Title of Invention

LINEAR MOTOR

Abstract The present invention provides an improved design of linear motor as well as an improved control strategy. The design allows for a shorted stator, where the armature magnets are controlled to reciprocate to a greater maximum displacement than for a equivalent conventional linear motor. The control strategy is such that a minimum of external sensors are required. The linear motor is driven at its resonant frequency ensuring optionally efficient operation. A determination of the maximum current is made based on a relationship with the resonant frequency and the evaporating temperature/pressure of the vapour entering the compressor. The current is then limited to control the maximum displacement to avoid damage.
Full Text "LINEAR MOTOR"
Technical Field
This invention relates to a compact linear motor for free piston
compressors (also called vibrating and linear compressors) for vapour
compression systems.
Background Art
Compressors, for example refrigerator compressors, are conventionally
driven by rotary electric motors. However, even in their most efficient form, there
are significant losses associated with the crank system that converts rotary
motion to linear reciprocating motion. Alternatively a rotary compressor which
does not require a crank can be used but again there are high centripetal loads,
leading to significant frictional losses. A Linear compressor driven by a linear
motor would not have these losses, and can be designed with a bearing load low
enough to allow the use of aerostatic gas bearings as disclosed in US Patent
5,525,845.
Linear reciprocating motors obviate the need for crank mechanisms which
characterise compressors powered by rotating electric motors and which
produce high side forces requiring oil lubrication. Such a motor is described in
US 4,602,174. US Patent 4,602,174 discloses a linear motor design that is
extremely efficient in terms of both reciprocating mass and electrical efficiency.
This design has been used very successfully in motors and alternators that
utilise the Stirling cycle. It has also been used as the motor for linear
compressors. However, in the case of compressors designed for household
refrigerators the design in US 4,602,174 is somewhat larger and more costly
than is desirable for this market.
The piston of a free piston compressor oscillates in conjunction with a
spring as a resonant system and there are no inherent limits to the amplitude of
oscillation except for collision with a stationary part, typically part of the cylinder
head assembly. The piston will take up an average position and amplitude that
depend on gas forces and input electrical power. Therefore for any given input
electrical power, as either evaporating or condensing pressure reduces, the
amplitude of oscillation increases until collision occurs. It is therefore necessary
to limit the power as a function of these pressures.
It is desirable for maximum efficiency to operate free piston refrigeration
compressors at the natural frequency of the mechanical system. This frequency
is determined by the spring constant and mass of the mechanical system and
also by the elasticity coefficient of the gas. In the case of refrigeration, the
elasticity coefficient of the gas increases with both evaporating and condensing
pressures. Consequently the natural frequency also increases. Therefore for
best operation the frequency of the electrical system powering the compressor
needs to vary to match the mechanical system frequency as it varies with
operating conditions.
Methods of synchronising the electrical voltage applied to the compressor
motor windings with the mechanical system frequency are well known. For a
permanent magnet motor used in a free piston compressor, a back electromotive
force (back EMF) is induced in the motor windings proportional to the piston
velocity as shown in Fig 8a. The equivalent circuit of the motor is shown in Fig
8b. An alternating voltage (V) is applied in synchronism with the alternating EMF
(av) in order to power the compressor. US 4,320,448 (Okuda et al.) discloses a
method whereby the timing of the applied voltage is determined by detecting the
zero crossings of the motor back EMF. The application of voltage to the motor
winding is controlled such that the current is zero, at the time at which the EMF
intersects with the zero level to allow back EMF zero crossing detection.
Various methods have been used to limit oscillation amplitude including
secondary gas spring, piston position detection, piston position calculation based
on current and applied voltage (US 5,496,153) measuring ambient and/or
evaporating temperature (US 4,179,899, US 4,283,920). Each of these methods
requires the cost of additional sensors or has some performance limitation.
DISCLOSURE OF THE INVENTION
It is an object of the present invention to provide a compact linear motor
which goes some way to overcoming the abovementioned disadvantages or
which will at least provide the public with a useful choice.
Accordingly in a first aspect the present invention may be said to consist
in an electric linear motor for driving a reciprocatable load comprising:
a stator having a magnetically permeable core with at least one air gap
and shaped to provide regions of substantially spacially uniform and spacially
non-uniform magnetic flux density in said at least one air gap;
an armature having a structure which supports at least one permanent
magnet of which at least a substantial portion is located in at least one of said at
least one air gap;
energisation means for producing an alternating magnetic flux in said
stator which interacts with the magnetic field of said at least one permanent
magnet to produce a reciprocating force on said armature such that at high
power levels at least one end of said at least one permanent magnet passes
outside the region of substantially spacially uniform flux density; and
said armature in use being connected to said load and thereby
reciprocating said load with respect to said stator.
Preferably wherein said energisation means comprises at least one coil
wound around a portion of said stator and a commutation circuit including a
direct current power supply, switching devices connected to said power supply to
supply current to said at least one coil and a programmed digital processor
including memory and input-output ports, at least one of said ports being
connected to said commutation circuit to supply switching control signals thereto.
Preferably wherein the displacement of said at least one permanent
magnet at which said at least one end of said at least one magnet passes
outside said region of substantially spacially uniform flux density is 67% of the
maximum displacement.
In a second aspect the present invention consists in a refrigerator which
uses a compressor characterised in that the compressor and compressor motor
are linear devices and said motor comprises:
a stator having a magnetically permeable core with at least one air gap
and shaped to provide regions of substantially spacially uniform and spacially
non-uniform magnetic flux density in said at least one air gap;
an armature having a structure which supports at least one permanent
magnet of which at least a substantial portion is located in at least one of said at
least one air gap;
energisation means for producing an alternating magnetic flux in said
stator which interacts with the magnetic field of said at least one permanent
magnet to produce a reciprocating force on said armature such that at high
power levels at least one end of said at least one permanent magnet passes
outside the region of substantially spacially uniform flux density; and
said armature in use being connected to said load and thereby
reciprocating said load with respect to said stator.
In a third aspect the present invention may be said to consist in a vapour
compressor comprising:
a piston,
a cylinder,
said piston being reciprocable within said cylinder, the vibrating system of
piston, spring and the pressure of said vapour having a natural frequency which
varies with vapour pressure,
a linear brushless DC motor drivably coupled to said piston having at least
one winding,
a DC power supply,
commutation means for electronically commutating said at least one
winding from said DC supply to provide a supply of current to said at least one
winding to reciprocate said piston,
resonant driving means which initiate commutation of said at least one
winding to thereby drive said piston at the resonant frequency of said vibrating
system,
current controlling means which determine the amount of said supply of
current supplied by said commutation means, said determined amount of current
being related to said resonant frequency, and which initiate commutation of said
at least one winding to thereby limit the amplitude of reciprocation of said piston.
In a fourth aspect the present invention may be said to consist in a
method for driving and controlling the amplitude of the piston in a free piston
vapour compressor wherein said piston reciprocates in a cylinder and wherein
the vibrating system of piston, spring and the pressure of said vapour has a
resonant frequency which varies with vapour pressure, said method using a
linear brushless DC motor having at least one winding and comprising the steps
of:
electronically commutating said at least one winding from a DC supply to
reciprocate said piston, with commutations timed to drive said piston at the
resonant frequency of said vibrating system, limiting the amount of current in
said at least one winding by limiting the value of a parameter which determines
current supply during commutation to a value which is a function of said resonant
frequency.
The "temperature of the vapour entering the compressor" is also referred
to in this specification as the "evaporator temperature". Likewise the "resonant
frequency" is also referred to as the "natural frequency".
To those skilled in the art to which the invention relates, many changes in
construction and widely differing embodiments and applications of the invention
will suggest themselves without departing from the scope of the invention as
defined in the appended claims. The disclosures and the descriptions herein are
purely illustrative and are not intended to be in any sense limiting.
BRIEF DESCRIPTION OF THE DRAWINGS
One preferred form of the present invention will now be described with
reference to the accompanying drawings in which:
Figure 1 is a cross-section of a linear compressor according to the present
invention;
Figure 2 is a cross-section of the double coil linear motor of the present
invention in isolation;
Figure 3 is a cross-section of a single coil linear motor;
Figure 4 is a comparison between a single window prior art linear motor
and a short stator linear motor according to the present invention;
Figure 5 is an illustration of the flux lines due to the coil current in a single
coil linear motor of the present invention;
Figure 6 is a graph of the motor constant versus magnet position for the
preferred embodiment of the present invention;
Figure 7 is a cross-section of a single coil linear motor with partially angled
pole faces;
Figure 8a shows motor piston displacement and back EMF waveforms for
a free piston compressor motor;
Figure 8b shows an equivalent circuit for such a motor;
Figure 9 shows an inverter for electronically commutating a single phase
free piston motor;
Figure 10 shows graphs of maximum motor current as a function of
frequency and evaporation temperature for a motor of the present invention;
Figure 11 is a block diagram of the motor control circuit;
Figure 12 is a graph of RMS motor current versus motor winding current
decay time;
Figure 13 is a flow chart of the motor control timing program;
Figure 14 is a flow chart of commutation time determination using
evaporator temperature and stroke time data; and
Figure 15 shows motor piston displacement and motor current waveforms.
MODE(S) FOR CARRYING OUT THE INVENTION
The present invention provides a linear motor with a number of
improvements over the prior art. Firstly it has a reduced size compared to the
conventional linear motor of the type described in US4602174 and thus reduces
the cost. This change keeps the efficiency high at low to medium power output at
the expense of slightly reduced efficiency at high power output. This is an
acceptable compromise for a compressor in a household refrigerator which runs
at low to medium power output most of the time and at high power output less
than 20% of the time (this occurs during periods of frequent loading and
unloading of the refrigerator contents or on very hot days). Secondly it uses a
control strategy which allows optimally efficient operation, while negating the
need for external sensors, which also reduces size and cost.
While in the following description the control strategy is described in
relation to a cylindrical linear motor it will be appreciated that this method is
equally applicable to linear motors in general and in particular also to flat linear
motors. While the invention is described in relation to a free piston compressor it
could equally be used in a diaphragm compressor.
A practical embodiment of the invention, shown in Figure 1, involves a
permanent magnet linear motor connected to a reciprocating free piston
compressor. The cylinder 9 is supported by a cylinder spring 14 within the
compressor shell 30. The piston 11 is supported radially by the bearing formed
by the cylinder bore plus its spring 13 via the spring mount 25.
The reciprocating movement of piston 11 within cylinder 9 draws gas in
through a suction tube 12 through a suction port 26 through a suction muffler 20
and through a suction valve port 24 in a valve plate 21 into a compression space
28. The compressed gas then leaves through a discharge valve port 23, is
silenced in a discharge muffler 19, and exits through a discharge tube 18.
The compressor motor comprises a two part stator 5,6 and an armature
22. The force which generates the reciprocating movement of the piston 11
comes from the interaction of two annular radially magnetised permanent
magnets 3,4 in the armature 22 (attached to the piston 11 by a flange 7), and the
magnetic field in an air gap 33 (induced by the stator 6 and coils 1,2).
A two coil embodiment of present invention, shown in Figure 1 and in
isolation in Figure 2, has a current flowing in coil 1, which creates a flux that
flows axially along the inside of the stator 6, radially outward through the end stator tooth 32, across the air gap 33,
then enters the back iron 5. Then it flows axially for a short distance 27 before flowing
radially inwards across the air gap 33 and back into the centre tooth 34 of the stator 6. The
second coil 2 creates a flux which flows radially in through the centre tooth 34 across the air
gap axially for a short distance 29, and outwards through the air gap 33 into the end tooth 35.
The flux crossing the air gap 33 from tooth 32 induces an axial force on the radially
magnetised magnets 3,4 provided that the magnetisation of the magnet 3 is of the opposite
polarity to the other magnet 4. It will be appreciated that instead of the back iron 5 it would
be equally possible to have another set of coils on the opposite sides of the magnets.
An oscillating current in coils 1 and 2, not necessarily sinusoidal, creates an
oscillating force on the magnets 3,4 that will give the magnets and stator substantial relative
movement provided the oscillation frequency is close to the natural frequency of the
mechanical system. This natural frequency is determined by the stiffness of the springs13,
14 and mass of the cylinder 9 and stator 6. The oscillating force on the magnets 3,4 creates
a reaction force on the stator parts. Thus the stator 6 must be rigidly attached to the cylinder
9 by adhesive, shrink fit or clamp etc. The back iron is clamped or bonded to the stator
mount 17. The stator mount 17 is rigidly connected to the cylinder 9.
In a single coil embodiment of the present invention, shown in Figure 3, current in
coil 109, creates a flux that flows axially along the inside of the inside stator 110, radially
outward through one tooth 111, across the magnet gap 112, then enters the back iron 115.
Then it flows axially for a short distance before flowing radially inwards across the magnet
gap 112 and back into the outer tooth 116. In this motor the entire magnet 122 has the same
polarity in its radial magnetisation.
In the preferred embodiment of the present invention the length of the armature
(tooth) faces only extends to, for example, 67% of the maximum, stroke (where the edge of
the magnet extends to at maximum power output) of the magnet. This is seen in Figure 4
where a conventional prior art linear motor is visually compared against the present invention
variable constant design of equivalent power output, both at maximum stroke. It can be seen
that the outer edge 200 of the stator tooth does not extend as far as the outer end of the
magnet 201. Similarly the inner edge 203 of the other stator tooth does not extend to the
inner end of the magnet 204. In contrast in the prior art design the edge of the magnet 205
SUBSTITUE SHEET (Rule 26)
docs match up. with the edges of the stator teeth 206,207 at maximum stroke.
At strokes less than, for example, 60% in the present invention the magnet 70 will be
in an area of uniform flux density as indicated by the region "a" to "b" in Figure 5, which
roughly corresponds where the stator teeth 71 extend to. As the stroke increases past 60%
the flux density encountered by the magnet edge 70 reduces as it enters the fringe portion
(non-uniform flux density) of the air gap magnetic field - the area outside "b" in Figure 5.
In a further embodiment shown in Figure 7, a stator for a linear motor is shown with
angled pole face 503. In its centre the pole face 503 has a flat section 500, which results in
the air gap facing that section having substantially uniform flux density. The end of the pole
face 503, is angled to give a more progressive transition from the uniform flux density of the
centre 500, to the fringe portion 502 (non-uniform flux density) at the end of the pole face
503. Similar to the proceeding embodiments the armature magnet 504, would be driven
outside me area of uniform flux density 500, and into the fringe portion 502 of non-uniform
flux density.
The "Motor Constant" is defined as the force (in Newtons) generated on the magnet
by one Ampere in the motor windings. The motor constant curve, shown in Figure 6 shows
how the Motor Constant 300 for the present invention varies with magnet position. Equally
the "Motor Constant" can be defined as the back EMF (in Volts) generated when the magnet
is moving at one metre/second. When the magnet is in the fringe field (outside "b" in Figure
5), because of the reduced magnetic coupling, more current will be required to generate a
given force when compared to that in the uniform flux region (from "a" to "b" in Figure 5).
This results in the "variable" motor constant curve 300 associated with the present invention
short stator linear motor as shown in Figure 6. This contrasts with the "constant" motor
constant curve 301, also seen in Figure 6, inherent in the conventional prior art linear motors.
With the motor constant curve 300 shown in Figure 6 at low and medium strokes
(corresponding to strokes of-3mm to +3mm) it will be apparent the present invention has
a high motor constant relative to an equivalent convention motor 301, (with less turns and
a greater volume of core material). A higher motor constant corresponds to more efficient
operation (due to lower invertor losses), therefore at lower power output the present
invention is more efficient than an equivalent conventional prior art linear motor. It also
SUBSTTTUE SHEET (Rule 26)
reduces the required cross sectional area of the core.
At high strokes the motor constant is low at the times when the current is increasing
most rapidly. This makes it possible to get more current into the motor and thus extract more
power from the motor at maximum strokes as compared to an equivalent conventional prior
art linear motor. Also such a design with a variable constant that is lowest at maximum
stroke tends to make motors driven by square wave voltages more efficient.
Control Strategy
Experiments have established that a free piston compressor is most efficient when
driven at the natural frequency of the compressor piston-spring system. However as well as
any deliberately provided metal spring, there is an inherent gases spring, the effective spring
constant of which, in the case of a refrigeration compressor, varies as either evaporator or
condenser pressure varies. The electronically commutated permanent magnet motor already
described, is controlled using techniques including those derived from the applicant"s
experience in electronically commutated permanent magnet motors as disclosed in US
4857814 and WO 98/35428 for example, the contents of which are incorporated herein by
reference. Those references disclose the control of a 3 phase rotating motor, but the same
control principles can be applied to linear motors. A suitable linear motor need only be a
single phase device and a suitable inverter bridge circuit for powering a motor can be of the
simple form shown in Figure 9.
By monitoring back EMF zero crossings in the motor winding current commutation
can be determined to follow the natural frequency of the piston. Since there is only a single
winding, the current flowing through either upper or lower inverter switching devices 411
or 412 must be interrupted so that back EMF can be measured. Controlling the current
through the motor winding in accordance with detected back EMF ensures current and back
EMF are maintained in phase for maximum system efficiency.
The frequency of operation of the motor is effectively continuously monitored as
frequency is twice the reciprocal of the time between back EMF zero crossings. Furthermore
according to WO 98/35428 the current decay time through free wheel diodes 413 and 414
after commutation has ceased is directly proportional to the motor current and thus a measure
of motor current is available.
The maximum motor current that can be employed before the piston collides with the
SUBSTITUE SHEET (Rule 26)
cylinder head of the compressor varies depending upon the evaporator temperature and the
natural frequency of the vibrating system
Figure 10 shows graphs of maximum permitted motor current against natural
mechanical system frequency and condenser temperatures for different evaporating
temperatures. These show the dependence of maximum motor current on both these
variables. They also demonstrate that condenser temperatures are proportional to mechanical
system frequency and thus maximum current control can be achieved without the need for
measurement of the third variable, condenser temperature.
The motor control circuit according to this invention is shown in Figure 11. It utilises
the observation that mechanical system frequency is related to condenser temperature. In
this invention the back EMF signal induced in the motor windings 1 is sensed and digitised
by circuit 402 and applied to the input of a microcomputer 403 which computes the
appropriate timing for the commutation of current to the motor windings to ensure that the
current is in phase with the back EMF. These commutation timing signals switch an inverter
404 (as shown in Figure 11) which delivers current to the motor windings 401. The
microcomputer 403 also measures the time between back EMF zero crossings and thereby
the period of the EMF waveform. The natural oscillation frequency of the mechanical
system is the inverse of the period of the EMF waveform. The microcomputer 403 therefore
has a measure of this frequency at all times.
The conventional temperature sensor 405 for measuring the evaporator temperature
for defrost purposes is utilised and its output is digitised and supplied as a further input to
microcomputer 403.
According to the present invention one method of limiting maximum motor current
and thus maximum displacement of the piston is for the microcomputer 403 to calculate a
maximum current amplitude for each half cycle of piston oscillation and limit the actual
current amplitude to less than the maximum. WO 98/35428)discloses a method of measuring
motor current in an electronically commutated permanent magnet motor by utilising the
digitised back EMF signal in an unpowered winding to measure the time taken for the current
in the motor winding to decay to zero. Use this technique in the present invention enables
Microcomputer 403 to limit maximum power without the need for dedicated current sensing
or limiting circuitry .The RMS moter current is directly proportional to the time duration of
SUBSTITUE SHEET (Rule 26)
current decay through the "freewheeling" diodes 413 or 414 after the associated inverter
switching device has switched off. The current decay results of course from the motor
winding being an inductor which has stored energy during commutation and which must be
dissipated after commutation has ceased. A graph of RMS moter current against current
decay duration (which is a simplification of Figure 6 is WO 98/35428 is shown in Figure

12.
Another preferred method is to limit the time that the current is commutated on
instead of limiting the maximum current value. Figure 15 shows the current waveform under
such control. This is in effect pulse width modulation (PWM) with only one modulated
current pulse per commutation interval. With this method a delay time from the back EMF
zero crossing is computed to minimise the phase angle between the Motor Current and the
back EMF for maximum efficiency. The invertor switch supplying current is turned off at
a time in the motor half cycle to allow, after a current decay period, time to monitor zero
crossing of the back EMF to determine the start commutation for the next half cycle. The
commutation time is also compared with a maximum commutation time appropriate to the
motor frequency and evaporator temperature to ensure maximum amplitude of the piston
stroke is not exceeded.
A flow diagram of the microcomputer control strategy to implement this method is
shown in Figures 13 and 14. Referring to Figure 13 when the compressor is first powered
(421), or is powered after sufficient time delay to ensure pressures are equalised in the
refrigeration system, the compressor runs at a minimum frequency. The stroke period of
this minimum frequency is measured as Run_Stroke and shown in the microcomputer as
Low_Stroke and a minimum Commutation Time is set for this value (428). For each
subsequent stroke the stroke period is measured and defined as the parameter Run_Stroke
(424). The difference between Run_Stroke and Low_Stroke is computed (431, Figure 14).
This difference is called Period_Index. The Period_Index is used in this sub-routine as an
index pointer in a lookup table of maximum commutation times for different stroke times
(frequencies). This table is called the Pulse_Limit_Value Table. In this instance there are
7 lookup tables (433 to 439) corresponding to 7 ranges of Evaporating Temperature (440 to
465).
The moter control circuit is typically included in a Temperature Control loop in the
SUBSTITUE SHEET (Rule 26)
conventional manner in order to maintain the temperature of the enclosed refrigerated space
of the refrigeration system. This control loop will be setting desired values for the power to
be applied to the motor windings depending on the operating conditions of the refrigeration
system. These values of desired power will correspond to values of commutation time.
These values of Commutation Time are compared on a stroke by stroke basis with the
Pulse_Limit_Value (440, Figure 14). If the Desired value of commutation time is greater
than the Pulse_Limit_Value then the commutation time is limited to the Pulse_Lirnit_Value.
This value sets the Commutation Timer (425) which controls the ON period of the relevant
inverter switching device. As previously explained, Motor current can also be used in a
similar manner to limit power applied to the motor to safe levels, but even where
commutation time is being controlled it is desirable to measure motor current in the manner
previously described and compare it with a stored absolute maximum value (426) which if
exceeded will cause the microcomputer program to reset (427).
Of course other methods of determining maximum commutation time and/or
maximum current value are feasible, for instance if the microcomputer is sufficiently
powerful, for example recent advances in DSP chip technology, these values can be
computed directly without the need for lookup tables.
If the DC power supply Voltage supplied to the inverter bridge of Figure 9 varies
significantly this will result in variation of Motor Current for any given commutation time
which should be allowed for. It may be desirable for maximum accuracy for the
microprocessor to sense this and compensate accordingly.
It will be appreciated that use of the present invention in a refrigerator reduces the
profile, size and weight of the motor compared to that of conventional designs. Also because
the mass of the moving parts is lower than that of a conventional refrigerator compressor:
the level of vibration is reduced,
the noise level is reduced,
the working stresses on the moving parts are reduced.
SUBSTITUE SHEET (Rule 26).
We claim
1. An electric linear motor for driving a reciprocatable load comprising:
a stator having a magnetically permeable core with at least one air
gap and shaped to provide regions of substantially spacially uniform and
spacially non-uniform magnetic flux density in said at least one air gap;
an armature having a structure which supports at least one
permanent magnet of which at least a substantial portion is located in at least
one of said at least one air gap;
energisation means for producing an alternating magnetic flux in
said stator which interacts with the magnetic field of said at least one
permanent magnet to produce a reciprocating force on said armature such that
at high power levels at least one end of said at least one permanent magnet
passes outside the region of substantially spacially uniform flux density; and
said armature in use being connected to said load and thereby
reciprocating said load with respect to said stator.
2. An electric linear motor as claimed in claim 1 wherein said energisation
means comprises at least one coil wound around a portion of said stator and a
commutation circuit including a direct current power supply, switching devices
connected to said power supply to supply current to said at least one coil and a
programmed digital processor including memory and input-output ports, at least
one of said ports being connected to said commutation circuit to supply switching
control signals thereto.
3. An electric linear motor as claimed in claim 1 wherein the displacement of
said at least one permanent magnet at which said at least one end of said at
least one magnet passes outside said region of substantially spacially uniform
flux density is 67% of the maximum displacement.
4. A refrigerator which uses a compressor wherein the compressor and
compressor motor are linear devices and said motor comprises:
a stator having a magnetically permeable core with at least one air
gap and shaped to provide regions of substantially spacially uniform and
spacially non uniform magnetic flux density in said at least one air gap;
an armature having a structure which supports at least one
permanent magnet of which at least a substantial portion is located in at least
one of said at least one air gap;
energisation means for producing an alternating magnetic flux in
said stator which interacts with the magnetic field of said at least one permanent
magnet to produce a reciprocating force on said armature such that at high
power levels at least one end of said at least one permanent magnet passes
outside the region of substantially spacially uniform flux density; and
said armature in use being connected to said load and thereby
reciprocating said load with respect to said stator.
5. A vapour compressor comprising:
a piston,
a cylinder,
said piston being reciprocable within said cylinder, the vibrating
system of piston, spring and the pressure of said vapour having a natural
frequency which varies with vapour pressure,
a linear brushless DC motor drivably coupled to said piston having
at least one winding,
a DC power supply,
commutation means for electronically commutating said at least
one winding from said DC supply to provide a supply of current to said at least
one winding to reciprocate said piston,
resonant driving means which initiate commutation of said at least
one winding to thereby drive said piston at the resonant frequency of said
vibrating system,
current controlling means which determine the amount of said
supply of current supplied by said commutation means, said determined amount
of current being related to said resonant frequency, and which initiate
commutation of said at least one winding to thereby limit the amplitude of
reciprocation of said piston.
6. A vapour compressor as claimed in claim 5 wherein further comprising:
a sensor for measuring a property of the vapour entering the
compressor which is an indicator of the pressure,
and wherein said determined amount of current also being related
to said measured indicative property.
7. A vapour compressor as claimed in claim 6 wherein said sensor measures
a property of the vapour entering the compressor which is an indicator of the
pressure on evaporation.
8. A vapour compressor as claimed in claim 7 wherein said resonant driving
means comprising:
back EMF detection means for sampling the back EMF induced in
said at least one winding when commutation current is not flowing and for
detecting back EMF zerocrossings and producing timing signals derived
therefrom, and resonant commutation means which initiate commutation of said
at least one winding in response to said zero crossing timing signals to thereby
drive said piston at the resonant frequency of said vibrating system.
9. A vapour compressor as claimed in claim 8 further comprising
current detection means for measuring the current flowing in said at
least one winding during commutation,
wherein said current controlling means terminates commutation
when said measured current reaches said determined amount of current.
10. A vapour compressor as claimed in claim 9 wherein said commutation
means includes switching devices connected to said DC power supply to supply
current to said at least one winding and unidirectional current devices which
supply a current path to dissipate energy stored in each winding after supply of
current through a switching device has terminated, and said current detection
means comprises:
a programmed digital processor including memory and input-output
ports, a first port being connected to the output of said back EMF detection
means and a second group of ports being connected to said commutation
means to supply switching control signals thereto,
software stored in said memory to cause said processor to
determine a measure of motor current based on intervals between those zero
crossings of said back EMF, which represent the duration of a current pulse
produced in said at least one winding due to dissipation of stored energy by said
unidirectional current devices after supply of current has been removed from
said at least one winding.
11. A vapour compressor as claimed in any one of claims 5 to 9 wherein said
current controlling means further comprises:
means for measuring said resonant frequency,
a memory which stores at least one look up table containing
maximum current commutation values for each of a plurality of resonant
frequencies for said vibrating system, and
value selection means for selecting the value in said table which
corresponds to said measured resonant frequency and for supplying same to
said commutation controlling means.
12. A vapour compressor as claimed in either claims 6 or 7 wherein said
current controlling means further comprising:
means for measuring said resonant frequency,
a memory which stores a plurality of look up tables stored in said
memory containing maximum current commutation values for each of a plurality
of resonant frequencies for said vibrating system, each look up table
corresponding to a nonoverlapping range of said indicative property,
table selection means for selecting a look up table to use on the
basis of the measured value of said indicative property, and

value selection means for selecting the value in said table which
corresponds to said measured resonant frequency and for supplying same to
said commutation controlling means.
13. A vapour compressor as claimed in any one of claims 5 to 9 wherein said
current controlling means includes a processor storing instructions which when
executed calculate said determined amount of current based on a
mathematically expressible relationship to at least said measured resonant
frequency and optionally said measured indicative property.
14. A method for driving and controlling the amplitude of the piston in a free
piston vapour compressor wherein said piston reciprocates in a cylinder and
wherein the vibrating system of piston, spring and the pressure of said vapour
has a resonant frequency which varies with vapour pressure, said method using
a linear brushless DC motor having at least one winding and comprising the
steps of:
electronically commutating said at least one winding from a DC
supply to reciprocate said piston, with commutations timed to drive said piston at
the resonant frequency of said vibrating system, limiting the amount of current in
said at least one winding by limiting the value of a parameter which determines
current supply during commutation to a value which is a function of said resonant
frequency.
15. A method as claimed in claim 14 further comprising the step of measuring
a property of the vapour entering the compressor which is an indicator of the
pressure, wherein said selected maximum current commutation value is also a
function of said measured indicative property.
16. A method as claimed in claim 15 wherein said measured indicative
property is an indicator of the pressure on evaporation.
17. A method as claimed in any one of claims 14 to 16 wherein said step of
driving said piston at the resonant frequency of said vibrating system comprises
the steps of:
unpowering said at least one winding at various intervals and detecting
zerocrossings of the back EMF induced in said at least one winding, using the
zero-crossing timing information to initiate commutation of said at least one
winding to thereby drive said piston at the resonant frequency of said vibrating
system.
18. A method as claimed in claim 17 wherein said step of electronic
commutation comprises using switching devices connected to said DC power
supply to supply current to said at least one winding and unidirectional current
devices which supply a current path to dissipate energy stored in each winding
after supply of current through a switching device has terminated, measuring
motor current based on intervals between those zero crossings of said back
EMF, which represent the duration of a current pulse produced in said at least
one winding due to dissipation of stored energy by said unidirectional current
devices after supply of current has been removed from said at least one winding,
and terminating commutation when said measured current reaches said
determined amount of current.
19. A method as claimed in either claims 15 or 16 further comprising a step of
measuring a property of the vapour entering the compressor which is an
indicator of the pressure on evaporation, wherein said maximum current
commutation value is selected from one of a set of look up tables containing
maximum current commutation values for each of a plurality of resonant
frequencies for said vibrating system and selecting the value which corresponds
to the measured resonant frequency, each look up table corresponding to a non-
overlapping range of said indicative property and being selected on the basis of
the measured value of said indicative property.
20. A method according to claim 19 wherein said parameter which is limited is
the magnitude of the current and said look up tables store maximum current
values.
21. A method according to claim 19 wherein said parameter which is limited is
the duration of commutation and said look up tables store maximum
commutation duration values.
22. A vapour compressor according to claim.5 wherein instead of said piston
and said cylinder said compressor is a diaphragm type compressor.
23. A method according to claim 14 wherein instead of said piston and said
cylinder said compressor is a diaphragm type compressor.
24. A method as claimed in claim 14 further comprising the steps of
measuring a property of the vapour entering the compressor which is an
indicator of evaporating pressure, and calculating said value of said parameter
which determines current supply during commutation from said resonant
frequency and said vapour property.
25. A method as claimed in claim 24 wherein said measured vapour property
is the vapour temperature.
26. A method according to claim 25 wherein said parameter which determines
current supply during commutation is the magnitude of the current in said at least
one winding.
27. A method according to claim 25 wherein said parameter which determines
current supply during commutation is the duration of the current in said at least
one winding.
The present invention provides
an improved design of linear motor as well as an
improved control strategy. The design allows for
a shorted staler, where the armature magnets are
controlled to reciprocate to a greats maximum
displacement than for a equivalent conventional
linear motor. The control strategy is such that a
minimum of external season are required. The
Linear motor is driven at its resonant frequency
ensuring optionally efficient operation. A
determination of the maximam current is made
based on a relationship with the resonant frequency
and the evaporating temperature/presure of the
vapour entering the compressor The current is then
limited to control the maximam displacement to
avoid damage.

Documents:

in-pct-2001-1278-kol-granted-abstract.pdf

in-pct-2001-1278-kol-granted-assignment.pdf

in-pct-2001-1278-kol-granted-claims.pdf

in-pct-2001-1278-kol-granted-correspondence.pdf

in-pct-2001-1278-kol-granted-description (complete).pdf

in-pct-2001-1278-kol-granted-drawings.pdf

in-pct-2001-1278-kol-granted-examination report.pdf

in-pct-2001-1278-kol-granted-form 1.pdf

in-pct-2001-1278-kol-granted-form 13.pdf

in-pct-2001-1278-kol-granted-form 18.pdf

in-pct-2001-1278-kol-granted-form 2.pdf

in-pct-2001-1278-kol-granted-form 3.pdf

in-pct-2001-1278-kol-granted-form 5.pdf

in-pct-2001-1278-kol-granted-gpa.pdf

in-pct-2001-1278-kol-granted-letter patent.pdf

in-pct-2001-1278-kol-granted-reply to examination report.pdf

in-pct-2001-1278-kol-granted-specification.pdf

in-pct-2001-1278-kol-granted-translated copy of priority document.pdf


Patent Number 213994
Indian Patent Application Number IN/PCT/2001/1278/KOL
PG Journal Number 04/2008
Publication Date 25-Jan-2008
Grant Date 23-Jan-2008
Date of Filing 04-Dec-2001
Name of Patentee FISHER & PAYKEL LIMITED
Applicant Address 78 SPRINGS ROAD, EAST TAMAKI, AUCKLAND
Inventors:
# Inventor's Name Inventor's Address
1 DUNCAN, GERALD DAVID 42 HENLEY ROAD MT. EDEN, AUCKLAND
2 BOYD, JOHN HENRY 57 FOREST HILLS DRIVE HOLLAND, MI 49424-2531
PCT International Classification Number H 02 K 33/00
PCT International Application Number PCT/NZ00/00105
PCT International Filing date 2000-06-21
PCT Conventions:
# PCT Application Number Date of Convention Priority Country
1 336375 1999-06-21 New Zealand