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BACKGROUND OF THE INVENTION
This invention relates to refrigerant compressors driven by an electrical
motor capable of operating at a low speed and a high speed and to the
method and means for controlling the refrigerant compressor utilizing a
two-speed electric motor having four separate windings, a two-pole
auxiliary and main, and a four-pole auxiliary and main winding, all wound
on the same core.
U.S. Pat. Nos. 1,609,310--Rodgers and 1,712,065--Baker teach two speed
motors wherein windings are arranged for two separate speeds of the motor.
Refrigerant compressor utilizing two-speed motors have been employed in
the past. One such arrangement is shown in U.S. Pat. No. 3,584,980--Cawley
et al, which discloses that a four-pole, two-winding electric motor can be
utilized to provide full capacity during two-pole operation and one half
capacity during four-pole operation and means for providing adequate
lubrication of the crankshaft bearing surfaces during both high speed and
low speed operation. U.S. Pat. No. 3,943,912 is directed to the method of
controlling a two-speed motor of the type disclosed in U.S. Pat. No.
3,584,980 wherein the operation of the two-speed motor compressor system
can be controlled such that the overall coefficient of performance
(efficiency) of the system at the lower speed, i.e., during part load
conditions, is equal to or greater than the coefficient of performance
(efficiency) of the system at the higher speed during peak load
conditions. A first torque load is imposed on the two-speed motor during
peak load operation and under a given set of evporating and condensing
conditions, and then a second torque load is imposed on the motor during
part load operation and under the same set of evaporating and condensing
conditions such that the ratio of the second torque load to the first
torque load is equal to or less than the ratio of the motor efficiency at
part load operation to the motor efficiency at peak load operation.
Other prior art teachings of a refrigerant compressor utilizing two-speed
motors are shown in U.S. Pat. No. 3,978,382--Pfarrer and
4,041,542--Pfarrer, wherein temperature sensors are located adjacent each
of the windings for detecting any change in temperature of the windings. A
current transformer is used in combination with the temperature sensor and
in the event the temperature or current conduction of any of the windings
exceeds a predetermined value, a control device operates a switch in a
pilot circuit that turns off power to the motor.
Another prior art teaching of a refrigerant compressor utilizing a
two-speed motor is shown in U.S. Pat. No. 4,064,420--Yuda et al, which
provides a control system for a pole-changing, motor-driven compressor
that stops the motor in case of the pole changing thereof and starts it
only after the operating conditions of the compressor change to permit the
restarting of the motor, whereby the reliable and dependable pole-changing
operation can be ensured and the motor may be prevented from being
damaged.
An object of the present invention is to provide a system and method of
controlling the operation of a hermetic two-speed, electric motor-driven
refrigerant compressor so as to attain an overall motor efficiency which
is substantially equal at either low or high speed operation, while
providing higher overall compressor efficiency at the lower speed.
Another object of the invention is the provision of a control system
wherein low speed operation of the compressor motor is controlled through
a first stage switch means of the indoor thermostat.
Another object is to provide for mandatory starting of the motor in low
speed and to continue low-speed operation for a predetermined period of
time.
Another object is to provide for instantaneous speed switching between
speeds without interrupting compressor operation.
Another object is to provide separate, internal, line break, motor
protectors for each speed winding that can be optimized for each winding
without affecting the other.
This and other objects and advantages of the present invention will become
apparent from the following description of the preferred embodiment of the
invention described in connection with the accompanying drawing.
SUMMARY OF THE INVENTION
In a hermetic refrigerant compressor of the type having a sealed outer
casing containing a compressor mechanism for receiving refrigerant gas
from a suction line, compressing the refrigerant gas and discharging the
compressed refrigerant gas through a discharge line, and a motor for
rotating drive shaft means in the compression mechanism at low speed and
at high speed from a source of single phase electrical power. The motor
comprises a slotted magnetic core in which a low-speed, four-pole main
winding is fitted substantially evenly therein and a four-pole auxiliary
winding which is arranged in a location adjacent the low speed main
winding. A high-speed, two-pole winding is also fitted in the slotted core
with a two-pole auxiliary winding arranged in a location adjacent the
high-speed, two-pole auxiliary winding.
A first line break motor protector is arranged in intimate thermal contact
with the low-speed, four-pole main and auxiliary windings and is connected
electrically in series between the power source and windings for
controlling power to the four-pole windings independent of said two-pole
windings. A second line break motor protector is arranged in intimate
thermal contact with the high-speed, two-pole main and auxiliary windings
and is connected electrically in series between the power source and
windings for controlling power to the two-pole windings independent of the
four-pole windings whereby the compression mechanism may operate at one of
said speeds when the line break of the other speed disconnects its
associated winding from the power source.
The control system includes a temperature-responsive switching means having
a first stage switch means and a second stage switch means. A speed
selection switch means is arranged in series with the first stage switch
means. The speed selection switch means being operable between a normal
low-speed position for connecting the four-pole windings to the power
source and to a high speed position for connecting the two-pole windings
to said power source. A high-speed switch means arranged in series with
the second stage switch means includes a delay means operable after a
predetermined time period for switching the speed selection switch between
the normal low-speed position and the high-speed position so that
operation of the motor is instantaneously switched between the four-pole
windings and the two-pole windings after the predetermined time delay
without interrupting operation of the compressor mechanism.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a side elevational view, partially in section with some parts
broken away, of a hermetic refrigerant compressor that is driven by a
two-speed electric motor;
FIG. 2 is a fragmentary view of a typical stator lamination;
FIG. 3 is a schematic showing the winding placement in the stator,
including relationship of the line break motor protectors; and
FIG. 4 is a simplified circuit diagram of the control system for the
two-speed compressor.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The two-speed compressor and control system in the present embodiment is
shown in conjunction with a cooling-only refrigeration system. It should
be noted, however, that it can be used in conjunction with a heat pump
system.
Referring to the drawing, there is illustrated a hermetic compressor unit
comprising a shell or casing 10 in which is resiliently supported a
motor-compressor unit 11. The unit 11 comprises a compressor block 12
defining a substantially closed crankcase 13 and at least one cylinder 14
opening into the crankcase. The compressor block 12 also includes upper
and lower axially aligned bearings 16 and 17 in which is mounted a
vertically extending shaft 18. The shaft 18 is connected at its upper end
to the motor rotor 62 and having an eccentric bearing portion 19 at its
lower end arranged between the bearings 16 and 17. A connecting rod 20
connects a piston 21 to the bearing 19. Thus, piston 21 reciprocates or
slides back and forth in the cylinder 14 in response to the reciprocating
forces provided by the eccentric 19 upon rotation of the shaft 18.
Means for driving the compressor 11 comprises an electric motor 24
positioned in the upper portion of the shell 10 above the compressor block
12. The motor 24 which will be described in detail hereinafter includes a
stator 61 supported within a casing 25 that is secured to the unit 11 and
the rotor 62 which is inductively connected to the stator 61.
The bottom of the shell 10 defines a sump for containing a body of
lubricating oil 27 used to lubricate the various bearings, as disclosed in
detail in U.S. Pat. No. 3,098,604--Dubberley, assigned to General
Electric, the assignee of the present invention. This body of lubricant is
preferably of a sufficient depth that the lower end of the crankcase,
including the bearing 17, is substantially immersed in the oil and is
lubricated by such immersion. For the purpose of providing lubrication for
the upper main bearing 16 and crank bearing 19 which are disposed above
the body of oil and below the motor 24, there is provided a centrifugal
pumping arrangement including a lubricant passage or duct in the shaft 18
having its lower or inlet end below the oil level in the reservoir or sump
and an upper outlet on the peripheral surface of the shaft encompassed by
and in frictional engagement with the upper bearing 16. This lubricant
passage comprises a horizontal, radially extending groove 29 which, with a
thrust plate 30, forms a radial passage and a vertical portion or passage
31 parallel to, but offset from, the vertical axis or center of rotation
of the shaft. The lower end of the passage 31 communicates with the
radially extending groove 29 and the upper end terminates within or
slightly above the portion of the shaft journalled in the upper unimmersed
bearing 16. To assure lubrication of the lower bearing under varying oil
level conditions, this bearing can be force fed by extending groove 29 to
the periphery of the shaft.
Adjacent the upper end of the vertical passage 31, there is provided an oil
port 34 extending from the passage to the peripheral surface of the shaft
within the bearing 16. By this arrangement, oil entering the lower passage
29 through an opening 32 in the thrust plate 30 is subjected to
centrifugal force set up by rotation of the shaft flows upwardly along the
passageway 31 and outwardly through the oil port 34 to lubricate the
bearing surfaces of the upper bearing 16. Intermediate the ends of the
shaft 18 and in line with the connecting rod 20 there may also be provided
one or more additional ports 35 which is adopted to furnish lubricating
oil to bearing 19 formed between the eccentric portion of the shaft and
the connecting rod 20.
The compressor is designed to form part of a hermetic refrigeration system
either of the type used for cooling only of an enclosure or the heat pump
type used for both heating and cooling. In the present instance, as
mentioned above, a cooling only system is shown diagrammatically including
a condenser 41, an expansion device which may be either an expansion valve
or, as shown, a capillary tube 42 and an evaporator 43 connected in closed
series flow relationship. During operation of the compressor, low pressure
or suction gas is withdrawn from the evaporator 43 through suction inlet
44 in the upper portion of the shell 10. This relatively cool suction gas
passes downwardly through the motor 24 between rotor 62 and stator 61 and
through a plurality of holes 45 into an annular suction muffler 46 formed
in the upper portion of the compressor block 12. The suction gas flows
from the muffler 46 into an annular cavity 48 surrounding the forward end
of the cylinder 14 and from this cavity through a plurality of suction
ports 49 and a suction valve into the interior or chamber 50 of the
cylinder 14.
Refrigerant compressed by the reciprocating piston 21 flows from cylinder
14 through a discharge valve (not shown), a discharge muffler 51, and into
a discharge line 52 which includes a plurality of loops 54 immersed in the
body of oil 27 and is thereafer discharged from the compressor unit
through an outlet 55 to the condenser 41.
In accordance with the present invention the motor and the means for
controlling operation, including the method thereof, will now be explained
in detail. The present motor for single-phase application includes four
separate windings, a first motor or set of windings 58 including a
two-pole auxiliary and main winding for high-speed compressor operation
and a separate motor or set of windings 60 including a four-pole and main
winding for low-speed compressor operation, all wound on the same core.
This approach to motor design allows the efficiency and torque levels of
each set of windings 58 and 60 to be optimized independently.
Typically, the motor 24 which in the present instance includes both sets,
the low and high-speed motor windings 58 and 60, respectively, provides a
stator 61 and rotor 62 that are constructed from a plurality of individual
laminations 63 and 64 respectively. Each of the laminations 63 of stator
61 has a central opening which, when the laminations are arranged in
stacked relationship, defines a stator bore 66. The bore 66 is sized to
receive the rotor and to provide a predetermined air gap 65 between the
inner diameter of the stator 61 and the outer diameter of the rotor 62.
The stator bore 66, as seen in FIG. 2, also has a plurality of receptacles
which define a plurality of winding receiving slots 68. Conventionally,
the slots extend radially outwardly from the bore 66 and communicate with
the bore along one end of the receptacle.
Referring to FIG. 3, there is shown schematically the arrangement of the
low and high-speed set of windings 58 and 60, respectively, wherein 70
represents the first or low-speed motor main winding of set 58 which in
the present case is shown as a four-pole winding, and 72 represents a
second or high-speed motor main winding of set 60 shown in the present
case as a two-pole winding. Cooperating with the main winding 70 is the
four-pole starting winding 74 and cooperating with the main winding 72 is
the two-pole starting winding 76.
It has been determined that under normal operating temperature that the
compressor motor will, in fact, operate in the low speed mode
approximately between 70% and 95% of the time. It is, therefore,
advantageous to design the four-pole low-speed motor windings 60 for
maximum efficiency. The operation of the compressor motor 24, driven by
windings 58 in the two-pole high-speed mode, is usually during the most
adverse conditions when maximum output is required of the system.
By the present invention means are provided to compensate for the fact that
the two-pole high-speed motor windings 58 do, in fact, operate in the most
adverse conditions. To this end, as shown schematically in FIG. 3, the
two-pole motor windings 60 are arranged inwardly in the stator slots
relative to the four-pole low-speed motor windings 58. As mentioned
hereinbefore, the relatively cool suction gas entering the compressor case
through inlet 44 flows through the motor and more, particularly, through
the air gap or spacing 65 between the stator and rotor. Accordingly, with
the two-pole winding in this inner position, a greater portion of the
relatively cool suction gas which is directed over the windings will flow
past the two-pole winding as the gas passes through the motor space or air
gap 65 between the rotor and stator. This extra degree of cooling allows
the present design balance of allocating more winding space to the
four-pole low-speed motor winding 58 with the result being optimal motor
efficiencies for both speeds. Since the four-pole low-speed motor 58
operates in the more favorable conditions wherein motor cooling is not
critical and, therefore, requires less cooling than the two-pole
high-speed motor, it can be arranged at the warmer outside position of the
stator slots 68 where it sees less of the cool suction gas as it passes
through the motor.
Referring now to FIG. 4, there is shown the motor control circuit for the
present two-speed compressor unit 11. While the present motor and control
system shown is of the single phase type, it should be noted that the same
principles of speed and control can be adapted to a three-phase power
system. Single phase electrical power from Lines L1, L2 is supplied to the
two-pole high speed windings 72,76 and the four-pole low speed windings
70,74 through high and low speed contactor switches 78 and 80
respectively. The operation of both contactor switches, as will be
explained hereinafter, is controlled by the low voltage control circuit,
including a two-stage comfort control thermostat 82.
Power to the low-speed windings 70,74 from Line 1 flows through conductor
84 to switch 86 of contactor 80, conductor 106, and thence through the
low-speed motor overload protector 88. From the overload protector 88,
power flows through start winding 74, conductor 91, switch 90, of
contactor 80 and conductor 92 and thence through run capacitor 94 to Line
L2. The circuit through low-speed start capacitor 96 is completed from
conductor 92 through the initially closed contacts 98 of a capacitor
switching relay 100. The circuit through the run winding 70 being
completed through conductor 102, switch 104, of contactor 80 to Line L2.
Relay 100 is energized through conductor 106, 108 to open contact 98 and
remove start capacitor 96 from the circuit momentarily after the motor
starts.
Power to the high speed windings 72,76 from Line L1 flows through conductor
110 to switch 112 of contactor 78, conductor 113, and thence through the
high-speed motor overload protector 114. From the overload protector 114,
the circuit to the run winding 72 is completed through a conductor 115,
switch 117, of contactor 78 to Line L2. Also from the overload protector
114, power flows through start winding 76, conductor 119, switch 116 of
contactor 78, conductor 92, thence through run capacitor 94 to Line L2.
The circuit, through high-speed start capacitor 118, is completed from
conductor 92 through the initially closed contacts 98 of relay 100, switch
120, of the high-speed switching relay 122 which closes switch 120 when,
as will be explained hereinafter, high-speed operation is called for by
thermostat 82. Operation of relay 122 also caused a circuit to be
completed through a high-speed run capacitor 124 through switch 125. It
should be noted that with regard to the start capacitors 96 and 118, that
the relay 122 will close its switch 120 to the start capacitor 118, before
the relay 100 opens switch 98 to remove capacitor 118. Both the start 118
and the run capacitor 124 will be in the circuit through the high-speed
switching relay contacts 120, 125 respectively only when the thermostat
calls for high-speed operation of the compressor. It should be noted that
while in the present instance two-run capacitors are in the circuit during
high speed motor operation, it may be advantageous in certain instances to
have two-run capacitors in the circuit during low-speed motor operation.
It should be apparent from the above description that the high and
low-speed windings in effect function as two separate motors sharing a
common stator.
Another feature of the present invention is the motor protection means
wherein because of the dual high and low-speed winding arrangement,
separate and independent line break protectors 114 and 88 are provided for
each of the two-pole and four-pole motor windings respectively. In effect,
each winding and protector sub-system can be optimized without affecting
the other. This is contrary to the use of temperature-only responsive
pilot duty systems that limit motor temperature by opening the incoming
power at the main line regardless of which winding is overheated because
the systems are not independent. With this present arrangement, it is
possible under certain operating conditions for one of the motor windings
to remain operational when the line break protector associated with the
other set of the motor windings opens its respective circuit. For example,
in the event the line break protector 88 associated with the low-speed
motor windings 70,74 sensing an abnormal condition opens the circuit
thereto, the compressor can operate in the high-speed mode through
windings 72,76 if temperature conditions are such that the thermostat, as
will be explained below, calls for high-speed operation. With the rotor 62
rotating clockwise, as indicated by the arrow in FIG. 3, the line break
protectors 88 and 114 are positioned tangentially relative to the suction
inlet 44 so that the high-speed breaker 114 is downstream of the suction
flow from the low-speed breaker 88. Due to the higher rotor speed of the
motor, the suction gas entering inlet 44 is carried further in the
direction of the arrow in high-speed relative to low-speed. With this
present arrangement, the high speed motor line break protector 114 sees
more of the relatively cool suction gas when the motor is operating at
high speed and, accordingly, allows for a balanced protection between
protectors since they see substantially similar boundary conditions such
as gas flow, motor temperatures and surrounding internal compressor
temperatures. Because of protector design, it is imperative that they be
cooled so that they remain closed under normal operating conditions;
otherwise, the internal energy they generate would be enough to cause them
to self trip.
Reference will now be made to the low voltage control portion of the
circuit wherein a transformer 126 has its primary connected to power Lines
L1, L2. Power to the two-stage thermostat 82 is fed from one side of the
transformer secondary to the first and second stage stationary contacts
128 and 130 of first and second stage switches 132 and 134 respectively.
In cooling, a rising temperature first causes movable contact 136 of first
stage switch 134 to engage contact 130. Closing of switch 134 completes a
low-voltage circuit through the switch 138 of the speed switching relay
140. The switch 138 is normally in the position shown and power flows
through stationary contact 142 to energize the low-speed contactor relay
coil 143 which causes all of the contacts 84, 90 and 104 of the low-speed
contactor 80 to close. Accordingly, power is applied to low-speed motor 60
and the compressor will operate at low speed through low-speed windings 70
and 74 as described hereinabove.
If the temperature continues to rise, it causes movable contact 144 of
switch 132 to engage contact 128. Closing of switch 132 completes a first
low-voltage circuit through a high-speed capacitor switching relay 122
which, as mentioned above, causes closing of switches 120 and 125 to
complete the circuits through the high-speed start and run capacitors 118
and 124 respectively to complete the circuits necessary for high-speed
operation of motor 58 so that the compressor will operate at high speed
through windings 72 and 76 as described. At the same time, a circuit is
also completed from switch 132 through a time delay relay 146. The delay
action of relay 146 causes its switch 148 to close after a predetermined
time delay which, in the present instance, is approximately between thirty
seconds and two minutes. Closing of switch 148 after the appropriate time
delay completes a circuit through the speed switching relay 140, causing
switch 138 to move to stationary contact 150 to energize high speed
contactor relay coil 152 which causes all of the contacts 112, 116 and 117
of the high-speed contactor 78 and, accordingly, the compressor will
operate through motor 58 at high speed through the high-speed windings
72,76 as described above.
The time delay relay 146 and its arrangement in the circuit relative to the
speed switching relay 140 provides a delay in switching from one speed to
the other and at the same time provides mandatory low-speed start
regardless of temperature or thermostat demands. Assuming with the system
shut down that temperature conditions are such that high speed operation
is required and the second stage switch 132 is closed. In this instance at
start up, a circuit is established through relay 146. In this mode the
system would still start at low speed through contact 142 until the
required delay period is over and switch 148 closes to energize relay 140
and move its switch 138 to the high speed contact 150. This mandatory
slow-speed start allows initial slower pressure reduction in the case 11
which results in less or slower boiling off of refrigerant out of the oil.
Starting the compressor at high speed can result in a sudden crank case
pressure drop which can cause a boiling action of the refrigerant. If the
boiling action is violent enough, it will carry off oil with the
refrigerant and/or cause a foaming action and, as a result, adversely
affect the lubricating system of the compressor. In either instance, this
lack of oil available to the compressor bearings can result in bearing
failure. In some instance, some of the oil by or mixed with refrigerant
can be drawn into the cylinder where its inability to compress can also
result in bearing failure.
In the case of two phasing between oil and refrigerant, the heavier
refrigerant is in the bottom of the sump and will be drawn into the
bearings rather than the oil. In the low-speed start, heat build up is
allowed to boil off refrigerant before bearing damage can result. In most
instances, the mandatory low-speed start will eliminate the liquid
refrigerant problems associated with compressor start up and reduce or
eliminate the need for sump heaters.
Another feature of the present control is the ability to switch between
speeds without stopping the motor operation of the compressor. The present
control allows instantaneous switching after a predetermined time delay
without stopping compressor operation. In operation with the compressor
operating at high speed, the second stage thermostat switch 132 may open
and control is through first stage switch 134 calling for low speed. At
this time, the time delay relay 146 is de-energized. Because of the time
delay built into relay 146, the compressor continues to operate at high
speed until the delay period expires, at which time switch 148 opens and
relay 140 is de-energized, causing switch 138 to move from contact 150 to
142 to complete the circuit to the low speed contactor relay 143, at which
time the motor is instantaneously switched to low speed without
interrupting operation of the compressor. Switching from low to high speed
is accomplished in the same manner, with the compressor operating during
the delay period without motor shutdown.
It should be apparent to those skilled in the art that the embodiment
described heretofore is considered to be the presently preferred form of
this invention. In accordance with the Patent Statutes, changes may be
made in the disclosed apparatus and the manner in which it is used without
actually departing from the true spirit and scope of this invention.
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