TECHNICAL FIELD

The present technical disclosure relates in general to devices and methods relating to electrical synchronous machines, and in particular to devices and methods for controlling changes in rotational speed of electrical synchronous machines.

BACKGROUND

Synchronous machines are electrical machines where a magnetized rotor is brought into a rotational motion by an alternating magnetic field created by a stator. Currents through stator windings give rise to a magnetic field that rotates around the rotor and by the interaction with the rotor magnetization, the rotor is exposed to a torque. At normal operation, the rotor is provided with alternating north poles and south poles along a circumference of the rotor, or put in other words, the rotor has north and south poles arranged one after the other along the circumference, either by permanent magnets attached to the rotor or by providing a current through rotor field windings. The stator windings are connected to a, typically multiphase, power source for providing, typically multiphase, currents through the stator windings. Typically, a three-phase system is used. The stator currents give rise to a stator magnetic field, at each time instant having alternating north and south poles along the gap towards the rotor, or put in other words, the stator field has at each time instant north and south poles arranged one after the other along the gap. The stator magnetic field does furthermore rotate. During normal motor operation, the south poles of the rotor are attracted to the north poles of the stator magnetic field and the north poles of the rotor are attracted to the south poles of the stator magnetic field, providing the driving torque for the motor, causing the rotor to follow the rotating stator magnetic field. At normal operation, the poles of the rotor and stator are rotating at the same speed, which explains the name synchronous machine.

At the start of the synchronous motor, the rotor does not initially rotate. The poles of the stator magnetic field will therefore pass the poles of the rotor with a relatively high rotational speed. In a situation where the rotor magnetization is kept at a same level as during normal operation, there might be difficulties in achieving an efficient start. The torque between the stator and rotor that is created initially is typically too small to give the rotor the synchronous rotational speed at once. A pole of the rotating stator magnetic field initially positioned behind, in the rotational direction, a corresponciing pole of the rotor causes a torque in the forward direction on the rotor. However, when the pole of the stator magnetic field passes the rotor pole, the torque caused by the magnetic interaction changes direction and acts for retarding the rotor rotation. The result is that there are alternating torques in the forward and backward direction, relative the direction of the stator magnetic field rotation, acting on the rotor. In a worst case scenario, the rotor may even be stuck in position and only presenting a shaking behavior. Particular measures have to be taken. For e.g. rotors provided with permanent magnets, a frequency converter may be provided, that reduces the frequency of the current through the stator windings during the start. This increases the time during which a positive torque is created, which allows the rotor to increase its rotational speed to adjust to the stator current frequency. In the published US patent US 5,315,225, a stator current with a lower frequency is applied during start-up and is gradually increased to higher frequencies up to the operating frequency. Such arrangements may be complex, in particular for large machines with heavy loads. Also machines based on electromagnets may be started in this way.

Another solution is to use an external assisting machine, which accelerates the rotor until the operating frequency is reached. This assisting machine is often referred to as a pony-motor. For electro-magnetized machines, a normal way to start them is to shut off, dampen or shorten the rotor field windings during the start-up phase. The torque created by interaction between magnetic fields generated by induced currents in the rotor iron core, the damping windings (so called armotisseur windings) or the shortened field winding then has to be larger than the starting torque of the rotor. Damping rods may be provided, in which a current passively is induced by the magnetic field of the stator. In the published US patent US 3,020,463, a typical example of a short- circuiting approach is disclosed. During start-up of the synchronous machine, the rotor field windings are shortened, whereby the synchronous machine is started in an asynchronous mode. When the rotational speed approaches the synchronous speed, the DC excitation of the rotor field winding is applied.

Torque oscillations and vibrations, as discussed further above, occur during start-up also for electro-magnetized machines without provision of field currents through the field-winding, primarily due to the changing equivalent reluctance path between stator and rotor. The rotating stator field magnetizes the salient poles of the rotor, causing an attractive force between stator and rotor. As the magnetic field of the stator rotates, this attractive force results in a torque which is alternatingly positive or negative. The resulting torque oscillation causes high mechanical stresses on the system in general, and on the shaft of the rotor in particular.

In the published US patent US 8,508,179, an asynchronous start of the rotor is performed by inductive current generation in the field windings. The published US patent application US 2014/0231217 Al, resistances are further provided for damping of the current generated in the field windings.

In the published US patent US 6,051,953, a synchronous machine having a brushless field system is disclosed. A damper winding is provided for start-up purposes. Furthermore, when initiating operation of a synchronous motor from a totally unpowered condition, the first magnetization of the stator occurring when the power to the stator is originally turned on requires a very large current, a so- called "inrush*. When the magnetization once is achieved, the steady-slate operation requires a much smaller current. Unfortunately, all power supply equipment on the stator side has to be dimensioned to the short lived, but large magnitude inrush current. Prior art start-up procedures for synchronous machines are in general not very efficient. A large portion of the energy supplied to the system is not used for causing any acceleration of the rotor. Therefore, the start-up of a typical synchronous motor, in particular when high loads are connected, takes a long time, requires large supplied power and causes large mechanical load on the rotor.

SUMMARY

A general object is to provide methods and devices for providing a more efficient control of a synchronous machine during periods of varying rotor speed.

The above object is achieved by methods and devices according to the independent claims. Preferred embodiments are defined in dependent claims.

In general words, in a first aspect, a method for controlling an electrical synchronous machine having a rotor, comprising rotor field windings, and a stator, comprising stator wmdings, comprises supplying of a rotor current to the rotor field windings for causing a magnetization of rotor magnetic poles. An alterrjating stator current is supplied to the stator windings for causing a rotating stator magnetic field. A quantity that is dependent on a relative angle, with respect to an axis of the rotor, between the rotor and the rotating stator magnetic field caused by the stator windings is measured, constantly or intermittently. An amplitude and/ or a direction of the rotor current is repeatedly varied as a response to a change of the measured quantity.

In a second aspect, a rotor magnetization control arrangement for an electrical synchronous machine comprises a measurement input for receiving a signal representing a measured quantity and a control output for providing control of a rotor current supplied to rotor field windings of the electrical synchronous machine. The measured quantity is a quantity that is dependent on a relative angle, with respect to an axis of a rotor of the electrical synchronous machine, between the rotor and a rotating stator magnetic field caused by stator windings of the electrical synchronous machine. The rotor magnetization control arrangement is configured for repeatedly varying an amplitude and/or a direction of the rotor current as a response to a change of the measured quantity.

In a third aspect, an electrical synchronous machine comprises a rotor and a stator. The rotor has rotor field windings. The stator has stator windings, is provided around the rotor and is arranged for allowing the rotor to rotate relative the stator. The electrical synchronous machine further comprises a rotor power supply and a stator power supply. The rotor power supply is arranged to supply the rotor windings with rotor current for causing a magnetization of rotor magnetic poles. The stator power suppty is arranged to supply the stator windings with alternating stator current for causing a rotating stator magnetic field. The electrical synchronous machine further comprises at least one sensor, arranged to measure, continuously or intermittently, a quantity that is dependent on a relative angle, with respect to an axis of the rotor, between the rotor and the rotating stator magnetic field caused by the stator windings. The electrical synchronous machine further comprises a rotor magnetization control arrangement according to the second aspect, communicationally connected to the sensor(s) for receiving a signal representing the measured quantity. One advantage with the proposed technology is that a smooth, fast and energy efficient start-up of synchronous machines can be achieved. Other advantages will be appreciated when reading the detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further objects and advantages thereof, may best be understood by making reference to the following description taken together with the accompanying drawings, in which:

FIG. 1 is a schematic illustration of an electrical synchronous machine;

FIGS. 2A-B are schematic drawings of a start-up situation of an electrical synchronous machine with active rotor field windings;

FIGS. 2C-D are schematic drawings of a start-up situation of an electrical synchronous machine with damping wmdings;

FIG. 3A is a diagram illustrating a varying measured stator flux density;

FIG. 3B is a diagram illustrating a torque during start-up of a machine having a reduced rotor field current;

FIG. 3C is a diagram illustrating a rotor frequency during start-up of a machine having a reduced rotor field current;

FIGS. 4A-B are schematic drawings of a start-up situation of an embodiment of an electrical synchronous machine with rotor field current control;

FIG. 4C is a diagram iUustrating a stator magnetic flux density and a rotor field current during start-up of an embodiment of an electrical synchronous machine with rotor field current control;

FIG. 4D is a diagram illustrating a torque during start-up using a rotor field current according to Fig. 4C;

FIG. 4E is a diagram illustrating a rotor frequency during start-up using a rotor field current according to Fig. 4C;

FIG. 4F is a diagram comparing average acceleration of the examples of

Figs. 3C and 4E;

FIG. 5 is a flow diagram of steps of an embodiment of a method for controlling an electrical synchronous machine; FIGS. 6A-F are diagrams variation of a relative angle dependent measured quantity and applied rotor field currents for starting of some embodiments of electrical synchronous machines;

FIG. 7 is a diagram illustrating one embodiment of measured quantity and applied rotor field currents as a function of time during an starting-up procedure;

FIGS. 8A-8E are diagrams iUustrating different embodiments of varying applied rotor field current amplitudes;

FIG. 9 is a schematic cross-sectional view of an embodiment of an electrical synchronous machine;

FIGS. 10A-G illustrates schematically different embodiments for rotor field current supply;

FIGS. 11A-B are schematic cross-sectional views of other embodiments of an electrical synchronous machine;

FIG. 12 is a diagram illustrating one embodiment of measured quantity and applied rotor field currents as a function of time during an braking procedure; and

FIGS. 13A-B are schematic drawings of a braking situation of an embodiment of an electrical synchronous machine with rotor field current control.

DETAILED DESCRIPTION

Throughout the drawings, the same reference numbers are used for similar or corresponding elements.

For a better understanding of the proposed technology, it may be useful to begin with a brief overview of a typical electrical synchronous machine. Fig. 1 illustrates a cross-sectional view of an electric synchronous machine 1 utilizing rotor magnetic poles 24 that are electromagnetically excited. The electric synchronous machine 1 comprises a stator 10 with stator windings 12, preferably multiphase stator windings. A rotor 20 is provided on a shaft 4 and is rotatable relative the stator 10 around a rotational axis A. A gap 8 is present between the rotor 20 and the stator 10. The term "airgap" is often used in this field of technology as a synonymous expression, even in cases where the gap actually may be filled with other substances than air or may be provided by a vacuum. In the present disclosure the terms "gap" and "airgap* will therefore be considered as synonymous term. The rotor 20 of this particular example has six rotor magnetic poles 24, however, the principles discussed in the present disclosures are applicable also to other number of poles. The poles are magnetized by sending a rotor current through rotor field windings 22 wound around the rotor magnetic poles 24. A rotor power supply

50 is therefore arranged to supply the rotor field windings 22 with current. The magnetization current around the rotor magnetic poles 24 gives rise to a magnetic field which interacts over the gap 8 with the stator 10. The interaction gives rise to a force between each rotor magnetic pole 24 and the stator 10.

A stator power supply 60 is arranged to supply the stator windings 12 with alternating stator current, preferably alternating multiphase stator current, for causing a rotating stator magnetic field. The rotating stator magnetic field has the same number of magnetic poles as the number of rotor magnetic poles.

This excitation of a rotating stator magnetic field by means of stator windings 12 is performed according to well-known prior art methods and will not be further discussed. The interaction between the rotating stator magnetic field and the rotor magnetic poles 24 gives rise to a torque that drives the rotor in a rotational motion. During normal operation, the rotor rotates with the same rotational speed as the rotating stator magnetic field rotates. This is the cause for the name "electrical synchronous motor". The steady-state operation is well-known in prior art and will not be discussed in detail. When an electrical synchronous machine is to be started, the alternating stator current may be provided to the stator windings, giving rise to a rotating stator magnetic field. However, since the rotor is standing still, the rotating stator magnetic field will rotate also with respect to the rotor. In Fig. 2A, a start-up situation is illustrated. The rotor field windings 22 are provided with a normal operation field current and thereby defines the rotor magnetic poles 24 as alternating south S and north N poles in the intentional rotational direction. The rotating stator magnetic field is illustrated by the arrows 14 and comprises also alternating south S and north N poles. In the situation depicted in Pig. 2A, each pole of the rotating stator magnetic field 14 is approaching a rotor magnetic pole 24 of a same polarity. This causes a torque T+ on the rotor directed in the same direction as the rotating stator magnetic field 14 rotates. Depending on the inertia of the rotor, this torque may initiate an acceleration of the rotor in the intended rotating direction. However, since the frequency of the rotating stator magnetic field 14 typically is relatively high and the rotor and any load connected thereto may have a considerable inertia, such an acceleration is typically small and the rotating stator magnetic field 14 will very soon pass the respective rotor magnetic pole 24.

In Fig. 2B, the rotating stator magnetic field 14 has rotated and passed the respective rotor magnetic pole 24. The acceleration of the rotor 20 is assumed to be so small and the time the positive torque acted on the rotor is short which means that also the change of position of the rotor is small. In this situation, the interaction between the rotor magnetic poles 24 and the rotating stator magnetic field 14 creates a torque T- on the rotor that is directed in a direction opposite to the direction in which the rotating stator magnetic field 14 rotates. The rotor 20 is therefore decelerated. For large rotors and/or for high loads, the result could be that the rotor 20 just vibrates around its original position without achieving any steady rotating speed.

As mentioned in the background, different measures are taken during startup to mitigate such kinds of problems. In Fig. 2C, the rotor field current to the rotor field windings 22 is turned off, typically by an open circuit or with a very high resistor connected in series, which results in that no magnetic poles are created on the rotor. Instead, damping windings are provided close to the rotor outer surface. In the situation depicted in Fig. 2C, the poles of the rotating stator magnetic field 14 approaches a respective damping windings 26 and induces a current in the damping windings 26. This interaction also gives rise to a relatively small torque t+ acting on the rotor and directed in the same direction as the rotating stator magnetic field 14 rotates. In Fig. 2D, the poles of the rotating stator magnetic field 14 has passed each respective damping windings 26. The strength of the magnetic field experienced by the damping windings 26 now decreases and the current induced in the damping windings therefore change direction, which in turn gives rise to a magnetic field with opposite polarity as compared to the situation in Fig. 2C. The interaction between the magnetic field generated by the damping windings 26 and the rotating stator magnetic field 14 now involves an attraction to the next pole, still giving a torque t+ on the rotor that is directed in the direction in which the rotating stator magnetic field 14 rotates.

Fig. 3A is a diagram illustrating the stator magnetic flux density 300 from the rotating stator magnetic field 14, at a static point on the surface of the rotor

20. Here, it is seen that the passings of north and south poles of the rotating stator magnetic field 14 are easily detected. At the times tl, t2, t3 and t4, south poles are passing, while north poles are passing at the peaks of the curves in-between.

Assuming a machine having a considerably reduced rotor field current in order to reduce the risk for having a rotor vibrating without starting-up, the torque t+ is relatively small compared to the torque available when the rotor field current is fully turned on. However, the total accelerating effect of the positive torque t+ is always positive (c.f. Figs. 2C-D). This is illustrated in the diagram of Fig. 3B. The operating torque 310 at normal operation is indicated as To. The torque on the rotor 311 oscillates between a positive and a negative value, once for each passing of a pole of the rotating stator magnetic field. However, the mean torque is slightly larger than zero, which means that the rotor in average slowly accelerates. Fig. 3C illustrates a possible course of events during start-up. The rotor frequency 320 is plotted as a function of time. Here, it is seen that the rotor in average accelerates slowly, but with large pulsations. The small increase in the rotor frequency is caused by the damping windings giving a small counter-directed field. These alternating acceleration and retardation periods also tend to give rise to vibrations as well as to increased mechanical load on the rotor shaft during the start-up period. One insight contributing to the present ideas is that it would be beneficiary if the counteracting or retarding torques could be reduced and preferably turned into accelerating torques instead. In electrical synchronous machines having rotor field current induced magnetic poles, there is a possibility to control the magnetic pole polarities. Since the position of the rotating stator magnetic field is easily detected, c.f. Fig. 3A, such a control can be performed in dependence of the relative position of the rotating stator magnetic field with respect to the rotor.

Pig. 4A illustrates an electrical synchronous machine 1 having active rotor field windings 22 during a start of the machine. As in Fig. 2A, the rotating stator magnetic field 14 interacts with the magnetic rotor poles 24 creating a torque T+ on the rotor directed in the same direction as the rotating stator magnetic field 14 rotates. The rotating stator magnetic field 14 continues to rotate relative to the rotor. When it is detected that a pole of the rotating stator magnetic field 14 passes a pole of a same polarity of the rotor 20, the rotor field current is changed. A current with an opposite direction is instead caused to be conducted through the rotor field windings 22, giving rise to a rotor magnetic pole of the opposite polarity. A situation, when such a pole polarity switch is made is illustrated in Fig. 4B.

Here, a pole of the rotating stator magnetic field 14 is situated slightly in front of, in the rotational direction, a rotor magnetic pole 24 of an opposite polarity. The result is then that a positive torque T+ still is applied on the rotor. In Fig. 4C, a diagram of the stator magnetic flux density 300 in front of a rotor magnetic pole is illustrated together with the rotor field current 330. The stator flux density is similar as in Fig. 3A presenting an essentially sinusoidal shape. At time tl , a south pole of the rotating stator magnetic field is situated in front of a rotor magnetic pole. The rotor magnetic pole is a north pole caused by a negative rotor field current. The torque on the rotor causes a slight acceleration, but the rotating stator magnetic field still rotates with respect to the rotor and a north pole of the rotating stator magnetic field approaches the rotor magnetic pole in question, at that moment still being a north pole. At the moment the north pole of the rotating stator magnetic field passes the pole of the rotor, i.e. when the stator flux density has a local maximum relative to the position of the rotor magnetic pole, the polarity of the rotor field current is switched. Since the rotor field windings have a certain inductance, the switch cannot be perfectly instantaneous, but is preferably switched as fast as possible. The rotor magnetic pole now becomes a south pole, and the rotor still becomes influenced by a positive, i.e. accelerating torque. A similar situation, but with opposite polarities, occurs for the south pole of the rotating stator magnetic field, giving rise to a torque in the same direction. At time t2, a south pole of the rotating stator magnetic field is once again situated in front of the rotor magnetic pole, and the rotor magnetic pole is switched back to become a north pole.

In Fig. 4D, a diagram illustrates the torque 312 on the rotor. The torque shows a positive curve from the time tl until half way to time t2, similar as was shown in Fig. 3B. However, since the rotor magnetic pole is created by a rotor field current, giving a stronger magnetic pole compared with the induced one in Fig. 3B, the peak value of the torque is much higher. In this particular example, a rotor field current that is 50% of the normal operating field current was used, and even this "reduced* field current gives a maximum torque that is higher than the torque 310 used during synchronous operation. Halfway to time t2, the polarity of the rotor magnetic pole is switched, and a new period of a positive torque on the rotor starts. In Fig. 4E, a corresponding diagram of rotor frequency 321 is illustrated. Here it is seen that since the torque is positive essentially all the time, the rotor frequency increases monotonically, without any substantial retardation periods. Due to the absence of retardation periods and the higher accelerating torque, the average acceleration of the rotor becomes very much quicker than illustrated in Fig. 3C. Note that the scale of Fig. 4E is very different from the scale of Fig. 3C. Fig. 4F illustrates the average acceleration for a typical electrical synchronous machine using damping windings for an asynchronous start 341 and the average acceleration for an electrical synchronous machine according to the above presented ideas 340. Here it can be seen that the acceleration of the rotor may be considerably higher when using the principles presented above. Furthermore, since the acceleration is monotonic, the mechanical load on the rotor shaft is estimated to be lower even though a higher mean torque is utilized.

When the rotor starts to rotate and accelerates, the frequency of the passings of the rotating stator magnetic field relative the rotor magnetic poles is reduced. This means that the switching of the polarity of the rotor magnetic poles is performed less frequently. When the rotor speed comes close to the synchronous speed, the poles are finally "locked" in a final polarity, which then is used during the synchronous operation.

The above presented ideas are possible for rotors provided with rotor field windings, i.e. where the poles of the rotor are created by excitation currents through the field windings. The solution is not applicable to synchronous machines based entirely on permanent magnets at the rotor. However, hybrid machines, i.e. electrical synchronous machines having both permanent magnets and rotor field windings, can also be used. The aim is to control the torque during starting or other non-synchronous operation by adapting the magnetic poles of the rotor so that they suit the polarity of the poles of the rotating stator magnetic field.

The rotor field windings are supplied with an excitation current or field current such that the relative position of the poles of the rotor are changed depending on the frequency of the rotating magnetic field generated by the stator and in dependence of the present rotational speed of the rotor. The excitation current is supplied to the field windings so that the polarity of the rotor poles changes, preferably between being north or south poles. The change is preferably performed in such a way that a positive torque is achieved at all instances. This is typically when a pole of the rotating stator magnetic field is positioned just in front of a pole of the rotor. This approach requires that there is a current control of the rotor field current to the rotor field windings.

Fig. 5 illustrates a flow diagram of steps of an embodiment of a method for controlling an electrical synchronous machine. The electrical synchronous machine has a rotor, comprising rotor field windings, and a stator, comprising stator windings, preferably multiphase stator windings. The method starts in step 200, In step 210, a rotor current is supplied to the rotor field windings for causing a magnetization of rotor magnetic poles. In step 212, an alternating stator current, preferably an alternating multiphase stator current, is supplied to the stator windings for causing a rotating stator magnetic field. In step 214, a quantity that is dependent on a relative angle, with respect to an axis of said rotor, between the rotor and the rotating stator magnetic field caused by the stator windings is measured, constantly or mtermittently. In step 216, an amplitude and/ or a direction of the rotor current is repeatedly varied as a response to a change of the measured quantity. The procedure ends in step 299.

In Fig. 6 A, rotor field currents 331 and measured quantity 301 associated with a stator magnetic field flux density of one embodiment are shown as a function of a relative angle between the rotating stator magnetic field and the rotor. The measured quantity 301 indicates the variation of the stator magnetic field flux density. In this embodiment, the rotor field current is changed every time the measured quantity 301 reaches a maximum or minimum value, which in this embodiment corresponds to the relative positions in which a pole of the rotating stator magnetic field is positioned just in front of a magnetic pole of the rotor. A broken line 332 indicates an ideal variation of the field current, however, due to the inherent inductiviry of the rotor field windings, curve 331 indicates a more realistic variation of the rotor field current.

In a preferred embodiment, the repeatedly varying of the amplitude and/or the direction of the rotor current is performed with a period that is essentially equal to a time between two latest consecutive instants when a north pole of the rotating stator magnetic field passes a reference point on the rotor or a time between two latest consecutive instants when a south pole of the rotating stator magnetic field passes the reference point.

In a preferred embodiment, the method is a method for controlling acceleration of the electrical synchronous machine. To this end, the repeatedly varying of the rotor current is performed to reduce torques between the stator and the rotor in a direction opposite of a rotating direction of the rotating stator magnetic field.

In Fig. 6B, rotor field currents 333 and measured quantity 301 associated with a stator magnetic field flux density of another embodiment are shown as a function of a relative angle between the rotating stator magnetic field and the rotor. Here the rotor field current 333 is present during half the period of the measured quantity 301 , During the other half period, the rotor field current is removed. This means that a torque is present between the rotating stator magnetic field and the rotor during half the period and essentially no torque at all is present during the other half. This embodiment does not give an equally efficient torque situation as the embodiment of Pig. 6A, but avoids at least torques of a not required direction. Furthermore, an alternating current is avoided.

When applying the rotor field currents as indicated in Fig. 6A at standstill, the torque ripple will be twice the stator field frequency. This results in very high frequency vibrations. This puts demands on the dimensions of the shaft of the machine. The embodiment of Pig. 6B offers one way to reduce such concerns. In this approach, the ripple of the torque is originally equal to the stator field frequency, since the polarity is not switched. The magnetizing of the rotor is instead turned off to prevent the high frequency torque ripple. This embodiment is therefore particularly suitable to use at the very first moments of a start-up procedure.

In one particular embodiment, the half-period switch off approach as illustrated in Fig. 6B is used at the beginning of the acceleration of the rotor of a synchronous machine. When the rotor has reached a certain speed, the frequency of the torque ripple is reduced and one may then switch over to a rotor field current changing polarity, e.g. as illustrated in Pig. 6A.

The approach of having only rotor field currents in one direction during the very first phase of a start-up procedure may also be applied to any of the embodiments presented in the present disclosure, e.g. the embodiments of Figs. 6C-F.

Even if the amplitude of the rotor field current in an alternative embodiment is not completely brought down to zero, a reduction of the amplitude of the rotor field during periods where rotor field currents of that direction give rise to unwanted torques, at least gives some improvements.

In a further preferred embodiment, the repeatedly varying of the rotor current comprises provision of rotor currents making a first rotor magnetic pole a south pole in a first time interval between a first instant, when a south pole of the rotating stator magnetic field passes the first rotor magnetic pole, and a second instant, when a north pole of the rotating stator magnetic field passes the first rotor magnetic pole.

In a further preferred embodiment, the repeatedly varying of the rotor current comprises providing of rotor currents making the first rotor magnetic pole a north pole in a second time interval between the second instant, when the north pole of the rotating stator magnetic field passes the first rotor magnetic pole, and the first instant, when the south pole of the rotating stator magnetic field passes the first rotor magnetic pole. In Fig. 6C, rotor field currents 334 and measured quantity 301 associated with a stator magnetic field flux density of another embodiment are shown as a function of a relative angle between the rotating stator magnetic field and the rotor. In this embodiment, the rotor field current 334 is varied according to an essentially sinusoidal shape as a function of the relative angle. Such a sinusoidal shape is typically easily achieved from a power supply. This means that the magnetic poles of the rotor varies in flux magnitude during each half period, not giving a maximum torque at every instant. However, when the poles of the rotating stator magnetic field are placed in the middle between two rotor magnetic poles, giving a highest interaction therewith, the amplitude of the rotor field current 334 is also at its maximum. This means that the most efficient parts of the acceleration of the rotor are not very much affected by the sinusoidal shape.

Pigs. 6D-F illustrate further embodiment, where the shape of the rotor field current is selected differently. In Fig. 6D, the rotor field current 335 has periods of constant currents connected with periods of linearly varying currents. In Fig. 6E, the rotor field current 336 has alternating linearly increasing and linearly decreasing currents. In Fig. 6F, the rotor field current 337 is a current caused by a constant voltage and a relatively high inductance.

In the diagrams of this disclosure showing rotor field currents, the magnetizing effect of the stator magnetic field on the rotor poles are neglected in order to illustrate the basic ideas in a clearer manner. Going into details, the magnetization of the rotor poles as caused by the stator magnetic field will create a reluctance torque between rotor and stator. At one point in time the stator field will magnetize the rotor poles creating a positive torque on the rotor. At a later point in time, the same magnetization will create a negative torque on the rotor. Thus in the first instant, less rotor current is required to achieve a certain torque. In the second instant, more rotor current is required to achieve the same torque.

As pointed out before, the diagram of Figs. 6A-F are plotted as functions of the relative angle. Since the rotating stator magnetic field typically is constant and the rotor change its rotational speed, the change of the relative angle between the rotating stator magnetic field and the rotor will vary with time. When the machine starts with a stationary rotor, the relative angle variation is equal to the frequency of the alternating stator current multiplied with the number of pole pairs of the rotating stator magnetic field. When the machine reaches the synchronous operation, the relative angle variation becomes zero, i.e. the relative angle becomes constant. In other words, the period of the variation of the measured quantity changes typically with time. A simplified illustration of this is seen in Fig. 7. Here, the measured quantity 301 changes quite rapidly in the beginning, i.e. in the left part of the figure. When the rotor accelerates, the variations of the measures quantity 301 becomes slower, i.e. the period of the measured quantity 301 increases. However, the variation of the rotor field current 338 still follows the pace of the measures quantity 301 and its period therefore also typically increases with time.

Thus, the rotor field windings are supplied with a time varying current, the frequency of which gradually decreases when the rotational speed of the rotor increases. When the rotor reaches the synchronous speed, that alternating current frequency will be zero and the magnetic poles of the rotor will then be locked into the final positions used for synchronous operation.

In other words, in a preferred embodiment, the repeatedly varying of the rotor current is terminated when the electrical synchronous machine has reached a synchronized rotor speed.

As discussed briefly above, the rotor field windings have an inherent inductance. This means that it is difficult to perform rapid changes of the current. When designing the change of amplitude and/or polarity of the current through the rotor field windings, one has to take the inherent inductance into consideration. The rotor field windings have to be fed with a voltage that is sufficient to cause the requested current change. In particular when the synchronous machine is started, the frequency of the current changes is high, and it might be difficult to achieve high rotor field currents within such short periods. Therefore, in some embodiments, the amplitude of the rotor field currents supplied to the rotor field windings may change depending on the frequency of the measured quantity. One embodiment is illustrated by Fig. 8A. In the beginning, when the frequency of the measured quantity and therefore the variation of the field current is high, the rotor field current 339 amplitude is kept on a relatively low level so that the requested rotor field current has time to reach the final value. However, when the frequency of the polarity changes decreases, there is more time available for changing the current and the final rotor field current amplitude is increased.

In Fig. 8B, one embodiment of how to control the rotor field current amplitude 340 as a function of rotor frequency is illustrated. At low frequencies, the rotor field current amplitude is low and increase towards the synchronous operation field current when the rotor frequency approaches the synchronous frequency.

In Fig. 8C, the rotor field current amplitude 340 is instead plotted as a function of the measured quantity frequency. At the start of the electrical synchronous machine, the measured quantity frequency is equal to the synchronous frequency times the number of pole pairs and the rotor field current amplitude 340 is reduced. At synchronous operation, the measured quantity becomes constant, i.e. has a zero frequency, and an increased rotor field current is allowed. The rotor field current amplitude can be varied in different ways. Figs. 8D-E illustrate two possibilities, but the person skilled in the art realizes that many other types of variations can be applied as well. In Fig. 8D, a linear increase of the rotor field current amplitude is given as a function of rotor frequency in a lower part of the frequency range, while the synchronous operation field current amplitude is applied above that frequency range. Pig. 8E illustrates an embodiment, where the rotor field current amplitude is increased stepwise with increasing rotor frequency.

In other words, in one embodiment, the repeatedly varying of the rotor current comprises increasing a highest amplitude of a cycle of the rotor current as a response to a decreasing change rate of the angle. Fig. 9 illustrates a schematic drawing of parts of an embodiment of an electrical synchronous machine 1. The electrical synchronous machine comprises a rotor 20 having rotor windings 22, and a stator 10 having stator windings 12, preferably multiphase stator windings. The stator 10 is provided around the rotor 20 and is arranged for allowing the rotor 20 to rotate relative the stator 10. A rotor power supply 50 is arranged to supply the rotor windings

22 with rotor field current for causing a magnetization of rotor magnetic poles 24. A stator power supply 60 is arranged to supply the stator windings 12 with alternating stator current, preferably alternating multiphase stator current, for causing a rotating stator magnetic field. The electrical synchronous machine 1 further comprises at least one sensor 30, arranged to measure, continuously or intermittently, a quantity that is dependent on a relative angle, with respect to an axis A of the rotor 20, between the rotor 20 and the rotating stator magnetic field caused by the stator windings 12. The sensors will be discussed more in detail further below. The electrical synchronous machine 1 also comprises a rotor magnetization control arrangement 40 that is communicationally connected to the sensor/ sensors for receiving a signal representing the measured quantity. The rotor magnetization control arrangement 40 is arranged for repeatedly varying an amplitude and/ or a direction of the rotor current as a response to a change of the measured quantity.

The rotor magnetization control arrangement 40 is thus suitable for operating with an electrical synchronous machine 1. To this end, the rotor magnetization control arrangement 40 comprises a measurement input for receiving a signal representing a measured quantity. The measured quantity is a quantity that is dependent on a relative angle, with respect to the axis A of the rotor 20 of the electrical synchronous machine 40, between the rotor 20 and the rotating stator magnetic field 14 caused by the stator wmdings 12 of the electrical synchronous machine 40. The rotor magnetization control arrangement 40 further comprises a control output for providing control of a rotor current supplied to the rotor field windings 22 of the electrical synchronous machine 40. The rotor magnetization control arrangement 40 is configured for repeatedly varying at least one of an amplitude and a direction of the rotor current, i.e. an amplitude and/or a direction of the rotor current, as a response to a change of the measured quantity. In a particular embodiment, the magnetization control arrangement 40 creates control signals on the control output aimed for causing the connected rotor power supply 50 to provide said repeatedly varying rotor currents.

In a preferred embodiment, the rotor magnetization control arrangement 40 is arranged for repeatedly varying the amplitude and/ or the direction of the rotor current with a period that is essentially equal to a time between two latest consecutive instants when a north pole of the rotating stator magnetic field passes a reference point on the rotor or a time between two latest consecutive instants when a south pole of the rotating stator magnetic field passes the reference point.

In one embodiment, the rotor magnetization control arrangement 40 is arranged to perform the repeatedly varying of the rotor current to reduce torques between the stator and the rotor in a direction opposite of a rotating direction of the rotating stator magnetic field.

In a further embodiment, the rotor magnetization control arrangement 40 is arranged to provide rotor currents making a first rotor magnetic pole a south pole in a first time interval between a first instant, when a south pole of the rotating stator magnetic field passes the first rotor magnetic pole, and a second instant, when a north pole of the rotating stator magnetic field passes the first rotor magnetic pole .

In a further embodiment, the rotor magnetization control arrangement 40 is arranged to provide rotor currents making the first rotor magnetic pole a south pole in a second time interval between the second instant, when the north pole of the rotating stator magnetic field passes the first rotor magnetic pole, and the first instant, when the south pole of the rotating stator magnetic field passes the first rotor magnetic pole.

In one embodiment, the rotor magnetization control arrangement 40 is arranged to perform the repeatedly varying of the rotor current to perform the repeatedly varying of the rotor current to increase a highest amplitude of a cycle of the rotor current as a response to a decreasing change rate of the measured quantity.

In a preferred embodiment, the repeatedly varying of the rotor current is terminated when the electrical synchronous machine has reached a synchronized rotor speed.

The rotor field currents can be provided in different ways, as such known in prior art. Figs. lOA-C illustrate some non-exclusive examples of such arrangements for supplying rotor field currents. In Fig. 10A, a schematic side cross-sectional view of an embodiment of an electrical machine 1 is illustrated. The rotor magnetization control arrangement 40 is here provided at a stationary part of the electrical machine 1. Also the rotor power supply 50 is provided at the stationary side. In this particular embodiment, individually controlled rotor currents are transferred over to the rotating parts. Therefore, in this embodiment, the electrical machine 1 further comprises at least three brushes 19 and at least three slip rings 29 connecting the rotor windings to the rotor current control arrangement 60 for provision of the respective rotor currents. This can be used in order to segment the rotor windings in groups and thereby control the inductivity of the used rotor windings during start-up. In other words, the rotor field windings are segmented rotor field windings and the supplying of a rotor field current to the rotor field windings for causing a magnetization of rotor magnetic poles comprises supplying of individually controlled rotor currents to the segmented rotor field windings during start, enabling control of the inductivity of the used rotor windings during start-up.

For instance, by only supplying rotor currents to a subset of groups of rotor windings, only a part of the rotor windings is used during the start-up phase. However, in alternative embodiments, all rotor field winding may be supplied with the same rotor current during the start-up.

However, the number of brushes and slip rings depends on the rotor winding connection design. For example, in Fig. 10A, there are four pairs of brushes and slip rings. Also two pairs of brushes and slip rings are feasible, if the current used in common for all rotor field windings is allowed to change direction in the same cable.

Fig. 10B illustrates a schematic side cross-sectional view of another, alternative, embodiment of an electrical machine 1. In this embodiment, the rotor magnetization control arrangement 40 is provided mechanically attached to the rotor 20. In a further alternative embodiment, the rotor magnetization control arrangement 40 may comprise one part provided at the rotor 20 and one part at the stationary side. In this embodiment, the rotor power supply 50 is still provided at a stationary part of the electrical machine 1. The rotor currents are controlled in the rotor magnetization control arrangement 40, but the supply of electrical energy from the rotor power supply 50 to the rotor magnetization control arrangement 40 has to be transferred from the stationary part of the electrical machine 1 to the rotor 20. Therefore, in this embodiment, the rotor magnetization control arrangement 40 is connected to the rotor power supply 50 by at least two brushes 19 and at least two slip rings 29. Fig. IOC illustrates a schematic side cross-sectional view of yet another, alternative, embodiment of an electrical machine 1. In this embodiment, the rotor magnetization control arrangement 40 is also provided, at least to a part, mechanically attached to the rotor 20. However, in this embodiment, the rotor power supply 50 comprises an excitation system 52 rotating together with the rotor 20. In such a way, the electric energy to be supplied to the rotor magnetic poles is produced locally directly on the rotor, and any electric transferring system therefore becomes unnecessary. In this embodiment, the rotor magnetization control arrangement 40 also comprises a rotor power storage 41, which can supply necessary rotor field current during the very first part of the starting, before the rotation of the rotor allows the excitation system 52 to produce any currents.

In other words, in one embodiment, the rotor power supply and/ or the rotor magnetization control arrangement is arranged at the rotor. In one embodiment, the rotor power supply and/ or the rotor magnetization control arrangement is arranged at the stator.

So far, the measurements of the position of the rotating stator magnetic field has been expressed as measuring of a quantity being dependent on a relative angle, with respect to an axis of the rotor, between the rotor and the rotating stator magnetic field caused by the stator windings. Here, the relative angle is understood to be defined relative a particular reference point at the rotor. Since it is the repetitive behavior of the measured quantity that is of importance, the exact position of the reference point at the rotor is of no particular importance. Any relative angle by use of a particular reference point will just be offset a certain difference angle relative to a relative angle by use of another particular reference point, in particular when the geometry is known.

Similarly, by selecting a particular point at the rotating stator magnetic field as reference point will just give a constant offset compared with selecting another reference point. There are several quantities that are depending on the relative position between the rotating stator magnetic field and the rotor. A first very obvious choice is the magnetic flux from the stator as experienced at a point fixed to the rotor. This fixed point has to be an off-axis point at the rotor. Preferably, the fixed point is positioned in a vicinity of the outer rotor surface. As indicated e _* g. in Fig. 4C, the stator magnetic flux density at a particular point at the outer rotor surface varies with the relative position of the magnetic field with respect to the rotor. It is therefore easy to determine when a north pole and a south pole, respectively, passes. In the embodiment of Fig. 9, a sensor 30, in this embodiment a magnetic field sensor, is placed at one of the rotor magnetic poles 24. It is thereby arranged at the rotor in a vicinity of the outer rotor surface. In a preferred embodiment, the magnetic field sensor is a Hall element _* Such a sensor will measure a magnetic field variation at an off-axis point at the rotor. The measured quantity will comprise a combination of the magnetic field created in the stator 10 and the magnetic field created by the rotor magnetic poles 24. However, since the sensor position is stationary relative to the rotor 20 and it is known how the rotor field current to the rotor field windings 22 vary, the passage of a pole of the rotating stator magnetic field is easily detected. The periodic behavior of the measured quantity, in this embodiment the magnetic flux density, thus directly corresponds to the relative position between the rotating stator magnetic field and the rotor 20.

In an alternative embodiment, also being based on measurements of the magnetic flux density, the sensor is a magnetic field sensor arranged at the stator in vicinity of a gap between the rotor and said stator. In other words, the measuring of a quantity comprises measuring of a magnetic field variation at a stationary point at the stator in vicinity of a gap between the rotor and the stator. Also here, in a preferred embodiment, the magnetic field sensor is a Hall element. In such a case, the measured quantity will be a combination of two periodic components, one from the rotating stator magnetic field and one from the passing of a rotor magnetic pole. Since the phases of the alternating stator current are known, the position of the rotating stator magnetic field relative to the sensor position is known. Furthermore, the stator contribution to the measured quantity can be compensated for. Since the control of the rotor field current to the rotor field windings is known, the variations of the rotor magnetic field amplitude and/ or direction as caused by the provided currents are known and any other variations in rotor magnetic field can be associated with the relative position between the sensor and the momentary rotor position. In such a way, the measured quantity can be directly associated with a relative position between the rotating stator magnetic field and the rotor.

In another embodiment, the sensor is responsive to the mechanical position of the rotor. In other words, the sensor is an angle meter arranged to measure a mechanical angle between the stator and the rotor. Such sensors are, as such, well known in prior art. In one particular embodiment, the sensor is an optical sensor arranged at the gap between the rotor and the stator, registering optical patterns or indications in the opposite surface. This gives a direct measure of the relative position between the rotor and stator. However, since the phases of the alternating stator current are known or can be measured, the position of the rotating stator magnetic field relative to the stator is known. In such a way, the measured quantity can be directly associated with a relative position between the rotating stator magnetic field and the rotor.

In Fig. 11A, yet another embodiment is illustrated. The sensor 30 is in this embodiment a voltage meter arranged to measure an induced voltage in the rotor field windings 22. Since the voltages provided to the rotor field windings

22 from the rotor power supply are known, any additional contribution to the measured voltages are associated with induced voltages in the rotor field windings 24. In other words, the measuring of a quantity here comprises measuring of induced voltage in the rotor field windings 24. Since these measurements can be considered as indirect measurements of the changes of magnetic flux at the rotor field windings 24, a similar behavior as for the direct magnetic measurements described here above is achieved. Also the rotor magnetic poles give rise to induced voltages in the stator windings. This opens up for using measurements of induced voltage in the stator windings 12. Such an embodiment is illustrated in Pig. 1 IB. The sensor 30 is here a voltage meter arranged to measure an induced voltage in the stator windings 12.

The rotor magnetization control approaches presented so far has assumed that the intended scenario is a start-up of an electrical synchronous machine. However, the same basic ideas can also be utilized also at other instances of a non-synchronous operation of an electrical synchronous machine.

One particularly interesting alternative use is to provide means for a fast retardation of electrical synchronous machines, e.g. at an emergency stop. If a stop is requested, one possibility is to remove the alternating stator current and the rotor field current and let the frictional forces between the rotor and the stator reduce the rotational speed. However, in particular with heavy loads, such a braking may take a long time. Alternatively, braking arrangements using induction braking by use of eddy currents may be applied. However, this typically requires additional arrangements.

Therefore, using the above presented ideas for an action to slow down a rotation of a rotor becomes interesting. In such an application, the general ideas of the technology presented here above can be used as a method for controlling retardation of the electrical synchronous machine. In such a case, the repeatedly varying of the rotor current is instead performed to increase torques between the stator and the rotor in a direction opposite of a rotating direction of the rotating stator magnetic field.

Fig. 12 illustrates a diagram of rotor field current 360 and the measured quantity 301. The rotor field current 360 has now an opposite polarity compared with the accelerating situation, c.f. Fig. 7. This rotor field current 360 thus gives rise to give torques on the rotor that operates for reducing the rotational speed. When the rotational speed of the rotor decreases, the frequency of the measured quantity instead increases, which can be seen in the figure.

As illustrated in Figs. 13A-B, in an electrical synchronous machine 1 operating according to these ideas, the rotor magnetization control arrangement is arranged to perform the repeatedly varying of the rotor current to increase torques T- between the stator 10 and the rotor 20 in a direction opposite of a rotating direction of the rotating stator magnetic field 14. In the situation illustrated in Fig. 13A, there is an attractive interaction between the poles of the rotating stator magnetic field 14 and the closest rotor magnetic pole 24, which gives rise to a torque in the opposite direction to the rotation of the rotor. In Fig. 13B, the pole of the rotating stator magnetic field 14 has passed the respective rotor magnetic pole 24, and the polarity of the rotor magnetic pole 24 has changed. This results in that there is an interaction between the poles of the rotating stator magnetic field 14 and the next rotor magnetic pole 24, which again gives rise to a torque in the opposite direction to the rotation of the rotor.

As mentioned briefly in the background, there is a general problem with inrush start currents when the power to the stator is turned on. AD electric equipment at the stator side, e.g. transformers, are typically dimensioned to stand potential high inrush currents. Such dimensioning of the electric equipment becomes unnecessarily powerful for the typical steady-state operation of the machine.

It would therefore be beneficial to find a method and arrangement for mitigating such large inrush currents.

The amplitude of the inrush current depends on the original direction of magnetization of the material in the stator and rotor parts, the length of the gap as well as the position of the rotor relative to the stator. If the magnetic domains that are present in the parts that are possible to be magnetized are directed in a direction opposite to the direction imposed by the stator windings current, the current needed to change the magnetization may be very large. It is not uncommon that the current increases 2-3 times when the generator is magnetized from the stator during the initial start-up compared to a normal operation current. On the other hand, if the remanence of the magnetized parts of the stator counteracts the flux that is intended to be built up by the field generated by the stator windings, the inrush current does not need to be very high. In an electrical motor having rotor magnetic poles excited by rotor field currents, the rotor field currents may also be used to provide a magnetic field that tends to act against the instantaneous flux from the stator windings. Inrush currents of only 20% more than a steady-state stator current would be feasible.

To this end, if the relative position between the rotor and stator is detennined, it is possible also to determine a rotor field current that will give rise to a magnetic field that counteracts the flux from the stator magnetic field when the stator current is turned on. In such a way, the inrush current can be reduced, which in turn allows for using electric equipment for the stator current with a smaller maximum capacity. Therefore, it may be wise to measure the relative angle even before magnetizing. If the position of the rotor is known, by geometry it is also known where the field from the rotor will be. The same applies to the stator, due to the winding distribution and number of poles. It is thereby known where the field will appear once magnetized. Due to symmetry, it does not matter which phase comes first, since one can predict if the rotor field should be north or south and which amplitude that is needed.

It is required to measure the rotor position relative the spatial position of the initial magnetomotive force from the stator so that the stator voltage meets the highest possible reactance and this gives the lowest possible inrush current.

Alternatively, if the wmding configuration is known, the stator should be magnetized when the different phase voltages have specified values. In other words, a method for initiating a start-up of an electrical machine having rotor field windings comprises determining a relative position between a stator of the electrical machine and the rotor. A rotor field current is applied to create a rotor magnetic field. A stator current is thereafter applied to the stator windings. The rotor field current is adapted to give a rotor magnetic field that in the determined relative position counteracts the flux from the stator magnetic field when the stator current is turned on.

As expressed in apparatus claims, an electrical machine comprises a rotor, having rotor field windings, a stator having stator windings, being provided around said rotor and being arranged for allowing said rotor to rotate relative said stator. The electrical machine further comprises a rotor power supply arranged to supply said rotor field windings with rotor field current for causing a magnetization of rotor magnetic poles. The electrical machine further comprises a stator power supply arranged to supply the stator windings with stator current. A position sensor is arranged to determine a relative position between the rotor and the stator prior to the start-up. A rotor magnetization control arrangement, communicationally connected to the position sensor for receiving a signal representing said determined relative position is arranged for controlling the rotor power supply to supply rotor field currents to the rotor field windings, prior to application of any stator current, which rotor field currents give rise to a rotor magnetic field that in the determined relative position counteracts the flux from the stator magnetic field when the stator current is turned.

There are a number of basic cases from which a start-up procedure will begin, depending on where the rotor is positioned mechanically.

In one case, the rotor and stator poles are aligned mechanically, and they have the opposite polarity at the time of connection. The d axis of the rotor magnetic field is positioned in front of a stator pole. This is the best case scenario, where a very small inrush current occurs. In another case, the rotor and stator are aligned mechanically, but they have the same polarity. In this case, there will be a large inrush that wants to move the rotor into the right position. In such a situation, the polarity of the rotor magnetic pole may be switched, thereby turning the situation into the best case scenario above.

In yet another case, the rotor and stator are positioned 90 electrical degrees apart, that is with the q axis of the rotor magnetic field in front of the stator pole. A large inrush will pull the rotor pole to try to lock it In such a situation, the rotor field current could initially be kept low and gradually increase until the best case scenario above is reached.

There are also the cases where the mechanical position between the rotor and stator is in between 0 and 90 degrees. In such cases, the right polarity and/ or magnitude of the rotor magnetization can be selected before the start-up in order to reduce the inrush as much as possible.

The sensor is giving the mechanical position between the rotor and stator. However, there is still need to have some additional knowledge from the stator magnetic field. Due to the knowledge of the construction of the machine, the position of the stator magnetic field poles for a particular instant of an applied voOltage is known. By additionally measuring or controlling the stator voltage, the instant angle of the stator magnetic field poles can be concluded.

The embodiments described above are to be understood as a few illustrative examples of the present invention. It will be understood by those skilled in the art that various modifications, combinations and changes may be made to the embodiments without departing from the scope of the present invention. In particular, different part solutions in the different embodiments can be combined in other configurations, where technicalry possible. The scope of the present invention is, however, defined by the appended claims.