Method and apparatus for vibration compensation in a piston compressor

The invention relates to a method and to an apparatus for vibration compensation in a piston compressor, the piston compressor of which is driven by means of a crankshaft by a three-phase motor controlled by a frequency converter, wherein the current position the crankshaft of the piston compressor is determined, and based on this the frequency converter, a torque (MM) for the three-phase motor is predetermined, which torque follows the load torque (ML) of the piston compressor in order to reduce the vibration stimulation of the entire piston compressor.

PRIORITY CLAIM

This patent application is a U.S. National Phase of International Patent Application No. PCT/EP2017/063769, filed Jun. 7, 2017, which claims priority to German Patent Application No. 10 2016 111 101.5, filed Jun. 17, 2016, the disclosures of which are incorporated herein by reference in their entirety. METHOD AND APPARATUS FOR VIBRATION COMPENSATION IN A PISTON COMPRESSOR.

FIELD

Disclosed embodiments relate to a method and an apparatus for vibration compensation in a piston-type compressor, the piston compressor of which is driven by means of a crankshaft by a three-phase motor or the like controlled by a frequency converter. More specifically, disclosed embodiments also relate to a piston-type compressor that is equipped with such an apparatus.

BACKGROUND

In transportation vehicles, in particular rail vehicles, in principle, the installation space in vehicles is limited; as a result, quite compact piston-type compressors are usually used for this, and an electric motor is directly flange-mounted on the usually multi-stage piston compressor, in order to drive the piston compressor. The load moment MLof a piston compressor, acting together with the torque MMof a driving motor, produces an excitation moment about the axis of rotation of the entire piston-type compressor, which leads to undesired rotational vibrations.

SUMMARY

Disclosed embodiments provide a method and an apparatus for vibration compensation in a piston-type compressor that allow effective suppression of vibration in every operating situation of the piston-type compressor by simple technical means.

Disclosed embodiments provide a process for vibration compensation, wherein a first the current position of the crankshaft of the piston compressor is determined, and, based on this, a torque MMthat follows the load moment MLof the piston compressor, i.e. corresponds to it, is prescribed by a frequency converter for the driving three-phase motor in order to reduce the vibration excitation of the piston-type compressor as a whole. Since the vibration excitation of the piston-type compressor arises from the difference between the torque MMof the driving motor and the load moment ML, the resultant vibration excitation can be eliminated by a closed-loop control based on the solution according to the disclosed embodiments. Flywheel masses between the motor and the piston compressor can be made smaller or can be dispensed with entirely.

DETAILED DESCRIPTION

Since in the case of the piston-type compressors of the type of interest here, the torque MMof the motor follows the load moment MLof the piston compressor with a time delay, the excitation moment increases in an unfavorable way.

DE 100 58 923 A1 discloses a piston-type compressor of the type in question, the multi-stage piston compressor of which is driven by an electric motor directly flange-mounted on it. The piston-type compressor is fastened upright on the chassis of the vehicle by way of a number of vibration-damping wire cable springs, in order to reduce the transfer of vibration from the piston-type compressor to the vehicle.

EP 1 242 741 A1 also describes the problem of vibration excitation of piston-type compressors due to the load moment MLand motor torque MMand measures for reducing vibration that lead to types of design of two-stage piston-type compressors with reduced vibration excitation. In order to minimize the influence of the motor on the vibration excitation, flywheel masses that counteract vibration excitation were used between the motor and the piston compressor. However, this technical solution causes a corresponding expenditure of material and produces an associated increase in weight.

In practice, piston compressors are usually operated by three-phase motors, to which a frequency converter is assigned. With the aid of the frequency converter, the piston compressor can be controlled with variable speed, in order in particular to obtain production of compressed air appropriate for requirements as part of a corresponding closed-loop control, while taking into consideration minimum switch-on times, intervals in intermittent operation and the like.

Frequency converters, in particular those designed for operating rail vehicles, have so far been quite complicated in their structural design and especially quite large. Furthermore, these so-called auxiliary power converters on rail vehicles do not just supply power to a single electrical load, but to a number of loads, such as for example also air-conditioning systems, traction fans, equipment fans, compressors and the like. It has therefore not been possible so far for such a commonly used auxiliary power converter to be designed just for one single load.

Further developments of converter technology and great availability of power electronic components used in this technology mean that there are currently boundary conditions that allow frequency converters to be assigned directly to a drive and also to be placed there.

With this understanding in mind, disclosed embodiments provide a method and an apparatus for vibration compensation in a piston-type compressor that allow effective suppression of vibration in every operating situation of the piston-type compressor by simple technical means.

A three-phase motor used as part of the solution according to the disclosed embodiments includes a three-phase asynchronous motor or a synchronous reluctance motor. Optionally, the torque MMprescribed for the three-phase motor corresponds to the load moment profile including a phase length. However, it is also conceivable that the torque MMprescribed for the three-phase motor corresponds to the first order of the load moment profile. Tests have shown that a vibration compensation method that is quite easy to implement but very effective is in fact that of just recreating the component of the first order in the motor torque MM. Higher orders are in this case ignored. The basis for this is the resilient mounting of the piston-type compressor. This mounting is designed such that excitations above a certain frequency are kept away from connecting structures. This has proven to be sufficient under these circumstances. Higher orders are largely kept away from the resilient mountings. For this reason, it is sufficient to eliminate vibration excitations up to and including the first order by the method according to the disclosed embodiments.

It is similarly sufficient if the deviation of the load moment MLof the piston compressor following the torque MMfor the three-phase motor is set in such a way that it is less than 30%. Within this deviation range, the torque MMof the three-phase motor only approximately follows the load moment MLof the piston compressor, which nevertheless produces effective vibration compensation. It has been found under all the structural boundary conditions that the entire vibration behavior can be improved by the electronic compensation according to the disclosed embodiments by up to 70%, while the vibration displacements of the piston-type compressor are significantly reduced, in particular at low rotational speeds.

According to a further optional measure that improves the disclosed embodiments, it is proposed that, to compensate for fluctuations in speed, the torque MMgenerated by the three-phase motor is produced by a variation of the feed voltage and/or a variation of the pulse width in the converter. Consequently, for example, an increase of the torque MMcan be achieved by the pulse width being increased for a short time. In this way, the pulsating load moments usually produced by the piston compressor are smoothed within the compressor, so that the vibration excitation caused by this is minimized further. Since the torque MMof the three-phase motor is proportional to the motor current, a torque compensation is achieved by a counteracting control of the motor current. The torque peak can be compensated by a corresponding control of the IGBT pulse width, and consequently by a motor current changed in this moment. Correspondingly quick control and a stable intermediate-circuit voltage are required for this so-called “space vectoring modulation”.

Optionally, an increase of the torque MMfor the three-phase motor can be carried out by the frequency converter in an easy way by a corresponding increase of the operating voltage. A control unit provided for carrying out the method according to the disclosed embodiments for vibration compensation may advantageously be integrated directly in the frequency converter. The frequency converter itself is optionally arranged directly on the three-phase motor in order to ensure easy connection to the three-phase source. Furthermore, this electronic structural unit may also have at least one sensor input, in order to connect to it a position sensor arranged in the region of the motor shaft or the crankshaft for measuring the current angular position. Optionally, the torque requirement that is to be adjusted according to the rotational speed is stored in the logic of the control unit implemented in the frequency converter.

FIG.1shows a piston-type compressor substantially consisting of a piston compressor1and a three-phase motor2. The piston compressor1is formed as a two-stage compressor unit and here comprises two low-pressure cylinders3a,3band a high-pressure cylinder4. Coming from the atmosphere, the compressed air is first pre-compressed in the low-pressure cylinder3a,3band then brought to an even higher pressure level by the high-pressure cylinder4, before this compressed air that is produced is passed on for further use in the vehicle.

For actuating the piston drive of pistons—not shown any further—of the cylinders3a,3band4, the piston compressor1has a crankshaft5, which is driven by the three-phase motor2. The electrical three-phase motor2is equipped with a frequency converter6, by way of which the connection to a three-phase system7is made. The frequency converter6is assigned an electronic control unit8, which is structurally integrated in it. On the input side, the electronic control unit8receives the measurement signal of a position sensor9, which is arranged in the region of the crankshaft5and prescribes the current angular position of the crankshaft5to the electronic control unit8.

FIG.2shows in a graphic representation the torque profile with respect to a complete revolution of 0 to 360° of the crankshaft of a piston compressor of the prior art. The average torque of the drive is at approximately 50 Nm (dotted line). It can be seen in the profile of the load moment MLthat, on account of a pressure peak at an angular position of the crankshaft of about 200°, it has a maximum of approximately 140 Nm. The profile of the load moment MLthat is shown is characteristic of two-stage piston compressors, as illustrated inFIG.1. The motor only responds to the dominant pressure peak after a time delay and, as can be seen, only builds up the motor torque MMwith a phase offset at an angular position of the crankshaft of about 0°. Consequently, the maximum motor torque MMof about 75 Nm only comes into effect when the load moment MLof the piston compressor has already fallen, here has even reached its minimum. Due to this effect, depending on their type of design, three-phase motors even increase the rotational vibration excitation in interaction with the piston compressors driven by them. The dominant pressure peak of the load moment MLof about 150 Nm results from the compression of the second stage, to be specific the high-pressure cylinder. The three-phase drive responds to this pressure peak and builds up its torque MMof the profile shown. The area between the load moment MLand the torque MMof the motor is marked here by hatching and represents a measure of the vibration excitation around the crankshaft of the piston compressor. Because of the hatched area having quite a large area content, a relatively great disadvantageous vibration excitation is to be assumed.

FIG.3shows the torque profile of the torque MMof the motor and of the load moment MLof the piston compressor for a full revolution of the crankshaft as a consequence of the vibration compensation according to the disclosed embodiments. In the case of this embodiment, the control of the motor takes place in such a way that its torque MMfollows the load moment MLof the piston compressor. This has the result that the area content of the area between the load moment MLand the motor torque MMis minimal as compared with the prior-art embodiment explained above, so that a very small vibration excitation takes place. This is so because, on account of the control according to the disclosed embodiments, the driving motor builds up its torque MMsynchronously and to this extent in a requirement-controlled manner with respect to the load moment MLof the piston compressor that is to be handled. Because there are only minimal non-uniformities, there is a similarly minimal vibration excitation.

FIG.4illustrates as a consequence of this a uniform profile of the rotational speed n of the crankshaft over the entire revolution. This also corresponds approximately to the average profile of the rotational speed n′.

FIG.5shows the time-based profile of the phase currents with respect to the three phases of the three-phase motor, which, on account of the almost complete control-system vibration compensation, also turns out here to be quite a uniform respective sine curve.

FIG.6illustrates with regard to the second embodiment the torque profile of the torque MMand of the load moment MLfor a full revolution of the crankshaft, though, by contrast with the embodiment described above, here there is only a compensation with regard to the first order of the load moment profile of the piston compressor by the torque MMof the three-phase motor. This has the result that, in comparison with the prior art explained above, a much smaller and uniformly distributed area content as a hatched area between the curves of the profile of the motor torque MMof the rotational speed motor and the load moment MLof the piston converter contributes to a vibration excitation. The vibration compensation achieved in this way can be regarded as sufficient for the application that is the subject of the disclosed embodiments.

FIG.7shows as a consequence of this that the rotational speed n of the crankshaft only fluctuates slightly about the average speed n′. A further uniformity of the speed profile can therefore be achieved here by the compensation of the first order of the load moment profile of the piston compressor.

FIG.8accordingly shows the time-based profile of the phase currents of the three phases of the three-phase motor, which, by contrast with the almost complete compensation of the disclosed embodiments that is discussed above, does in fact reveal a slight non-uniformity. Nevertheless, the phase current profile stays within narrow limits, which demonstrates the effect of the solution according to the disclosed embodiments according to the second embodiment.

The disclosed embodiments are not restricted to the specific embodiments described above. Rather, modifications thereof that are included within the scope of the following claims are also conceivable. For example, instead of a two-stage piston-type compressor, it is also possible to also equip a single-stage piston-type compressor with the control-system vibration compensation according to the disclosed embodiments.

LIST OF DESIGNATIONS