Contrary to direct current and synchronous machines, an asynchronous machine utilises both the stator and rotor windings as power winding and field winding. Both the windings operate with the same type of current, i. e. alternating current. The rotor of the asynchronous machine has rotor rods which, together with two opposing short-circuiting rings, form a cage. The asynchronous machine is most commonly used as a motor but can also be used as a generator. When supplying an asynchronous motor with three-phase alternating current a synchronous rotating flux is obtained between the rotor and stator, which rotating flux induces a stator electromotive force and a rotor electromotive force. When the rotor winding is connected, e. g. short-circuited, a rotor current flows through the rotor winding which, together with the rotating flux, provides a momentum which in turn produces rotation. The direction of rotation of the rotor coincides with the direction of rotation of the rotating flux, which in turn is dependent on the phase sequence of the applied three-phase voltage. Altered phase sequence thus results in altered direction of rotation. The magnitude of the voltage induced in the rotor is

dependent on the number of revolutions of the rotor. It decreases linearly with increasing number of revolutions and becomes zero if the rotor achieves synchronous number of revolutions because it does not intersect any field lines and thus does not induce any rotor electromotive force. The frequency of the rotor voltage is influenced in similar manner. At the number of revolutions of zero the frequency of the rotor voltage is equal to the power frequency and approaches zero when the number of revolutions approaches the synchronous number of revolutions. Short-circuited asynchronous motors are the most usual type of electric motors.

In order to control the number of revolutions of asynchronous motors the following relationship applies: s = ns-n/ns i. e. the slip s is equal to the synchronous number of revolutions ns minus the operating number of revolutions n divided by the synchronous number of revolutions ns, where the synchronous number of revolutions ns is equal to 60 times the supplying frequency f divided by the pole pair number p, i. e. ns = 60xf/p. The synchronous number of revolutions is thus effected by frequency and pole pair number. Frequency control is a preferred method of controlling the number of revolutions of asynchronous motors. The frequency control requires special equipment of frequency changer which, for lower powers makes use of transistor technology, IGBT (Insulated Gate Bipolar Transistor) and for higher powers makes use of thyristor technology, GTO (Gate Turn Off). The transistor technology is being developed so that it is expected that it will be possible to use it for higher powers also. In this way the number of revolutions can be continuously controlled from zero up to the nominal number of revolutions of the motor, and sometimes even

3 super-synchronously. The voltage emitted by a frequency changer is far from sinus-shaped. The problems of harmonic are thus common and cause considerable iron losses and noise problems, among other things. The noise problem occurring at frequency changer operation arises when the magnetic domains in the stator/rotor laminations are turning in.

As a result of skin effects, iron losses and eddy current losses, caused by the superimposed frequencies of the applied voltage and the fact that a vector-controlled frequency changer does not have any symmetrical pattern in its modulation, an asymmetrical rotor flux may occur.

When an asymmetrical rotor flux occurs the motor shaft will be intersected by the asymmetrical rotor flux and voltage is induced over the motor shaft so that a current will flow through the motor shaft. The shaft current thus obtained flows from the motor shaft, through the second bearing and back to the shaft. Such shaft currents are dependent on the load and change character if the load changes. Another reason for shaft currents occurring is that the stator/rotor lamination package does not have a homogenous magnetic quality, i. e. the virgin curve does not have the same size in all domains around the laminations in said package and asymmetry in the flux is therefore obtained, resulting in shaft currents. The high-frequency voltage components existing in the stator frame are formed because the power semiconductors have an extremely steep derivative.

Damages occur in run tracks and on rolling bodies in motor bearings through which an electric current passes, which may cause demolition of the bearings long before the normal service life has come to an end.

Before the existence of bearing currents was noticed and the fact that the bearing currents could cause damage to

4 the bearings, motors and gearboxes were extensively replaced in an attempt to eliminate vibrations in the drive systems. The vibrations remained even after such replacements. Moreover, problems arose with pulse transducers that vibrated. The pulse transducers were found to be out-of-line, but the vibrations remained even after they had been centred. The problems were instead found to be due to damages in the motor bearings and could be temporarily alleviated by replacing the damaged motor bearings with new ones. The cause of the bearing damages was subsequently found to be due to electric currents passing the bearings. The cause of these currents has been explained above. A plurality of different efforts have been performed to reduce the effect of the harmful bearing currents on the bearings.

The asynchronous machines have been earthed to protect them from capacitive discharge currents. An insulted coupling has been arranged between motor and gearbox. An earthing brush has been mounted on the pulse transducer.

None of these measures has been successful in preventing damages to the motor bearings caused by bearing currents.

The object of the invention is to considerably reduce the above mentioned problems of bearing currents and the resulting bearing damages.

The asynchronous motor according to the invention is characterized in that a conductor is arranged between the shaft and the short-circuiting ring on each side of the rotor, that connectors connect the conductor electrically to the shaft and the short-circuiting ring, respectively, and that in order to connect the conductor to the shaft the connector is located at a point between the bearing and the rotor cage.

In one embodiment of the invention each conductor consists of a plurality of longitudinally extending

objects in the form of tapes, wires, strips, bars, rods and the like, which objects are distributed uniformly around the shaft.

In another embodiment, currently most preferred, each conductor consists of an annular body with a central opening for the shaft, the annular body may have a cylindrical part forming said opening and a radial flange facing the short-circuiting ring for attachment thereto.

The connectors may consist of screws, bolts, rivets, electric adhesive joints, welded joints or the like.

It is preferred for the connector for connecting the conductor to the shaft to be arranged immediately adjacent to the bearing.

The frequency changer is preferably pulse-width modulated (PWM).

The invention is described in more detail in the following with reference to the drawings.

Figure 1 shows schematically an asynchronous motor provided with a conductor according to the invention.

Figure 2 is a sectional view along the line II-II in Figure 1.

Figure 3 shows another embodiment of a conductor seen in section in the same position as that in Figure 2.

In Figure 1 an asynchronous motor is shown schematically, said asynchronous motor being supplied with alternating current via a frequency changer (not shown) which is pulse-width modulated (PWM). The asynchronous machine has a machine or motor housing 14, a rotor 1 with a rotor

winding and a stator 2 arranged stationarily in the motor housing 14 and having a stator winding. Further, the asynchronous motor has a shaft 3 supporting two bearings 4,5 and extending through the rotor 1 and rigidly connected to it. With the two bearings 4,5 the shaft 3 and rotor 1 are rotatably journalled in the motor housing. The rotor 1 has a plurality of rods 6 arranged adjacent each other around its circumference, and two opposing parallel short-circuiting rings 7,8 between which the rotor rods 6 extend and to which the rotor rods 6 are permanently rigidly joined. The rotor rods 6 and short-circuiting rings 7,8 form together a rotor cage 9.

The reference number 10 designates a synchronous, rotating flux which during operation is produced between stator 2 and rotor 1. On each side of the rotor 1 is a conductor 11 which is arranged between the shaft 3 and the short-circuiting ring 7,8 of the rotor cage 9.

Connectors 12,13 connect the conductor 11 electrically to the shaft 3 and the short-circuiting ring 7,8 so that the shaft 3 is short-circuited to the rotor cage 9. The connector 12 which connects the conductor 11 to the shaft 3, is located at a point between the bearing 4,5 and the rotor cage 9. In the asynchronous motor described the rotating flux 10 through induction generates a voltage in the shaft 3 which gives rise to an electric current through the shaft 3, as explained previously. Without the two conductors 11, this current would circulate in the circuit comprised by the shaft 3, the one bearing 4, the motor housing 14, the other bearing 5 and back to the shaft 3. Thanks to the present invention, the current is instead conducted from the shaft 3, through the one conductor 11, through the rotor cage 9, through the other conductor 11 and back to the shaft 3. Depending on the design of the conductors 11 and their connectors 12 from the majority, i. e. 51% and above, to practically all current, can be conducted away from the shaft bearings 4,5 which results in a corresponding, substantial

reduction in damages to the shaft bearings 4,5 caused by electric currents through the shaft bearings. The conductors 11 and their connectors 12,13 can be considered to form bridges between the motor shaft 3 and the rotor cage 9.

In the embodiment illustrated in Figures 1 and 2 each conductor 11 consists of three rods distributed uniformly around the shaft 3 and screwed to the shaft and the short-circuiting rings 7,8 by means of connectors in the form of screws or the like.

In the embodiment shown in Figure 3 the conductor 11 consists of an annular body with a central opening for the shaft 3. The annular body has a cylindrical part forming said opening and a radial flange facing towards the short-circuiting ring for attachment thereto by means of screws, bolts 13 or the like.