Disc-geometry homopolar synchronous machine - IET Digital Library

Discussion on
Disc-geometry homopolar synchronous machine
B. Capaldi (Pendar Technical Associates Ltd) (supplementary
presentation): Sealed Motor Construction Co.
Ltd has been manufacturing disc induction motors for 20
years for use with hydraulic pumps, and two of the major
factors that have influenced the design philosophy have been
the introduction of new materials and new manufacturing
techniques.
The existence of large unbalanced magnetic forces between
the rotor and shaft has meant that care must be taken when
designing bearings. It is usually possible for the axial force
produced by the impeller operating under the required head/
flow conditions to offset some of the UMP. A common
bearing material mix in the early 1960s was Polypenco running
on hardened-steel shafts. The hardened-steel shaft was quickly
ground away by magnetite, present in the water, becoming
embedded in the soft Polypenco. To alleviate this problem,
the disc equivalent of induction motor was replaced by a
copper disc running between a laminated stator and a castiron
return path. This improved the bearing life but clearly
reduced the overall performance: (a) by reducing the motor
efficiency, and (b) by increasing the hydraulic losses. The
advent of ceramic materials, mass produced to reduce losses,
meant that, by the late 1970s, the induction motor was beginning
to replace the eddy-current type motor.
In a similar fashion, the ability to cast copper into
machined or punched slots now means that greater attention
can be paid to slot shape and distribution of conducting
material on the rotor.
H.R. Bolton (Imperial College, London): Disc motors
are now an important part of the rotating-machines scene
and the authors are to be congratulated on their interesting
and valuable contributions. I would be interested in further
information on a number of points. Paper 895B deals with
a solid-rotor, brushless alternator possessing a novel and
ingenious configuration. Could the authors express an opinion
about the suitability of the machine (or improved forms
of it) for aerospace applications? Could the flux barrier
on the rotor in Fig. 2b be made any wider in order to reduce
X q further? Is operation at unity or leading power factor
feasible with the existing excitation design? What tooth
construction is employed to give the semiclosed slots shown
on Fig. 1 c? Is it possible to design for reasonably high values
Of mean gap flux density, given the possibility of flux throttling
in or near the centre boss? Finally, could the authors give
a brief idea of how the new machine compares, e.g. in power/
weight terms, with previously developed solid-rotor brushless
alternators?
Dr. Capaldi's contribution adds considerably to the rather
scanty literature dealing with the widely used single-phase,
capacitor-run, sealed-rotor, disc induction motor. Could he
state the method of dealing with axial forces which, although
made fairly small by appropriate pump and motor design
features, may still be significant? Is the stainless-steel rotor
cap magnetic? Is the use of simple solid-iron rotor feasible or
would torque levels be too low at reasonable speeds? Have
the possibilities offered by permanent magnet synchronous/
induction or reluctance/induction rotor designs for performance
per cost improvement been examined?
C.F. Amor (Design Council): I am interested in the
method of stator-core production by punching strip and
Paper 895B by EVANS, P.D., and EASTHAM, J.F. [See IEEPROC. B
Electr. Power Appl., 1980, 127, (5), pp. 299-307.] Read before IEE
Power Division Professional Group PI, 9th March 1981
coiling, described by Dr. Capaldi. I believe that an overseas
development uses this type of core with a disc-geometry
squirrel cage for rather larger powers than the central heating
pumps, and I should be pleased to know if he or another
speaker has any knowledge of this.
Another speaker questioned the economics of this construction,
and it appears that a parallel may exist in transformer
production, where, for many years, toroidal cores
have existed but have generally proved more costly than
stacked laminations. Recently, however, some small toroidal
transformers seem to be competitively priced with, hopefully,
some attendant advantage in weight or losses.
N. Bridle (Lee-Dickens Ltd): My employers have a
current interest in electric drive systems for road-going
vehicles. During the presentation, Dr. Evans suggested and
illustrated an application using the disc motor as the traction
motor for a road vehicle. The machine described appeared
to be optimised as a constant-speed device, yet no fully
variable-speed gearbox was illustrated in this application.
Could Dr. Evans describe the application in more detail,
particularly the type of speed control system envisaged?
B. Capaldi (Pendar Technical Associated Ltd): Could Dr.
Evans please describe the techniques used to measure the
attractive force between rotor and stator. Could he also
indicate how accurately he was able to predict the forces,
their magnitudes, and how account was made for the effects
of saturation?
R.E. Steven (Southampton University): My early
interest in disc machines was about 10 years ago, but lapsed
owing to limited workshop facilities. Lately I have a renewed
interest. My first AC disc motor proved to be very similar
to, and contemporary with, Prof. Nasar's axial-flux machine
[A]. My machine gave a performance of 62.5% at a power
factor of 0.85 lagging and a power rating of about 900 W.
The shortcomings were complexity of manufacture and
cost of production. There was concern also about unbalanced
magnetic pull. In bulk, it hardly compared favourably with
a conventional cylindrical induction motor of the same
rating. An industrial observer commented, 'Have you costed
this design?'
With the intention of producing, hopefully, a disc profile
machine, I considered possible 'printed circuit' techniques
for prominently complex features, e.g. the 3-phase stator.
I examined a commercial 'printed circuit' DC motor armature.
To obtain conductor thickness, the 'printed' winding is, in
fact, punched from copper foil. The wave winding provided
a convenient repetitive profile for a cutting tool.
It may be noted, (a) this armature has no iron core, (b)
the machine (c. 1 kW) has efficiency of 80 + %, and (c) a
DC machine is an AC machine with a commutator.
My present design, untested at this date, has a central
'printed' circuit 3-phase stator winding in disc form. It is
readily adaptable to 1-phase, (split phase), operation. Rotor
plates (induction), or permanent magnets (synchronous),
are located on both sides, giving an overall disc shape.
Comments on this approach to the design of small AC
disc machines would be appreciated.
P.D. Evans and Prof. J.F. Eastham (in reply): In reply
to Dr. Bolton, much of the existing literature on this general
type of machine, which appears to have been most prolific
in the 1950s and 1960s, seems to be related to aerospace
IEE PROC, Vol. 128, Pt. B, No. 5, SEPTEMBER 1981 0143-7038/81/050269 + 02 $01.50/0 269

applications. The absence of brushes and the solid rotor
has made these machines very suitable for high-speed and
high-frequency operation. It is probably fairer to say, therefore,
that we have been learning from aerospace practice
rather than the other way round.
Some further optimisation of the flux barriers in Fig. 2b
is probably possible, but the gains achieved by increasing
their width become progressively smaller. This is because
the flux path across this barrier is in parallel with a path
through the central boss, via the auxiliary airgap.
The prototype machine was designed to operate at a
high lagging power factor. Further improvement of this
power factor, or achievement of a leading power factor
appears to be a matter of applying normal synchronousmachine
design methods.
The teeth are made from L-shaped laminations, as shown
in Fig. la. The outer three laminations on each side of a
tooth were made longer, and bent to produce the effect of
a tooth tip. A similar effect could probably have been achieved
by the use of saturable wedges.
The main constraint of the possibility of flux throttling
in the centre boss is the total amount of flux that is carried
by it. For a given amount of flux, any reasonable airgap
flux density can be achieved in the airgap, provided that
the core width is adjusted.
The machine described is of the homopolar variety in that
flux density excursions in the iron core are unidirectional.
It is necessarily, therefore, potentially heavier than conventional
machines of a similar rating. However, its form
of construction is such that it can be run at high speeds —
from a solid-state variable-frequency supply — so that its
power/weight ratio can be considered to be improved by
this means.
In reply to Dr. Capaldi, the attractive forces between
stator and rotor were measured by means of piezoelectric
force transducers and a charge amplifier. Three such transducers
were placed between the base of the stator and its
casing. The theoretical method of predicting these forces,
which is described in the paper, applies the principle of
Maxwell's stresses to a salient-pole machine with field
excitation. The axial force is expressed in terms of the d-
and #-axis reactance model of the machine. It is therefore
valid so long as the model is valid. Results presented in the
paper (Fig. 9) show that close agreement is achieved for
unsaturated conditions, but that the quality of predictions
deteriorates at higher levels of saturation (Section 5).
Refinements to the method described could probably be
made to improve these predictions, in the same kind of way
that allowances are made in the 2-axis model for the prediction
of torque etc. under saturated conditions. Even as it
stands, however, the model enables relatively reliable force
predictions to be made by simple calculations. It would
be expected that improved calculations could be made using
computer-based methods, but these would be expensive to
implement.
In reply to Dr. Steven, it was interesting to hear of his
own disc-motor activities, and we look forward to hearing the
outcome of his present work.
In reply to Mr. Bridle the disc machine described may
be thought of as a synchronous machine with an unusual
geometry. It may, therefore, be incorporated in a drive system
in the same sort of way that a synchronous machine might
be. Typically, therefore, it could be operated with a variable
frequency controller to give a variable speed output, probably
in conjunction with a gearbox that has discrete speed ratios.
Alternatively, it could be driven at a fixed — and probably
high — speed, with the output being taken through a gearbox
that provides a continuously variable speed ratio.
B. Capaldi (in reply): In reply to Dr. Bolton, in the
pump's designed by Sealed Motor Construction Co. Ltd, it
is possible to ensure that magnetic attraction and hydraulic
repulsion are exactly equal for one load condition. In
practice, effort is made to ensure that some resultant force
is present, and this is catered for by arranging that hydrodynamic
lubrication exists on the appropriate bearing surfaces.
The stainless-steel rotor cap is magnetic and the disc motors
manufactured invariably incorporate solid iron rotors. Other
motor types have only received superficial attention, though
the exarnination has not been restricted to design procedures
alone. The results to date do not suggest any improvements
! in costs; although, as with radial flux machines, exception
undoubtedly exists.
In reply to Mr. Amor, Sealed Motor Construction
Co. Ltd now produce stators by punching the strip and coiling
in the same sequence. Three other firms in Switzerland,
Italy and Australia are known to produce disc motor stators
in a similar way. Specific enquiries in relation to these techniques
should be addressed to the above Company.
On the economics of manufacturing this type of motor/
pump combination, it is difficult to divide the costs. Indeed,
this product is a prime example of the need to suit the motor
design to the local requirements; particularly the type of
load.
References
DC107 B
A NASAR, S.A.: 'An axial airgap, variable speed eddy-current motor',
IEEE Trans., 1968, PAS-87,pp. 1599-1603
270 IEEPROC, Vol. 128, Pt. B, No. 5, SEPTEMBER 1981