The present invention relates to a system and a method that enable one to control the stopping (braking) behavior of a reciprocating compressor.
Hermitic compressor of reciprocating type comprise rod-crank-and-piston type with reciprocating movement and are widely used in the cooling-equipment, household and commercial industry.
Reciprocating compressors may be of the fixed-capacity type, wherein the control of two fixed-velocity states (ON/OFF) is carried out upon turning on the compressor at a maximum temperature and turning off the compressor at a minimum temperature, or varying-capacity compressors, wherein the control is carried out by some electromechanical device or electronic circuit, capable of responding to a programming dependent upon variables to be controlled on the cooling equipment, as for instance the inner temperature of the compartments, wherein the compressor acts in reciprocating operation cycles at varying velocities and stop.
During the periods of operation, the reciprocating compressors are responsible for circulating the cooling gas through the cooling circuit, the rod-crank-and-piston mechanism being responsible for carrying out cyclic movements in which the piston raises the gas pressure during its advance and the cooling gas applied a contrary stress onto the mechanism and to the turning axle. This stress on the piston and the consequent reaction on the mechanism and turning axle varies significantly throughout a turn of the turning axle, the variation being directly proportional to the values of cooling-gas pressure (the greater the difference between the pressures of evaporation and of condensation of the cooling circuit, the greater it is).
Thus, with cooling equipment that uses reciprocating compressors, at the moments when the compressor is turned off the mechanism still turns due to the inertia of the assembly, mainly the inertia of the motor rotor, which imposes the turning movement. The inertia movement causes a jolt during the stopping of the compressor due to a contrary impulse on the piston, caused by the different in pressure of the gas. The impulse is caused by the abrupt stopping of the axle or by the turning movement in an opposite direction at the last turn of the axle because the piston is not capable of overcoming the pressure. Thus, the gas is compressed and uncompressed in an alternating movement, which may cause problems to the reciprocating compressor.
Because of this, the stopping jolt is typical in reciprocating compressors for cooling. Generally, one designs suspension-spring systems inside the compressor, which support the whole assembly, so as to absorb impulses and attenuate them, and not cause problems, such as spring breaks or stopping noises due to shocks between parts. The greater the difference in pressure under which the compressor is operating, the greater the stopping impulses will be.
One of the engineering solutions to the jolt problem when the compressor is stopping is a balanced design of the suspension springs. The main function of the suspension springs is to attenuate the transmission of the vibrations generated during the normal operation in the pumping system due to the reciprocating movement of the piston, thus preventing these vibrations from passing on to the outer compressor body and, as a result, to the cooler, which causes noises. In this way, the springs should then be soft enough to attenuate the normal-functioning vibration, besides absorbing the stopping impulse. On the other hand, the springs should not be designed to be excessively soft to the point of allowing a long displacement of the assembly during this stopping impulse, since this may cause shocks at the mechanical stops, raising noises. Similarly, the design should be adopted so as not to cause excessive stress on the springs to the point of causing fatigue or breakage thereof.
It is possible to note that the stopping jolt is more intense on compressors that operate with greater differences in pressure and on compressors that have smaller inner mass of their components. Besides, factors linked to the pressure condition and to the assembly mass make it difficult to design the suspension springs, and the more one wants to attenuate the normal-operation vibration the higher this project will be, especially in operation at low rotations. Because of this, one encounters even more severe contour conditions, which are difficult to be met.
In deigns where there are severe pressure conditions, optimization of the assembly weight and the need to reduce considerably the vibration level in low-rotation operation, the solution to the spring design may not meet all the desired conditions.
Therefore, it is a first objective of this invention to provide a system and a method for reducing the rigidity of the springs of the suspension system, thus minimizing the vibration level during normal operation.
It is another objective of this invention to provide a system and a method that are capable of reducing the demand for robustness of the suspension system, maintaining the level of reliability and useful life of the springs, by preventing breakage thereof.
A further objective of this invention is to provide a system and a method that are capable of enabling the compressor to operate in conditions of high difference in pressure, under which it can be turned off without undesired impacts and noises being generated.
The objectives of the invention are achieved by means of a control system for cooling compressors, the system comprising at least one electronic control and one reciprocating compressor, which comprises at least one mechanical assembly that has at least one compression mechanism and one motor, the control system being configured to detect a rotation velocity of the compression mechanism and apply a braking torque to the mechanical assembly after detecting that the turning velocity is below a velocity level.
Additionally, one further proposes a control method for a hermetis compressor for cooling, comprising the steps of:
(a) detecting a turning velocity of a mechanical assembly, which comprises at least the compression mechanism and a motor;
(b) comparing the turning velocity with a velocity level; and
(c) applying a braking torque for decelerating the mechanical assembly if the detection indicates that the turning velocity is below a velocity level.
The present invention will now be described in greater detail with reference to the following figures:
As represented in
The mechanical vibrations generated by the compression mechanism 8, due to the unbalancing and torque variation, are filtered by the suspension springs 11. For this reason, the suspension springs 11 are projected so as to have a low elasticity coefficient (that is, as soft as possible), in order to increase the effectiveness of vibration filtration. However, this design increases the amplitude of the oscillation transient and displacement of the mechanical assembly 12 during the stop of the reciprocating compressor 3, if the suspension springs 11 are made to soft, being capable of causing mechanical shocks between the mechanical assembly 12 (drive and compression) against the housing 17 of the reciprocating compressor 3, generating acoustic noise and possible fatigues or breaks of the suspension springs 11.
As can be seen in
Thus, the turning axle 10 loses turn velocity 23 quickly, that it, a high deceleration (rpm/s) takes place, which causes a reverse impulse in the compression mechanism 8 at the impulse moment 24. The deceleration of the compression mechanism 8 in a very short period of time drives the whole mechanical assembly 12 and may cause the turning axle 10 to turn in the opposition direction. The kinetic energy of the turning axle 10 depends on the rotation (squared) and on the inertia of the turning axle 10. The reverse impulse that takes place at the abrupt stop causes a strong impulse on the mechanical assembly 12 and, in this way, causes a large displacement and possible mechanical shock between mechanical assembly 12 and housing 17, thus causing noise and fatigue of the suspension springs 11.
Preferably, this detection is made by the electronic control 2, which detects the time between the changes of rotor position. As can be seen ion
The application of the braking torque 36 may be made in various ways. Preferably one employs the methods of adding a resistance between the windings of the motor 9, which causes the current generated by the movement of the motor 9 to circulate ion a closed circuit and generates a torque contrary to the motion (which may also be carried out by means of a PWM modulation of the inverter that controls the motor 9), or the application of a current contrary to that applied to the motor 9 when it is in operation.
This following 35 following the velocity level 34 comprises much of the last turn of the turning axle 10, beginning a braking period 37 of the turning axle 10. In this way, one prevents the last compression cycle from taking place, thus preventing also a strong reverse impulse on the compression mechanism 8. In this way, the deceleration of the turning axle 10 takes place and is distributed throughout the last turn in a controlled manner, resulting in a deceleration value (rpm/s) that is substantially lower than the one observed in the present-day art. In order for this event to take place, the rotation velocity level 34 of the turning axle 10 should preferably be sufficient for the kinetic energy stored on the turning axle 10 of the reciprocating compression 3 to be capable of completing a complete compression cycle, thus preventing the sudden deceleration and jolt of the compression mechanism 8.
Thus, the present invention enables the suspension springs 11 of the mechanism 12 to be designed so as to have low elasticity coefficient, being very effective to filter vibration, and still prevents shocks of the mechanical assembly 12 with the housing 17 of the reciprocating compressor 3. Besides, the present invention prevents high displacement of this mechanical assembly 12 during the stopping transient, minimizing the mechanical stress and fatigue caused to the suspension springs 11.
Therefore, the present invention defines a system and a method that reduces significantly (or even eliminates) jolts on the mechanical assembly of the compressor during its stop, by means of controlled deceleration of the rod-crank-and-piston assembly throughout the last turn of the turning axle, this preventing the piston from decelerating abruptly during the last incomplete gas compression cycle and also preventing the production of a high impulse with torque.
A preferred example of embodiment having been described, one should understand that the scope of the present invention embraces other possible variants, being limited only by the contents of the accompanying claims, which include the possible equivalents.
Number | Date | Country | Kind |
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1100026 | Jan 2011 | BR | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/BR2012/000014 | 1/25/2012 | WO | 00 | 11/12/2013 |
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WO2012/100313 | 8/2/2012 | WO | A |
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Number | Date | Country | |
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20140072451 A1 | Mar 2014 | US |