** DESCRIPTION

This invention relates to dynamometers and is intended to provide such apparatus that can also be used as a motor. According to the present invention, there is provided a rotary electric machine in which heat is generated due to 5 electrical eddy currents when mechanically driven and can also be used as a multiphase motor when supplied from a source of alternating current, and means by which the machine can be supplied with alternating current at a frequency that can be varied at will. 10 When the frequency is high relative to the rotor speed, the rotor will be driven and the frequency of the alternat¬ ing current supply may be reduced so that the rotor speed will be reduced.

By way of example an embodiment of the invention will 15 now be described with reference to the accompanying drawing, in which:

Figure 1 shows a rotary electric machine in side view, partly cut away;

Figure 2 shows a section through that machine on the 20 line II - II of Figure 1; and

Figure 3 shows in an isometric view of a stator included in the machine shown in Figures 1 and 2.

The machine illustrated provides a horizontal shaft 1 extending across a casing 2 that is preferably made of 25 a non-magnetic material with a high eleqtrical resistivity. The casing is mounted between trunnion bearings 2a so that

4) it is able to rotate about the rotor axis. The rotation is prevented by means of a load measuring device e.g. a load strain gauge load cell, and thus the torque reaction between 30 the rotor 3 and stators 4 (which are to be described) may be measured.

Mounted on the shaft 1 within the casing 2 is a plain disc rotor 3 that is magnetically and electrically homo¬ geneous and made of a material with a low electrical resis¬ tivity and high magnetic permeability. The rotor 3 lies parallel to, and mid-way between, two electrically magnetis- able stators 4.

The bearings 5 on which the shaft is mounted are cooling fluid shaft seal assemblies which provide seals between the shaft 1 and the stators 4 and act to keep the water by which the machine is cooled away from the bearings. The cooling water enters the casing 2 through the inlets 6, flows over the rotor 3 and the stators 4 to escape to the outlet 7 through the external passage 8.

Each stator 4 comprises a ring 9, spirally laminated, or otherwise constructed so as to minimize the formation of electrical eddy currents within it. Radial slots 10 are formed in one face of the ring 9 and an electrical winding 11 ^" is laid in the slots 10 and at the inner and outer circumference of the ring 9. The winding 11 is connected into three electrical phases (although more or less are envisaged) and distributed so that pairs of magnetic poles of appropriate polarity (one pair provides one "north" pole and one "south" pole) are produced when the windings are fed with electric current. A control unit indicated at 12, is provided whereby the winding may be supplied with alternating current at a frequency and current that is variable from zero to the desired maximum. When the winding is fed from a source of alternating current (with the same number of phases) then the magnetic poles (or "primary magnetic field") will rotate about the axial face of the stator.

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As viewed from this axial face the rotation of the magnetic poles is in the opposite direction in each stator 4 (i.e. in one stator the poles rotate clockwise, in the other they rotate anticlockwise). Therefore when the stators 5 4 are placed face to face either side of the rotor, as shown, the primary magnetic fields will both be rotating in the same direction.

Now if the rotational speed of the primary magnetic field is different from the rotational speed of the rotor 3

10 then electrical eddy currents will be induced to flow within the rotor 3. These eddy currents produce their own, secondary, magnetic field which interacts with the primary magnetic field to produce a torque on the rotor 3.

If the primary magentic field rotates faster than the

15 rotor 3 and in the same direction then there will be a motoring torque on the rotor 3. If the primary magnetic field rotates slower than, or in the opposite direction to, the rotor 3 then there will be a retarding torque on the rotor 3 and the machine absorbs power from the prime mover

20 and will act as a dynamometer.

Suppose that the rotor 3 is motoring and it is desired to apply a retarding force, the frequency of the alternating current supplied from the device 12 is varied so that the rotational speed of the primary magentic field is reduced

25 to below that of the rotor. In this case some of absorbed power is returned to the power supply and some is dissipated as resistive heating in the rotor. The retarding torque on the rotor is controlled by varying the magnitude of the alternating current in the windings (the higher the current,

30 the higher the torque) and its frequency. As the frequency is decreased to reduce the rotational speed of the primary

4 magnetic field in relation to the rotor speed, the torque will vary in accordance with the natural torque/speed characteristic of the machine.

With the rotor retarded by the change in frequency, additional retardation could be provided by the cessation of the A.C. supply and the substitution of a D.C. supply only. In such circumstances, most of the absorbed power is dissipated as resistive heating in the rotor. Power not dissipated in this way is dissipated in drag losses in the cooling fluid, bearing loss, noise, vibration etc.

The control unit 12 is electrically governed and able to vary the polarity, frequency and magnitude of the alter- nating current continuously e.g. from a polarity which gives clockwise rotation of the primary magnetic field at maximum frequency through zero frequency (DC) to a polarity which gives anticlockwise rotation of the primary magnetic field at maximum frequency, whilst also controlling the magnitude of the current from zero to the desired maximum.

The current's polarity, frequency and magnitude may be changed whilst the rotor 3 is rotating i.e. the machine's function may be changed from absorbing to motoring and vice-versa without bringing the rotor to rest.