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BACKGROUND OF INVENTION
The ubiquitous use of a.c. induction motors in major appliances and air
conditioning equipment is well known. The simple, inexpensive design of
such motors makes their choice very cost-effective from the designers
point-of-view.
Low cost fractional horsepower induction motors are also known to waste a
lot of electric energy as heat and to have not particularly high operating
efficiency, especially when they operate at less than full load. It is not
unusual for a common induction motor to draw nearly as much line current
amperage when unloaded as what it consumes under full load. Although the
unloaded power factor is lower, due to the lagging current draw due to the
highly inductive load characteristic, with the result that some of the
apparent wasted power is "returned" to the power line by the action of the
back e.m.f. of the motor, a good portion of the loss persists as a
combination of eddy current heating loss brought on by relatively high
magnetic field densities, combined with copper losses due to the choice of
minimal wire size in the windings. These loss factors are mostly brought
on by cost reduction practices of the so-called modern motor, wherein a
very high temperature rise can be tolerated due to improvements made in
insulating materials over the past number of years.
A motor's temperature rise is a sure sign of electrical inefficiency. All
the electrical losses convert directly into heat. It is not unusual for
ordinary induction motors to operate with a surface temperature of 150 to
well over 200 degrees Fahrenheit. Motors made with class A insulation are
commonly rated for 50 degree centigrade temperature rise above ambient,
while class B insulation permits a 75 degree centigrade temperature rise.
Thus a lot of energy is intentionally thrown away as thermal loss in an
effort to produce a cheap product.
In most induction motor applications in major appliances, the load driven
by the motor varies over a wide range. A common washing machine is often
equipped with a 1/2 horsepower motor, such as a General Electric Company
model 54KH46JR15S which draws about 7.9 amperes under full load, and with
a power factor of about 85%. Since one horsepower represents 745 watts
(550 ft. lb. per second), this represents a full load efficiency of
merely:
((745W/2)/(115V.times.7.9A.times.0.85PF)).times.100=48% efficiency.
As a result of this, about half of the electricity consumed by the motor
under full load is thrown away as wasted heat:
(115V.times.7.9A.times.0.85PF)-(745W/2)=400 watts wasted,
and the manufacturer admits of a 70 degree centigrade temperature rise
above ambient (e.g., about 95 degrees centigrade operating temperature).
It is also known that full load motor power is seldom required in the usual
washing machine. With a heavy load of clothes, the spin dry cycle and the
agitate cycle may draw nearly full power: but when washing lighter loads,
nowhere near the full amount of available motor power is needed. During
the pump-out cycle very little power is needed, and it is also noteworthy
that during the spin dry cycle the most driving torque and therefore the
highest power demand occurs during the start-up of the cycle.
The result of the variable load presented by the washing machine's ordinary
operation is that the motor drive seldom operates anywhere near its full
capacity. However, low-cost design practices dictate that the motor torque
rating must be sized for the worst case load in order to have sufficient
reserve power to run the machine without stalling.
When an induction motor is operating at less than full-load, its actual
electrical efficiency is miserable. For example, the mentioned General
Electric 5KH46JR15S motor continues to draw nearly 5.7 amperes under
NO-LOAD conditions (e.g., load disconnected from motor shaft), which
simply means that over 655 apparent watts are thrown away to do nothing!
The irony is, horrible dictu, the less the motor "works", the more power
the motor "wastes". Even if a power factor of as low as 64% is allowed,
the wasted power 655W.times.0.64F=420 watts is still very high. Under
no-load, practical motors seem to assume a power factor of about 70% to
80% due to eddy current loss and copper (resistance) loss. Furthermore,
motors with aluminum windings suffer somewhat more loss (but "better"
power factor) than those with copper windings.
The National Electric Code has given a listing of nominal load currents
which may be expected to be drawn by a.c. motors running under full load,
as:
______________________________________
Single-Phase
3-Phase AC
Horsepower
115 V 230 V 110 V 208 V 277 V
______________________________________
1/6 4.4 A 2.2 A
1/4 5.8 A 2.9 A
1/3 7.2 A 3.6 A
1/2 9.8 A 4.9 A 4.0 A 2.1 A 1.6 A
3/4 13.8 A 6.9 A 5.6 A 2.9 A 2.2 A
1 16.0 A 8.0 A 7.0 A 3.3 A 2.8 A
11/2 20.0 A 10.0 A 10.0 A 4.7 A 3.9 A
2 24.0 A 12.0 A 13.0 A 6.1 A 5.2 A
3 34.0 A 17.0 A 9.5 A 7.1 A
5 56.0 A 28.0 A 15.9 A 11.9 A
10 50.0 A 28.5 A 21.4 A
______________________________________
From REFERENCE DATA FOR RADIO ENGINEERS, 5th Edition, Howard W. Sams &
Co.; Library of Congress No. 4314665; page 41-13.
The principal causes for energy loss brought on by wasteful motor heating
is eddy current loss in the magnetic path and "copper" loss in the
windings. Cheap motors often use aluminum windings (instead of copper),
and the result is even worse winding loss ("aluminum" loss!?), due to the
somewhat higher resistivity of the aluminum wire. Designers often run the
flux densities in the magnetic structure (particularly the stator) near
saturation in order to obtain a desired level of performance with the
least amount of material, and in a smaller and lighter configuration. High
flux densities merely serve to aggravate heating caused by eddy current
losses, and of course increased temperature aggravates the winding losses.
In the mentioned General Electric 5KH46JR15S motor, such losses due to
near-saturation of the magnetic structure is very evident when the
operating voltage is reduced. Yes! an unloaded induction motor can be run
at much less than full rated voltage AFTER it starts, and it will continue
to spin at near full speed. This particular motor runs well at 50% applied
voltage under light load: and the fully unloaded (nothing connected to the
shaft) current draw dips to merely about 2.1 amperes (with about 60 volts
applied to the RUN winding, which is equivalent to about 1.1 amperes at
full 115 line volts). Thus the wasted power becomes only about 126 watts
(or merely 88 watts for a 70% PF) under no-load, whereas when it is run at
the nameplate voltage the wasted power soars to about 458 watts (with 85%
power factor), representing a whopping 520 percent increase in wasted
energy. In this particular motor embodiment, real power waste begins to
soar when the applied voltage reaches about 85 volts or so. The no load
current quickly zooms up to the 5.7 ampere level when the applied voltage
is increased above 90 volts up to the rated 115 volts. With 125 volts
applied, the no load current jumps up to 7.3 amperes, or about 640 watts
(with 70% power factor) as wasted energy! This does not mean that the
motor should be run at reduced voltage except when the load is reduced,
i.e. the applied motor power should be matched with the mechanical loading
demand placed on the motor. It is well known that the power that an a.c.
motor develops varies directly with the square of the applied voltage, and
therefore a reduction to 85 volts results in a considerable reduction in
available torque to be only about half that available with 115 volts
applied: but even that is usually more than enough running torque for many
portions of an appliance's operating cycle. Conversely, during a full-load
operation of the appliance full voltage must definitely be applied, in
order to avert stalling and possible burn-out of the motor. Similar
performance is found with the Kenmore (Sears Roebuck & Co.) washing
machine motor model C68PXDBZ3290 (part no. 62556) which is rated for 1/2
horsepower with a full-load current of 9.8 amperes (e.g., over 900 watts
with an 80% power factor). It also appears that the =newer" a major
appliancemotor design is, the more wasteful of energy it becomes when only
lightly loaded.
A clothes dryer motor such as the General Electric Company model
5KH47ER150X which is rated 1/3 horsepower draws about 6.4 amperes from the
115 volt a.c. mains, hence running with about 34% efficiency when fully
loaded. The same motor also continues to draw nearly 6 amperes under no
load, and therefore (even with a 70% power factor), "wastes" over 480
watts of energy. It is also obvious that in a clothes dryer application,
the load difference between that of drying a heavy wet blanket or two, and
a different lighter load consisting of no more than a few pieces of
lingerie demands considerably different torque from the drive motor. As in
the earlier mentioned washing machine motor, this motor wastes
significantly more energy when lightly loaded. This motor operates
satisfactorily with only about 85 volts applied, albeit with less torque,
and under this reduced voltage condition it draws only about 3 amperes
(178 watts with 70% power factor) under no load (or a very light load),
while with 60 volts applied it draws merely 2.2 amperes (92 watts with 70%
power factor).
What is now shown is that the washing machine using a General Electric
5KH46JR15S or 5KH47KR223B (or equivalent) motor and the clothes dryer
using a motor like the General Electric model 5KH47ER150X represent a
typical laundry combination which together can readily "waste" about a
kilowatt of electric power when running with less than full load. Taking
into consideration that the typical laundary machine may operate for 25 to
60 minutes per day on the average, the overall amount of energy merely
wasted by the cummulative energy consumption of the hundreds of millions
of laundry machine operation performed each day is enormous.
The continuous duty rated Dayton model 5K461B motor produces 3/4 horsepower
and is intended for two-speed belt-driven exhaust fan applications. From
this table of measured performance you will see that a lot of energy can
be saved by reducing the motor voltage when the load is light.
______________________________________
Applied
RUN Line Current Amps (Watts)
Output
Winding
High/Low Operating Speed
Loading
Volts 1,725 RPM 1,140 RPM Condition
______________________________________
115 11.0 A 1,075 W) 7.4 A 723 W) Full
115 8.2 A (660 W) 7.0 A (563 W)
None
85 5.0 A (297 W) 4.5 A (267 W)
None
60 3.2 A (134 W) 3.0 A (126 W)
None
______________________________________
Assumed Power Factor: 85% full load, 70% no load.
Hence there is a whopping 802 percent difference in the amount of energy
consumed (at 1,725 RPM) between full load at 115 volts and no-load, which
can run just as well at 60 volts (half-voltage). More importantly, the
reduction in wasted energy obtainable with no-load (e.g., light load) on
the motor between the condition of full excitation and partial excitation
is nearly 500 percent!
Another wasteful appliance is the common dishwasher. A 1/3 horsepower motor
is commonly used, but the torque developed by the motor is only needed
during short portions of each running cycle. Since the typical dishwasher
motor, such as the Emerson Electric Co. industry standard type 4093, sold
by the W. W. Grainger Company as their model 4K180, draws about 6.5
amperes at 3,450 r.p.m. under full load about 748 watts (or 598 watts for
an 80% PF) are consumed while running with about 41% efficiency. Worse
however is the consumption of about 375 watts (with 70% power factor)
under no load. This wasteful performance continues throughout the
machine's operating cycle even when little motor torque is needed, such as
during pump-out and light-load washing.
Refrigeration equipment, and in particular air-conditioners, operate over a
rather wide load range, depending upon ambient temperatures, humidity and
so forth. The typical sealed refrigeration compressor unit employs an
integral 1/4 horsepower induction motor running at 3,450 r.p.m. and
drawing about 4.0 amperes. The Kelvinator model A044-1 is such a combined
1,050 B.T.U. rated motor and compressor unit. The resulting full-load
efficiency with 80% power factor is:
((745W/4)/(4.0A.times.117V.times.0.8PF)).times.100=49.8% motor efficiency.
The lost power under full load is:
(4.0A.times.117V.times.0.8PF)-(745W/4)=188 watts.
It is well known that a refrigeration compressor works harder under some
conditions of ambient humidity and temperature than what it does under
other conditions. The compressor motor is again sized for the worst case
condition to minimize the liklihood for stalling, while much of the time
the motor is actually working a lesser load. The same kinds of light load
inefficiencies occur in the compressor motor as were mentioned for the
washing machine motor, because cheap design practices prevail and thus
high magnetic flux densities and high winding current densities are
allowed by the manufacturer.
Reducing the motor voltage as the compressor load decreases can save
considerable energy and it can also reduce unecessary hum noise produced
by the motor and as a result a more quiet and more energy efficient
product results.
Reduction of the magnetic flux density in the induction motor's stator has
a two-fold effect. Eddy current losses are reduced, and the inductance of
the windings increases somewhat, when the magnetic path flux density is
decreased. The combined effect is an increase in the impedance of the
winding and a resultant lowering of current through the winding. Flux
density can be reduced by increasing the amount of iron in the magnetic
path, or by increasing the winding turns. For a given motor design,
however the only practical method for reducing the flux density is to
reduce the current flow through the winding, such as by lowering the
terminal voltage. Such a simple-minded approach usually merely results in
lowered motor performance and probable stalling, unless the load is also
much reduced. For this reason, common motor design practice intentionally
tolerates wasting a lot of energy as heat build-up in order to produce an
economical, essentially cheap motorized product which has sufficient
reserve horsepower or running torque to operate under the worst case load
scenario. The result is that the consumer pays out in wasted electricity
more than what is saved by not having a more energy efficient motor
running the appliance in the first place. The most far-reaching and
probably most devastating fact, however, is the enormous waste of
non-renewable natural resources this usual kind of cheap and low
efficiency motor engineering incurs.
SUMMARY
A.c. induction motors afford a very economical and time proven source of
power for driving major appliances, air conditioners, and other kinds of
domestic and commercial machines. The dependability of induction motors is
exceptional, and years of product engineering have resulted in a simple
and cost effective configuration using few parts.
Production of electric power in America is reaching a point where the
utility companies in many parts of the country will soon be nearing 100%
capacity. Unless more generating capacity is soon built, brown-outs, power
grid failures, and other cataclysmic power distribution events are likely
to occur because no reserve power capacity is available.
The unecessary waste of electrical energy can be reduced through improved
utilization of whatever electricity is now used for operating major
appliances, air conditioners, and other kinds of common machines. The
ordinary a.c. induction motor is notorious for being inefficient, as
evidenced by the high temperature rise inherent in any common motor. Since
most motors operate at least part of the time with much less than full
load, it behooves the designer of a modern product to somehow adapt the
motor to match the dynamics of the load and thereby waste less energy. Any
appliance which is trully energy efficient will soon have to have a better
match between the driving motor and the load under all conditions of
loading.
A.c. induction motors could be supplanted by other kinds of drives such as
"brushless" d.c. motors. However, quick changeover to an alternate motor
design is unlikely, due to the long and proven product history of a.c.
induction motors. The induction motor has become so highly refined in
mechanical design that it is unlikely that any other contender will soon
even approach the economics of that design in the bigger appliance sizes,
say from 1/4 horsepower and up.
The most surprising improvement that can be implemented with the common
induction motor design involves mere modification of the motor winding
procedure to include two parallel RUN windings. The first RUN winding is
sized with turns and wire guage selected to produce just sufficient
magnetic field to meet the running needs of the motor under its lead-load
condition, while the second RUN winding is in a like way sized with turns
and wire guage to provide additional magnetic field such that when it is
directly in parallel with the first RUN winding, the two combine to meet
the running needs of the motor under full-load. The actual improvement is
obtained through the expedient of connecting the first RUN winding
directly with the source of a.c. power, while the second RUN winding is
modulated with more or less of the available a.c. power depending upon
instantaneous motor loading. For example, under minimum load only the
first RUN winding is excited, and the flux density in the motor is
relatively low thereby reducing the eddy current losses. As the load
increases, a controller increases the second RUN winding excitation in
proportion and the motor stator magnetic field density is always
maintained at a level which is just sufficient to provide the needed
rotational torque in the rotor.
In my earlier application Ser. No. 07/075,990 ("Electric Induction Motor
Power Control Method and Apparatus") I taught the variable control of the
a.c. power connected with a singular RUN winding of an a.c. induction
motor as a way to obtain energy conversation under less than full load
conditions. This earlier controller effectively varied the applied RUN
winding power with a phase controlled thyristor, with the result that
during at least part of each a.c. power half-cycle no electrical energy
was coupled with the winding (excepting, of course, under condition of
full load), with the result that no magnetic flux was produced in the
motor field during that part of the a.c. power half-cycle. The turn-ON of
the thyristor at some later point during the power half-cycle then
abruptly turned the RUN winding power on, with the delay being related
with the instantaneous loading of the motor. The distortion of the a.c.
current waveform cycle brought forth by such abrupt turn-ON of current
presented a load having a poor power factor to the a.c. source. Current
rush caused by sudden thyristor firing can produce spiking, and of course
a considerably larger thyristor device is needed to handle the full motor
current. In this earlier invention, I utilized the "slip" in speed which
is related to loading in the common induction motor as a signal for
setting the control parameters. This is a sensitive and instantaneous
reflection of motor loading, and is therefore applicable in part with my
present invention.
Through the use of two RUN windings, with one permanently excited by the
a.c. source I now achieve less distortion of the a.c. power current
waveform as seen by the a.c. source, and superior control of the motor's
performance. The magnetic path in the motor (i.e., the stator) is always
excited to the extent needed for the minimum load condition (or at least a
lessened load condition), whereas the controller merely adds to this
initial magnetic excitation to the extent needed to produce increased and
perhaps full power capability from the motor. The result is that the
abruptness of current change caused by thyristor firing is very
considerably lessened, and the current flow in the first RUN winding which
precedes the firing of the thyristor that excites the second RUN winding
serves to somewhat swamp the deleterious effect of abrupt current
switching the by the thyristor. A smaller thyristor can also be used to
switch power to the second RUN winding, since it draws a lesser portion of
the total running current of the motor.
Through the expedient of modifying an ordinary motor, like a 1/3 horsepower
General Electric model 5KH42ER355S design and using essentially the same
design "factors", results in the winding of a first RUN winding for a
calculated voltage rating of 150 volts, and then actually operating it on
115 volts. This design modification results in the desired operational
equivalence of having reduced the power applied to the original 115 volt
winding to about 88 volts. The immediate result is that of greatly
reducing the no-load current draw, to about 1 ampere or less (i.e., about
80 watts with a 70% power factor). The second RUN winding is then added to
the stator to be physically as well as electrically in parallel with the
first RUN winding. The calculated sizing of the second RUN winding number
of turns and wire guage is such that under FULL load, the total magnetic
flux present in the stator as provided by the two RUN windings being
coupled in parallel is established to be about equivalent to that of the
unmodified motor when its sole RUN winding was connected directly with the
115 volt power source. The field produced by the second RUN winding is
usually a lot stronger than that produced by the first RUN winding when
the motor is operating near full load.
An objective of my invention is to provide an energy efficient induction
motor having improved electrical operation, and less energy waste, under
any condition of less than full load.
It is another objective of my invention to enable practical implementation
of energy efficiency improvements in the electrical performance of
induction motors having ordinary physical design merely through the
modification of the RUN winding portion of the motor structure.
Still another objective for my invention is to provide just enough magnetic
flux in the motor field to produce the rotating member torque necessary to
sustain satisfactory motor performance under various amounts of loading.
A further objective is to teach the use of a novel induction motor having
more than one RUN winding, whereby during motor operation one of the
windings continuously draws a.c. current from the power line while the
other RUN winding is variously modulated with a.c. power in prompt
response to any changes sensed in motor loading.
Still another objective is to provide control of the induction motor's
applied power over a range, between that of minimum load and full load, in
order to reduce power consumption, while providing such energy efficient
control of the motor with semiconductor devices having smaller power
rating size than ordinarily needed.
Yet another objective is to provide a motor controller that immediately
senses any change in the rotational speed slip of the motor and therefrom
derives a control signal which modulates the power applied to the second
RUN winding, whereby an increase in slip results in an immediate increase
in power coupling with the second RUN winding.
The instantaneous sensing of changes in rotational speed slip of the motor
serves to provide a fundamental performance signal which can serve to
signal how much applied RUN winding power is needed for the motor's
satisfactory operation, with the objective being that less load results in
less speed slip which results in a reduction in a.c. power coupling with
the motor and a significant reduction of electrical energy waste.
An important objective of my invention is that of introducing art which
will show existing motor designs can be merely modified by the
manufacturer to advantageously include the significant improvements in
energy conserving performance, without necessitating any further cost for
re-tooling or the like.
These and other advantages of my invention will now be revealed to the
skilled artisan, and it is anticipated that applications for and ways of
using the elements of my invention will vary from those which are now
presented herewithin, since my examples are given merely for the
illustrative purpose of providing a clear understanding of the underlying
essence of my invention. Such broader application of my invention's
teachings to benefit other commonplaces usages for induction motors shall
also be deemed to be obvious to any practicing artisan and within the
general scope of my invention's reading.
DESCRIPTION OF DRAWINGS
Eleven sheets of drawings showing twelve figures serve to illustrate the
substance of my invention.
FIG. 1 Block diagram showing a motor having two RUN windings, one of which
is variably controlled in response to motor load variation.
FIG. 2 Plot showing NO-LOAD line current and power dissipation for a
typical induction motor relative with applied voltage variation.
FIG. 3 Plot showing improvement in energy saving obtained through the use
of the invention with a conventional motor design having a wide variation
in loading.
FIG. 4 Block diagram for elements which make up a performance computer
suitable controlling the power coupled with an induction motor.
FIG. 5 Plot showing changes in the a.c. power conduction angle produced by
the thyristor trigger drive signal provided by the circuit of FIG. 4
relative with sensed motor speed.
FIG. 6 Additional circuit which cooperates with the elements of FIG. 4 to
obtain integral cycle power variation control.
FIG. 7 Hookup for a variable power switch using a thyristor suitable for
driving the RUN winding of an induction motor from the control circuitry
of FIG. 4.
FIG. 8 A.c. power cycle waveforms for several control conditions.
FIG. 9 Four-pole induction motor of conventional design, whereby two poles
are constantly excited by the line power, and the other two poles are
variably excited in response to any sensed changes in motor loading.
FIG. 10 Electrical diagram for a typical window air-conditioner wherein the
hermetic motor/compressor unit is provided with an additional RUN winding
to attain energy conservation through load-responsive a.c. power control.
FIG. 11 Three phase motor is shown to have separate pairs of RUN windings
for each phase leg in order to obtain reduction of light-load power
consumption.
FIG. 12 Hermetic motor/compressor of the sort used with common
refrigerators and air-conditioners, showing connection of compression
impulse sensors to obtain motor speed sensing.
DESCRIPTION OF MY INVENTION
My invention provides for the energy-efficient operation of ordinary a.c.
induction motors through the modulation of the magnetic flux in the field
(or stator) of the motor in response to variations in loading of the
motor. Such load variation has been found to be instantly measurable
through the expedient of sensing any even small variation in the
sub-synchronous motor speed slip which is characteristic of all common
types of induction motors. In FIG. 1 I show a controller 10 which includes
a performance computer 20 having circuits which compare the instant motor
speed signal, as provided from the speed sensor 30, with other
predetermined parameters. The sensor 30 is coupled 32 with the rotor 56 of
the induction motor 50 to measure the motor's instant speed. The
functional elements which make up this performance computer 20 portion of
the invention work together to set the instant effective value of the
power control signal which couples with the power regulator 40. The power
regulator produces a variation in the amount of a.c. power coupled with
the power line L2. The other RUN winding 54-1 (R1) is directly coupled
with the a.c. power source and therefore provides constant excitation so
long as a.c. power is applied.
The induction motor 50 also includes a START winding 52-1, a starting
capacitor 52-2 and a starting switch 52-3 which may be a usual kind of
centrifugual speed responsive set of contacts, coupled with the motor's
rotor 56 which is closed at rest, and which opens when the motor attains a
substantial speed of about 70% or so of FULL speed. Another initially
closed contact set 54-3 also serves to shunt the a.c. line L2 directly
with the second RUN winding during the starting period, thereby producing
full starting torque from the motor and bypassing the initial current
surge around the power regulator thereby allowing for a smaller, more
economical thyristor to be used in the power regulator. The variable load
58 is driven by the rotor 56, and may undergo a wide fluctuation in
instant torque demand such as that produced by appliances like washing
machines, or by the compressor in a refrigeration or air conditioning
system.
A.c. power consumption for an induction motor of common design is depicted
in the plot of FIG. 2. With curve 2-1 I show how an ordinary 1/3
horsepower split-phase induction motor, such as the General Electric type
5KC45HR1S, which uses about 6.8 amperes of 115 volt a.c. power under FULL
load still continues to draw about 5.7 amperes of 115 volt a.c. power
under NO load, resulting in the WASTE of about 458 watts (with 70% power
factor). My curve 2-2 also show how dramatically the NO-LOAD energy
consumption drops off as the applied a.c. line voltage is reduced (not
including power factor correction). Since the mechanical power that an
induction motor develops varies in direct relationship to the square of
the applied voltage, 85 volts represents abo | | |
