AC generator

Details AC generator AC generator

2001-04-23

2004-11-09

Nguyen, Tran (Department: 2834)

Electrical generator or motor structure

Dynamoelectric

Rotary

C310S156210, C310S156310

Reexamination Certificate

active

06815864

ABSTRACT:

This application is based on Application No. 2000-223896, filed in Japan on Jul. 25, 2000, the contents of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
This invention relates to an ac generator and, more particularly, to a vehicular alternating current generator driven by an engine.
FIG. 12
is a sectional side view showing one example of a conventional vehicular ac generator. As shown in
FIG. 12
, the generator comprises a case
3
composed of a front bracket
1
and a rear bracket made of aluminum, a shaft disposed within the case
3
and having a pulley
4
secured to one end portion, a, Randell-type rotor
7
secured to the shaft
6
, a fan
5
fixed to the opposite ends of the rotor
7
and a stator
8
secured to an inner surface of the case
3
.
The generator further comprises a slip ring
9
attached to the other end of the shaft
6
for supplying an electric current to the rotor
7
, a pair of brushes
10
sliding on the slip ring
9
, a brush holder
11
housing the brushes
10
therein, a rectifier
12
electrically connected to the stator
8
for rectifying an alternating current generated in the stator
8
into a direct current, a heat sink
19
fitted over the brush holder
11
and a regulator
20
attached to the heat sink
19
and regulating the magnitude of the ac voltage generated in the stator
8
. The front bracket
1
and the rear bracket
2
each has an exhaust window
17
which serves as a ventilation port for a cooling wind.
The rotor
7
comprises a cylindrical rotor coil
13
through which an electric current flows for generating magnetic fluxes and a pole core
14
disposed to cover the rotor coil
13
for generating a magnetic core.
The stator
8
comprises a stator core
15
and a stator coil
16
wound on the stator core
15
and generating an alternating current due to the change in magnetic fluxes from the rotor coil
13
upon the rotation of the rotor
7
.
The pole core
14
comprises a pole core member
22
including a pair of first pole core member
21
and a second pole core member
22
meshing with each other. The pole core member
21
and the pole core member
22
are usually made of iron and comprises cylindrical portions
21
e
and
22
e
to which the rotor coil
13
is wound and base portions
21
k
and
22
k
from which the cylindrical portions
21
e
and
22
e
are projected. Disposed respectively at the outer edges of the base portions
21
k
and
22
k
and between the outer circumference of the rotor coil
13
and the inner circumference are plurality of paw-like magnetic poles
23
and
24
meshing with each other.
The pawl-like magnetic poles
23
and
24
have a large thickness and width at the base
21
k
and
22
k
and smaller thickness and width toward the tip end. The inner circumferential surfaces
23
a
and
24
a
of the pawl-like magnetic poles
23
and
24
have thinner thickness at the tip portion and the outer circumferential surfaces
23
b
and
24
b
are curved in an arc along the inner circumferential surface of the stator
8
. The pawl-like magnetic poles
23
and
24
have two trapezoidal side surfaces
23
c
and
24
c
in relation to the circumferential direction of the rotor
7
. Since the respective pawl-like magnetic poles
23
and
24
are placed in an alternatingly meshing relationship with their tip opposing to each other, the inclined faces of the inner circumferential surfaces
23
a
and
25
a
of the pawl-like magnetic poles
23
and
24
are arranged in a circumferential raw in a alternating relationship. Also, the side surfaces
23
c
and
24
c
of the pawl-like magnetic poles
23
and
24
are inclined toward the centers of the pawl-like magnetic poles
23
and
24
so that they become gradually thinner at the tip portion than at the root portion.
Secured between the adjacent pawl-like magnetic poles
23
and
24
are permanent magnets
30
A of a substantially rectangular parallelepiped configuration so magnetized that reduces the leakage of the magnetic flux between the opposing side surfaces
23
c
and
24
c.
The operation will now be described. When an electric current is supplied to the rotor coil
13
from the unillustrated battery through the brush
10
and the slip ring
9
, a magnetic flux is generated to magnetize the pawl-like magnetic pole
23
of the first pole core member
21
into the N pole and the pawl-like magnetic pole
24
of the second pole core member
22
into the S pole. On the other hand, the engine rotates the pulley
4
and the shaft
6
rotates the rotor
7
, so that an alternating electromotive force is generated at the stator coil
16
. This alternating electromotive force is regulated into a direct current through the rectifier
12
and is regulated at its magnitude by the regulator
20
, thereby to charge the unillustrated battery.
The magnet
30
A of a substantially rectangular parallelepiped configuration secured between the pawl-like magnetic poles
23
and
24
is a plastic magnet. As for the magnet material, a ferrite magnet is advantageous from the viewpoint of cost, but this material is seldom used because of the mechanical brittleness, the low magnetizable residual magnetic flux density and the heat sensitive properties. Therefore, as for the magnet material, because of the advantages of the large degree of freedom in the magnet configuration and the high residual magnetic flux density, plastic magnet is often utilized. As for the plastic magnet, neodymium-iron-born group (Nd—Fe—Co—B bond magnet) and Samarium-iron group (Sm—Fe—N bond magnet) have been used.
The temperature coefficient of the residual magnetic flux density Br of the Nd—Fe—Co—B bond magnet is −0.1%/K (negative temperature coefficient) and the temperature coefficient of the residual magnetic flux density of the Sm—Fe—N bond magnet is −0.07%/K (negative temperature coefficient), so that the magnet effect is reduced to lower the generator output when the ac generator is at an elevated temperature condition.
Generally, a typical magnet exhibits the phenomenon of the nonreversible demagnetizing, in which phenomenon the magnetic flux (magnetic force) does not recover to the initial property value after the magnet heated to an elevated temperature is returned to the room temperature, and such the rate of change is referred to as the non-reversible demagnetizing factor. Here, the non-reversible demagnetizing factor where the magnet is heated to 373K and the heating time is 2 hours is referred to as 2-hour non-reversible demagnetizing factor, and the one that the heating time is 300 hours is referred to as 300-hour non-reversible demagnetizing factor, then the 2-hour non-reversible demagnetizing factor (373K×2 hr) of the Nd—Fe—Co—B bond magnet is −4.4% and the 300-hour non-reversible demagnetizing factor (373K×300 hr) is −5.4%. The 2-hour non-reversible demagnetizing factor (373K×2 hr) of the Sm—Fe—N bond magnet is −4.0% and the 300-hour non-reversible demagnetizing factor (373K×300 hr) is −5.3%. Therefore, when the ac generator is continuously used at an elevated temperature, the magnetic property of the magnet is deteriorated and the power of the ac generator is decreased as compared to that at the initial value.
On the other hand, the oxygen content of the Nd—Fe—Co—B magnetic powder after heating (373K×300 hr) is 0.8 wt % and the oxygen content of Sm—Fe—N magnetic powder after heating (373K×300 hr) is 0.4 wt %. The larger the oxygen content, the more easily rust is generated on the magnetic powder and the magnetic poles due to the ingress of moisture or the like. When the rust is generated, the magnet strength and the bonding strength between the magnetic poles and the magnet is decreased, significantly reducing the rotor strength at a high speed rotation. Particularly, the oxygen content of the Nd—Fe—Co—B bond magnet is as high as twice of that of the Sm—Fe—N bond magnet and is inferior in the oxygen-resistance, so that the surface treatment such as an epoxy coating or plating is necessary and costly.
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