The invention relates to an electric motor comprising a rotor, a
stator, at least two pole pairs at the rotor and an integral resolver or the like
transducing means. One particularly preferred application of the electric motor
according to the invention is small-size accurate servomotors of high capacity.
At present, feed-back information required by servomotors is generally
obtained from at least partially separate transducing means attached to the motors,
such as an optical or magnetic coders, a tachogenerator, or resolver (see GB-A-1382670).
A drawback of these known solutions is that the volume and weight of the motors
is high. In addition, the adjusting system is liable to disturbances, which is
due to the clearances and flexibility of the coupling.
The object of the present invention is thus to eliminate the drawbacks
described above. According to the invention it has been found that the resolver
can be integrated in the basic structure of the motor if the number of pole pairs
in the motor is at least double as compared with that of the resolver. With this
kind of motor, the above-described drawbacks of the prior art are eliminated by
the features that the resolver is of the type with a rotating transformer so that
the primary winding of the resolver is arranged in the rotor of the motor and
that a secondary winding of the resolver is wound round the stator of the motor
in such a manner that the winding wire goes alternately inside and outside the
As is known, the absolute angle position, velocity, and direction
of the rotor can thus be determined on the basis of signals generated in the secondary
winding of the resolver. These can be used for further controlling the commutation,
velocity, and parking of the motor.
According to the invention it is possible, if desired, to integrate
a synchro or some other transducing means of the type with a rotating transformer
in the basic structure of the motor. Amongst these, however, the resolver is the
most useful alternative in view of the control of the servomotor, for instance,
wherefore it is used in this particular case. As used herein, the term "resolver",
however, has to be considered to have a wider meaning in such a way as described
A motor effected according to the invention is clearly smaller and
lighter than known motors comprising a separate resolver for obtaining feed-back
information. Consequently, it is particularly suited for uses in which a small
size and lightness are among the basic requirements set for the motor. Such uses
include e.g. aeroplanes. In addition, the structure according to the invention
eliminates the liability to disturbances caused by the flexibility and clearances.
One more major advantage is that the motor is cheaper than previously.
In the following the invention and its preferred embodiments will
be described in more detail with reference to the examples of the attached drawings,
- Figure 1 is a partial longitudinal cross-section of a brushless permanent magnet
motor when the number of pole pairs in the resolver is one,
- Figure 2 is a cross-sectional view of the motor of Figure 1 in the direction
of the line A-A,
- Figures 3a and 3b illustrate one way of winding the secondary windings of the
resolver in the motor shown in Figures 1 and 2,
- Figures 3c and 3d illustrate an alternative way of winding for the way of winding
of Figures 3a and 3b,
- Figure 4 is a simplified view of the secondary winding of the resolver as a
planar view for the demonstration of noise voltage,
- Figure 5 illustrates a noise voltage occurring in the secondary winding of
the resolver in the case of a quadripole motor, and
- Figure 6 illustrates a noise voltage occurring in the secondary winding of
the resolver in the case of a bipolar motor.
Figures 1 and 2 illustrate a hexapolar brushless permanent magnet
motor the rotor and stator of which are indicated with the reference numerals 1
and 2, respectively. The rotor periphery is formed by six magnetic poles N1 to
N6, of which the three upper poles N1 to N3 are visible in the sectional view of
Figure 2. The primary or rotor winding of the resolver is formed by two windings
4a and 4b which, in the specific motor shown in the figures, are wound round pole
magnets disposed at an angle of 180 degrees with respect to each other. Thus only
the upper winding 4a is visible in Figures 1 and 2; however, for the sake of clarity,
the lower winding 4b positioned at an angle of 180 degrees with respect to the
winding 4a is provided with its own reference numeral. The alternating reference
voltage of the resolver is applied to the windings 4a and 4b by means of an inductive
coupling. In this particular motor, this takes place simultaneously by means of
inductive switches (windings) 5 positioned at both ends of the stator. From these
inductive switches, the reference voltage is induced in windings 7 positioned at
the ends of the rotor; the windings 7, in turn, are connected in series with the
two rotor windings 4a and 4b of the resolver. In this particular motor the inductive
coupling is designed so that it utilizes a shaft 8 of the motor as a part of the
magnetic circuit. Thereby the shaft has to be of special construction. Within the
area of the rotor, it has to be ferromagnetic and magnetically soft, within the
area of the shaft ends preferably non-ferromagnetic. Of course, other solutions
for obtaining an inductive coupling are also possible as well as other ways of
applying a reference voltage to the rotor windings 4a and 4b of the resolver, e.g.
slip-rings. The inductive coupling operates without brushes, as is desireable
in all parts of a brushless motor.
A stator winding 10 is arranged in winding slots 9 formed in the
stator 2 in a manner known per se. The secondary or stator winding of the resolver
is wound round the stator 2, which secondary winding is formed by two windings
11a and 11b positioned at an angle of 90 degrees with respect to each other. In
the cross-section of Figure 2, the winding wires are visible in pairs so that
one wire belongs to the winding 11a and the other to the winding 11b. The winding
wire in both windings goes alternately on the stator 2 and in the winding slot
9. Figure 1 shows two wire pairs of the secondary winding of the resolver in the
upper half of the motor in a cross-section so as to show that they extend in an
Figures 3a and 3b illustrate in more detail the way of winding of
the secondary windings 11a and 11b of the resolver in this particular tooth-type
motor in which the winding slots 9 are bevelled in the longitudinal direction
over one spacing. Figure 3a illustrates the winding 11a, and Figure 3b the winding
11b. Both windings are shown in a planar view; the winding has been cut in the
middle, and the halves are coupled in such a manner that one half in the coupling
is the reverse of the other. The winding wire portion positioned in the winding
slot 9 is indicated with a broken line and the winding wire portion positioned
on the stator 2 with a continuous line. As appears from Figure 3a, the winding
wire of the winding 11a first goes round through 180 degrees in every other winding
slot 9, returns thereafter to the starting point and goes in every other winding
slot 9, whereafter it goes over to go round the other half of 180 degrees in a
similar way. The winding wire of the winding 11b shown in figure 3b goes in the
corresponding way, however, with a displacement of 90 degrees with respect to
the winding 11a. When the winding is carried out as described above, the winding
is distributed evenly in this specific dc motor in which the winding slots are
bevelled over one spacing. In principle, a single winding turn of one winding
of 180 degrees shown in Figures 3a and 3b is enough for obtaining a resolver signal,
the rest only amplifies the signal.
Figure 3c shows an alternative for the way of winding shown in Figure
3a. In this case, the winding wire first goes round through 180 degrees in every
other winding slot 9, then goes over to the other half of 180 degrees, round which
it goes twice in every other winding slot 9, whereafter it goes back to the first
half of 180 degrees and goes round it in every other winding slot 9. A corresponding
alternative for the way of winding of Figure 3b is shown in Figure 3d. The winding
wire of a winding 12b shown in Figure 3d goes similarly as the winding wire of
the winding 12a in Figure 3c but with a displacement of 90 degrees with respect
to it. As to the windings shown in Figures 3a to 3d, it should be noted that their
starting point may be positioned arbitrarily along the periphery of the stator
whereby the way of presentation may change correspondingly. It is further to be
noted that irrespective of the way of winding the winding wires may be wound several
times, that is, a single winding wire may be replaced with a skein comprising several
In the following, possible noise voltages and the requirements they
set to the motor will be dealt with. A noise voltage may occur in the secondary
winding 11a, 11b or 12a, 12b of the resolver due to (i) the rotation of the permanent
magnetic field within the stator 2, and (ii) alternating magnetic fields caused
by stator currents. Their effect on the secondary winding of the resolver can be
illustrated by an imaginary planar view of the secondary winding of the resolver
shown in Figure 4 and by simplifying the winding to one typical winding in each
winding half (a and b) which are connected in series as shown in Figure 4. As
known, the electromotoric force E induced in the winding can be calculated by the
E = - N d&phis; / (dt)
wherein N is the number of winding turns and &phis; is magnetic flux. When the
winding turns are positioned evenly round the stator, the area of the graph of
change of the magnetic field illustrates the induced total voltage within ranges
a and b. When the winding halves are coupled as described above, a condition for
a value zero of the pole voltage is that the total change of the field within
the range a reduced with the total change of the field within the range b is equal
to zero. A special case of this condition is that the total change of the field
within both the range a and the range b is equal to zero.
Case (i) will be discussed first, that is, the noise voltage induced
in the secondary winding of the resolver by the permanent magnet field rotating
within the stator. It is assumed that the field caused by the poles is sinusoidal,
whereby the change of the field, too, is sinusoidal, though 90 degrees out of
phase therewith. Accordingly, the value of change (d&phis;dt)
of the field has alternatively the minimum and the maximum value at shift points
between the poles, at which points the value (&phis;) of the field has zero crossings.
Figure 5 shows a situation when a quadripolar permanent magnet field rotates within
the stator 2 at a given moment. The curve thus illustrates the value of change
d&phis;dt of the field; for the sake of simplicity, the
curve is not shown in the sinusoidal form; instead, it is formed by rectilinear
parts because the shape of the field does not affect the final result in this
case. However, the fields have to be equal in shape at each pole (N1, N2...). The
horizontal axis in Figure 5 represents the length of the periphery of the stator,
and the section of each pole is indicated with the respective reference N1 to N4.
As appears from the figure, the areas of the graphs of change compensate each
other separately both within the range a and the range b, whereby the induced pole
voltage is zero. This will happen when the number of poles is even and at least
In case (ii), the magnetic fields caused by stator currents are analogous
with the above-described permanent magnetic fields, and so are the changes thereof.
As compared to case (i), the only difference is the phase shift of 90 degrees when
the motor operates ideally. Accordingly, the magnetic fields caused by the stator
currents do not cause an additional voltage in the secondary winding of the resolver
when the number of the poles is even and at least four.
Figure 6 further illustrates a situation corresponding to that of
Figure 5 when a bipolar magnetic field alternates within the stator 2. As appears
from the figure, the area of the graph of change has a determined total value
(unshadowed area) both within the range a and the range b, depending on the position
of the poles with respect to said ranges a and b. Consequently, a voltage is induced
in the secondary winding of the resolver. Correspondingly, it appears from the
figure that a voltage is induced in the secondary windings of the resolver if a
magnetic field having an even number of poles alternates within the stator. An
absolute prerequisite for the motor according to the invention is that it comprises
an even number of poles which are at least four.
Similarly as described above, it is also possible to prove that when
the magnetic fields and the graphs of change thereof are asymmetrical in shape,
no additional voltage occurs in the secondary windings of the resolver if the
number of poles in the field is at least two. A prerequisite is that all the fields
are similarly asymmetrical.
The combined effect of the reference current and the rotatory movement
of the rotor 1 also causes a noise signal proportional to the speed of rotation
of the rotor in the secondary windings 11a and 11b or 12a and 12b of the resolver.
This is undesirable because the amplitude of the resolver signal should not be
dependent on the speed of rotation. However, it can be proved that this noise
signal is fairly insignificant, and its effects can be compensated for in known
manners, if this is considered necessary.
Disturbances may occur for the following reasons:
- A. The permanent magnetic poles (N1, N2...) have unequal flux values with respect
to each other, which causes an imbalance therebetween. Unequal flux values may
be due to e.g. a slightly different magnetization of the magnets.
- B. The mounting of the rotor is eccentric and the size of the air gap may vary
due to manufacturing tolerances.
- C. The secondary winding of the resolver is not distributed quite evenly round
However, the above sources of disturbance can be minimized in a suitable
way if they distort the output signals of the secondary windings of the resolver
too much. This minimizing can be carried out e.g. by magnetic balancing of the
rotor and electronic compensation of the disturbances.
A noise voltage may possibly also occur when the winding currents
and magnetic fields act on the inductive coupling of the reference voltage of the
resolver, which coupling is indicated with the reference numerals 5 and 7 in Figure
1. The effect of the currents of the stator winding 10, however, can be compensated
for by applying a reference voltage simultaneously to both ends of the rotor,
as shown in Figure 1. The directions of the currents should thereby be chosen
so that the stator current amplifies the reference voltage at one end and weakens
it at the other end. The shape and dimensions of the stator winding should be
equal at both ends.
In principle, it is also possible that a noise voltage is induced
in the primary windings 4a and 4b of the resolver if an external magnetic field
moves with respect to them. Fields caused by permanent magnets do not, however,
induce a voltage in the primary windings of the resolver because their position
with respect to the primary windings is fixed. On the other hand, it can be proved
that the total voltage induced by the stator windings 10 in the primary windings
4a and 4b of the resolver is zero if the windings 4a and 4b are connected in series.
To sum up the above discussion on noise voltages, it is obvious that
the magnetic fields of the motor do not in principle affect the resolver signals.
However, disturbances caused by manufacturing inaccuracies may occur but these
can be eliminated by careful planning. In addition to that, an insignificant additional
signal proportional to the speed of rotation of the rotor is formed similarly
as in other resolvers.
The above discussion refers to a resolver integrated in the basic
structure of the motor and having one pole pair. A resolver having a higher number
of pole pairs than this is obtained in the following way: it is imagined that
a combination of a one-pole-pair resolver and a motor is split at one side thereof;
the stator and the rotor are spread into a sector having an angle of 360o
divided with a desired number of pole pairs in the resolver and this is attached
to other sectors obtained in the same way. Couplings between these windings are
carried out according to the same principles as what is disclosed above concerning
the couplings of a resolver comprising one pole pair. It follows from the above
that an absolute prerequisite for integrating a resolver comprising several pole
pairs in the basic structure of the motor in that the number of pole pairs in
the motor has to be at least double with respect to the number of pole pairs in
The motor according to the invention can be applied particularly
to small-size accurate servomotors of high capacity, because the volume and weight
of the feed-back transducing means of such motors form a substantial part of the
entire motor. As to the different types of motor, the following may be stated.
In principle, this particular structure is suitable both for synchronous and asynchronous
motors irrespective of whether they comprise a tooth-type or toothless stator.
The rotor may be of a cylinder, ring or disc type; cylinder and ring type rotors,
however, are perhaps the most practical in this respect.
One application is a stepping motor type salient pole motor if it
is constructed so that at least four poles are magnetized concurrently and identically
except that successive fields have to have different directions.