SYNCHRONOUS MACHINE

The object of the invention is a synchronous machine according to the preamble part of Claim 1.

AC motor drives controlled with frequency converters have become more common in industrial drives. Their adjustment features and straightforward, low-cost implementation have made them a real alternative in industrial line drives, too. The adjusted operation of three-phase electric motors is increasingly based on the use of frequency converters. The availability of permanent magnets of preferable performance has replaced former adjusted asynchronous motor drives with a synchronous machine magnetised with permanent magnets, whose technical advantage, compared with its size, is high continuous torque with small losses, even at low speed. In a frequency converter drive, it basically does not matter whether the electric machine acts as a motor or as a generator, in which case the electric machine brakes, if only the braking energy is handled in a suitable manner. The motor of the frequency converter drive is a single-speed motor and its speed is set by means of the frequency converter.

In large-scale processes it is cost-effective to feed several frequency converters from the same DC system. Both asynchronous machine drives and permanent magnet synchronous machine drives can be connected to the system. Drives can be disconnected from the joint DC system for maintenance. The drive to be maintained is always disconnected from the DC system. In a known technique, the inputs of frequency converters controlling permanent magnet synchronous machines are provided with disconnectors, which are opened when maintaining control devices, as described below with reference to Figure 1. One implementation of this kind is disclosed e.g. in the conference publication 'Safety Aspects of Permanent Magnet Motors in Paper Machine Applications', Tero Huhtanen, Ilkka Erkkila; IEEE Pulp & Paper Conference Victoria, Victoria Island, BC, Canada 2004- 06-27....2004-07-01.

When drives with AC motors are used in different applications, situations occur where the electric motor is disconnected from the mains and/or from the input source controlling the motor, such as a frequency converter, while the motor still continues to rotate due to a mechanical load. Industrial line drives, for instance, contain large masses and rollers that cannot be stopped immediately. If the electric motor is rotating due to a load affecting the

axle, and motor magnetisation is in operation, a voltage is generated in the motor windings. As a result, the motor terminals may contain a voltage equal to the operating voltage even if the frequency converter had been switched off.

A voltage is generated in the poles in synchronous machines if machine magnetisation is on. If the magnetisation of a synchronous machine has been implemented by means of electric magnets, the magnetisation is usually controllable and also switched off by a suitable means when the input voltage is cut off. In a permanent magnet synchronous machine, the situation is problematic because the magnetisation is always on and an electromotive force is generated in the stator windings of the synchronous machine when the machine is rotating. When the motor has been switched off, it is usually assumed that there is no voltage in the terminals of the motor and the frequency converter feeding it. The maintenance and operating staff often have to work in an area where the machines are located so they may unintentionally expose themselves to dangerous voltage. In addition, the rotating mechanical roller itself may have been separated from the motor by means of a wall or partition in which case the personnel are not directly aware of the cause of danger.

A voltage is always induced onto the stator winding of a three-phase electric machine, or synchronous machine magnetised with permanent magnets, when the rotor runs. Several percentages of rated voltage are already induced at small rotating speed when mechanically-linked bodies are moved.

The said voltage may be harmful, even dangerous, when the frequency converter is serviced with the machine stopped or other checks are made to electric control devices and the rotor is still rotating due to an uncontrollable cause. The voltage may directly endanger personal safety or prevent maintenance activities due to a danger of component damage. This drawback materialises, especially in equipments that are so large that the various parts of the electric drive, the motor, the control devices, such as the frequency converter and its supplementary parts, and the motor cables are far away from each other, typically in different rooms, especially the motor and the control devices, hi many industrial processes and other major controlled processes, the equipment must be located in the above manner. It is then not always possible to reliably control how the parts of the machine combination are moving or are moved, or whether a motor is rotating with the moving parts.

Another feature typical of a synchronous machine magnetised with permanent magnets is the ability to constantly feed power to short-circuit in the case of a fault, until the axle has stopped. The short-circuit may occur in the control equipment or the motor cable as a result of failure. This behaviour significantly differs from an asynchronous machine. In an asynchronous machine, magnetisation and the ability of the motor to feed power disappears at failure in a few seconds down to insignificant residual magnetisation, as the magnetising current ceases when the feeding three-phase system is eliminated, either as a result of a direct fault or when the frequency converter is stopped. A known technique only discloses a partial solution to this problem encountered with a synchronous machine.

The aforementioned adverse effects caused by voltage in the control equipment can be prevented according to a known technique by means of supplementary parts of the frequency converter within the same switchgear at the motor cable starting point, such as output-disconnector, three-phase coil short-circuit of the machine, especially terminal short-circuit, or by earthing temporary the machine terminals.

The last two methods may give rise to extensive heating in the machine, depending on cooling, if unintentional rotor rotation continues longer, as the winding then typically contains a current corresponding to the rated current. The coil short-circuit and the temporary earth are implemented as a structural combination with the output-disconnector in accordance with a known technique.

The object of the present invention is to develop a solution by means of which harmful, dangerous voltages can be prevented in the motor terminals when the permanent magnet motor is disconnected from the input source and when it is possible that the motor will continue to rotate. In order to achieve this, the invention is characterised by the features specified in the characteristics section of Claim 1. Certain other preferred embodiments of the invention are characterised by the features specified in the dependent claims.

According to the invention, connections are performed in the actual motor so that the ability of the motor to feed power upon the rotation of the rotor remains very small and the terminal voltage is essentially reset to zero.

The solution, according to the invention, eliminates the adverse effects of the electromotive force induced onto the stator coils when the rotor is rotating, in cases where the synchronous machine has been disconnected from the frequency converter feeding it. The

sub-coils constituting the stator winding of the synchronous machine are connected in such a way that no harmful or dangerous voltages are present in the coil terminals. The stator winding of the synchronous machine is divided into sub-coils, which can be connected in such a way that the phase winding and its effect corresponds to the machine's normal operation and that the sub-windings are opposite to each other, in which case their joint voltage is essentially zero.

According to some preferable embodiments of the invention, the sub-coils of each phase are, in normal operation, series-connected, and the sub-coils are wound in the same direction, in which case their joint effect is the sum of the effects of the sub-coils. When the sub-coils are connected oppositely, according to the invention, the voltages induced onto the sub-coils cancel each other out and the voltage shown in the machine's external connections is essentially zero.

According to another preferable alternative, the sub-coils are connected in parallel during normal operation so that they magnetise in the same direction. When the machine is disconnected from the input source, the sub-coils are series-connected so that the voltages induced onto them cancel each other out and the voltage in the machine's connectors is essentially zero.

In one of its preferable embodiments, the switching device is remote-controlled and timed according to the frequency converter's control. It is then also possible to connect the auxiliary contact of the isolator on the input side of the frequency converter to the switching device control. This ensures that the frequency converter has been reliably disconnected when the switches of the synchronous machine are in the rest position. The isolator's actuating lever can preferably be separately lockable.

According to another embodiment, the switching device is locally controlled so that the desired connections can be made by means of local control. The switching device still receives its control energy from the control device.

In a third embodiment of the switching device, the control method is local and mechanical. Information on the position of the switching device can also be transferred to the control device.

In the following, the invention will be described in detail with the help of its embodiment examples by referring to the enclosed drawings, where:

- Figure 1 illustrates a diagram of a line drive, which is one invention application environment,

- Figure 2 illustrates a coupling arrangement according to the invention,

- Figure 3 illustrates another coupling arrangement according to the invention,

- Figure 4 illustrates a third coupling arrangement according to the invention, and

- Figure 5 illustrates a fourth coupling arrangement according to the invention.

The line drive of an industrial plant is schematically composed of a system shown in Figure 1. The DC bus bar 2 is fed from the electric network 4 by means of the rectifier 6. The rectifier can be disconnected from the supply power system by means of the switch 8. Several AC motors 10 have been connected to the DC bus bar, which AC motors are controlled by means of the inverters 12. The inverters 12 can be disconnected from the DC bus bar 2 by means of the switches 14 and from the motors 10 by means of the switches 16. The motors 10 of the line drive can be mechanically disconnected from or connected to each other. Although the motors are separate, a mechanical load with a large rotating flywheel mass is often connected to their axles. Thus the release of the switches 14 and 16 does not prevent the motor from rotating, so an electromotive force at a frequency depending on the speed of rotation is induced onto the windings of the disconnected motor.

A solution according to the invention is described below with reference to a synchronous motor magnetised with permanent magnets, which is provided with a three-phase stator winding, and the winding of each phase is composed of two essentially identical sub- windings. The stator coils have been connected to form a star or delta configuration and the sub-coils are connected either in series or in parallel with each other in normal operation. In all cases, the stator winding is conventionally connected to the motor's mains connectors. In addition, the ends of the sub-coils are connected to the switch contacts.

In the current embodiment of the invention, the motor's phase coils consist of two sub- coils, which in normal operation magnetise in the same direction. The ends of the sub-coils have been connected by means of connectors to switches. When the switches are in the

active position, the motor operates in the normal mode. When the switches are turned to the rest position, the sub-coils are connected so that the electromotive forces induced onto them compensate each other in phases. The switches are preferably located in the motor connection box or some other place outside the motor, which is accessible to the service technician. As voltage is induced onto the coils of a permanent magnet motor when the motor is rotating, the use of the reset switches must be supervised and prevented so that the circuit cannot be closed during normal use.

Figure 2 illustrates a solution according to the invention, in which the voltage induced on the coils has been eliminated in the motor connectors. The motor's stator winding has been connected to form a delta configuration and each phase coil U, V and W is made of two sub-coils U _p1 and U _p2 , and correspondingly V _pl and V _p2 and W _pl and W _p2 . The winding directions are marked with a black dot at the other end of the coil in a known manner. The sub-coils U _p1 and U _p2 are wound in the same direction. When the switches K ₁ , K ₂ and K ₃ are in the rest position, their contacts are in the position illustrated in Figure 2 and the sub- coils U _p1 and U _p2 are connected oppositely in series with each other. Due to the opposite winding direction, the voltage induced onto the sub-coils carries an opposite charge even though the sub-coils are penetrated by a parallel magnetic flux and thereby the voltage in the connectors L ₁ and L ₂ of the phase coil U is zero. Correspondingly, the voltages induced onto the sub-coils V _pl and V _p2 of the phase coil V and the voltages induced onto the sub- windings W _pl and W _p2 of the phase coil W cancel each other out, in which case the voltage shown in all the connectors L ₁ , L ₂ and L ₃ is zero. When the switches K ₁ , K ₂ and K ₃ are turned to the active position, the sub-coils are connected in parallel, they magnetise in the same direction and the voltage induced onto them is parallel. The motor is then in normal working condition.

Figure 3 illustrates a situation where the winding of one phase is composed of two sub- coils, which are connected in series in normal operation of the synchronous machine. Both the sub-coils of the phase, such as U _pl and U _p2 , are wound in the same direction so their magnetic impact is parallel when the reset switches K ₁ , K ₂ and K ₃ and K ₁ ', K ₂ ' and K ₃ ' are in the active position. When the motor is disconnected from the feeding network or a power supply, the switches K ₁ , K ₂ and K ₃ are turned in the rest position and the switch contacts will then be in the positions illustrated in Figure 3. The sub-coils of each phase are connected oppositely in series and the voltage over the entire phase coil is zero. In the same way as in Figure 2, the voltage in the connectors L ₁ , L ₂ and L ₃ of the machine is zero.

In the case of a motor with a star connection, the coils are connected in accordance with Figures 4 and 5. Figure 4 illustrates a connection where the coils of one phase are connected in parallel with each other when the machine is in the normal mode of operation. When the switches K ₁ , K ₂ and K ₃ are turned to the rest position, the phase coils are in series with each other, as illustrated in Figure 4, so their joint electromotive force in the machine's connectors is zero, due to the opposite winding direction of the sub-coils of the phase.

In the case illustrated in Figure 5, the phase sub-coils are connected in series in normal operation when the switches K ₁ , K ₂ and K ₃ are in the active position. Their winding direction is then the same and the connectors contain a voltage that equals the sum of the voltages of the sub-coils. When the switches are turned to the rest position, as illustrated in Figure 5, the voltages induced onto the sub-coil affect in opposing directions and the voltage shown in the connectors L ₁ , L ₂ and L ₃ of the machine is zero.

A contactor or corresponding switching device is mounted in the junction box of the permanent magnet synchronous machine, which contactor or corresponding switching device is always in the closed position, as illustrated in the Figures 2-5, when the frequency converter is stopped. The switching device switches the main circuit part of each phase coil to the opposite direction. The part of the coil which will be shifted to the opposite direction is chosen from the sub-coils in such a way that the voltage induced onto it is the same as the voltage induced onto the rest of the phase winding of the said phase.

This can be achieved when the winding revolutions of oppositely-connected sub-coils are the same. The revolutions remaining in series are typically half of the phase winding revolutions. If the rotor is now rotated, the motor's terminal voltages remain zero so permanent magnetisation is effectively prevented from extending its influence to outside the motor. When the motor is run, the switching device is turned to the active position as a result of which it first activates the desired motor connection, i.e. a delta connection or star connection, which complies with the standard connection described above.

Although the above description only concerns a three-phase motor, in which each stage has two sub-windings, the operation according to the invention can equally well be implemented using a phase winding consisting of a larger number of phases and/or a phase winding consisting of a larger number of sub-windings. The only requirement is that the

switch arrangement is such that the sub-coils of each phase can be connected in such a way that the voltage in the terminals is zero.

According to the invention, an extra sub-coil, whose revolutions match the revolutions of the actual winding of the synchronous machine, can also be added to the synchronous machine. The said extra sub-coil is connected oppositely in series with the coil of the actual winding, in which case their sum voltage is zero. The extra sub-coil can be essentially wound of a thinner line wire, as the current passing in it is low. In normal operation, the extra sub-coil can be put out of the circuit.

In the above, the invention has been described with the help of certain embodiments. However, the description should not be regarded as limiting the scope of patent protection; the embodiments of the invention may vary within the scope of the following claims.