GB2440400A

GB2440400A - Starting a rim driven pm motor by an associated induction motor

Info

Publication number: GB2440400A
Application number: GB0614802A
Authority: GB
Inventors: Stephen Mark Husband; Alexander Charles Smith; Nigel Schofield
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 2006-07-26
Filing date: 2006-07-26
Publication date: 2008-01-30
Also published as: GB0614802D0

Abstract

In situations such as marine propulsion the advantages of a rim driven permanent magnet motor propeller for providing a motor propulsion arrangement have been identified. However, it is difficult to initiate drive of such permanent magnet motors without use of a cumbersome and complicated converter. By providing a combination of a permanent magnet motor with an induction motor in association initial driving of the propeller and therefore the permanent magnet motor up to synchronisation can be achieved by the induction motor. Once near synchronisation has been achieved the permanent magnet motor will then continue operation and drive of the propeller in the normal way. The induction motor association may be provided as a line-starting induction motor in association where there is a conductive can about the permanent magnets to create magnetic interaction with circulating magnetic fluxes created by electrical currents passing through coils about the conductive can. Alternatively, the inductive motor association may be provided by an appropriately rated separate inductive motor coupled to the permanent magnet motor through a drive train and appropriate gearbox to allow a reduced rating for the induction motor.

Description

PROPULSION MOTOR

The present invention relates to propulsion motors and more particularly to permanent magnet propulsion motors utilised in ships and other marine vehicles.

Marine applications are increasingly considering electrical propulsion solutions, for not only auxiliary propulsion but also primary propulsion. The primary solutions are either the provision of a converter driven variable speed propulsion motor solution or a fixed speed and fixed voltage (Direct On Line [DOLl) solution requiring a controllable pitch propeller (CPP) . Typically, an in-board motor via a shaft creates drive through a propeller hub and via a gearbox within the propeller hub.

An alternative is to provide so called rim driven motors using permanent magnet technology. Rim driven motors are more efficient in terms of on-going operation.

Typically, the propeller arrangement comprises a number of blades fastened between a hub and a rim, such that by rotation of the propeller formed by the blades between the hub and rim thrust is provided. The rim, generally in accordance with known electrical propulsion solutions, provides a location for electromagnetic interaction in order to drive the rotation and therefore thrust to the propeller in use. The propeller is generally secured within a thrust tunnel or water duct in order to direct thrust as required. With a permanent magnetic motor arrangement electrical coils are appropriately pulsed and energised in order to interact with permanent magnets secured about the rim to drive rotation. In such circumstances, there are significant advantages with respect to control of thrust in use.

In accordance with certain aspects of the present invention there is provided a propulsion motor arrangement comprising a permanent magnet motor and an induction motor in association coupled to a drive mechanism, the association comprises a conductive can about the permanent magnet motor to generate an inductive magnetic field in the conductive can to develop a start torque to drive the permanent magnet motor into synchronism.

Possibly, the drive train includes a gearbox.

Advantageously, the gearbox provides mechanical advantage to drive the drive mechanism by an otherwise unacceptable induction motor association in terms of torque capacity.

Possibly, the induction motor association comprises a permanent magnet stator arrangement and the conductive can comprises an iron layer. Additionally, the conduction can provides an environmental containment for the motor arrangement.

Also in accordance with the present invention there is provided a propulsion arrangement for a ship comprising a propulsion motor arrangement as described above and a propeller secured to the drive mechanism.

Typically, the permanent magnet motor is formed by having a rim about the propeller having a plurality of permanent magnets and a motor stator having electric coils mounted in the propeller duct (or shroud) to drive movement by electrical power presented to the electrical coils interacting with the permanent magnets associated with the propeller.

Possibly, the propeller incorporates controllable pitch blades to facilitate starting of the drive mechanism by varying the load upon the propeller.

Embodiments in accordance with certain aspects of the present invention will now be described by way of example only, with reference to the accompanying drawings in which: -Figure 1 is an exploded view of a rim driven motor arrangement; Figure 2 is a schematic front view and side cross-sectional view of a propeller arrangement; Figure 3 is a schematic side cross section of the propeller depicted in Figure 2; and, Figure 4 is a schematic side cross section of an alternate motor arrangement in accordance with certain aspects of the present invention.

Rim driven technology involves the installation of the motor rotor with permanent magnets to the outside of the propeller blades and the installation of the stator into the surrounding duct. To fulfil this requirement all the stator and rotor parts must be protected from the environment and sufficient mechanical clearance must also be provided. This necessitates a large electromagnetic air gap such that a permanent magnet motor solution is preferred. Conventional permanent magnet propulsion motors require a converter to start and control the motor. The use of this converter not only increases the overall in-board footprint, but more significantly the cost. A motor connected directly to the power system is significantly cheaper and in terms of in-board space requirement far less demanding. However, the converter does minimise the overall starting current requirement that would otherwise be required for fixed speed motors which is typically 6x rated electrical current.

As indicated above it is known to provide rim driven propulsion mechanisms utilising permanent magnet motors.

Examples of such rim driven propulsion motor arrangements are provided in Patent Nos. US5252875 and US6837757.

However, the problem with such permanent magnet propulsion arrangements are the difficulties with regard to synchronising coil and magnetic interaction.

It should be understood that a motor arrangement is typically for a propeller drive mechanism. At a base level this propeller drive mechanism may simply comprise directly coupling a propeller to a shaft driven by the motor arrangement or a propeller itself driven through a peripheral shroud. In addition the mechanism may have the propeller including magnets which are part of a permanent magnet rotor driven by appropriate excitation of coils in a stator rim about the propeller shroud. An induction motor provides a driving motion by applying a voltage/current to the stator in order to turn a rotor by induced electromagnetic effects whilst a permanent magnet motor comprises a large number of permanent magnets secured to the periphery of a rotor and a stator arranged around that rotor and the stator incorporating electrical coils. The stator coils are normally arranged in an opposed relationship and are provided with pulses of alternate electrical energy in order to cause sequential excitation and therefore produce resultant torque forces for rotation of the rotor. The operational performance of an induction motor is directly related to the slip (synchronous speed -actual speed divided by synchronous speed) and therefore an induction motor is capable of generating torque at all speeds except synchronous speeds. For an effective large air-gap machine the operational performance of an induction motor is inefficient and a permanent magnet motor solution is much preferred. A permanent magnet motor is a synchronous machine being able to generate torque only at synchronous speeds. In such circumstances by combining permanent magnet motor and induction motor technology a DOL starting permanent magnet solution can be realised. This solution will reduce the overall cost and volume associated with the motor arrangement by removing the need for a converter for the permanent magnet motor. Reducing the starting current also minimises the impact on marine generating systems which can provide further savings and performance benefits.

In accordance with certain aspects of the present invention there are provided two embodiments. In one embodiment a hybrid arrangement is provided whereby possibly a significantly underrated in-board induction motor is used along with a rim drive permanent magnet motor. In a second embodiment there is provided a line-starting rim driven permanent magnet motor. Line starting permanent magnet machines offer the opportunity to operate at fixed voltage and frequency direct from the power supply system. The design of such a motor utilises induction motor technology for starting and permanent magnet technology for normal operation. Line starting permanent-magnet motors have been provided in earlier patents TJS5548172, US5952757 and W002061918, but these have not been applied to rim driven propulsion technology for marine applications. In addition patents US5548172 and US5952757 utilise a squirrel cage rotor design that is undesirable in that it interferes with the requirements for the permanent magnets. The preferred design incorporates a solid rotor design. Patent W00206l918 incorporates such a solid rotor design but the solid rotor construction concentrates on providing a high corrosion resistance solid rotor to enable pump operation in harsh environments. It will be understood that a thick corrosion resistant rotor is desirable for harsh pumping environments but the losses through such a thick section would be unacceptable for motor propulsion efficiency. This does not provide the design freedom to minimise starting current requirements as required for marine operation.

By aspects of the present invention the advantages of a permanent magnet motor for ongoing operation are combined with the advantages of an induction motor during initial starting of a drive mechanism. As outlined above generally an induction motor in the form of an association can be provided either in line with a permanent magnet motor or the induction motor associated through a drive chain or train with the drive mechanism and permanent magnet motor.

As indicated above operation of permanent magnet motors and induction motors is well known, and reference to such operation is incorporated into the present

description.

Figure 1 provides an exploded illustration of a rim driven permanent magnet motor arrangement 1 in which a rotor 2 housing incorporates permanent magnets in a shroud portion about blades 3. The blades 3 are fastened on a hub and a rim of the rotor housing 2. In use, there is a water gap 4 between the rotor 2 and a stator 5 which includes a plurality of electrical coils 6. In operation these electrical coils 6 interact with the permanent magnets (not shown) in the rotor 2 in order to drive rotation of the blades 3. It will be appreciated that the coils 6 are appropriately coupled to wiring 7 to enable operation of the coils 6 to drive rotation of the rotor 2. It will be understood that generally an interface 8 will be provided to allow association of the arrangement 1 within a water duct or tunnel for propulsion. Appropriate protection in terms of a sleeve 9 will be provided to protect the rotor 2. The blades 3 are fastened to a rotor hub that is surrounded by the rotor rim on which are placed the permanent magnets. The permanent magnets are enclosed by a rotor rim for the purpose of protecting the rotor iron and permanent magnets from the sea water. This complete structure 2 forms the rotor. As will be described later this sleeve 9 in accordance with aspects of the present invention can take the form of a magnetically conductive and inductable can which is utilised in order to provide an induction motor association to generate a torque to drive initial rotation of the rotor 2 until the permanent magnets (not shown) in the rotor 2 latch with the appropriate synchronised driving forces provided by the coils 6 when sequentially activated by alternating electric charge pulses. Other elements are provided within the arrangement 1 in order to provide sealing and bearing surfaces appropriately.

Referring to Fig. 2, a feature is that the conducting can 24 enables the generation of an inductive magnetic field circuit through the can 24 to provide a starting torque as an induction motor. This starting torque enables the propeller 20 to be accelerated from standstill to near synchronism where the permanent magnets 27 are able to latch into synchronism with electrical induction provided by sequentially executed coils.

The conducting can depth should be minirnised to reduce normal operating losses across the can and the overall electromagnetic airgap requirement.

The can depth needs only to provide starting torque and may thus be significantly underrated. The ability to underrate the design is also enhanced by the improved cooling provided by seawater.

Some design freedom is provided with regard to the rotor can resistivity. A higher electrical resistance rotor can leads to a reduced starting or in-rush current value.

Current rim driven permanent magnet propulsion motor rotors are environmentally sealed around the periphery by a water proofing composite can. The conducting can in accordance with aspects of the invention replaces the composite can.

Upon initial start up the permanent magnets within the rotor interact with localised rotating magnetic fields induced in the conductive can by electrical currents in the stator coils. As indicated previously these rotating magnetic fields in the conductive can draw high electrical voltages and currents as an induction association to create a torque force for initialising rotation of the rotor.

Once the rotor, through the induction association starting torque, has been accelerated to near synchronisation, it will be understood that the motor arrangement will be arranged to operate substantially as a permanent magnet synchronous motor with electrical alternating current pulsing in appropriate phasing in the coils to drive rotation. In such circumstances, as described above, as the induction association in the conductive can in accordance with aspects of the present invention is only operated for a relatively short period of time, this association can use high voltages and currents to generate a high starting torque as the induction association will only use such levels of electrical power for short periods of time. It will be appreciated that longer term operation of such underrated inductive motor associations would be inefficient and unacceptable.

An alternative embodiment of aspects of the present invention is shown in Figure 4. This embodiment utilises a more conventional propulsor motor arrangement whereby a DOL in-board inductive starting motor starts the propeller via a drive shaft through the stator vanes and propeller hub and via a gearbox within the propeller hub. Once the propeller is at rated speed then the rim driven propulsion motor can be operated by synchronisation of electrical pulses to electrical coils for interaction with the permanent magnets.

As the in-board motor is only required for starting it can be significantly underrated in that it is only required to develop a very high torque during starting and not during normal operation. Consequently the volume occupied by the in-board motor and its cost is reduced. Further as the machine rating is reduced (typically by a factor of 3 without CPP and by a factor of 10 with CPP) then the starting current requirement is correspondingly reduced, removing the requirement for a starter unit.

The performance requirement for the shaft and gearbox connecting the in-board motor to the propeller is also reduced. Additionally the in-board motor provides redundancy for emergency propulsion and can be used nominally in an and'' configuration to reduce the rim driven requirements thus saving weight, volume and cost for the rim driven motor.

As indicated above Figure 4 provides an illustration showing a motor arrangement in accordance with certain aspects of the present invention in which there is essentially provided an induction motor 41 with a permanent magnet in association with a propeller. A drive train 43 is provided to enable the induction motor 41 to initiate drive of the propeller until the permanent magnet motor 42 is able to synchronise between the permanent magnets and the excitation coils to provide an ongoing driving force to the propeller.

The induction motor association 41 comprises a conventional construction which has a reduced rating in view of the gearing effect provided by the drive train 43 enabling a starting torque to be provided to the propeller and therefore the permanent magnet motor 42. The induction motor association 41 has a conventional form. Thus, a shaft 44 is directly coupled to a rotor 45 with a stator housing 46 about the rotor 45. In such circumstances, an electrical current is passed through an end winding 48 around the stator housing 46 through a terminal block 47.

In such circumstances through magnetic interaction between the rotor 45 and the stator housing 46 rotation of the rotor 45 is achieved and therefore the shaft 44. The whole assembly is secured in a frame which is mounted into a ship's hull or a structure by a mounting 50 and a bearing 51 is provided at either end of the shaft 44.

The drive train 43 as indicated generally transmits rotation from the shaft 44 to a gearbox 52 which in turn has a shaft 53 to drive rotation of a propeller (not shown). This propeller will have a shroud incorporating permanent magnets and a surrounding stator with excitation coils to drive that propeller through a permanent magnet motor combination in accordance with convention and practice. In such circumstances it will be appreciated that electrical power will be provided through the cables within the propeller duct housing to the stator coils for the permanent magnet motor. In order to avoid drag typically the shaft 53 will be disengaged by an appropriate clutch above a rotational speed at which the permanent magnet motor will be operational.

As can be seen the drive train 43 will typically take the form of a hydro pod for appropriate orientation. This pod may be rotatable about its mounting to vary the direction of the propeller.

Normally, the conductive can will be made of a metal or metal alloy but it is possible the can could be formed from graphite or some other suitably conductive material.

Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

Claims

Claims: - 1. A propulsion motor arrangement comprising a permanent

magnet motor and an induction motor in association coupled to a drive mechanism, the association comprises a conductive can about the permanent magnet motor to generate an inductive magnetic field in the conductive can to develop a start torque to drive the permanent magnet motor into synchronism.

2. An arrangement as claimed in claim 1 wherein the induction motor association comprises a separate induction motor coupled to the drive mechanism through a drive train.

3. An arrangement as claimed in claim 2 wherein the drive train incluces a gearbox.

4. An arrangement as claimed in claim 1 wherein the induction motor comprises a permanent magnet stator and the conductive can comprises an iron layer.

5. An arrangement as claimed in claim 4 wherein the conductive can provides an environmental containment for the motor arrangement.

6. An arrangement as claimed in any preceding claim wherein the induction motor is utilised as an auxiliary drive for the drive mechanism should the permanent magnet motor fail.

7. An arrangement as claimed in any preceding claim wherein the conductive can is made of a high electrical resistance material to limit electrical current of the induction motor in association.

8. A propulsion motor arrangement for a marine vehicle substantially as hereinbefore described with reference to the accompanying drawings.

9. A propulsion arrangement for a ship comprising a propulsion motor arrangement as claimed in any preceding claim and a propeller secured to the drive mechanism.

10. An arrangement as claimed in claim 9 wherein the permanent magnet motor is formed by having a rim about the propeller having a plurality of permanent magnets and a motor stator having electric coils to drive movement by electrical power provided to the electrical coils interacting with the permanent magnets associated with the propeller.

11. An arrangement as claimed in claims 9 or claim 10 wherein the propeller incorporates controllable pitch blades to facilitate starting of the drive mechanism by the induction motor in association.

12. A propulsion arrangement for a ship substantially as hereinbefore described with reference to the accompanying drawings.

13. Any novel subject matter or combination including novel subject matter disclosed herein, whether or not within the scope of or relating to the same invention as any of the preceding claims.