Brushless DC motor

Details Brushless DC motor Brushless DC motor

2000-04-20

2002-01-01

Ramirez, Nestor (Department: 2834)

Electrical generator or motor structure

Dynamoelectric

Rotary

C338S03200R

Reexamination Certificate

active

06335578

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a brushless DC motor using a giant magnetoresistive element as a magnetoelectric converting element which detects magnetic poles of a magnetic rotor.
2. Description of the Prior Art
In recent years, brushless DC motors are widely used as a capstan motor for forwarding a tape in a videotape recorder (VTR) or an audiotape recorder, a spindle motor for driving a floppy disk or a compact disk, and the like. Likewise in the field of such applications as factory automation (FA) and robot, brushless DC motors which are smaller in size and larger in torque are used.
A conventional brushless DC motor, as illustrated in
FIG. 1
, is composed of a magnetic rotor (hereinafter referred to as “rotor magnet”)
10
formed of a permanent magnet and accommodated rotatably in a housing, exciting oils
11
disposed around the rotor magnet
10
as opposed thereto across a prescribed gap, and rotational angle detecting magnetic sensors
12
interposed between the exciting coils
11
. The rotor magnet
10
is rotationally driven by orderly exciting the exciting coils
11
externally depending on the position of magnetic poles of the rotor magnet and the speed of rotation can be controlled with the signal of excitation. Heretofore, a Hall sensor has been employed as the magnetic sensors for discriminating N-S poles of the rotor magnet
10
.
An example of the conventional brushless DC motor is shown in FIG.
2
.
In
FIG. 2
, a cylindrical frame
13
is provided at its lower end with a flange part
14
formed integrally therewith, on which a stator core
15
is set in place through the medium of a spacer
16
. A circuit substrate
17
including a driving circuit, etc. is also placed on the flange part
14
and fixedly secured thereto together with the stator core
15
and the spacer
16
with screws
18
. The stator core
15
is provided at its outer periphery side with a plurality of projected poles on which the exciting coils
11
of each phase are wound respectively. A rotor magnet
10
a
is disposed around the stator core
15
in such a manner that the inner peripheral surface of the rotor magnet is opposed to the outer peripheral surface of the stator core across a suitable gap. The rotor magnet
10
a
is fixedly secured to the inner peripheral surface of a rotor casing
19
having generally dish-like contours and magnetized so as to possess a plurality of magnetic poles. The center portion of the rotor casing
19
is fixedly secured to a boss
21
into which a rotating shaft
20
is fitted. The rotating shaft
20
is rotatably supported by the upper and lower pairs of ball bearings
22
and
22
which are arranged inside the frame
13
mentioned above.
Three Hall elements
12
connected to the circuit substrate
17
are respectively disposed in the proximity of the rotor magnet
10
a
so as to face to the lower surface thereof (see FIG.
1
). The Hall elements
12
detect the magnetic poles of the rotor magnet
10
a
and the detection output is inputted to the aforementioned driving circuit, which in turn passes the electric current through the exciting coils
11
of respective phases at proper time intervals. Thus, by passing the electric current through the exciting coils
11
according to the position of the magnetic poles of the rotor magnet
10
a
, the rotor magnet
10
a
is urged to rotate as well known in the art. To each of three Hall elements
12
, four sensor leads, i.e. two current input terminals and two output terminals are connected, these terminals being connected to a motor driving circuit through the medium of exciting coils
11
of the stator core
15
.
In the brushless DC motor the rotation of the magnets constituting the rotor is detected and the output signal is used as a rotating signal for controlling the rotation of motor. To this end, the sensor is required to have responsiveness to a magnetic field of several kOe and the ability to discriminate the polarity (N/S). Further, it is required to possess the high frequency characteristics in order to detect the N/S poles of the rotor magnet which rotates at high speed. The Hall element has been heretofore used as a sensor which satisfies these requirements. When the electric current flowing through the exciting coil is increased to obtain high output power, however, the sensitivity of the Hall element using a semiconductor such as InSb which is unstable at an elevated temperature decreases because a temperature of the environment in the motor is elevated. This poses the problem that the Hall element is unserviceable at elevated temperatures and the sensor determines the output of the motor. Moreover, since the Hall element requires four lead wires, two for input and two for output, and three elements are usually used in one motor, it is necessary to use twelve lead wires in total. As a result, the layout of wires is complicated, which gives the largest cause to prevent the miniaturization of the motor.
To overcome the problem mentioned above, the idea of utilizing a magnetoresistive element in place of the Hall element is proposed in published Japanese Patent Application, KOKAI (Early Publication) No. 5-207721 and No. 6-245464, for example.
The term “magnetoresistance (MR) effect” as used herein means a phenomenon that the electric resistance offered by a given material is varied by applying a magnetic field to that material. Generally, a ferromagnetic material is used as an MR element. A CoFe alloy having a rate of change of about 5% and a permalloy having a rate of change of about 2%, in magnetoresistance, are typical examples of the MR element. The rate of change of the magnetoresistance effect (magnetoresistance ratio, MR ratio) is expressed by the following formula (1):
Magnetoresistance ratio(%)=[
R
(
O
)−
R
(
H
)]/
R
(
O
)×100  (1)
wherein R(O) represents the electric resistance in the absence of a magnetic field and R(H) represents the electric resistance in the presence of application of a magnetic field.
The utilization of the magnetoresistance effect is effective in realizing miniaturization of a motor as by reducing the number of necessary sensor leads to two and simplifying the layout of wires, for example. Since the brushless DC motor uses a magnet (having a surface magnetic field of not less than 100 [Oe]) as a rotor thereof and the exciting coil thereof for driving the rotor has a strong magnetic field (some hundreds of Oe), however, the sensor which utilizes a magnetoresistive element formed of a soft magnetic material represented by permalloy has the problem that it cannot detect the rotational angle because its detectable magnetic field (not more than some tens of Oe) is surpassed. Further, the magnetoresistive element made of an alloy can not be used under the environment in which the magnetic fluctuation is large, because the magnetic range to which it can respond is narrow.
SUMMARY OF THE INVENTION
The magnetic sensor for use in the small brushless DC motor described above is required to fulfill the following four requirements. The sensors of the conventional class, however, have the problem that none of them cannot satisfy all these requirements.
(1) The sensor should be capable of being easily miniaturized (miniaturization).
(2) The sensor should not suffer the detecting sensitivity of the magnetic field thereof to vary notably with temperature (temperature characteristics).
(3) The sensor should be capable of detecting a magnetic field of up to several kOe (magnetic field characteristics).
(4) The sensor should be capable of detecting an AC magnetic field of up to several kHz (frequency characteristics).
To satisfy these requirements, the present invention provides a brushless DC motor which comprises a stator provided with a plurality of coils, a rotor magnet magnetized so as to possess a plurality of magnetic poles and rotatably disposed as opposed to the coils of the stator, and a sensor capable of detecting the magnetic poles of the rotor magnet, in which the electric cu

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