GB2472020A - A drum motor including a magnetic gear arrangment - Google Patents

Abstract

A drum motor 200 comprises a mounted drive element 204, the mounted drive element 204 comprising an electrical machine with integral magnetic gearing having a wound stator with permanent magnets surrounding the high speed low torque permanent magnet rotor 212 with the low speed, high torque rotor 210 comprising magnetic pole pieces being arranged between the stator and the high speed rotor and being connected to an output drive shaft 216. The drum shell 202 may alternatively be connected directly to the rotor 210 via a flange. The drive element may alternatively use a wound high speed rotor which may be supplied by slip rings or an exciting alternator. Cooling is enhanced by the wound stator being located on the outside of the drive element.

Description

DRUM MOTOR

The present invention relates, in general, to a drum motor.

In a standard drum motor construction, an electric motor is built into a drum and is mechanically coupled to the drum such that the drum is driven by the output torque of the motor such that the drum turns, while the stator part of the motor remains stationary. In general the output shaft of the motor is coupled to a gearbox in order to provide the torque required by the application. The drum can in principle be of any diameter and/or length with the restriction that the smaller the diameter and the shorter the length, the lower the available power as the diameter and length of the electric machine must be reduced accordingly.

The gearbox is in general necessary as the output torque of an electric machine is fundamentally a function of the motor current, and the power loss in the machine is a function of the square of the current. Thus any attempt to increase the torque output of the electric motor results in severe power losses and heating of the motor. Furthermore the gear box is typically of a planetary construction in order to reach high transmission ratios in a small length and volume. The gear is thus an expensive component of the drum motor which must be carefully designed and increases the length of the drive motor as it must be axially mounted to the motor within the drum. Furthermore, the gearbox has a limited life span dictated by the output torque, thus it is important that each drum motor application be accurately "sized" to avoid mechanical destruction of the gearbox or a severely reduced operational life time.

In order to overcome or at least mitigate the problems identified above other constructions of drum motor have been proposed and adopted within the industry. The other constructions use external rotor motors where the outer rotating part of the motor becomes the drum; these types of designs avoid the use of a gearbox. However, they are still fundamentally limited in torque by the power (copper) losses in the stator of the motor which is in this case on the inside of the drum making cooling even more difficult. Furthermore such designs have significant electro-mechanical limitations due to the distortion of the air gap which occurs when a load is placed on the drum (rotor) of the motor. This type of direct drive inside-out motor can not replace the gear motor design due to the severely limited torque output and is only useful in high speed designs which are rare.

In light of the above problems, the applicants have developed an alternative approach to the industry pull and provide a high torque electrical machine which overcomes many of the disadvantages outlined above. From the terminals the machine operates as a conventional permanent magnet brushless motor, but has a magnetic gearing mechanically and magnetically integrated within the machine.

According to a first aspect of the present invention, there is provided a drum motor comprising a mounted drive element, the mounted drive element comprising an electrical machine with integral magnetic gearing having a magnetic pole piece rotor connected to an output drive shaft of the drum motor.

Preferable the drive element is mounted radially.

Such a drive element converts a high speed rotation of one rotor (which is caused to rotate by currents in a stator winding) into a lower geared speed on a secondary rotor. In gearing down speed, the torque is geared up and therefore a combination of the primary excitation rotor and stator winding only needs to provide a low torque. The currents and hence losses in the machine element are then minimised (reducing the requirement for cooling).

The integral magnetic gearing within the drive element exploits a magnet-to-magnet coupling principle which is capable of providing very high airgap shear stress, and is capable of bearing the large output torques without requiring large active airgap area. The overall machine then has very high torque density leading to a compact machine for a given torque.

The resulting machine for a given torque output is significantly smaller than a conventional direct drive machine, using a similar cooling regime to remove the losses in the windings. The resulting machine is capable of operating with a high power factor (0.8 -0.95 being typical) allowing for a correctly rated power electronic converter to be employed.

Preferably, the electrical machine with magnetic gearing comprises three members, a first member of which comprises a first set of permanent magnets and forms a high speed, low torque rotor, a second member of which comprises a stator provided with a second set of permanent magnets, the two sets of permanent magnets having different numbers of magnetic poles; wherein the third member is the magnetic pole piece rotor and forms a low speed, high torque output rotor relative to the first member and a multi-phase winding is arranged to interact with the fundamental space harmonic of the magnetic field created by first set of permanent magnets of the first member.

In an alternative embodiment, the electrical machine with magnetic gearing preferably comprises three members, a first member of which comprises a first rotor with an electrical winding arrangement and forms a high speed, low torque rotor, a second member of which comprises a stator provided with a set of permanent magnets having a respective number of pole pairs; wherein the third member is the magnetic pole piece rotor and forms a low speed, high torque output rotor relative to the first member; and a multi-phase winding is arranged to interact magnetically with a fundamental space harmonic of a magnetic field created by the electric winding arrangement associated with the first member.

The electric winding arrangement on the high speed rotor is supplied via slip rings or preferably by a small permanent magnet alternator and a rotating rectifier arrangement similar to a conventional wound field synchronous machine.

The internal gearing is derived from a non-contacting magnetic gear instead of an external gearbox and has the advantages of reduced wear, lubricant-free operation and reduced maintenance costs.

Also if necessary the gear torque transmission capability can be designed such that if a pre-determined torque is exceeded the gear will slip which can be used to protect the transmission system. When the abnormal load torque is removed the magnetic gear automatically re-engages.

Further preferred features are described in the dependent claims.

One embodiment of the invention will now be described, by way of example only, and with reference to the accompanying drawings in which: Figure 1 is a schematic diagram of an electrical machine with integral magnetic gearing as known in the prior-art; and Figure 2 is a schematic diagram of a drum motor comprising a radially mounted drive element according to a first embodiment of the present invention.

Thus referring to Figure 1, a schematic diagram of an electrical machine according to the prior art comprises a first or inner rotor 102 having a support 104 bearing a first plurality of permanent magnets 106. In the embodiment shown, permanent magnets having 8 poles are used.

However, embodiments are not limited to using such a number of permanent magnets. Embodiments can be realised that use some other number of permanent magnets. The electrical machine 100 comprises a second rotor 107 in the form of a plurality of ferromagnetic pole pieces 108. The pole pieces 108 are arranged to magnetically couple the permanent magnets 106 of the first/inner rotor 102 to a plurality of second permanent magnets 110 that are fixed to a periphery of the wound stator in the region of the pole shoes 112 of respective teeth 114 thereby forming a wound stator 116. In the embodiment shown, a number of permanent magnets are fixed to the periphery of the wound stator 116. The embodiment comprises 12 teeth 114 with the respective periphery of the wound stator 116. It will be appreciated that embodiments are not limited to such a number of permanent magnets and teeth. Embodiments comprising some other number of permanent magnets and teeth can be realised.

The pole pieces 108 form a rotatable array of pole pieces 108. That is, they form the second rotor 107. The inner rotor 102 is rotatable. The teeth 114 and associated permanent magnets 110 are fixed. The coupling between the permanent magnets 106 and the permanent magnets 110 is realised using the rotatable pole pieces 108. Associated with the stator 116 is a plurality of 3-phase windings 117. The 3-phase windings, and associated currents, are arranged to create magnetic fields that couple with or form magnetic circuits with the first or fundamental harmonic of the permanent magnets 106 associated with the inner rotor 102. The inner rotor 102 rotates when a correct current sequence flows in the windings 117. Although the pole pieces 108 have no effect on the interaction between the windings 117 and the inner rotor 102, the presence of the pole pieces 108 produces an asynchronous harmonic with the same number of poles as the plurality of the static second permanent magnets 110. As the inner rotor 102 rotates, the second rotor 108 comprising the array of pole pieces 108 rotates at a geared speed because the second permanent magnets 110 are static.

The pole pieces 108 are usually connected to an end plate or end plates to transmit the torque to a central shaft. Although in the present case a plurality of 3-phase windings are associated with the stator 116, in general any multi-phase winding can be used such as a 5-phase winding.

Additionally, a winding may comprise a number of independent coil circuits to enable redundancy or fault tolerance, known as duplex or triplex windings (in the event of one winding becoming open-circuit or short circuit the machine can still function).

The electrical machine described above comprises a permanent magnet excitation. Alternatively, the electrical machine with integral magnetic gearing can comprise a wound field excitation as disclosed in the applicant's co-pending GB patent application GB0807388.4, the contents of which are incorporated herein by reference in their entirety.

One skilled in the art understands how to select and design the pole pieces given the first 106 and second 110 permanent magnets, to achieve the necessary magnetic circuit or coupling such that gearing between the first 102 rotor and the pole piece rotor 108 as can be appreciated from, for example, K. Atallah, D. Howe, "A novel high-performance magnetic gear", IEEE Transactions on Magnetics, Vol. 37, No. 4, pp. 2844-2846, 2001 and K. Atallah, S. D. Calverley, D. Howe, "Design, analysis and realisation of a high performance magnetic gear", lEE Proceedings -Electric Power Applications, Vol. 151, pp. 135-143, 2004, which are incorporated herein by reference in their entirety.

Referring to Figure 2, a schematic diagram of a drum motor 200 comprising a radially mounted drive element 204 according to a first embodiment of the present invention comprises a drum shell 202 housing a drive element shown generally at reference 204. The drive element 204 is an electrical machine with integral magnetic gearing of a type discussed in respect of Figure 1 above having a permanent magnet excitation arrangement although a wound field excitation can alternatively be used.

The drive element 204 is radially mounted between a front shaft 206 and rear shaft 208 and comprises a low-speed, high torque output rotor 210 and high-speed, low torque rotor 212. A front housing 214 and end housing 216 seal the drive element 204 and complete outer shell of the drum motor 200.

According to the present invention therefore, the standard mechanical gearbox/motor combination has been replaced by a novel single highly integrated drive element consisting of a high efficiency permanent magnet synchronous machine and an magnetic gear, the gear being constructed such that it is an integral part of the motor, and of a cylindrical construction, radially mounted, thus radically reducing the length of the drive element.

Furthermore, the current carrying winding of the drive element is located on the "outside" of the drive element similar to the stator of a standard permanent magnet machine, thus allowing heat to be removed effectively from the drive element.

The component or components that make up the magnetic gear and also act as the mechanical output of the drive element are located between the outer winding and an inner permanent magnet rotor producing a very compact design.

This allows the construction of very short drum motors which have high torque outputs, which is otherwise impossible to achieve.

This drive element has a number of other important advantages when incorporated into a drum motor. The drive unit incorporates a PM synchronous machine which provides the basic motive force and has a very high efficiency (of the order of 9O%), while the magnetic gear has no mechanical components other than two bearings which themselves have minimal power losses. Thus the magnetic or electromagnetic gear itself has a very high efficiency resulting in a drive element that is not only very compact but also has a very high efficiency, a combination that it is usually impossible to achieve as the smaller the components the higher the losses for a given torque output.

A further advantage of the drum motor described in the present invention is its ability to survive mechanical overloading with no adverse affects.

The drive element utilises a novel electromagnetic gear with no mechanically coupled parts, thus if the drum motor is overloaded, i.e. if the maximum rated peak load torque of the drum is exceeded for any reason, then the magnetic coupling element or elements simply lose synchronisation with the rotating magnetic field of the drive unit and rotates freely. Once the load torque falls below the rated torque of the drive element then the magnetic gear element(s) re-synchronise with the magnetic field and generate the output torque required by the load. This is a purely magnetic effect which acts as a novel torque limiter protecting the mechanical components of the drum motor. This is an extremely useful feature as it prevents the drum from being damaged which would otherwise necessitate the replacement of the drum which is in itself a complicated task usually requiring the conveyor system to be disassembled and reconstructed. This torque limiting feature without mechanical damage also means that there can be no gradual accumulated damage to the drum motor that would result in its ultimate mechanical failure at some unknowable time, i.e. a severe reduction in the expected life of the drum.

A further advantage of the present invention lies in the significantly reduced complexity of the manufacturing process. The drive unit itself is of an entirely electromagnetic construction which results in components that are physically similar in nature, i.e. magnetic materials, cylindrical forms, which can be manufactured using simplified and similar processes which lend themselves to standardisation. This is in stark contrast to a gear motor using mechanical gears which are produced using processes that differ radically from the processes required to manufacture the electric machine.

Further, the electromagnetic gear element or elements of the drive unit can be designed so as to incorporate the mechanical coupling required to transfer torque to the drum, the drum itself being similarly constructed to accept the drive element, such that the novel drive element can be mechanically incorporated into the drum in a very simple manufacturing process requiring no further mechanical couplings.

No doubt many other effective alternatives will occur to the skilled person. It will be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the spirit and scope of the claims appended hereto.

Claims (8)

1. Claims 1. A drum motor comprising a mounted drive element, the mounted drive element comprising an electrical machine with integral magnetic gearing having a magnetic pole piece rotor connected to an output drive shaft of the drum motor.
2. 2. A drum motor as claimed in claim 1, wherein the mounted drive element is radially mounted.
3. 3. A drum motor as claimed in claim 1 or 2, wherein the electrical machine with magnetic gearing comprises three members, a first member of which comprises a first set of permanent magnets and forms a high speed, low torque rotor, a second member of which comprises a stator provided with a second set of permanent magnets, the two sets of permanent magnets having different numbers of magnetic poles; wherein the third member is the magnetic pole piece rotor and forms a low speed, high torque rotor relative to the first member and a multi-phase winding is arranged to interact with the fundamental space harmonic of the magnetic field created by first set of permanent magnets of the first member.
4. 4. A drum motor as claimed in claim 1 or 2, wherein the electrical machine with magnetic gearing comprises three members, a first member of which comprises a first rotor with an electrical winding arrangement and forms a high speed, low torque rotor, a second member of which comprises a stator provided with a set of permanent magnets having a respective number of pole pairs; wherein the third member is the magnetic pole piece rotor and forms a low speed, high torque rotor relative to the first member; and a multi-phase winding is arranged to interact magnetically with a fundamental space harmonic of a magnetic field created by the electric winding arrangement associated with the first member.
5. 5. A drum motor as claimed in claim 3 or claim 4, wherein the high speed, low torque rotor is, in use, a rotor driven by the multi-phase winding and the magnetic pole piece rotor is, in use, a rotor for magnetically gearing down speed and increasing torque relative to the high speed rotor.
6. 6. A conveyor belt system comprising a drum motor as claimed in any one of the preceding claims.
7. 7. A drum motor substantially as hereinbefore described and/or with reference to Figure 2 of the accompanying drawings.
8. 8. A gear torque transmission system for a drum motor, the system comprising a magnetic gear having a magnetic pole piece rotor connected to an output drive shaft of the drum motor.