The present invention relates to a speed-regulated helical screw rotor compressor that is adapted to work against a pressure container whose pressure P lies within the working range of the compressor and which is allowed to vary between a lowest pressure and a highest pressure. The compressor is driven by an electric motor.
Small pressure variations are desirable in such a pressure container or accumulator tank. In the case of a large accumulator tank this can be achieved with a highly frequent start-stop control facility or by regulating the speed (r.p.m.) of the motor.
Compressor speed control is generally used in respect of air compressors that are driven by a high power motor down to a power of 10-30 kW. The compressor speed is controlled with the aid of electronic control means. With regard to compressors that are driven by weaker motors, such as motors of lower power, or much lower power, than 10-30 kW the use of compressor speed control based on electronic circuits cannot be defended economically. This is because the control electronics are extremely expensive in relation to the energy savings resulting from compressor speed control. The aforesaid lower limit of about 10-30 kW in respect of the power of a speed control compressor can, however, be lower with increased energy costs.
A smart way of controlling the pressure in the pressure container is to use in the container a pressure sensor which, via appropriate control means, functions to switch-off the compressor motor when the pressure in the container has reached its maximum value and to switch-on the motor when the container pressure has reached a pre-determined lowest value.
When using typical asynchronous motors, such control will result in rapid filling of the pressure container to a maximum pressure. When consumption is high or when the pressure container is relatively small, the motor will be frequently switched on and off. These frequent motor-starts will shorten the useful life of the motor quite considerably.
In addition to saving energy, the aim of speed control is to enable the buffer tank against which the compressor works to be made much smaller than would otherwise be the case. A compressor whose speed is not controlled will thus require a larger buffer tank and larger tank accommodating space, therewith incurring higher investment costs.
There is a need for small, speed-regulated compressors that can be used in simpler applications, such as with screwdrivers, paint sprays and various other hand tools.
Accordingly, the aim of the present invention is to provide a motor-driven compressor whose motor has a much smaller power than the aforesaid lowest power and the speed of which can be controlled at least within one working range in the absence of expensive control equipment.
This aim is achieved in accordance with the invention with a compressor that is driven by a motor whose speed is significantly dependant on the torque or moment index within a given working range. Preferred embodiments will be apparent from the dependent claims.
The present invention will now be described in more detail with reference to the accompanying drawing, in which
A brief description of the construction and working principle of a helical screw compressor will now be given with reference to
A pair of mutually engaging screw rotors 101, 102 are mounted for rotation in a working space delimited by two end walls 103, 104 and including a barrel wall 105 that extends between said end walls. The barrel wall 105 has a form which corresponds generally to that of two mutually intersecting cylinders, as evident from
The gas to be compressed, normally air, is delivered to a working space of the compressor through an inlet port 108 and is then compressed in V-shaped working chambers formed between the rotors and the walls of the working space. Each working chamber moves to the right in
The pressure in the tank T shall vary between a highest pressure P1 and a lowest pressure P2. The motor M drives the compressor K until the pressure in the tank has reached said highest pressure P1, whereupon the motor M is switched off. When the pressure in the tank T has fallen to the lowest pressure P2, the motor M is restarted and again drives the compressor and therewith deliver compressed air to the tank T. The check valve 3 prevents compressed air from flowing from the tank T back through the compressor K and the inlet line 2.
As a result of this property of the asynchronous motor, the motor will be started when the tank pressure has fallen to the pressure P2, wherewith the compressor begins to compress air. Because of the small increase in speed required to raise the motor torque from M2A to M1A, the compressor will work at almost maximum capacity in this torque range. This results in a rapid increase in tank pressure. A compressor driven by an asynchronous motor will thus result in a short compressor operating time in achieving the desired highest pressure in the tank T. Only a relatively small volume of air responsible in lowering the tank pressure will be consumed during this relatively short period of time. This will result in frequent starting of the motor, in order to keep the tank pressure within the desired pressure range. These moments of frequent starting and stopping of the motor will significantly shorten its useful life, for instance as a result of overheating of the motor windings.
Similar to
As a result of this property of the commutator motor the tank pressure will have fallen to P2k when the motor is started (see
According to one preferred embodiment of the invention there is used a compressor that has a relatively low internal volume factor. By internal volume factor is meant the relationship between the minimum and maximum thread volume enclosed in the helical rotor compressor used. The internal volume factor will preferably be such that the pressure of the compressor K will be less than P2+0.85*(P1−P2) when the thread volume of the working chamber that commences communication with the tank T has its minimum volume. This means that the compressor outlet pressure in given working chamber will be at most equal to the lowest pressure of the tank plus 85 percent of the difference between the highest and the lowest pressure of the tank. The compressor will preferably be optimised for an internal volume factor at which the compressor pressure at the opening instance will be equal to the lowest working pressure P2 in the pressure container. It is particularly preferred that the compressor is optimised in respect of an internal volume factor at which the compressor pressure at the opening instance is lower than the lowest working pressure P2 in the pressure container.
Number | Date | Country | Kind |
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0302739-8 | Oct 2003 | SE | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/SE04/01390 | 9/30/2004 | WO | 11/10/2006 |