AN ARRANGEMENT FOR DRIVING PUMPS AND LIKE LOADS

Abstract: An arrangement for driving a pump, fan or like load (1), which only temporarily needs to be driven at a maximum speed, comprises a short-circuited asynchronous drive motor (2), which can be connected alternatively to a first a.c. power supply network (N1) having a first frequency (f1) corresponding to the desired maximum speed of the drive motor, and a second a.c. power supply network (N2) having a second frequency (f2) which is lower than the first frequency. The second network is supplied from the first network via frequency converting means (5). The frequency converting means advantageously includes a slip-ring asynchronous machine whose stator winding is connected to the first network and whose rotor winding is connected to the second network, and a shortcircuited asynchronous machine, whose rotor is mechanically coupled to the rotor of the slip-ring asynchronous machine and whose stator winding is connected either to the first network or the second network. The two asynchronous machines are coupled together via a speed gear means and/or have a mutually different number of poles corresponding to the desired ratio between the frequencies of the two networks.

Description: An arrangement for driving pumps and like loads The present invention relates to an arrangement for driving pumps, fans and like loads. Pumps, fans and like loads are, to a large extent driven by short-circuited asynchronous motors connected to the external a.c. mains available at the installation con¬ cerned. The load will thus be operated at a substantially c stant speed, and consequently it is necessary to design or dimension the asynchronous motor in a manner to produce th desired maximum speed for the load in question. Loads of th kind, such as pumps and ans, however, are often used in co nections where the need to drive the object at a maximum speed, e.g. the need for a maximum flow fr.om a pump or fan corresponding to said maximum speed, is only temporary, whi during a substantial part of the total operating time only a minor flow is required from the fan or pump. In present day systems of this kind, the desired reduction in flow, required over a greater part of the total operating time, i obtained by correspondingly throttling the maximum flow de¬ livered by the fan ' or pump operating at maximum speed. As will be understood, this results in considerable power losse Tests carried out on typical installations having large num bers of pump drive systems, such as pulp and paper mills, have shown that these losses may often reach 50 % of the total sum of the rated input power of the drive systems. On way of eliminating these losses would be to enable the speed of the load, e.g. the fan or the pump, to be varied a required, by supplying the asynchronous drive motor with a variable frequency. In principle, such a variable frequency for supplying an asynchronous motor can be obtained by-mean of a controlled variable frequency converter. The high cost of such an arrangement is unacceptable in practice, however especially since a separate frequency converter would be re quired for each single load in the installation or plant. Consequently, an object of the present invention is to provide a simple and economically acceptable arrangement by means of which loads of the aforementioned kind can be driven by means of short-circuited asynchronous motors with greatly reduced power losses. ^ This object is achieved in accordance with the inven¬ tion in that. he asynchronous drive motor can be connected electrically alternatively to a first a.c. power supply network having a first frequency corresponding to the desired maximum speed of the drive motor, normally to the external a.c. mains available at the installation in question, and to a second a.c. power supply network having a second frequency which is lower than said first frequency, the second supply network being supplied from the first supply network through a frequency converter. As will be understood, the second a.c. power supply network need not be a network in the normal meaning of this term, when con¬ cerned with driving only a single load. However, in the case of an installation or plant incorporating a plurality of similar loads driven by short-circuited asynchronous motors, the second a.c. power supply network of the lower frequency can be used for all of these motors, whereby only one frequency converter is required for the whole installation. By means of an arrangement according to the invention, the asynchronous motor can be driven alternatively at two different speeds, as desired, namely the desired maximum speed and a second lower speed obtained by connecting the motor to the second supply network of lower frequency. The motor is switched to the supply network of lower frequency as soon as the operational conditions for the object in question are such that the object need not be driven at a higher speed than that corresponded by the lower frequency of the second supply network. It has been found that this simple arrangement will often provide a significant reduc¬ tion in those power losses which would otherwise occur if the drive motor was constantly driven at its maximum speed. Tests carried out on typical installations and plants have shown that a reduction in power losses of 50 % can be readily achieved. Thus, if the power losses experienced in continuous operation at maximum speed reach about 50 % of the total sum of installed rated power, it is possible by means of an arrangement according to the invention to reduce the power losses by a figure corresponding to about 25 % of the total sum of installed rated power, which results in considerable savings with respect to energy costs. The relationship between the lower frequency and the higher frequency can be selected at a suitable value, depending upon the operating conditions of the driven load. Normally, however, the lower frequency is between 6Q % and 85 % of the higher frequency, and in many cases an advantage figure with respect to the lower frequency is 75 % of the higher frequency. In accordance with a.particularly advantageous embodi¬ ment of the invention, the frequency converting means for supplying the second a.c. supply network of the lower frequency from the first a * ,c. supply network of the higher frequency comprises a slip-ring asynchronous machine having its one winding connected to the first a.c. supply network and its second winding connected to the second a.c. supply network, and a short-circuited asynchronous machine, whose rotor is mechanically coupled to the rotor of the slip- ringed asynchronous machine and whose stator winding is connected either to the first or to the second a.c. supply network. The desired relationship between the two frequencies is obtained herewith either by driving the two asynchronous machines in the converter arrangement at mutually the same rotor speed but providing them with mutually different numbers of poles , which provide the desired relationship between the two frequencies, or by providing the two asynchronous machines with mututally the same number of poles but mechanically coupling the machines together through gear means having a speed ratio which provides the desired relationship between the two fre¬ quencies. As will be understood, a combination of these two measures can also be applied. The invention and further advantages afforded thereby will now be described in more detail with reference to the accompanying drawing, which illustrates by way of example a number of embodiments of the invention and in which OMPI Figure 1 is a simplified circuit diagram illustrating a drive system according to the invention; Figure 2 is a circuit diagram of a first embodiment of the frequency converter in a system according to the inven- tion: and Figure 3 is a circuit diagram of a second embodiment of the frequency converter in a system according to the invention. Figure 1 illustrates schematically a pump, fan or like load 1, * which is driven by a short-circuited asynchronous drive motor 2. In accordance with the invention, the drive motor 2 can be connected alternatively to a first a.c. power supply network Nl of frequency f, and a second a.c. power supply network N2 or frequency f 2 , by means of switch means 3 operated through a suitable control means 4. The first power supply network Nl is suitably the external a.c. mains available to ' the system, said mains having a frequency f.. - 50Hz. The asynchronous drive motor 2 is de¬ signed so that when connected to the supply network Nl the load operates at its desired maximum speed. The second supply network N2 has a frequency f 2 which is lower than the frequency f-, of the network Nl, so that when the drive motor 2 is connected to the second supply network N2 the load 1 will be driven at a correspondingly lower speed. For example, the frequency „ may be 0.75 f, , so that the load 1 can alternatively be driven at maximum speed or a speed which is 75 % of this maximum. The network N2 is supplied from the power supply network Nl through a frequency con¬ verter 5. As will be understood, a plurality of drive motors 2 for different loads 1 can be connected to the two power supply networks Nl and N2 through individual switching means 3. The frequency converter 5, through which the network N2 is supplied from the power supply network Nl, may have many different forms. One simple, inexpensive and advantage¬ ous embodiment of the frequency converter 5 according to the invention is illustrated in Figure 2. The illustrated fre¬ quency converter comprises a slip-ring asynchronous machine 6, the stator winding of which is connected, in the illustrated embodiment, to the a.c. power supply network Nl and its rotor winding is connected to the network N2. Alter natively, the windings of the machine may be connected to the two networks the other way round. The rotor of the machine 6 is coupled mechanically to the rotor of a short- circuited asynchronous machine 7, via a gear means 8. The stator winding of the short-circuited asynchronous machine 7 is connected to the power supply network Nl. When the two asynchronous machines 6 have the same number of poles and the gear means 8 has the gear or speed ratio 1:4, as indi¬ cated in the drawing, so that the machine 7 rotates at a speed four times greater than the machine 6, it will be seen that the frequency f on the network,N2 will be 0.75 f, It will also be seen that the same result can be obtained by coupling the rotors of the two asynchronous machines 6 and 7 together, to rotate at the same speed, and by provid¬ ing the machine 7 with- four times the number of poles of the ' machine 6. It will also be understood that other ratios between the two frequencies f r- and η can be ob ained, by using other ratios between the speeds of the two ' machines 6 and 7 or other ratios between the number of poles provided in.the two machines. It will also be apparent that a combina tion of different speeds of the two machines and different numbers of poles therein can also be used. When using the frequency converting means illustrated in Figure 2, the power output to the network N2 will, in theory, be 75 % of the power supplied to the machine 6 from the network Nl. The residual 25 % of said power will be re- turned to network Nl through the machine 7. Thus, the machine 6 is utilized relatively poorly. A considerable improvement is obtained in this respect when the frequency converting neans have the form illustrated in Figure 3. The frequency converting means illustrated in Figure 3 also comprise a slip-ring asynchronous machine 6 whose stator winding is connected to the network Nl and whose rotor winding is connected to the network N2 and the rotor of which is mechanically coupled to the rotor of a short- circuited asynchronous machine 7 via a gear means 8. In the Figure 3 embodiment, however, the stator winding of the short-circuited asynchronous machine 7 is connected to the network N2. It will be seen that if, in this case, the gear means 8 has a gear or speed ratio of 1:3, in the indicated manner, so that. the speed of the machine 7 is three times that of the machine 6, and the two machines are provided with the same number of poles, the frequency fr, on the net¬ work N2 will be 0.75 f,. In this case, however, the network N2 will, theoretically, be supplied with the whole of the power-supplied to the machine 6 from the network Nl, namely 75 % directly from the machine 6 and 25 % via the machine 7. It will also be seen that when using the arrangement illustrated in Figure 3 the rotors of the ,two machines 6 and 7 may instead be coupled directly together, to rotate at the same speed, and the machine 7 provided with a greater number of poles than the machine 6. It will also be apparent that a " combination of speed difference between the two machines and mutually different numbers of poles in respective machines may also be used. In the arrangemnets illustrated in Figure 2 and Figure 3, as beforedescribed, i.e. in the absence of the capaci¬ tance 9 , the active power trans erred to the second net¬ work N2 is nowhere near a value corresponding to the rated power of the slip-ring asynchronous machine 6. The reason for this is that the reactive power requirements of the pump motors connected to the second metwork N2, and in "the arrangement according to Figure 2 also the reactive power requirement of the short-circuited asynchronous machine 7 , * must be supplied via the stator and rotor windings of the machine 6 with a considerably impaired power factor as a result. A very substantial improvement in this respect can be obtained, however, by connecting a capacitance 9 of suitable magnitude to the network N2. It has been found possible, in this way, to greatly improve the loading capa¬ city of the machine 6. In the arrangement illustrated in Figure 2, it is possible to increase the loading capacity of the machine 6 by a factor of approximately 1.6 by mean - of the capacitance 9. With the arrangement illustrated in Figure 3, however, a still better result is obtained, since the loading capacity of the machine 6.can be increased by a factor of approximately 2 by means of the capacitance 9. A frequency converting means of the form illustrated in Figure 3 and provided with a capacitance 9 has there¬ fore be found extremely advantageous. A frequency convertin means of this design is relatively simple and inexpensive besides being reliable in operation. Ft will be understood that an arrangement according to the invention can be readily installed in already existing, installations or plants, to substantially reduce power losses when operating such loads as pumps, fans and the like.

Claims: C l a i m s 1. An arrangement for driving a pump or a like load (1) , comprising a short-circuited asynchronous drive motor (2), characterized in that the short-circuited asynchronous drive motor (2) can be electrically connected alternatively to a first a.c. power supply network (Nl) having a first frequency (f-.) corresponding to the desired maximum speed of the drive motor, and a second a.c. power supply network (N2) having a second, frequency (f ) , -which is lower than said first frequency ^) , said second network (N2) being supplied from the first network (Nl) through a frequency converting means (5). 2. An arrangement according to claim 1, characterized in that said second frequency (f 2 is between 60 % and 85 % , preferably about 75 %, of the first frequency (f-,). 3. An arrangement according to claim 1 or claim 2, characterized in that the frequency converting means (5) includes a slip-ring asynchronous machine (6) having .a stator winding and a rotor winding, one of said windings being connected to the first network (Nl) and the other being connected to -the second network (N2) ; and a short- circuited asynchronous machine (7), whose rotor is mechani¬ cally coupled to the slip-ring asynchronous machine (6) and whose stator winding is connected to the irst network (Nl) . 4. An arrangement according to claim 1 or claim 2, characterized in that the frequency converting means (5) includes a slip-ring asynchronous machine (6) having a stator winding and a rotor winding, one of said windings being connected to the first network (Nl) and the other being connected to the second network (N2) , and a short- circuited asynchronous machine (7), whose rotor is mechani¬ cally coupled to the slip-ring asynchronous machine (6) and whose stator winding is connected to the second network (N2). 5. An arrangement according to claim 3 or claim 4, characterized in that a capacitance (9) is connected to the second network (N2). 6. An arrangement according to any one of claims 3 to 5 , characterized in that . the slip-ring asynchronous machine (6 and the short-circuited asynchronous machine (7) have mutua ly the same number of poles , and in that the rotors of said machines are coupled together through a speed gear means (8 having a speed ratio corresponding to the ratio of the difference between the two frequencies (f, , f„) of the two networks (Nl, N2) to the frequency of the network to which the short-circuited asynchronous machine (7) is connected. 7. An arrangement according to any one of claims 3 to 5, characterized in that the rotors of the slip-ring asyn¬ chronous machine (6) and the short-circuited asynchronous machine (7) are coupled together to rotate at the same speed, and in that the short-circuited machine has a greate number of poles than the slip-ring machine in correspondance to the ratio of the difference between the frequencies (f, , fr, . of the two networks (Nl, N2) to the frequency of the ne work to which the short-circuited asynchronous machine (7) is connected. 8. An arrangement according to any one of claims 3 to 5, characterized in that the rotors of the slip-ring asynchro¬ nous machine (6) and the short-circuited asynchronous machine (7) are coupled together through a speed gear means and have mutually different numbers of poles, the speed ratio and the difference in the number of poles being selected so as to commonly correspond to the ratio of the difference between the frequencies (f, , f„) of the two net¬ works (Nl, N2) to the frequency _ of the network to which the short-circuited asynchronous machine (7) is connected.

Inventors: HALVARSSON PER GOERAN, TOEROEK VILMOS, HALVARSSON, PER, GOERAN, TOEROEK, VILMOS
Applicants: HALVARSSON PER GOERAN, TOEROEK VILMOS, HALVARSSON, PER, GOERAN, TOEROEK, VILMOS
Application year: 1982
Main classification: H02M5
IPC codes: H02P23, H02P25, H02P27