Description

An electric drive and the method for controlling the drive

Technical Field

This invention relates to an electric drive and to the method for controlling the drive. The invention relates in particular to an electric multiple-motor drive comprising brushless motors and to the method for controlling the drive.

Background Art

As is known, a brushless motor is a motor whose rotation period is synchronized with the frequency of the supply voltage and, for this reason, requires electronic control circuitry capable of powering it even at variable frequency, especially when it is started.

When started, a brushless motor is powered by a voltage of defined amplitude and frequency which are gradually increased until it reaches the desired rotation speed which is always synchronous with the frequency used. hi all its applications therefore, a brushless motor is combined with an electronic control circuit that enables it to function and which, together with the motor itself, form a system known as a brushless drive, that is to say, a self- contained system for converting electrical energy to mechanical energy.

In many applications, such as for example, radiator cooling in the automotive industry, it is common practice to use a plurality of fans, where each fan is driven by a respective brushless drive.

These applications are relatively onerous precisely because of the presence of the electronic circuits: in fact, the higher the number of motors present in a system, the higher the cost of the electronic circuitry required.

The presence of electronic circuits for all the motors also leads to problems of size and overheating, since the drives are often installed in confined spaces and because the motors generate heat when in use. hi other known applications, schematically illustrated in Figure 1, a plurality of brushless motors without individual electronic circuitry are powered in

parallel by a single modulation control unit which applies the same voltage and frequency to all the motors, irrespective of the actual load conditions of each motor.

These applications are generally unsatisfactory and unreliable since, for example when one motor is overloaded, the electronic circuits are unable to control the power supply in such a way as to create the best compromise for the drive system as a whole because there is no information as to what is happening to each individual motor.

Disclosure of the Invention

In this context, the main purpose of the present invention is to propose an electric multiple-motor drive and a method for controlling the drive which can overcome the above mentioned disadvantages.

One aim of this invention is to provide an electric multiple-motor drive which is more economical than prior art solutions and which has a relatively simple constructional architecture.

Another aim of the invention is to provide an electric drive and a method for controlling the drive which can be relied upon to guarantee efficient operation for specific applications.

Yet another aim of the invention is to provide an electric drive and a method for controlling the drive which can be relied upon even in the event of overloads in one or more of the motors.

The stated technical purpose and at least the specified aims are substantially achieved by an electric drive with the characteristics described in claim 1 and in one or more of the claims dependent thereon. The invention also relates to a method of controlling the drive comprising the operating steps described in claim 10 and in one or more of the claims dependent thereon..

Brief Description of the Drawings

Further features and advantages of the present invention are more apparent in the description below, with reference to a preferred, non-limiting embodiment of an electric drive as illustrated in the accompanying drawings, in which:

- Figure 1 schematically illustrates an electric multiple-motor drive according to the current state of the art;

- Figure 2 shows a diagram of an electric drive according to the invention.

Detailed Description of the Preferred Embodiments of the Invention

With reference to the accompanying drawings and in particular Figure 2, the numeral 1 denotes an electric drive according to this invention.

The drive 1 comprises three synchronous motor 2, 3, A, and more specifically, brushless motors, and a power line 5 which the motors 2, 3 and 4 are connected to.

A control unit 6 controls the power supply of the line 5 and at least partly includes the latter.

Although the power line 5 represented in the diagram is a three-phase line, in alternative embodiments that are not illustrated, it may advantageously have any other number of phases.

By way of example and without limiting the scope of the invention, three motors are illustrated but the drive 1 may comprise any number of brushless motors.

In practice the motors 2, 3, 4 are connected in parallel to the line 5, each through a respective branch 2a, 3a, 4a of the line 5, and they are powered by the control unit 6 which modulates the frequency and amplitude of the supply voltage, enabling them to function.

The main purpose of the control unit 6 is to power the motor 2, 3 e 4 through the line 5 in order to start them and take them to synchronous speed all together. As is known, the control unit 6 is connected up to a direct current power line 6a.

In other words, the control unit 6 powers the motors, which, according to the invention, do not have sophisticated electronic circuitry of their own, at frequency and amplitude that are variable but instantaneously equal for all the motors, as described in more detail below.

The drive 1 comprises a feedback circuit 7 through which the control unit 6 manages a data set relating to the operation of the motor 2, 3 and 4.

As illustrated in Figure 2, the circuit 7 comprises, for each motor 2, 3 and 4, a corresponding transmission channel or line 8, 9 and 10 through which the data relating to a single motor 2, 3 and 4 is processed in the control unit 6,

The data transmitted are preferably defined by one or more feedback parameters relating to the operation of the corresponding motor 2, 3 and 4 and are represented by respective signals labelled S2, S3 and S4 in the diagram.

In particular, the feedback circuit 7 is active between the line 5 and the separate power supply branches 2a, 3a, 4a of the individual motors.

The feedback signals are at least partly detected by suitable sensors 14, 15, 16

located on the power lines of the respective motors.

It should be noticed that the feedback circuit 7 is powered by the line 5 and that each branch 2a, 3a and 4a passes through the corresponding sensor 14, 15, 16 and powers the respective motor 2, 3, 4.

More in detail, the sensor 14, 15, 16 are current sensors and are located on at least one of the phases of each motor 2, 3, 4.

Advantageously, in embodiments that are not illustrated, the drive comprises a current sensor for each phase of each motor.

Advantageously, the drive has three accessible nodes A, B and C from which a data item relating to the supply voltage of the motors 2, 3 and 4 can be obtained.

The supply voltage integrates the data relating to absorbed current in the respective signals Sl, S2, S3.

It should be stressed that by processing the data relating to absorbed current and supply voltage, the control unit 6 can derive the electromotive force of each motor.

The control unit 6 comprises means, schematically represented as a block 17, for modulating the supply voltage of the line 5 as a function of the data received from the motor 2, 3 and 4 through the feedback circuit 7.

In practice, the feedback signals circulate in a feedback loop between the power lines 2a, 3a, 4a of the individual motors and the modulation means 17. hi the drive control method according to the invention the power supply of the motors 2, 3 and 4 is optimized as a function of the parameters of the motor that is working under the most unfavourable conditions. hi other words, since the motors 2, 3, 4 are powered in the same way, in particular at the same voltage and frequency, the drive 1 is controlled in such a way that the power supply is weighed on the basis of the operating conditions of the motors and preferably weighted on the motor that is operating under the most onerous conditions, as indicated by the above mentioned feedback data. hi practice, the operation of a plurality of synchronous motors powered in parallel would be ideal under all conditions if all the motors had the same inertia and the same load at all times. In automotive applications and, more generally, for driving electric fans, which this specification expressly refers to but without limiting the scope of the invention, the load of one or motors may increase for example on account of variations in the working conditions of one motor compared to the others. This creates a situation where continuing to power the line as if all the motors were

operating under ideal and identical conditions, that is to say, with a prior art system like the one represented schematically in Figure 1, would lead to damage to the entire system.

The drive 1 according to the invention optimizes motor operation under unpredictable conditions. hi the event of an overload of one of the motors 2, 3 or 4, detected by the control unit 6 through the circuit 7, the control unit 6, through the means 17, modulates the power supply of the line 5 on the basis of the overload, thus protecting both the motor and the drive 1 as a whole.

The drive according to the invention as described herein allows considerable savings since it has a single electronic control system, that is, the control unit, for a plurality of motors while at the same time offering good resistance to possible differences in the load conditions of the motors.

The feedback lines, not present in prior art drives, optimizes the use of resources and improves the reliability of the drive as a whole.

The invention described above may be modified and adapted in several ways without thereby departing from the scope of the inventive concept, as defined in the claims herein.

Moreover, all the details of the invention may be substituted by technically equivalent elements.