Cobalt-Platinum Rotors for Small Synchronous Devices
Cobalt-Platinum Rotors for Small Synchronous Devices
Economics of Design and Construction
The unique magnetic properties of the cobalt-platinum alloy Platinax II enable small permanent magnets to be made which will produce a high magnetic flux across a large air gap. A small cylinder of this alloy magnetised across its diameter, produces a two pole magnet with an ideal field geometry for the rotor of a synchro-transmitter. Used in conjunction with Hall effect elements the design of such devices can be simplified as the need for windings of very fine insulated copper wire on laminated rotors, used in conventional motors, is eliminated. The increasing demand for miniaturisation of electronic components may make it necessary to reduce still further the size of servo components, and the ductility and machining properties of cobalt-platinum make it an ideal material from which to make the permanent magnet rotors.
It is usual to consider the use of cobalt-platinum permanent magnets only in very small devices. There are increasing circumstances in which it may be economical to use them in the rotors of very small machines. One such machine being studied is a small synchro-transmitter using a permanent magnet rotor and Hall elements instead of stator windings.
Most rotating machines require a flux density in the air gap which approximates very closely to a sine function of position. Rotor windings excited by d.c. or a.c. are commonly used to produce the flux, but in many cases permanent magnet rotors are better. When very small two-pole rotors are required to produce a sine shaped field of great accuracy, cobalt-platinum has distinct advantages.
It is well known (1), but remarkable, that certain symmetrical bodies attain constant magnetic conditions throughout when placed in a uniform magnetising field. Of these the ellipsoid is one, if one of its axes is in the direction of the magnetising field. The sphere is a special case of the ellipsoid. The cylinder is another, provided that its axis is at right angles to the magnetising field. This is of great practical importance, because accurate cylinders are easily produced. Fig. 2 shows a cross-section of a cylinder of ferromagnetic material under the influence of a uniform magnetising field. Although the resultant field just outside the cylinder is greatly distorted, that within the cylinder is quite uniform.
Two-pole Permanent Magnet Rotors
If the material of a cylinder has permanent magnet properties and the external magnetising field is removed, the cylinder will remain uniformly magnetised, although at a reduced level. Thus a two-pole permanent magnet results, in which the flux density B and the magnetic field strength H are constant at every point inside the cylinder. The cylinder must be long enough for end effects to be neglected. Fig. 3 shows the new condition for the flux lines with the resulting point P(B1H1) on the B-H curve of the material.
The magnetic potentials appearing at the surface of the cylinder are proportional to the number of molecular magnets in series, and therefore to the lengths of the chords parallel to the diameter in the direction of magnetisation. In Fig. 4 let F1 be the magnetic potential at the point N, then if r is the cylinder radius and if Fmax is the maximum potential,
This is precisely the condition required to give the most desirable form to the external magnetic field (2), both for a small uniform air-gap machine and for a very large air-gap machine.
The equi-atomic cobalt-platinum alloy is a high-priced material. Its use can only be justified in rotors of the smallest size, and in applications where it has distinct advantages over other permanent magnet materials. The application described here is of interest in that the properties of this alloy are used to some advantage.
Some of the smallest electrical machines are to be found in the field of control. Position control systems employ synchronous devices to transmit angular position over a distance. In a conventional synchro-transmitter the wound rotor is excited by a.c., usually 50, 60 or 400 cycles per second. The stator may have either a two-phase or a three-phase winding. Ideally the output of a two-phase synchro-transmitter would consist of exact sine and cosine waves of equal amplitude if r.m.s. voltages were plotted versus rotor position.
Conventional synchros with insulated copper windings are being produced in which the overall diameter is only ½ inch. The difficulties encountered in winding these machines, and their consequent high costs, can well be appreciated. It would be a great simplification if the insulated copper windings and the slots could be replaced by more basic structures. In the Hall synchros being investigated, slotted laminations, windings, slip rings and brushes have been eliminated and replaced by Hall elements and permanent magnets. The construction is shown in Fig. 5.
The drawing shows two stationary Hall elements lying in the field of the permanent magnet rotor. The supporting stator material is non-magnetic and is not shown. Hall voltage is proportional to the instantaneous product of the control current passing through the element and the magnetic flux cutting the element. It does not depend on the rate of magnetic flux-cutting. If the control current (say 400 c/s) is held constant, then a 400 c/s Hall voltage is produced, which is proportional to the flux alone. By placing the two Hall elements at 90º, two 400 c/s voltages will be produced whose magnitudes will be sine and cosine functions of rotor position, exactly as described above for a conventional wound synchro. However, one difference is that the output of the Hall synchro-transmitter will only be about one-hundredth that of a wound synchro of similar size. This may be allowed for by providing an additional gain of 100 in the servo amplifier—a practical proposition due to the low noise level of Hall voltage output.
In order to check the accuracy of the two-phase output of the Hall synchro-transmitter, it is convenient to replace the 400 c/s control current by d.c., and to turn the permanent magnet rotor at constant speed (3). An oscillogram obtained in this way is shown in Fig. 6.
A wave analyser was used to investigate the harmonics present. A typical phase showed 0.7 per cent second harmonic, 0.6 per cent third harmonic, 0.1 per cent fourth, 0.2 per cent fifth, and 0.1 per cent or less for all higher harmonics. The importance of these harmonics is mentioned below.
Advantages of Cobalt-Platinum
Several advantages stand out for cobalt-platinum in the Hall synchro application being considered.
The output sensitivity of a conventional synchro is of the order of I volt per degree. The output of a Hall synchro is of the order of 10 millivolts per degree. To achieve a reasonable output it is desirable to keep the flux density in the air gap as large as possible. However, for reasons of overall synchro performance it has been found best to use very large air gaps (2) in spite of their high reluctances.
Fig. 7 shows that permanent magnetic materials with the highest remanence produce the greatest flux density in magnetic circuits of low reluctance, but that high coercive force is most important in circuits of high reluctance. Cobalt-platinum has a coercive force approximately three times that of any common material, and the flux produced in a large air gap is about three times that possible with other materials.
Another consideration concerns the accuracy of performance of synchro systems. When synchro-transmitters give accurate sine outputs from each phase (r.m.s. voltage versus rotor position) synchro receivers can follow to within ten minutes of arc of rotation. The magnitudes of the harmonics in each phase output are closely related to the accuracy of machining of the rotor. In contrast with the Al-Ni-Co series of permanent magnets, cobalt-platinum is ductile in the quenched state and easily cold worked and machined. In the heat treated state it can be ground, hence greater machining accuracy may be attained.
Some examples of cobalt-platinum rotors are shown in the illustration on page 115. The largest is built up from a cobalt-platinum centre piece and duralumin end pieces, the latter forming the shaft. In the smaller sizes there is no step from the rotor diameter to the bearing diameter, one solid piece of cobalt-platinum being used. The weight of the smallest rotor, inch in diameter and 1.5 inches long, is only ounce. The cost of this in a small synchro makes its use a possibility on economic grounds.
The work described above indicates that cobalt-platinum is a suitable material for use in very small two-pole permanent magnet rotors. In synchro applications, looking to the future, it may be possible to use rotors needle-like in size and dimensions, and cobalt-platinum would be a good material of which to make them. For it has been shown that when a cylinder is magnetised across a diameter it forms an almost ideal two-pole rotor, whose angular position may be uniquely registered and transmitted by Hall elements suitably placed in its field.
The investigation was initiated in the laboratories of Muirhead & Co Ltd, and is continuing at the University of New South Wales.
J. C. Maxwell, “A Treatise on Electricity and Magnetism”, Oxford University Press, 1873
B. Inglis Ph.D. Thesis (in preparation): A Study of Hall Effect and Related Phenomena, with Particular Reference to Position Control Servo-mechanisms (University of NSW)
G. W. Donaldson, The Application of Hall Effect to Control Synchros, Electronic Engng, 1963, 35, No. 423, May