What is VVD VFD
When regulating the speed
1 Maamar Bouchareb John Philip Gibson Frank Lubich Speed regulation of refrigeration compressors with intelligent frequency converters Design criteria and experience The regulation of the suction pressure through stepless speed adjustment of standard refrigeration compressors has many advantages. With a correct design, features such as improved storage quality of the refrigerated goods, energy savings, improved behavior in the partial load range and increased service life of the compressors can easily be achieved. Variable-speed operation of refrigeration compressors with intelligent frequency inverters design considerations and experience The control of suction pressure by varying the speed of refrigeration compressors provides many advantages. If the installation is designed correctly then important considerations such improved quality of stored goods, energy saving, improved performance at light refrigeration loads and increased working life can be easily achieved. This part contribution deals mainly with the electrical and control considerations. A following contribution will deal with important refrigeration aspects of the design of the refrigeration installation. Keywords: refrigeration, compressor, frequency inverter, inverter, control, compressor pack, energy M. Bouchareb, Dr. J. P. Gibson, KIMO Industrie-Elektronik GmbH, Erlangen, F. Lubich, Lubich Kälte-Klima-Umwelttechnik-Solar, Pfaffenhofen / Ilm When regulating the speed, refrigeration compressors are operated at an operating point for which they were initially not intended by the manufacturer. The detailed observation of electrotechnical and refrigeration design criteria is very important for reliable operation. In this essay, compound systems with a variable speed compressor are considered as a master compressor in parallel with some fixed speed compressors, see Figure 1. For a better overview, the following abbreviations are used: VvD: variable speed compressor VfD: fixed speed compressor FU: frequency converter ( frequency) inverter) In the systems described here, the speed change is carried out by an electronic frequency converter. An explanation of how it works is e.g. described in . This concept is based on the use of intelligent FUs. These FU types are able to take over regulation and control tasks in the system network. The main function of the intelligent frequency converter is to keep the suction pressure constant by continuously adjusting the speed of the VvD. As soon as the cooling capacity of the VvD is insufficient, a VfD is switched on, see Figure 2. The system adjusts itself automatically according to the cooling requirement. A typical system with this technology is shown in Figure 4. It is basically possible to provide two or more VVDs in one refrigeration circuit. Such systems do not have any significant advantages in practice, so we will not consider such systems here. Advantages The infinitely variable adjustment of the performance of a compound system by regulating the speed of the VvD essentially brings the following advantages: Improved cooling quality by maintaining a constant suction pressure * (see Fig. 3) Larger setting range for regulating the cooling capacity + (see Fig. 5) Increasing the output by increasing the speed of the VvD + (see Fig. 6) Energy savings * Increase in the service life of the compressors + Better options for monitoring, remote setting and diagnosis + Setting range for regulating the cooling capacity with a frequency converter regulated master compressor) + 2 x VfD. In almost all operating points, it is possible to provide the required cooling capacity without the need for frequent cycling of the compressors. This has decisive advantages: Fluctuations in the suction pressure due to switching on and off a VfD are significantly minimized, see Figure 3. The switching frequency of the compressors is significantly reduced, the service life and maintenance intervals of the compressors are extended accordingly + Will be explained in more detail in this article * Is in explained in detail in a following article KI air and refrigeration technology 1 /
2 Fig. 1: Refrigeration circuit with one variable speed compressor (VvD) and two compressors with fixed speed (VfD). Right: FrigoPack intelligent frequency converter (FU) Fig. 2: Function of a compound system with speed-controlled master compressor Fig. 3: Measurement curves of a real system in operation with suction pressure control using an intelligent frequency converter compared to operation with a conventional compound control with step switchgear The evaporation temperature in the system can be increased The same control quality when using a smaller number of larger compressors (minimization of the installation effort). A setting range of% would be optimal, but about% is economically feasible. In practice, the setting range can be positively influenced by the following configurations: Use of three or more compressors in a network Use of a VvD with a low minimum speed / frequency Use of VvDs with the highest possible maximum speed / frequency in order to use the smallest possible compressor size to achieve the specified performance This problem is explained in more detail in the following sections. Minimum speed / frequency of a VvD A few years ago it was difficult to get technical application data for the operation of compressors at different speed / frequency from compressor manufacturers. This is understandable, since the complex type approval measurements are usually only carried out at 50 or 60 Hz. The data available for operation at other speeds were conservative blanket data that applied to all compressors in a type range. The precise assessment of the permissible minimum speed of a specific compressor requires consideration of the following questions: Is the lubrication system able to guarantee the required lubrication? Is the oil transport in the refrigeration circuit sufficient with a reduced volume flow? Is the cooling of the motor winding of a semi-hermetic VAD sufficient? To assess the winding cooling of a semi-hermetic VVD at reduced speed, detailed measurements were carried out on typical compressors. The evaluation of the measurements of a compressor with medium power is shown in Fig. 7. The increase in the winding temperature (twind) due to the reduced volume flow at low speed / fre- 26 KI Luft- und Kältetechnik 1/2003
3 sequence is clearly visible. Measures to ensure sufficient cooling of the winding are described in the following section. In the meantime, important manufacturers have specified the minimum speed for VvD operation of their reciprocating compressors in the Hz range. If the application points for refrigeration are known, low speed limits can often be implemented on request. These reduced minimum speeds prove to be very advantageous. Minimum cooling capacity of the compound system Fig. 4: Typical compound system with intelligent frequency converter Fig. 5: Comparison of a compound system with: 3/4 x VfD (conventional step switchgear) VvD + 2 x VfD (FU regulated master compressor) The use of the VvD combined with power regulation by means of a Cylinder bank shutdown. However, this requires careful design of the refrigeration circuit in coordination with the compressor manufacturer. Fig. 6: Increasing the cooling capacity when operating a compressor at 60 Hz with a frequency converter Maximum speed / frequency of a VvD (increase in capacity) The use of a frequency converter to increase the maximum speed is recommended. Since almost all compressors are mechanically designed for operation on 60 Hz networks, there are no problems with the resulting increase in output by approx. 20%, see Figure 6. However, depending on the manufacturer, further increases in speed are usually permissible or at least are tolerated. In this case, it is advisable to ask the manufacturer individually, stating the system data. Operation up to 70 Hz with approx. 40% overspeed is usually permissible. The application limits are mostly in the area of thermal and fluidic stress in the compressed gas area of the compressor. Figure 7 shows the measurement results of a semi-hermetic compressor with a Mo- The minimum output of the compound system is of great importance, especially in the supermarket sector. In winter operation with covered refrigerated counters, the required cooling capacity is relatively low. If the refrigeration system is oversized compared to the required refrigeration capacity, even when the VvD is operated at the minimum speed / frequency, the required low refrigeration capacity can only be achieved by frequent cycling of the master compressor. Fig. 7: Measurements on a typical semi-hermetic reciprocating compressor Operating data: R404A, t o = 10 C, t c = +40 C, t oh = 25 C KI air and refrigeration technology 1 /
4 The increased cooling capacity provides the necessary reserves to ensure operation at peak cooling capacity (especially in summer) without having to oversize the compressors of the compound system Avoiding oversizing of the compound system is especially important for operation in the partial load range (see previous Section). Fig. 8: To select the compressor gate winding for 400 V at 50 Hz. Please note the following: The speed (Fmech) above 50 Hz increases slightly less than the electrical frequency due to the decreasing magnetic flux in the motor (magnetic field weakening) The refrigeration output (Prefr) also increases slightly less than proportionally (approx. 30% increase in output at 70 Hz compared to operation at 50 Hz) The temperature of the winding (Twind) when operated at 60 Hz is even lower than when operated with 60 Hz due to the higher volume flow 50 Hz. However, this is a property of the measured compressor and cannot be used as a general statement.With the measured compressor, the temperature at 70 Hz is insignificantly higher than at 50 Hz despite the magnetic field weakening of the motor the magnetic field weakening above 50 Hz is problematic. The comparison of the thermal behavior of a semi-hermetic compressor motor with the thermal behavior of an industrial motor is wrong. Careful consideration of the maximum permissible upper speed / frequency is important for the following reasons: Selection of the VvD compressor There have been positive experiences with the following compressor types: Semi-hermetic reciprocating compressors Screw compressors Fully hermetic reciprocating compressors from some manufacturers Scroll compressors from some manufacturers Open and separating hood compressors. The widespread reciprocating compressors are discussed in more detail below. Almost all manufacturers offer two motor variants for each mechanical size: Size with small motor (motor 1) for operation with restricted suction pressure or limited evaporation temperature Size with large motor (motor 2) also for operation with high evaporation temperature Figure 9: Relative electrical power consumption Pe [pu] of a compressor as a function of the suction pressure Po when using motors M1 (small) and M2 (large) 28 KI Luft- und Kältetechnik 1/2003
5 Fig. 10: Overview circuit diagram of the control technology P o: Evaporation pressure; P c: condensing pressure Figure 8 shows the refrigeration performance data of a typical semi-hermetic compressor with a differentiation according to motor size. The operation of a compressor with speed / frequency control represents an increased thermal load on the motor in the limit range. At low speed the volume flow of the refrigerant is lower, at higher speed the electric current is higher due to the magnetic field weakening. Using the larger motor eliminates these potential problems entirely. This can be explained as follows: In the critical operating points, the electrical power consumed by the smaller motor 1 is significantly greater than that of the larger motor 2, see Figure 8 The larger motor (motor 2) has a larger internal surface for cooling with refrigerant (suction gas Cooling) At a given operating point (see Fig. 8), the larger motor is not so heavily loaded, so the temperature increase in the winding is significantly lower. Starting up a VvD is of crucial importance for a trouble-free system. This is especially true for small compressors with two cylinders that require a high starting torque. According to statements , torque reserves of 60% are required at the start. In practice, the compressors have to start up even at high evaporation pressure; it is not enough to consider the intended operating point alone. Example: The voltage is interrupted for a few minutes, the evaporation pressure rises rapidly, the condensation pressure is still high - the required starting torque is now considerable. If the start does not succeed, the motor winding is not cooled, the winding temperature and thus the resistance of the winding rise sharply with every start attempt. This usually ends in shutdown by the thermal thermistor monitoring of the motor. The load on the motor winding is considerable. The authors strongly recommend using this technology only when using compressors with the larger motor. The additional costs of the compressor compared to the annoyance of start-up problems are negligible. Selection of the rated output of the frequency converter The following criteria are to be used: Consideration of the necessary starting torque depending on the type or number of cylinders, see e.g.  Consideration of possible measures to reduce the starting torque, e.g. start-up relief (when starting, a solenoid valve between the pressure and suction side of the compressor is opened) Pressure limiter in the suction gas line or on the evaporator The use of an apparently oversized frequency converter is essential for reliable operation required, ie the dimensioning is primarily dependent on the provision of the starting torque. There's nothing worse than a compressor that won't start. Therefore, the approach of dimensioning according to the nominal point based on fan drives is out of place. The widespread use of this new technology is only possible if the required rated output of the frequency converter is clearly defined. For this purpose, the authors have drawn up so-called compressor assignment lists . The necessary FUs for all common compressor types, taking into account field experience with problem compressors, is summarized as a database. Control-related aspects The integrated suction pressure control sets the speed of the VvD according to the refrigeration requirements. A VfD is only switched on when the cooling capacity of the VvD is insufficient. The integrated cooling software of the FrigoPack system can control up to 3 VfDs. An external network control is neither necessary nor permitted (otherwise competing control loops will arise). The minimum running and switch-off times required by the compressor manufacturer are taken into account in the software. Figure 10 shows an overview of the regulation and compound control. To increase the availability of the system, a high-pressure control system is available as an optional function. This is very useful in the following cases: Condenser output with high cooling output in summer not sufficient Condenser dirty Fan on the condenser failed Icing of the evaporator during heat pump operation Noise protection restrictions allow operation of the condenser, depending on the time of day, only at reduced speed If a limit pressure is exceeded, the speed of the VvDs automatically reduced. AI ventilation and refrigeration technology 1 /
6 Concept of the combined compressor and condenser control It makes sense to integrate the condenser control into the compressor control using the existing signal from the high pressure sensor. This arrangement has the following advantages: Only one pressure transmitter is required for the high pressure The high pressure limitation and the condensing pressure can be set together in the settings menu of the compressor control software The monitoring of the condenser can be integrated in the compressor control Electrotechnical installation aspects The use of modern electronic frequency converters brings new ones Requirements for the system installation. The current situation is comparable to the general introduction of frequency converters in fan technology approx. 8 years ago. Here are two important examples: The electrical wiring of the control cabinet and the system must be carried out carefully in accordance with EMC aspects.The best control only works well if the sensor values for the suction and high pressure are available as actual values at the controller input without interference. The use of pressure transducers of high quality and reliability is very important. Special EMC precautions are necessary so that the signals from the pressure transducers are not disturbed by the frequency converter. In summary, there is a series of measures that must be carefully considered. This requires deepening electrical engineering know-how in the refrigeration trade. Remote diagnosis and remote optimization The use of an intelligent FU offers new options for system support by means of remote diagnosis and remote optimization. Use of field bus systems such as LonWorks, which enable remote transmission via modem or the Internet. These systems are particularly suitable for the integration of the monitoring of refrigeration points. Use of web server-based systems for monitoring the compound system and, if necessary, the condensing unit.The LonWorks field bus system is particularly interesting for future systems, as all leading manufacturers of refrigeration components and control systems work together to define a LonMark world standard. This work is very advanced. The new technology of speed-regulated compressors is already taken into account in this standard. Experience The first steps with this technology are now over 10 years old. At that time the available FUs were very expensive, prone to failure and only partially had the necessary regulation and control functionality. For about 5 years this technology has been used increasingly by some experienced companies with great success. There are now over a thousand refrigeration and air conditioning systems in Germany and worldwide that work with KIMO FU s to the full satisfaction of their operators. In the meantime, a very close cooperation has emerged between leading compressor manufacturers and the advocates of this technology. Thus there is a joint effort to carry out the necessary training measures for a widespread use of this technology. The use of this new technology requires a lot more systematic thinking than with previous conventional technologies. The advantages of the new technology can only be achieved with the correct design and installation of the refrigeration, control and electrical engineering. That is why technically skilled and experienced system partners and wholesalers play an important role. Summary and outlook Frequency converter (FU) technology is an essential part of modern refrigeration technology. The open-minded and interested user will quickly become familiar with the requirements for system planning and execution. Experiences and additions to conventional refrigeration technology are dealt with in the following articles. Literature  Arndt, A. Jantsch, U .: Digital control of VRF-Multisplit. KI Luft- und Kältetechnik 38 (2002) 10, p. 468  Hendriks, M, R: Capacity control of reciprocating and screw compressors. Kälte Klima aktuell, 21 (2002) 6, S  KIMO Industrie-Elektronik GmbH: Compressor assignment lists (available on request) Keywords refrigeration, compressor, frequency converter, regulation Verbund Energie 30 KI Luft- und Kältetechnik 1/2003
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