Philips tech. Rev. 33, 244-248, 1973, No. 8/9
244
Moving-coil motors
J_ H_ M. Hofmeester
Small d.c. motors are used in very large numbers in
playback equipment, toys, measuring recorders, etc.
These motors owe their wide applications to a number
of features which can be made use of singly or in combination, depending on the particular requirements.
For example, the efficiency is high, even for very small
motors; the delivered torque is uniform, and there is
ample scope for selection and contra I of the motor
speed. Further features which may be of importance
are the rapid start, the ease with which the direction of
rotation can be reversed and the possibility of using a
battery as power supply. The rotor of such motors
often consists of a laminated ferromagnetic
material,
with the rotor windings wound round the stack of
laminations. Fig. 1 shows by way of example a simple
version with three rotor lobes. The stator generally
consists of a permanent magnet in the form of a hollow cylinder, which is sometimes divided into a number
of segments.
Despite the utility of this type of motor for many
applications, it has certain disadvantages when a very
uniform torque is required, e.g. in tape recorders. Since
there are slots in the rotor, it has anum ber of preferred
positions,
sometimes called 'reluctance
positions'.
The corresponding magnetic reluctance torques can be
felt when the shaft of the motor is turned by hand. It
will be clear that these reluctance torq ues give fluetuations in the torque delivered by the motor; this has
an adverse effect on the quality of audio recording and
playback. The reluctance torques can also cause motor
noise, since there is always a little play in the bearings.
The reluctance torques can be reduced by increasing
the number of rotor lobes, but for given motor dimensions this can only be done at the expense of the efficiency, since the space factor of the winding is then
decreased. Another solution is to minimize the width
of the rotor slots, but this also can only be done within
certain limits since the slot has to be wide enough for
the winding process.
The impossibility of eliminating the reluctance torques from motors with an iron rotor while continuing
to satisfy all the other requirements involved led us to
design three d.c. motors with no iron in the rotor. Such
motors, which are called moving-coil motors, offer the
following advantages:
J. H. M. Hofmeester and J. P. Koutstaal are with the Philips Audio
Division at the 'Johan de Wilt' Works, Dordrecht, the Netherlands.
and J. P. Koutstaal
1. Reluctance torques cannot arise since there is no
iron.
2. Since there are no discrete rotor lobes, the rotor can
be wound uniformly, which makes it possible to divide
the winding into a large number of coils. This also helps
to reduce the fluctuations in the delivered motor torque,
since a large number of coils with corresponding commutator segments reduces the torque pulses resulting
from commutation.
This feature makes this type of
motor very suitable if a low and uniform speed of
rotation is required.
3. The moment of inertia of the rotor can be made
smaller than that of a rotor with an iron core in a comparable motor (same power consumption, same torquespeed characteristic).
4. Iron losses (hysteresis losses and eddy-current losses)
cannot occur in the rotor. This is especially important
in small motors, where the delivered motor torque can
be of the sa me order of magnitude as the loss torque
caused by an iron core (a situation which leads to a
very low efficiency).
5. The inductance of the rotor is low, which improves
the commutation:
there is less electrical interference,
and the life ofthe commutator (and hence ofthe motor)
is increased.
We shall now discuss the basically simple principle
of the d.c. moving-coil motor, with reference to jig. 2.
Fig. 1. Example of a d.c. motor with iron rotor. H housing.
M cylindrical permanent
magnet with poles Nand
S. R rotor,
consisting of a stack of laminations.
The winding W is situated
in the slots between the rotor lobes (three in this example).
Philips tech. Rev. 33, No. 8/9
MOVING-COIL
MOTORS
245
The stator consists of a permanent magnet M, which
produces a magnetic flux density B in the air gap d
between the magnet and the housing H (which is made
of a ferromagnetic
material). The rotor winding W
rotates round a vertical axis in this air gap (only one
turn of the winding is shown). If the flux density B
and the current i in the turn have the directions shown
in the figure, the rotor will rotate anticlockwise under
the influence of a Lorentz force F applied to one side
of the turn; the magnitude of this force is liB, where
I is the length of the side of the turn in question. The
torque delivered per turn, Te, is equal to 2Fb (b is the
radius of the winding).
Construction
Fig. 2. Principle of the moving-coil
motor. H housing. M permanent magnet (stator). Wane turn of the coil (rotor). The side
of the coil is of length I. The magnet M gives a magnetic flux
density B in the air gap d. When a current i flows in this turn,
a Lorentz force F is exerted on it. The radius of the winding is b.
4
H
w
fv1
Fig.3 shows a cross-sectional view of one of our
movi ng-coil motors. The stator consists of a permanent
magnet M mounted on the steel motor housing H by
means of a support 7. This steel housing is also a part
of the magnetic circuit; the magnet has two diametrically opposite poles.
The rotor is formed by a winding W wound on a coil
former 5 with cover 6. The coil former is fixed on a
shaft 8. The winding consists of a large number ofturns
distributed uniformly round the circumference, and is
divided into five or nine coils, depending on the type
of motor.
The commutator 2 is built up of flat segments (see
also fig.4b), one per rotor coil. These segments are
gold-plated by a special technique.
The brush unit consists of two brush springs J,
mounted in an injection-moulded plastic bridge 4. The
5
Fig.3. Cross-sectional
view of a
practical version of the movingcoil motor.
J brush spring.
2
commutator
segments. 3 damping compound.
4 plastic bridge
carrying
the brush springs.
H
motor housing.
W coil. M permanent
magnet.
5 coil former
with cover
6. 7 support
for
mounting
the magnet
on the
housing. 8 motor shaft.
J. H. M. HOFMEESTER
246
brush springs are silver-plated at the place where they
make contact with the commutator.
Damping compound 3 is applied to damp the vibrations of the brush
springs.
Winding and magnet
It can be seen from fig. 3 that the coil former has
central projections at each end: these are necessary for
the insertion of the magnet support and for attaching
the coil former to the motor shaft. This arrangement
does not permit purely diametral winding, and the
winding method shown in fig. 4a has to be used, in
which the plane of the winding is slightly skewed with
respect to the centre-line of the coil former.
A regular distribution of the turns around the circumference is obtained by rotating the coil former
through a small angle Cl. after each turn of the coil has
and J. P. KOUTSTAAL
Philips tech. Rev. 33, No. 8/9
centrated at a small cross-section through the winding.
As a result, the total length of the winding in a motor
of a given size can be made less than in a moving-coil
motor. Since the motor losses are mainly the sum of
copper-resistance losses and iron losses, the higher dissipation in the coilof the moving-coil motor could
partially cancel out the advantage of the absence of
iron losses.
This difficulty can be avoided by making the permanent magnet of material with very good magnetic
properties. This makes it possible to obtain a motor
with good characteristics (in particular, high efficiency
and short starting time). In the moving-coil motors
described here we used 'Ticonal' 550 [*1, a magnet
steel which is given an anisotropic structure by means
of a special heat treatment; this material combines a
reasonably high remanence with a high coercivity (see
b
a
Fig. 4. a) Principle
of the 'ball-winding'
method which has to be used to leave the central
space in the coil free. The turns are distributed
over the circurnference
by rotating the coil
former through an angle (J. after each turn. b) A rotor (coil + commutator)
wound in this
way.
been wound; the value of Cl. is chosen so that the coil
former will have rotated through exactly 360 after the
desired number of turns. Because of the skewed position of the turns, successive turns will cross previously
wound ones during the winding process. The coil obtained in this way looks rather like a ball of string and
this winding method is therefore called 'ball winding'.
A photograph of a wound rotor complete with commutator is shown in fig. 4b.
The air gap between the magnet and the housing is
considerably wider than the thickness of the winding,
since room must be left for the coil former and for
clearance on each side. Since this space can only be
provided at the expense of the magnetic flux, the
moving-coil motor is at a disadvantage here compared
with the iron-rotor motor (see fig. I). Moreover, in the
motor with iron rotor the magnetic flux can be con0
jig. 5). 'Ticonal'
550 has a (BH)max product of about
4.4x 104 J/m3 (5.5x 106 GsOe). In the operating range
of the motors, the product BH lies between 4.4 X 104
and 3.5 x 104 J/m3. The high magnetic flux produced
in the air gap as a result of the use of this material,
and which is not of course limited in this type of motor
by magnetic saturation of the core, compensates for
the above-mentioned
disadvantages.
A further advantage is that with the geometry of coil and magnet
used here, the ends of the coil are situated in the stray
field of the magnet, so that the winding wire there does
not merely increase the resistance of the coil, it also
makes an appreciable
contribution
to the motor
torque.
'Ticonal' 550 magnets are expensive compared with
magnets made of the materials conventionally used in
d.c. motors, such as ferroxdure. The winding method
Philips tech. Rev. 33, No. 8/9
MOVING-COIL
MOTORS
247
that the 'motor approaches the zero-load speed IlO asymptotically;
at the speed IlO it will rotate with a drive torque Te = O.
The time constant Ts is defined for the unloaded motor, so
that only the moment of inertia of the rotor is of significance.
If we denote the latter quantity by I, the changes in speed are
given as a function of time by:
Tc
= 2nl
d/l
dr '
B
r
Combining the above two equations, we find that the speed a
time t after switching on is given by:
lI(t) = /10(1 -
e -liT") ,
where
=
Ts
-1
-0.5
-H
Fig. 5. Demagnetization curve of 'Ticonal' 550. B is the flux
density in teslas (I T = 104 Gs). H is the demagnetizing fieldstrength in A/m (I A/m = 4n x 10-3 Oe). The load lines for the
magnets of the motors described in this article fall inside the
shaded area.
used for the coil, and the complete construction of the
motor are also more expensive. It is therefore likely
that the application of the moving-coil motor will be
restricted to cases where high-quality operation is
required - in particular, uniform rotation, a low loss
torque and a small moment of inertia. The last point
is of importance for rapid motor starting, and rapid
speed changing; the 'starting time constant' of the
motor, which is a measure of its performance in this
respect, should be kept low.
The starting time constant Ts depends on a number of characteristic features of the motor. To illustrate the relation between
this constant and the features in question, we shall derive an
expression for Ts. If we neglect the electrical time constant, equal
to the ratio L/R of the inductance L and the resistance R of the
motor, we can start by considering the linear relation between
the motor torque Te and the speed n found for a d.c. motor
(jig. 6). This linear relation is represented by the equation
Te
=
Ts(1 -
11/110),
where Ts is the starting torque and 110 the speed at zero load.
Mechanical and electricallosses are neglected here, which means
Te
1
The starting time constant Ts is thus proportional to the moment
of inertia I, which can be made much smaller for an ironless rotor
than for an iron rotor. If we want to derive the full advantage
from this, we must ensure that the ratio 1I0/Ts for the moving-coil
motor is no larger than the value for an iron-rotor motor.
The zero-load speed is defined by:
V
=
2nC/lo<Prs,
where V is the applied voltage, C is a motor constant and <Prs
the flux enclosed by the rotor coils [IJ. The starting torque is
given by:
where is is the current immediately after switching on. The current is therefore entirely determined by the motor resistance R
and the applied voltage:
is
=
VIR,
so that
110
Ts -
R
2nC2<Prs2
•
The time constant is thus inversely proportional to the square
ofthe enclosed flux. Ifwe want to take full advantage ofthe small
moment of inertia of the moving-coil motor, we must provide a
powerful magnetic field. As mentioned above, we are not limited
here by the magnetic saturation of the rotor core as in the case
of a motor with iron rotor.
Practical realization
Fig. 7 gives the dimensions and other data for three
of our moving-coil motors, together with a number of
applications. Motor A was designed for use in a new
version of the Pocket Memo dictating machine [21.
The good performance of the motor is reflected in the
following four points:
1. Since the motor is small (the delivered torque is only
2 X 10-4 Nm), the low loss torque of the moving-coil
motor (0.5 X 10-4 Nm) is particularly useful. As a result
of this, the mechanical efficiency is as high as 80 %,
while the overall efficiency (54 %) is also relatively high.
[IJ
[2J
Fig. 6. Torque-speed characteristic of a d.c. motor. The motor
torque Te is plotted as a function of the speed 11. T. is the starting
torque, and IlO the zero-load speed.
2nlIl0/Ts•
[*1
See E. M. H. Kamerbeek, Electric motors; this issue, p. 215.
An earlier version of the Pocket Memo is described in P. van
der Lely and G. Missriegler, Audio tape cassettes, Philips
tech. Rev. 31, 77-92, 1970.
'Ticonal' is a registered trade mark of N.V. Philips' Gloeilampenfabrieken.
248
MOVING-COlL
MOTORS
The current required for driving this machine is only
20mA.
2. The Pocket Memo has no capstan and hence no flywheel, so that a motor with a uniform speed is vital
here to avoid wow and flutter.
3. The motor noise must be kept to a minimum, since
the built-in microphone is very close to the motor. An
important source of noise - bearing chatter due to the
reluctance torques is completely absent in the
moving-coil motor.
4. As the motor is also very close to other electrical
Philips tech. Rev. 33, No. 8/9
The more powerful motor C is used in tape recorders
with separate reels and in professional equipment such
as measuring recorders and computers. The application
in tape recorders is again based on the uniform rotation, even at low speeds where the effect of the flywheel
is slight. The small time constant (Ts = 19 rns) is a
great advantage for professional equi pment.
These examples show that a motor with a movingcoil rotor may be the best choice in cases where uniform
rotation, minimum motor noise and a small starting
time constant are of importance.
,w
29
ÎTl
I
-tt.,
395
I
i
A
Te
=
2x 10-4 Nrn
efficiency
Ts
54 %
5 rotor
TS
coils
pocket dictating
cameras
Te
=
=
64
%
efficiency
23 ms
9 rotor
machines
c
Tc = 10-2 Nrn
5 X 10-3 N m
efficiency
= 56 ms
U
B
Ts
coils
components in the dictating machine, the low rotor
inductance - and therefore the low electrical interference level - is a useful feature.
Motor 8 was developed for cassette recorders (for
both audio and digital data). The diameter is chosen
so that four motors can be mounted under one cassette
if required, thus permitting both cassette reels and both
capstans to be driven directly (for tape transport in
both directions). In tape recorders the direct drive is
an added advantage; the moving-coil motor does not
produce wow and there is no chance of wow arising
from gearing.
The small starting-time
constant
(r, = 23 ms) is important for digital applications.
82 %
19 ms
9 rotor
digital cassette recorders
audio cassette recorders
Fig. 7. Illustrations
and data for three moving-coil
motors. The
constant.
The photograph
for motor A shows this motor used in
shows a model of a digital cassette recorder developed
at Philips
tape transport
in both directions,
this photograph
shows (on the
control. The photograph
for motor C shows this motor mounted
=
coils
tape recorders
professional
equipment,
such as
measuring recorders and computers
torque Te is given lor a speed of 3000 rpm. Tg is the starting time
a Philips Pocket Memo dictating machine. The photo for motor B
Electrologica.
Apart from two type B motors, which are used for
right) a type C motor with a tachosystem
designed for tape-speed
in a Philips tape recorder.
Summary.
Moving-coil
motors contain a rotor consisting merely
of a coil with commutator
rotating about a permanent
magnet
(the stator);
conventional
small d.c. motors
have rotor coils
wound on an iron core. The advantages
of the ironless rotor
include the absence of reluctance torques (so that it is easier
to obtain uniform, low-noise motion) and the small moment of
inertia,
permitting
short
start and stop times.
Moving-coil
motors also have a high efficiency (particularly
in the low-power
models) because there are no iron losses. However, these motors
have the disadvantage
that the flux density inside the coil is lower
than in an iron-rotor
motor of comparable
dimensions.
In the
motors described in this article, this disadvantage
is minimized
by making use of the permanent-magnet
material 'Ticonal'
550,
which has a large (BH)max
product.
Moving-coil
motors are
suitable
for use in equipment
where uniform
motion,
rapid
reaction and high efficiency are required, and where the greater
expense is not a serious drawback. A number
of practical versions are discussed,
with their applications
in tape recorders,
pocket dictating machines,
measuring recorders,
etc.