WO1988008636A1

WO1988008636A1 - Eddy-currents machine for drive and braking purposes

Info

Publication number: WO1988008636A1
Application number: PCT/GB1988/000332
Authority: WO
Inventors: Martin Boughtwood; Adam Alasdair Pride
Original assignee: Froude Consine Ltd
Priority date: 1987-04-30
Filing date: 1988-04-28
Publication date: 1988-11-03
Also published as: CN1030674A

Abstract

The invention provides an electrical machine having a rotor (5) rotatable on a shaft (4) between two stators (21). The stator can be energized by A.C. passed through coils (24) so that the machine will act as a motor and by D.C. passed through the coils (25) so that the machine will act in a braking mode.

Description

Eddy-currents machine for drive and braking purposes

DESCRIPTION

This invention relates to apparatus that can be used either as a dynamometer or as a motor. It seeks to provide an apparatus which can absorb rotational energy (i.e. act as a dynamometer) or produce rotational energy (i.e. act as a motor) as required. 5 As the actual performance and exhaust emissions of an engine become increasingly important given the rapidly more restrictive legislation which is coming into force throughout the world, there is an ever increasing need for a dynamometer which is able to load the engine in a manner which can correspond to actual 0 operational conditions. In particular it means that the dynamometer should be capable of motoring the engine (to simulate, for example, when a car is going down hill) and retarding, i.e. loading, the engine (to simulate, for example, when a car is accelerating). 5 According to the present invention there is provided an electrical machine comprising a magnetic disc rotor mounted for rotation between a pair of stators, each stator comprising coils which may be energized by alternating current so that the machine acts as a motor and further coils which may alternatively be 0 energized by direct current so that the machine acts in a braking mode, with absorbed energy being dissipated in the rotor as heat.

An embodiment of the invention will now be described with reference to the accompanying drawing which shows a section in a plane that contains the axis of a rotor.

The apparatus comprises a bedplate 1 carrying two trunnions 5 2 between which, and mounted in bearings 3, there extends a horizontal shaft 4. Mounted on the shaft 4 is a plane, right circular, disc rotor 5 that is magnetically and electrically homogeneous and made of a material with a low electrical resistivity and high magnetic permeability. In use, the shaft 4 0 is coupled to an engine that is to be tested. Boundary walls, indicated generally at 10, define a coolant space 11 in which the rotor 4 rotates. The boundary walls 10 include two annular shims 10a, parallel to the rotor 5. At their inner edges they are connected to walls 10b that enclose the 5 shaft 4 and an inlet 12 for cooling water discharges into the part of the coolant space that lies within the walls 10b. The coolant flows thence over the rotor 5, between the shims 10a and discharges into the annular duct 14 that surrounds the rotor 5, from which coolant discharges through the outlet 16. The shims

10 10 are provided with radial grooves to guide the cooling fluid and give rigidity to the barriers that are formed by the shims.

In this particular embodiment, the rotor 5 lies between two stators 20 in the form of a plurality of cores 21 uniformly distributed around the axis, with a core on one side of the rotor

1.5 aligned with a core on the other side. Annular boundary walls 22a and 22b which may be of stainless steel, and define a stator space 23 are sealed to the cuter ends of the cores 21 and, at their inner edges, the shims 10a. The inner edges of the boundary walls may be turned in, and the inwardly-turned margins

20 connected to the shims 10a by electron beam welding. By having the shims and the boundary walls to which they are welded of the same material, their connection is facilitated and thermal stresses are minimised-.

Each core 21 is provided with a coil 24 that is connected to 25 be energized from a variable source of A.C. and a coil 25 that is conneted to be energized from a variable source of D.C. The stator space 23 is filled with a heat transfer material; oil and sand are suit.able such materials, and heat may be removed from the stator space by conduction through the shims 10a into the 30 coolant. When the apparatus that has been described is used in a retarding mode eddy currents are set up in the rotor 5, creating considerable heat. The heat is removed by the coolant, and by efficient cooling the diameter of the rotor may be small because it is feasible to dissipate very high power in the rotor. This means that the mechanical strains on bearings and other elements of the machine are greatly reduced with the result that the machine can operate at high speeds. Moreover, the rotor has a low inertia which permits rapid acceleration and deceleration. The shims 10a may be stiffened in regions where they are not in direct contact with other boundaries by toughened epoxy resin applied to the stator side of the shims 10a.

The stator may be formed of laminated steel, or cast from a resin material heavily loaded with iron particles which are substantially electrically insulated one from another. By such constructions, it is possible to lessen the formation of eddy currents within the stator.

The coils that are fed by A.C. are divided into two or more electrical phases and distributed so that one or more pairs of magnetic poles of opposite polarity (i.e. one pair comprises one 'north' pole and one 'south' pole) are produced when the windings are fed from a source of alternating current (with the same number of ph.ases); then the magnetic poles (or 'primary magnetic field' ) will rotate around the axial face of the stator. As viewed from this axial face the rotation of the magnetic poles is in the opposite sense in each stator (i.e. in one stator the poles rotate clockwise, in the other they rotate anticlockwise). Therefore when the stators are placed face to face either side of the rotor, the primary magnetic fields will both be rotating in the same direction.

The frequency of the alternating current may be varied from zero to the desired maximum.

The frequency is controlled so that the rotational speed of the primary magnetic field is equal to or higher than the rotor's speed. When there is a difference in speed between the two electrical eddy currents will be induced to flow in the rotor. The eddy currents produce their own, secondary, magnetic field, which interacts with the primary magnetic field to produce a motoring torque on the rotor.

The motoring torque is controlled by varying the magnitude of the alternating current flowing in the windings (the.higher the current the higher the torque), and its frequency. As the frequency is increased the torque produced will vary in accordance with the natural torque/speed characteristic of the machine. The rotor speed is controlled by varying the frequency of the alternating current - the higher the frequency the higher the speed. The machine is bi-directional and so with suitable phase relationship of supply can be made to rotate in either direction.

The D.C. winding comprises a series of coils distributed so that one or more pairs of magnetic poles of opposite polarity (i.e. one^"pair comprises one 'north' pole and one 'south' pole) are produced when the windings are fed with electric current. The windings are fed with direct current and so the magnetic poles (or 'primary magnetic field' ) are stationary with respect to the stator.

As the rotor rotates any particular section will experience an alternating magnetic field as it passes the magnetic poles and this will induce eddy currents to flow within the rotor. The eddy currents produce their own, secondary, magnetic field, which interacts with the primary magnetic field to produce a retarding torque en the rotor. This retarding torque is controlled by varying the direct current in the windings. The eddy currents also produce resistive heating in the rotor and this heat is carried away by the cooling fluid flowing over the faces of the rotors.

An electronic control unit may be provided that is able to vary the polarity, frequency and magnitude of the alternating current continuously e.g. from a polarity which gives clockwise rotation of the primary magnetic field at maximum frequency through zero frequency to a polarity which gives anticlockwise rotation of the primary magnetic field at axiitum frequency, whilst also controlling the magnitude of the current fr n zero to the desired maximum. The control unit may be simultaneously capable of controlling the D.C. current/voltage so that dynamic changes frcm a motoring mode to a braking, or absorbing, mode is possible at any speed.

Claims

Claims:

1. An electrical machine comprising a magnetic disc rotor mounted for rotation between a pair of stators, each stator comprising coils which may be energized by alternating current so that the machine acts as a motor and further coils which may be energized by direct current so that the machine acts in a braking mode, with absorbed energy being dissipated in the rotor as heat

2. An electrical machine as claimed in claim 1 in which the coils that may be energized by direct current are uniformly distributed around the .axis, with a coil of one stator opposed to a coil of the other stator, and are such that on energization - axially aligned poles are of opposite polarity across the rotor.

3. An electrical machine as claimed in either of the preceding claims in which the rotor is contained in a coolant space through which coolant can flew from an inlet to an cutlet.

4. An electrical machine as claimed in claim 3 in which the boundaries of the coolant space include two thin annular walls each parallel, and close, to the rotor.

5. An electrical machine as claimed in claim 4 in which the coils are included in a stator space of which the boundaries are in part constituted by'the thin walls.

6. An electrical machine as claimed in claim 5 in which the connections between the thin walls and other boundaries of the stator space are effected by an electron beam or other precision welding technique.

7. An electric machine as claimed in any of claims 4 to 6 in which the thin walls are radially fluted.

8. An electric machine as claimed in claim 5 or any claim appendent to claim 5 in which the stator space contains oil, sand, or other heat transfer material by which the coils are surrounded.