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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a self-starting synchronized induction
motor using a permanent magnet rotor.
The synchronized induction motor of the present invention may be used, for
example, in a winding system in a spinning plant.
2. Description of the Prior Art
In the winding system in a spinning plant, since a plurality of
synchronized induction motors are arranged in parallel and driven in
synchronism, the motors should not be of external starting type but they
should be of self-starting type and also they should be able to pull in a
large load inertia.
Heretofore, in the electric motor of this type, a special rotor which
operates as an induction motor at the start of the motor and can be
operated as a synchronous motor while making use of a permanent magnet of
the rotor after pull-in has been used.
A structure of the permanent magnet rotor is disclosed, for example, in
U.S. Pat. No. 3,967,827. In the rotor of the U.S. Patent, a ring-shaped
permanent magnet assembly having magnetic poles on the periphery is fitted
to one end of a conventional cage rotor. In this type of synchronized
induction motor, when the rotor speed approaches a synchronous speed, the
rotor constitutes an induction motor together with a stator. Therefore, no
substantial current flows into a secondary conductor and about 80% of
total fluxes pass through the rotor of the induction motor while only
about 20% of total fluxes pass through the ring permanent magnet.
Consequently, it is difficult to attain a synchronized induction motor of
large capacity such as 100 watts or higher.
A large capacity synchronized induction motor of 100 watts or higher is
disclosed in the Japanese Utility Model Application Laid-Open No.
114,612/76 entitled "A Rotor of a Magnet Type Synchronous Machine". In the
rotor of this application, a number of cage secondary conductors are
embedded in the vicinity of an outer surface of a rotor core, magnets are
incorporated inside the rotor core, and slits for preventing the short of
magnetic fluxes of the permanent magnets when the motor is operated as a
synchronous motor are radially formed in a magnet housing. In this type of
synchronized induction motor, when the rotor speed approaches the
synchronous speed, most fluxes do not pass through the slits but pass
through the permanent magnet. Consequently, a large capacity motor can be
attained and the drawback encountered in the synchronized induction motor
disclosed in the U.S. Pat. No. 3,967,827 is overcome.
As described above, this type of synchronized induction motor is used in
spinning machines or the like, and in that case, a number of such motors
are powered from a power supply such as an inverter having a fixed
frequency and driven in synchronism. When such motors are used in spinning
machine, the speed of the spinning machines is proportional to a speed of
a roll directly coupled to the synchronized induction motors. In order to
optimize manufacture it is desirable to increase the speed of the motors.
If the speed of the motors is doubled, a yield of the system can be
doubled, and for a given yield the scale of the system can be reduced to
one half. For this reason, it is required that the synchronized induction
motor have enough strength to withstand high speed operation. The
structure shown in the Japanese Utility Model Application Laid-Open No.
114,612/76 suffers from a limitation to the above requirement.
Namely, firstly, since the permanent magnets are housed in the core, slits
for preventing the short of the magnetic fluxes of the permanent magnet
are necessary. Consequently, the strength of the core cannot be high
enough and hence the speed of the synchronized induction motor cannot be
increased greatly.
Secondly, a problem in increasing the speed resides in the need of
increasing a danger speed of the rotor which is a rotational speed
corresponding to a primary resonance frequency of a bearing span of a
rotor shaft. In order to raise the danger speed of the rotor, a diameter
of the rotor shaft need be increased. In this case, a sectional area of
the core defined between a starting secondary conductor which, together
with a stator, constitute the induction motor and the rotor shaft, that
is, a sectional area of core back is decreased. As a result, a magnetic
circuit is apt to be saturated at the start of the synchronized induction
motor and a starting current increases.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a synchronized
induction motor which minimizes a starting current.
It is another object of the present invention to provide a synchronized
induction motor which is mechanically strong enough to withstand high
speed operation.
In order to achieve the above objects, according to the present invention,
permanent magnets are arranged axially of a rotor shaft around an outer
circumference of a rotor core, with the permanent magnets having a
coercive force such that the magnet is not demagnetized by rotating
magnetic field of the air gaps when the motor operates as an induction
motor. With the arrangement of the present invention, the slits for
preventing the short of the magnetic fluxes of the permanent magnets are
not necessary. As a result, the core back sectional cures can be increased
and the magnetic circuit will not be saturated and hence the starting
current can be reduced. Furthermore, since the slits for preventing the
short of the magnetic fluxes of the permanent magnets are not necessry in
the present invention, a higher rotor breakdown speed can be established
and a structure suited for a high speed synchronized induction motor can
be attained.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross sectional view of a rotor of a synchronized induction
motor in accordance with a preferred embodiment of the present invention,
in which the rotor is cut perpendicularly to an axial direction of the
rotor.
FIG. 2 is a longitudinal sectional view of the rotor and a stator of the
synchronized induction motor of the preferred embodiment of the present
invention, in which the rotor and the stator are cut axially.
FIG. 3 shows a mounting method of a magnet reinforcing member in accordance
with the present invention.
FIG. 4 shows an embodiment of a permanent magnet in accordance with the
present invention.
FIG. 5 shows a cross sectional view of a rotor in accordance with another
embodiment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 shows a cross sectional view of a rotor in accordance with a
preferred embodiment of the present invention, and FIG. 2 shows a
longitudinal sectional view of the rotor and a stator of the preferred
embodiment of the present invention. In FIG. 2, numeral 30 denotes an
armature core and numeral 31 denotes an armature winding for generating a
rotating magnetic field in the armature core 30. A stator having the
armature core 30 and the armature winding 31 is fixed inside a cylindrical
housing 32, opposite open ends of which are covered with end brackets 33a
and 33b. A rotor shaft 10 is coaxially arranged along an axis of the
armature core 30. The rotor shaft 10 is rotatably supported by bearings
34a and 34b mounted to the end brackets 33a and 33b. Numeral 11 denotes a
rotor core which is coaxially mounted on the rotor shaft 10 and
magnetically coupled to the armature core 30 through an air gap 40.
Numeral 12 denotes cage secondary conductors which extend through end
plates 13a and 13b mounted in the vicinity of the outer circumference of
the rotor core 11 to sandwich the rotor core 11. Numerals 14a and 14b
denote end rings for shorting the ends of the secondary conductors 12.
Numeral 15 denotes a cylindrical permanent magnet which is made of high
coercive force samarium-cobalt (Sm-Co) and fitted around the outer
circumference of the rotor core 11. Numeral 16 denotes a cylindrical
reinforcing member fitted around the outer circumference of the magnet 15.
The opposite ends of the magnet 15 and the reinforcing member 16 are held
by the end plates 13a and 13b. As shown in FIG. 1, the magnet 15 is
magnetized after it has been mounted. Numerals 35a and 35b denote balance
rings mounted on the rotor shaft 10 in the vicinity of the end brackets
33a and 33b, and they serve to dynamically balance the rotor.
The synchronized induction motor having the rotor described above operates
an an induction motor at the time of the start of the motor by the
function of the rotating magnetic field of the air gap of the stator and
the secondary conductors of the rotor, and after the pull-in, it operates
as a synchronous motor by the function of the rotating magnetic field of
the air gap of the stator and the permanent magnet 15 on the outer
circumference of the rotor.
The permanent magnet 15 is preferably made of a material having a coercive
force iHc of higher than 15 K oersteds for the following reason. Namely,
when the synchronized induction motor operates as the induction motor, the
magnetic flux at the air gap 40 is expelled to the external of the
secondary conductors 12. (This is referred to as a magnetic shield effect
of the cage induction motor.). Accordingly, the magnetic flux generated,
when the motor operates as an induction motor, directly passes through the
permanent magnet 15 mounted around the rotor core 11. Unless a magnetic
flux density when a current flows in the secondary conductors 12 is around
10,000 Gausses at a tooth position of the rotor between two adjacent
secondary conductors, a sufficient starting torque and synchronizing
torque cannot be attained. When a magnetic flux on the outer circumference
of the secondary conductors 12, that is, on the outer circumference of the
rotor core 11 is 7,000 Gausses, and the permanent magnet 15 has a
remanence magnetic field density Br of 8,000 Gauss and a permeability .mu.
of 1, a subtractive magnetic field of 15,000 oersteds at maximum is
applied to the permanent magnet 15. In the next half cycle of an A.C.
voltage applied to the armature winding, an additive magnetic field is
applied to the permanent magnet 15. Accordingly, in the above case, the
permanent magnet 15 must have a corecive force of 15 K oersteds. So long
as the above requirement is met, the permanent magnet 15 may be made of
any material. It is convenient to use Sm-Co for the permanent magnet
because it has highest coercive force iHc and relatively easily available
among high coercive force materials.
A rare earth-cobalt magnet may be used as a high corecive force permanent
magnet. In an experiment, when Sm-Co is used for the permanent magnet 15
of the synchronized induction motor with P=1.0 kW, GD.sup.2 =0.16
Kgm.sup.2, T.sub.st =10 sec., pull-out torque=3 KW, pull-in torque=1.8 KW,
rotor rotation speed=12,000 r.p.m. and rotor outer diameter=104 mm, a
thickness of Sm-Co may be 1.5 mm, iHc may be 25 K oersteds and Br may be
8,000 gausses. The reinforcing member 16 may be a stainless ring having a
thickness of 1.5 mm.
In the structure shown in FIGS. 1 and 2, since no other parts than the
rotor shaft 10 and the secondary conductors 12 are required inside the
rotor core 11, that is, since the permanent magnet need not be arranged
inside the stator core as is needed in the Japanese Utility Model
Application Laid-Open No. 114,612/76, the core back sectional area can be
made large enough to prevent the saturation of the magnetic circuit and
reduce the starting current. Furthermore, since the core back sectional
area is large, deep groove cage conductors or double-cage conductors,
which have been difficult to adopt in the prior art structure, can be
readily adopted so that the synchronized induction motor with a small
starting current and a high acceleration capability are provided. In this
type of motor, it has been difficult in the prior art structure to assure
a core back sectional area when the number of poles is small. In the
present invention, this can be readily attained.
In the above embodiment, the gap length when the motor operates as the
induction motor is substantially equal to a sum of the gap and the
thicknesses of the permanent magnet 15 and the reinforcing member 16.
Accordingly, one might consider that a larger exciting current would be
required. According to an actual test, a starting current in the prior art
synchronized induction motor having the slits in the rotor core is about
30 times as high as a rated current. However, according to an experiment
of the present invention, the starting current is less than 20 times as
high as the rated current. Accordingly, in the present invention, the
increase of the exciting current due to the thicknesses of the permanent
magnet 15 and the reinforcing member 16 is of no importance.
Furthermore, according to the present embodiment, the synchronized
induction motor having an enough strength to withstand the high speed
operation can be provided for the following two reasons.
First, since no slit is formed in the rotor core 11 of the present
structure, the rotor strength is much higher than the prior art rotor
having the slits in the rotor core, and hence no reinforcing member such
as a reinforcing pin need be provided in the rotor core.
Second, since no permanent magnet is arranged inside the rotor core 11 of
the present invention unlike the rotor core of the prior art synchronized
induction motor, the diameter of the rotor shaft 10 can be increased.
Accordingly, a higher shaft danger speed may be set and a structure
suitable for high speed motor can be provided. In order to increase the
speed of the motor, it is necessary to increase the rotor danger speed. In
the prior art rotor of more than 100 watts capacity having a magnet, If
the diameter of the rotor shaft is increased, the sectional area of the
secondary conductors necessarily decreases, which leads to the reduction
of the synchronizing torque. Consequently, for a large load inertia, the
synchronization is difficult to attain. In addition, when the diameter of
the rotor shaft is increased, the sectional area of the magnet decreases,
which leads to the reduction of the output in the synchronous operation
and the reduction of power factor. Therefore, the diameter of the shaft
cannot be increasec greatly. On the other hand, in the present structure
in which no permanent magnet is contained inside the rotor core, the
sectional areas of the secondary conductors 12 and the permanent magnet 15
are not reduced even if the diameter of the rotor shaft is increased.
Therefore, the diameter of the rotor shaft can be increased relatively
easily so that the shaft danger speed can be increased and the structure
suitable for high speed motor can be attained.
Furthermore, in accordance with the present embodiment, since the permanent
magnet 15 is arranged around the outer periphery of the rotor, the
magnetic flux generating sectional area of the permanent magnet 15 can be
increased and hence the amount of magnetic flux emitted from the magnet
can be increased. Therefore, according to the present invention, the
output in the synchronous operation can be increased and the power factor
can be raised.
Furthermore, according to the illustrated structure, the motor size can be
reduced because the rotor of the illustrated structure can increase the
core back sectional area and increase the sectional area of the magnet. In
an experiment, when the structure of the present invention is used to
attain the same output as is provided by a motor having a rotor of a prior
art structure with a diameter of 105 mm and a length of 155 mm, a
small-size rotor having a diameter of 80 mm and a length of 80 mm is
sufficient.
Furthermore, in the structure of the present embodiment, when the number of
poles is to be changed, only the number of magnetizing points of the
magnet need be changed without changing a blanking die for the rotor core.
Therefore, the assembling work is easy and the reinforcing member 16 need
only function to hold the magnet 15. In a 12,000 r.p.m. motor, for
example, a stainless steel plate of 1.5 mm thickness is sufficient. In
addition, since the permanent magnet need not be fitted in the rotor core
as is done in the prior art rotor, the blanking die for the rotor core may
be simple and hence the synchronized induction motor which is cheaper, as
a whole, than the prior art motor can be provided.
Furthermore, in accordance with the structure of the present invention,
since no slit for blocking the magnetic flux is formed in the rotor core,
the permeance of the rotor does not change from rotating position to
rotating position of the rotor. Therefore, the pulsation of a primary
current is reduced and the effect upon the components such as inverter
connected between the terminals of the stator is reduced.
Furthermore, in accordance with the present invention, since the starting
current is reduced, the components such as the inverter may be of small
capacity and hence the cost for the facilities can be reduced.
In FIG. 3, a reinforcing member is made of a web-like plate member 17
instead of the stainless steel ring. The ring-shaped permanent magnet 15
is mounted on the outer circumference of the rotor core 11 (not shown in
FIG. 3), and the plate member 17 is wound around the outer circumference
of the permanent magnet 15 to hold the magnet 15. With this structure, the
assembly work is easy and automatic asssembly of the synchronized
induction motor is facilitated. The plate member 17 is preferably welded
to the permanent magnet 15 intermediate the magnetic poles, that is, at
intermediate points 18a and 18b of N and S poles, because the
deterioration of the characteristic of the magnet will not raise a
significant problem when the interpole points are welded while the
characteristic of the magnet changes as the temperature of the magnet
rises by welding because layer transition of the material takes place.
A split-type magnet comprising arcuate magnets 19 shown in FIG. 4 may be
used in place of the ring-shaped permanent magnet 15. The split-type
magnet is easier to manufacture than the ring-shaped magnet. In order to
mount the split-type permanent magnet 19 around the outer circumference of
the rotor core 11, a tape made of epoxy resin impregnated glass fiber is
used as the reinforcing member 16 and the split-type permanent magnet 19
is bound on the rotor core 11 by the tape. Alternatively, a stainless
steel may be used as the reinforcing member 16.
For an application where certain deterioration of performance is allowed in
the starting time and the synchronizing torque, a rotor structure as shown
in FIG. 5 may be used. Namely, a plurality of axial bores 21 each having
an arcuate cross section and extending axially with a substantial length
are formed in the outer circumference of the rotor core 20, and the
arcuate magnets 19 as shown in FIG. 4 are inserted in the bore 21. With
this structure, since the magnetic fluxes generated by the magnets 19 are
shorted by portions 22 of the core between adjacent magnets, the amount of
available magnetic flux is reduced but the reinforcing material 16 shown
in FIGS. 1 and 2 is not necessary. Accordingly, the manufacturing process
can be simplified.
In the present invention, the number of poles of the motor and the shape of
the secondary conductors may be arbitrarily selected.
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