Alternator having a starter with non-uniformly pitched slots

Details Alternator having a starter with non-uniformly pitched slots Alternator having a starter with non-uniformly pitched slots

2000-01-28

2001-09-11

Nguyen, Tran (Department: 2834)

Electrical generator or motor structure

Dynamoelectric

Rotary

C310S193000, C310S216006

Reexamination Certificate

active

06288471

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an alternator in which an alternating voltage is generated in a stator by rotation of a rotor.
2. Description of the Related Art
FIG. 9
is a cross-section of a conventional automotive alternator (hereinafter simply “alternator”), and
FIG. 10
is a perspective view of the rotor in
FIG. 9
,
FIG. 11
is a front elevation of the stator core in
FIG. 9
, and
FIG. 12
is an electrical circuit diagram of the alternator in FIG.
9
.
The alternator includes: a case composed of an aluminum front bracket
1
and an aluminum rear bracket
2
; a shaft
5
rotably disposed in the case having a pulley
4
secured to a first end thereof; a Lundell-type rotor
6
secured to the shaft
5
; fans
7
secured to both axial ends of the rotor
6
; a stator
8
secured to the inner wall of the case
3
; slip rings
9
secured to a second end of the shaft
5
for supplying electric current to the rotor
6
; brushes
10
sliding in contact with the slip rings
9
; brush holders
11
accommodating the brushes
10
; first and second rectifiers
12
a
and
12
b
in electrical contact with the stator
8
for converting alternating current generated in the stator
8
into direct current; a heat sink
13
fitted over the brush holder
11
; and a regulator
14
fastened to the heat sink
13
by adhesive for adjusting the magnitude of the alternating voltage generated in the stator
8
.
The rotor
6
includes a field coil
15
for generating magnetic flux on passage of electric current, and a pole core
16
covering the field coil
15
in which magnetic poles are produced by the magnetic flux. The pole core
16
includes a first pole core assembly
17
and a second pole core assembly
18
which are mutually intermeshed. The first pole core assembly
17
and the second pole core assembly
18
are made of iron and have claw-shaped magnetic poles
19
and
20
at their ends. Spaces
21
are formed between adjacent claw-shaped magnetic poles
19
and
20
in order to prevent magnetic flus from leaking from between the claw-shaped magnetic poles
19
and
20
, and also to function as cooling passages for cooling the field coil
15
.
The stator
8
includes a stator core
22
, and two three-phase stator coils
23
a
and
23
b
composed of wire wound onto the stator core with a phase difference of 30 electrical degrees. The stator core
22
shown in
FIG. 11
is constructed by punching thin sheets of steel plate into an evenly-spaced comb shape and winding or laminating the comb-shaped plates into a ring shape. Slots
25
and teeth
24
extending in a radial direction are formed on an inner circumferential portion of the stator core
22
.
This example has two sets of stator coil, namely the first three-phase stator coil
23
a
and the second three-phase coil
23
b,
and the rotor
6
has
12
poles with two three-phase portions corresponding to each pole, and there are therefore seventy-two slots
25
and teeth
24
. The stator core
22
is annular, but is shown flattened out in
FIG. 13
, and the annular stator core
22
is formed with slots
25
at an even pitch of five mechanical degrees (360 degrees/72). Thus, because the seventy-two slots correspond evenly to the twelve poles, the slots
25
are formed at an even pitch of 30 electrical degrees.
The two star-connected three-phase stator coils
23
a
and
23
b
are disposed in the slots
25
with a phase difference of 30 electrical degrees from each other, and are electrically connected to the first rectifier
12
a
and the second rectifier
12
b.
In an automotive alternator of the above construction, current is supplied by a battery (not shown) through the brushes
10
and slip rings
9
to the field coil
15
, generating magnetic flux, whereby the claw-shaped magnetic poles
19
of the first pole core assembly
17
are polarized with north-seeking (N) poles and the claw-shaped magnetic poles
20
of the second pole core assembly
18
are polarized with south-seeking (S) poles.
At the same time, the pulley
4
is rotated by an engine, and the rotor
6
rotates together with the shaft
5
. Consequently, a rotating magnetic field is imparted to the stator coils
23
a
and
23
b
and electromotive force is generated. This alternating electromotive force is converted into direct current by means of the rectifiers
12
a
and
12
b,
its magnitude is regulated by the regulator
14
, and the battery is recharged.
In this automotive alternator, there is one slot
25
in the stator core
22
for each set and phase of the stator coil
23
and for each pole of the rotor
6
, and flux leakages formed between adjacent claw-shaped magnetic poles
19
and
20
of the rotor
6
rarely form within the same tooth
24
, making flux leakage time in the teeth
24
short. Consequently, decreases in the amount of effective flux acting on the stator coil
23
due to flux leakages are rare and flux surges are reduced.
Moreover, the same technical content as the above automotive alternator is disclosed in Japanese Patent Laid-Open No. HEI 4-26345.
In a conventional automotive alternator, as shown in
FIG. 3
which was prepared by the present inventors based on electromagnetic field analysis (A slot opening pitch of 24 degrees, for example, on the horizontal axis means that the electrical angle is at an even pitch of 30 degrees, and the physical angle is at an uneven pitch of 24 to 36 degrees. The vertical axis represents the ratio of harmonic frequencies to the fundamental frequency of the stator magnetomotive force), when the pitch of the slot openings is constant at an electrical angle of 30 degrees, the fifth and seventh spatial harmonic frequencies do not appear in the flux density waveform. However, one problem has been that when the eleventh and thirteenth spatial harmonic frequencies in the magnetomotive harmonic frequencies of the stator
8
are large and the eleventh or thirteenth harmonic frequencies are present in the magnetomotive harmonic frequencies of the rotor
6
, suppression of magnetic flux surges is inadequate due to interference between the frequencies, making the suppression of variations in the generated voltage inadequate, and magnetic attraction arises between the claw-shaped magnetic poles
19
and
20
and the claw-shaped magnetic poles
19
and
20
or the rotor
6
vibrate, giving rise to a sound which is unpleasant to passengers in the vehicle.
Similarly, another problem has been that when the slot harmonic frequencies of the stator
8
are the eleventh and thirteenth spatial harmonic frequencies and the eleventh or thirteenth harmonic frequencies are present in the magnetomotive harmonic frequencies of the rotor
6
, suppression of magnetic flux surges is inadequate due to interference between the frequencies, making the suppression of variations in the generated voltage inadequate, and magnetic attraction arises between the claw-shaped magnetic poles
19
and
20
or the rotor
6
and the stator
8
, making the stator
8
, the case
3
, etc., resonate, or the claw-shaped magnetic poles
19
and
20
of the rotor
6
vibrate, giving rise to a sound which is unpleasant to passengers in the vehicle.
SUMMARY OF THE INVENTION
The present invention aims to solve the above problems and an object of the present invention is to provide an alternator capable of reducing variations in the generated voltage and reducing noise by enabling the reduction of higher order stator magnetomotive harmonic frequencies and stator slot harmonic frequencies which greatly affect flux surges and magnetic attraction between the stator and the rotor.
To this end, according to the present invention, there is provided an alternator comprising: a stator having a stator core formed with a number of slots extending longitudinally on an inner circumference, and two sets of three-phase stator coils installed in the slots; and a rotor rotably disposed inside the stator, the rotor having a field coil for generating magnetic flux on passage of electric current, and pole cores covering the field coil, a

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