The object of the invention is a drive for multiple permanent magnet synchronous motors - PMSM with a series connection to one frequency converter (inverter).

It is known that fans are mostly driven by asynchronous motors. Lately, they are powered by frequency converters in order to save energy. Some applications include several identical fans in one place, the speed, i. e. revolutions, of which need to be constantly changed. A first option is that each fan has its own frequency converter. This is a simple and reliable variant, yet a relatively expensive one. Another option is that several fans are driven simultaneously by a single frequency converter. This variant is considerably cheaper, yet technically much more demanding and subject to limitations. Primary limitation is the total length of cables. Each motor must be provided with its own current protection. A parallel connection of several asynchronous motors is described in the article BFI Applications Support Library, Appl. Note AN-BEI-P2-045 of 07/03/2012. Instructions for use of a majority of manufacturers of frequency converters include instructions for a drive with several asynchronous motors connected in parallel with one frequency converter.

Latest technical embodiments frequently use permanent magnet synchronous motors for the drive of fans (DC brushiess motors, EC motors, PMSM). These motors have a very high yield in the entire range of operation. Their operation (always) requires commutation electronics or a frequency converter provided with a programme for sensorless control of permanent magnet motors. To reduce the price of the drive, certain manufacturers have developed a motor with integrated electronics, which is considerably cheaper than a motor and a separate frequency converter. This is a predominant solution in certain applications. However, the prior art motors with integrated electronics have a relatively low motor power. Only one manufacturer offers a 10-kW drive. This is why a solution, in which several smaller fans (Fan Wall) are used instead of one single fan, is gaining ground. One can find in the newspaper CCIZEITUNG dated 28/10/2016 that several manufacturers present a Fan wall with fans with EC motor and integrated electronics. Many applications are known, in which several identical fans have been used on one device, particularly for cooling systems (chillers), cooling towers, roof fans, in clean rooms. A drive provided with integrated electronics also has certain drawbacks. In case of failure of the electronics, the entire fan or at least the motor with the electronics needs to be replaced. The electronics is arranged in a working air flow which can be hot, even 70°C or even more, and this reduces the life span of the electronics. It is often exposed to huge temperature fluctuations, rain and snow.

It would be ideal for many applications of this type if several PMS motors were driven by a single frequency converter. This is principally possible, yet there are no known technical solutions in practice for a reliable drive for multiple parallel (series) connected PMS motors with one frequency converter. In ideal circumstances, if the motors were identical, the fans identical, the cable lengths identical, the fan load identical, this would in principle work. However, the circumstances are not ideal in real time applications. It may happen that one of the motors loses synchronisation with the others. In such an event the current of this motor can no longer be controlled. This motor normally gets demagnetised, which is especially encountered in ferrite magnet motors, moreover, other motors may stop. Demagnetisation is less likely in motors with rare-earth magnets. A motor that stopped can get overheated. Each motor should therefore have a temperature switch at least in two windings (phases). Moreover, motor operation at very low revolutions is rather unstable due to additional currents between individual motors. This is called hunting in English. A similar problem occurs if one of the motors is mechanically blocked. This motor will get overheated. There is considerable likelihood that the remaining motors will not rotate. A further problem is that the total permitted length of cables is easily reached. In fact, this can be solved by using a Du/dt filter, yet this incurs a further cost. Moreover, an additional connection socket is needed, where the cables of individual motors are connected to a cable that is guided to a frequency converter. This represents additional work and higher cost. One of the manufacturers at the SPS IPC Drives fair presented a drive for two parallel connected PMS motors driven by one electronics.

The task and goal of the invention is such a connection that will allow a simple and reliable use of one frequency converter for driving several PMS motors.

The invention will be described by way of drawings, in which:

Fig. 1: shows a series connection of several PMSMs to one frequency converter, where the phase ends of the last motor are connected to a star point;

Fig. 2: shows a series connection of several PMSMs to one frequency converter, where the phase ends are guided to a converter and connected to a delta connection;

The task of the invention is solved by a series connection of PMS motors. The current in all motors is always the same. The problem of current oscillation among individual motors is herewith eliminated. The maximum current generated by the frequency converter must be identical or smaller than the smallest nominal current of each motor connected in series. Optimally, the nominal current of all motors connected in series is identical. In this way, none of the motors in the series can get demagnetised and overheated. Even if one of the motors loses synchronisation, it will stop, yet no demagnetisation and overheating will occur. Moreover, if one motor stops, the remaining motors keep working. In the event that one of the motors is mechanically blocked, the other motors can normally rotate. The motor which is mechanically blocked does not get demagnetised nor overheated. Motors of various sizes, even with different numbers of rotor poles can be used. A minimum number of motors connected in series is two. A maximum number of motors is, in fact, not limited. To optimally exploit the frequency converter and individual motors it is important that the nominal current of each individual motor is identical to or up to about 20% higher than the nominal output current of the frequency converter and that the total nominal voltage of all motors connected in series is about 20% smaller than or identical to the output voltage of the frequency converter. The frequency converter can detect failure of one of the motors in several ways: reduced input power, reduced generator voltage (BEMF). Even if one of the motors is mechanically blocked, the other motors can be normally driven. Even more, connection of motors is very simplified. The embodiment and the drawings illustrate a variant with three phases. The invention covers any number of phases of a frequency converter and of motors but the number of phases of the frequency converter and of all motors must be identical. Three phase wires U, V and W and a grounding conductor PE are guided from the frequency converter to phase starting ends and the grounding contact of the first motor Ml. The ends of phases U, V and W of the first motor Ml and of the grounding conductor PE are guided to the phase starting ends and the grounding contact of the second motor M2. The phase ends of the last motor Mn are connected to a starpoint - short- circuited among each other. This wiring is shown in Figure 1. This is an optimum wiring for the operation of the PMS motors. The output current of the frequency converter and the currents in the individual phases U, V and W are identical. This is moreover the simplest and the cheapest way of connecting motors connected in series. In the event that there is too high nominal voltage due to technological limitations of winding in connection to a star, a delta connection can be applied. This wiring is shown in Figure 2. In the embodiment, the ends of the phases U, V and W of the last motor Mn are connected to the starting ends of the phases U, V and W of the first motor Ml. The end of the U phase of the motor Mn is connected to the beginning, i. e. the output of the frequency converter, of the phase V, the end of the phase V is connected to the beginning of the phase W, while the end of the phase W is connected to the beginning of the phase U. In general: the end of the phase of the last motor Mn is connected to the next phase of the output of the frequency converter. Individual phase windings of the motors Ml to Mn are connected in series and to an individual phase output of the frequency converter. Of course, both cables can be guided to the frequency converter and the connection made in the frequency converter. A delta connection is not optimal for the PMS motors. In the event that the voltages of individual phases do not have an ideal sine shape and an identical amplitude, additional equalising currents flow between the phases, which can increase noise and reduce the yield of the motor. The sum of the nominal voltages of the motors must be adapted to the nominal output voltage of the frequency converter. In series connection, the number of motors which can be connected to one converter is not limited. A limitation of the length of the cable to the first motor is the same as with one single motor. The cable which connects the subsequent motors does not have any influence on the frequency converter because the first motor operates as a dU/dt filter.

The invention further includes a variant, in which the first motor is equipped with integrated electronics, i. e. with a frequency converter of higher power, while the rest of the motors are connected in series. The power of the integrated frequency converter of the first motor must correspond to the total power of all the motors connected in series. The same holds true when a separate frequency converter is used to control several PMS motors connected in series.

Given the fact that mostly three-phase frequency converters are commercially available, the figures show a three-phase motor connection. A similar principle of connection can be used for any other number of phases from one and multiple.

The advantages of the solution of the invention are as follows: one electronics/converter for a large number of motors, therefore more economical and less burden to the environment; the total length of cables can considerably exceed the limitations for a cable length for one motor; a considerably smaller total length of cables compared to a parallel connection of motors; no distribution box is needed between the converter and the motors; the frequency converter can be arranged at a user-friendly place and in climatically stables conditions; perfectly synchronous rotation of all motors without additional sensors; savings in terms of cables and needed frequency regulators.

The drive of the invention is industrially applicable and has been successfully tested in practice on motors from line production.