loading increases, motor slip speed increases or
conversely power factor increases, signalling for an increase in applied
a.c. power. The increase is instantly produced by shunting the voltage
drop developed across the reactor by turn-ON of a triac that is in
parallel with the reactor. I do show that the reactor might have one or
more taps, and as such the level of instant motor power might be tailored
to suit the immediate motor loading conditions. Ordinary practice of this
invention requires the use of an inductor (e.g., a reactor or choke)
separate from the motor, and it is the reactive voltage drop which
develops across the reactor due to current flow to the motor RUN winding
which produces a reduction of motor terminal voltage. As a result, the
instant level of motor terminal voltage may undesirably decrease in
response to increases in motor loading.
In a co-pending application Ser. No. 07/237,045 filed Aug. 29, 1988 for
"Energy Conserving Electric Motor Control Method and Apparatus" I continue
to describe an induction motor having a main RUN winding and a
supplementary RUN winding. The main winding is fully excited by a.c.
power, providing just enough magnetic flux in the field to operate the
motor while driving a minimal level of mechanical load. A program
ordinarily is defined for the motor's operating cycle and proportionately
more or less power is simultaneously fed to the supplementary load winding
to provide additional field flux necessary to overcome anticipated changes
in load. Ordinarily, a microprocessor or mechanical timer device may be
used to operate the motor through any predetermined cyclic sequence, while
some modulation of instant levels of the programmed power changes may
further be obtained by sensing real-time fluctuations in motor load.
In each of these earlier patents as well as in one of the co-pending
applications, my inventions entail novel utilization of an induction motor
having two usually parallel-wound mutually coupled disimilar sets of RUN
windings. Ordinarily, the first or main RUN winding set is wound with
10-20% more turns than the secondary or supplementary RUN winding set.
This results in increased inductance in the first RUN winding set, and
reduced current. The net result is substantially reduced ampere/turn
excitation of the field by the first RUN winding set. Fabricating multiple
windings in an induction motor is not unusual manufacturing practice in
that multi-speed motors such as a General Electric model 7HR144S (1/2 hp
1725/1140 rpm 2-speed) are well known. However in these kinds of earlier
designs, each RUN winding set is wound in a relatively different position
(i.e., wound with angular displacement between the winding sets). For
example, in this mentioned General Electric motor one winding set is
positioned every 90 degrees as a 4-pole motor configuration, while the
other winding set is positioned every 60 degrees as a 6-pole motor
configuration. Clearly it is unusual to over-wind more than one disimilar
turns count RUN winding in the same position as called for in my earlier
inventions. I do strive to keep the accumulative amount of copper about
the same, because my duplex main and secondary RUN windings are wound with
wire having a guage substantially smaller than what a usual monowinding
requires.
In smaller motors (fractional horsepower induction motors for example like
those used in refrigerators, window size air conditioners, and major
appliances) utilizing multiple RUN windings such as described by my
earlier inventions it became apparent that it is sometimes difficult to
stuff a sufficient number of multiple winding wire turns through stator
corepieces of ordinary design. This condition is particularly aggravated
by "insulation buildup" on the additional turns of wire, albeit the actual
amount of copper involved remains about the same in either case.
Recognizing this drawback, particularly in relation to mimimal redesign of
pre-existing motor structure designs, I have found that utilizing a
singular winding which is initially "over wound" with sufficient
end-to-end turns to operate from about 10-15% higher than design center
voltage results in a motor configuration which may be readily provided
with at least one tap that matches the motor to the available a.c. power
voltage level and manufacture may proceed without undue complication.
Alternatively, of course a motor of standard design may merely have about
10% more turns added to one end of the original winding, with the juncture
serving as the "tap", and preferably with the additional turns evenly
distributed over each of the several field poles. Realized also is that
the wire guage in either of these configurations, at least between the
common end and the tap location, must be sized sufficient to carry the
full operating current of the motor drawn with line voltage applied to the
tap position while the remaining turns between the tap position and the
end extreme from the common end of the winding may be of substantially
smaller gauge. Common art practice teaches parallel connection of motor
field windings, making tapped winding facture merely an extension of old
practice. Take for example ordinary 2-pole induction motors rated for 117
or 234 volts: when connected for 117 volt operation, the two field
windings are parallel connected. Primarily the advantage of this
configuration over my earlier work using multiple RUN windings is that
much less insulation build-up is encountered in the winding core windows
and obviously less turns-count is required.
Induction motors having tapped field windings are well known in the art,
but for purposes alien to the fundamental purpose of my invention. Such
motors, like a McGraw-Edison model 203PEG, Emerson Electric model
RAK-5107, Westinghouse model 322P490, and General Electric model
5KCP39DGA931T all have tapped field windings intended to obtain speed
adjustment ordinarily with the motor directly coupled with a fan blade.
Since torque demand of a fan changes in proportion to speed, reducing
available motor torque causes increased slip in the motor that eventually
reaches a point of equilibrium where fan speed matches available motor
torque. Such motors are most common in 4 and 6-pole permanent split
capacitor (PSC) arrangements, as in the case of an Emerson Electric model
RAK-8558 used in Whirlpool air conditioners which runs about 1,075 r.p.m.
at "full speed". This represents about 10% slip and is typical of these
kind of known motors. Ordinarily, engineering goals have designed these
kinds of motors to normally operate with "high slip", illustrated by trade
motors such as a 4-pole General Electric model 5KCP39PGB810S 4-speed PSC
motor having about 10% slip as rated for 1,625 r.p.m. full-speed or a 1/3
hp Emerson Electric model K-4340 PSC motor rated for 1,420 r.p.m. having
about 21% slip as used in certain Frigidaire applications.
In contrast, good quality "constant speed" motors like a Westinghouse model
312P417, Emerson Electric model 3874, and General Electric model 37NN6X
operate with merely about 4% slip. Aside from these general purpose
motors, "low slip" motors are also widely used in "sealed" air conditioner
and refrigerator hermetic compressor unit applications, where they
commonly operate around 1,725 r.p.m. and 3,450 r.p.m. from 60 hertz a.c.
power.
The astute artisan recognize that I have found a novel combination of the
advantages afforded by several of my earlier efforts. I bring additional
reactance into play in this invention which is much like having the
external reactor of my earlier copending '079 application, but without the
bulk and inconvenience together with expense of the separate inductor. By
switching between the motor RUN winding taps as I now do, I alter the
effective motor RUN winding circuit inductance (much like selectively
shunting the reactor in the copending application) and I obtain truly
efficient motor power flow under a wide range of external load conditions.
SUMMARY
My invention fundamentally reflects the novel combination of low slip motor
design with multitapped RUN field windings (fabricated somewhat like those
provided in high slip multi-speed motors) to obtain adjustment of field
excitation and resulting rotor torque in proportion to instant motor
loading, with the motor speed remaining about constant and ordinarily at
its most efficient point.
Motor loading is constantly sensed and changes in motor loading are used to
determine which tap is selected thereby producing continual variation in
the magnitude of the portion of the total field winding that is excited.
The result of this action is to produce modulation of motor flux in about
direct proportion to motor load. When lightly loaded, the motor field is
less fully excited which substantially reduces intrinsic losses due to
eddy currents in the field core and copper losses in the field windings.
Usual practice of my invention includes a motor having one or more taps
intermediate of the field winding ends. When minimum load is encountered,
a.c. power is applied between the winding ends resulting in the most
number of winding turns and highest impedance. As load increases, a.c.
power is alternatively coupled between one end furthest from the tap and
the tap connection which represents fewer overall turns being excited and
lower impedance thereby increasing field flux density. Usually, my
invention works well with a motor having field connections with taps
located around 80% and 90% of the overall winding extent. It shall be
realized however that more or fewer taps may be used at other tap
locations depending upon the operational objectives of a specific motor
design.
My invention comprises three essential functions:
a substantially constant speed and ordinarily low-slip induction motor
having a multitapped field winding;
a motor load level sensing or predetermination arrangement;
a power control determination function which variously couples a.c. power
to differing portions of the field winding in immediate response to
changes in sensed or predetermined levels of motor loading.
I have already said that the motor portion of my invention may be provided
as a unique hybrid of a low slip constant speed motor structure combined
with a tapped field winding structure typical of high slip multi-speed
motor designs. As such, physical manufacture of a motor suitable for my
invention's application is not at all unusual, requiring no exceptional
production practices and costs.
In one preferred embodiment, load sensing is obtained through determination
of changes in motor current phase relative to motor voltage phase (so
called power factor change). When lightly loaded, the current phase
considerably lags the voltage phase. A lag of 60 degrees or more is
unexceptional. In contrast, when the motor is fully loaded the lag is much
less, oftimes being on the order of 15 to 30 degrees depending upon the
particular design of a motor's structure. In any event, this change in lag
is proportional to motor loading, with lag becoming greater as motor load
lessens (and the motor appears more inductive). Although the exact design
particulars as to how I obtain this sensed determination of power factor
change is not the central issue of this inve | | |
