The present invention relates to a method for internal access in an electric machine comprising a motor housing with a stator and a rotor equipped with permanent magnets.

Background of the invention

One type of electric machine is a permanent magnet motor, PM motor, which is a type of brushless electric motor that uses permanent magnets rather than windings in the rotor.

In recent years permanent magnet motors have been used for anchor-handling winches, where relatively large loads are being handled. Traditionally hydraulic motors were used to drive winches. The advantage with using a permanent magnet motor in relation to a standard hydraulically driven winch is, among other things, a considerable reduction in energy consumption.

Other advantages with the use of a permanent magnet motor are higher speed and dynamic winch control, low noise level and maximum torque immediately after starting up.

Permanent magnet motors for use in anchor-handling operations can be sizeable motors with a large diameter and a considerable torque. An example of such motors can be a motor with an outer diameter of 2 m, power of 1 .4 MW and with a nominal torque of 140 kNm.

Assembly procedure of an electric machine such as the PM motor is rather complicated. The same applies to the dismantling. The rotor contains large permanent magnets, and inserting it into the stator is difficult due to high attraction forces between the rotor and the stator. Bearing failure on electrical machines is rather common, so changing bearings may be necessary.

Disclosure of the state of art

WO2007051895 A1 discloses a method for installing a rotor excited by permanent magnets into an electrical machine using a support, the electrical machine including a stator and a rotor. The support contains a shaft supporting the rotor. The rotor is rotated, or the shaft supporting the rotor is rotated, whereupon the rotor is moved to its operating position with the help of mutually corresponding threads arranged on the rotor and the shaft supporting the rotor.

It is known to use wedges in electric machines, usually for holding parts of the machine together. Examples of this are EP 2626975 A2, US2015333585 A1 and US2006028083 A1 .

EP 2626975 A2 disclose a rotor assembly for a motor in which separation type rotor cores having the other polarity are disposed between core members of integral type rotor cores having one polarity and the separation type rotor cores and the integral type rotor cores are fixed to each other using wedges or injection molding materials that are formed of a non-magnetic material, such that movement of magnetic fluxes between the integral type rotor cores and the separation type rotor core may be minimized.

US2015333585 A1 discloses a wedge mechanism including at least one wedge element and a guide member. The guide member is attached to the second assembly and adapted for receiving the wedge element in such a way as to define at least an inactive position and an active position of the mechanism after the mechanism has been received in the guide member. In the inactive position the first and second assemblies can be detached from each other and in the active position one of the wedge element and the guide member presses against two different sectors of the first assembly such that the first assembly and the second assembly remain attached to each other.

US2006028083 A1 discloses a rotor using nonmagnetic beams. The rotor includes a magnetic steel rim connected to a main generator shaft by a hub. The magnetic rim supports the components of the rotor, which includes a plurality of magnets and pole pieces. The pole pieces are connected to the rim with non-magnetic standoffs and nonmagnetic fasteners. The magnets are supported radially by nonmagnetic beams. The magnets are retained tangentially by pole pieces and radially by wedges. The components of the rotor are further retained axially between plates coupled to the rim and a shoulder on the pole pieces. Objects of the present invention

It is an object to provide a method for internal access in an electric machine, such as a PM motor, in order to be able to change for instance bearings and other parts in the electric machine without dismantling the complete motor, and without need for special equipment.

The method is applicable on all electric machines with magnets on the rotor, motors, generators, etc., but the mechanical design must allow for the method to be used. Summary of the invention

The above objects are achieved with a method for internal access in an electric machine, said electric machine comprising a motor housing with a stator and a rotor equipped with permanent magnets, said rotor being connected to a central shaft by bearings, and the motor housing comprises end plates and bearing housing covering said bearings supporting the shaft, wherein the method comprises the following steps: inserting a number of locking wedges through holes in the end plate, and into an air gap between the rotor and the stator; firmly locking the rotor with respect to the stator using said locking wedges in the air gap; and removing the bearing housing for access to internal parts of the machine.

The method may further comprise the step of changing the bearing after the bearing housing has been removed.

The method may further comprise the step of removing the end plate after the bearing housing has been removed.

The method may further comprise the step of servicing internal cooler or other internal parts in the motor after the end plate has been removed. The method may comprise use of non-magnetic locking wedges.

The method may comprise the step of inserting non-magnetic locking wedges of decreasing thickness across the length of the wedges, to fit with possible variations in the air gap between the rotor and the stator.

The steps of the method may be performed on the drive end side of the motor, or the steps of the method can be performed on the non drive end side of the motor. The steps of the method can be reversed and all the parts be put back into respective positions.

The method may comprise the step of covering the wedges with a protective and possible non-magnetic material, such as a rubber covering, to prevent damage to the permanent magnets on the rotor.

Description of the diagram

Embodiments of the present invention will now be described, by way of example only, with reference to the following diagram, wherein:

Figure 1 shows schematically a cross section of a typical permanent magnet motor.

Description of preferred embodiments of the invention

The method according to the invention is applicable on all electric machines with magnets on the rotor, motors, generators etc. In the following the method is described in relation to a typical permanent magnet motor.

As shown in figure 1 a typical permanent magnet motor 10 comprises a motor housing 12 with a stator 14 and an internal rotor 16. All details about a PM motor are not explained in the present disclosure, as such details are regarded as known to a skilled person.

The rotor 16 is supported on a central shaft 24, and the shaft 24 is supported in respective bearings 20 on both sides of the motor housing 12. The bearings 20 are embedded in a bearing housing 22.

An end plate 18 is mounted on respective sides of the motor housing 12, and part of the bearing housing 22 covers respective end plates 18. It is therefore normally not possible to remove one of the end plates 18 without removing the bearing housing 22 for internal access to the motor 10. However, other mutual arrangements of the bearing housing 22 and the end plates 18 are possible. For instance, the end plate and the bearing housing may be integrated with each other. In such a case, removing the bearing housing 22 also means removing the end plate 18. The stator 14 is placed internally in the motor housing 12, and is equipped with coils 32. The rotor 16 is equipped radially with magnets 30. During operation of the electric motor 10 torque is transferred to the central shaft 24 for operation of external equipment. Between the stator 14 and the rotor 16 is an air gap 28 formed. Such an electric motor 10 may further comprise a cooling system for cooling the motor. For instance a closed cooling system in the form of a rotor cooling system and a stator cooling system, where the closed rotor cooling system may comprise an internal fan system in the motor housing 12, to force air to flow through the air gap 28 between rotor 16 and stator 14, and the stator cooling system may comprise a cooling jacket in the motor housing 12, and which surrounds the stator 14.

Cooling systems will require maintenance at regular intervals. Bearing failure on electrical machines requires changing bearings when necessary. At sea, without proper equipment, such operations may be difficult.

The method according to the invention is also applicable on other PM motors. For instance on a PM motor with a stationary shaft, and with the outer drum as the rotating part of the motor, in contrast to standard motors, on which the shaft rotates. The permanent magnets are fastened to the inside of the rotor drum, and the electrical windings are in the stationary part. The motor housing will similar as explained above have end plates and bearings for supporting the shaft. The method according to the invention shall now be described. The steps are described performed on the drive end side of the motor 10, but may also be performed on the non drive end side.

Several access holes 26 or apertures are provided in one or both of the end plates 18. The access holes 26 are normally predrilled, but may also be drilled on site. The access holes 26 are preferably provided in a circular pattern on the end plate. It is important that the holes 26 are located where the air gap 28 between the rotor 16 and stator 14 is. Thereafter are several preferable non-magnetic locking wedges 40 inserted and knocked firmly into the airgap 28, such that the rotor 16 is locked in respect to the stator 16. In figure 1 a lower wedge 40 is shown partially inserted in the hole 26 in the end plate 18, while an upper wedge 40 is shown fully inserted through the hole 26 and into the air gap 28. The holes 26 are plugged when not in use and the motor is sealed. The bearing housing 22 is thereafter removed. When the bearing housing 22 is removed, the rotor 16 is no longer supported in the end plate 18. At this point, only the locking wedges 40 prevent the rotor 16 from crashing into the stator 14. Access to the bearing 20 is now possible when the bearing housing 22 has been removed. By pumping up pressure under the bearing 20 and by using standard tools, it is now possible to remove the bearing 20.

The cooler is normally bolted to the end plate 18 inside the motor 10. The end plate 18 can now be removed to inspect the cooler. The end plate 18 can thus be removed without any complications.

The whole process is reversed and all the parts are put back into respective positions, and the motor 10 will again be ready for use.

By locking the rotor 16 respectively to stator 14 it is possible to gain access to the bearing 20 and also to the internal cooler and various other parts inside the motor 10. This is a major advantage and it is possible to change bearings (or cooler) even onboard a ship. No special tools except for the locking wedges 40 are needed in the process.

The locking wedges 40 are preferable non-magnetic, as it is convenient to use nonmagnetic wedges since there are permanent magnets on the rotor 16. But the wedges 40 can be of whatever material as long as they can be applied and safely removed afterwards. Thus, the locking wedges 40 may also be magnetic or semi- magnetic wedges.

The locking wedges 40 are specially designed. They can be designed with various thickness across the length, to fit with possible variations around the airgap 28. They can be of circular, square or rectangular cross section. Further, the wedges 40 can be covered by a protective and possible non-magnetic material, such as a rubber covering, to prevent damage to the permanent magnets on the rotor.