ARRANGEMENT IN A SYNCHRONOUS MACHINE AND METHOD OF CONNECTING A SYNCHRONOUS MACHINE

The object of the invention is an arrangement according to the preamble part of Claims 1 and 11 in a synchronous machine and a method according to the preamble part of .Claim 21 for disconnecting a synchronous machine.

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 the former adjusted asynchronous motor drives with a synchronous machine magnetised by 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 does not basically matter whether the synchronous machine acts as the rotor or generator, in which case the synchronous 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 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, hi a known technique, the inputs of frequency converters controlling synchronous permanently magnetised 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 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 without delay. 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 has 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 adjustable and also switched off by a suitable means when the input voltage is cut off. hi a permanently magnetised 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 poles of the motor and 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 synchronous machine magnetised by permanently magnets when the rotor is rotating. Several percentages of rated voltage are already induced at low rotating speeds 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 on 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 particularly materialises in equipment that is 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 control devices. In 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 by 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 direct result of a fault or when the frequency converter is stopped. A known technique only discloses a partial solution to this problem encountered with synchronous machines magnetised by permanent magnets.

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 grounding for work are implemented as a structural combination with the output-disconnector in accordance with a known technique.

The object of the present invention is develop a solution by means of which harmful, dangerous voltages can be prevented in the motor poles 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 arrangement according to the invention is characterised by the features specified in the characteristics section of claims 1 and 11. The method according to the invention is characterised by the features specified in the characteristics section of claim 21. Certain other preferred embodiments of the invention are characterised by the features specified in the dependent claims.

According to the invention, the motor is disconnected from the input source and then the connections between the motor's phase coils are cut off, as a result of which the coil circuits will remain open and no current passes through them. When the motor is started, the phase coils are first connected to each other and the motor is then connected to the input source.

According to one preferable embodiment, the stator of the motor is star-connected, in which case the motor's star point connection is always opened by means of a contactor

from the motor junction box when the motor is stopped. When the motor is started, the contactor is first controlled so as to form a star point connection. Only two-phase windings have to be connected in order to control the star point connection. To form the star point, the contactor or some other means first has to be activated when starting the machine. Disconnecting the star point will cut off the circuit, the power circuit, and the voltage generated in the windings through the circuit can affect the motor control device, such as a frequency converter.

According to another preferable embodiment, the phase winding of the synchronous machine is delta-connected and the switching devices, such as contactors, are fitted in the joints of the phase windings and used to disconnnect the phase windings from each other. There is a contactor at the other end of each phase coil, which opens the phase coil circuit when the circuit affecting the frequency converter has been cut off.

According to yet another embodiment, the ends of the phase windings have been protected against contact. It is impossible to become unintentionally exposed to dangerous voltage induced onto the coils when the machine is rotating. The protection is preferably implemented by means of a lockable junction box. The junction box cannot be opened if a voltage has been induced onto the phase windings.

According to one embodiment of the invention, the switching device is remote-controlled and timed according to the frequency converter's control. In this embodiment it is also possible to connect the switching device control to the auxiliary contact of the disconnector located at the side of the frequency converter input. The disconnector operating lever is preferably separately lockable.

According to yet another preferable embodiment of the switching device, the switching device is also locally controlled so that the machine's main circuit can be opened through 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 of the switching device is local and mechanical. Information on the position of the local control device can also be transferred further 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 the machine's star connection;

- Figure 3 illustrates an arrangement according to the invention in which the machine has been star-connected;

- Figure 4 illustrates the machine' s delta connection;

- Figure 5 illustrates an arrangement according to the invention in which the machine has been delta-connected; and

- Figure 6 illustrates one connection of discharge resistors. 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 isolated from the supply power system by means of the switch 8. Several AC motors 10 have been connected to the DC bus bar, with 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.

The star connection of the motor coils is illustrated in Figure 2. The second ends U ₂ , V ₂ and W ₂ of the phase coils U, V and W have been connected to each other at the star point T. The first end U ₁ of the phase coil U has been connected to the motor's phase connector L ₁ , which is connected to the first phase of the feeding network or control device. Correspondingly, the first ends V ₁ and W ₁ of the phase coils V and W have been connected to the phase connectors L ₂ and L ₃ , which are connected to the first and third phases of the feeding network or control device. The winding directions of the phase coils are marked with a black dot in the known manner.

Figure 3 illustrates a coupling arrangement in which a switching device, such as a contactor, has been fitted in the star point of the motor in order to disconnect the second ends of the phase coils U, V and W. Two switching bodies K ₁ and K ₂ have been arranged

at the star point in such a way that K ₁ has been connected to the end U ₂ of the phase coil U and K ₂ to the end V ₂ of the phase coil V. In the coupling situation illustrated in Figure 3, the contact surface S _a of the switching body K ₁ has been connected to the grounding resistance R _u and, correspondingly, the contact surface S _b of the switching body K ₂ has been connected to the grounding resistance R _v . The second end of the phase coil W has been connected to the ground resistance R _w via the contact surface S _c of the switching body K ₃ . The phase coils have been disconnected from each other in the manner illustrated in Figure 3 and a closed power circuit is not formed through them via line terminals to the control device, such as a frequency converter. The grounding resistances ensure that the voltage induced onto the coils remains at a safe level. At the same time, it is ensured that long-term capacitive voltages are not formed between the coils or between the coils and protective earth. The discharge resistors are connected to earth or to form a star-shaped configuration, whose star point has been grounded, in the manner illustrated in Figure 3. The discharge resistors can also be connected to form a four-branch star, as illustrated in Figure 6, in which star the discharge resistors Ru, Rv and Rw to be connected to the ends of the phase coils have been connected to a joint star point at their second ends, which star point is further connected to protective earth through the discharge resistor RP _E .

It must be taken into consideration when dimensioning the discharge resistors that the discharge resistors must restrict the current so that it is not dangerous at the rated voltage.

In addition, the capacitive voltages induced when the rotor is rotating must be prevented.

The capacitive voltage is generated when the voltages induced onto the windings when the rotor is rotating are divided into distributed capacitances, which exist between the motor winding and frame and in the motor cable. In addition, the discharge resistors must be dimensioned in such a way that the discharge time constant of the RC circuit formed by the discharge resistor and distributed capacitance is sufficiently short at the capacitance values typical of the said synchronous machine. Furthermore, it must be taken into consideration when dimensioning the power handling capacity of the discharge resistor that the air temperature and motor temperature in the junction box are markedly above the ambient temperature. As a result, the power handling capacity of the discharge resistor must be three times higher than the rated power, even higher. If it is not necessary to arrange grounding, the grounding resistors and switching body K ₃ can be omitted. In that case,

only two switching bodies are required for disconnecting the star point from the phase coils.

When the motor is started, the switching bodies K ₁ , K ₂ and K ₃ are opened and the phase coil ends U ₂ , V ₂ and W ₂ of the star point side are connected to each other by means of the contacts of the said switching bodies. The motor is then started in the usual manner.

The connection of a delta-connected motor is illustrated in Figure 4, in which the phase coils and motor phase connections are termed in the same way as in Figures 2 and 3. Contactors have been connected to the first ends U ₁ , V ₁ and W ₁ of the phase coils, which contactors are used to form a delta connection in normal use and to disconnect the phase coils, as illustrated in Figure 5. The contact surfaces S _a , S _b and S _c are connected to the grounding resistors R _u , R _v and R _w , in which case the switching bodies K ₁ , K ₂ and K ₃ connect the motor's connections and the first ends of the phase coils to the grounding resistors, and the second ends of the phase coils have been disconnected. When starting the motor, the switching bodies are turned to the closed position, in which case the contact surfaces connect the phase coils to a delta connection.

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.