|Title of Invention||
"A MAGNETLESS BRUSHLESS DC MOTOR"
|Abstract||The invention relates to a magnetless brushtess DC motor comprising an unmagnetized salient pole rotor and wound stator wherein the stator windings Wl and W3 corresponding to stator salient poles PI and P3 are wound in the same direction and connected in series, stator windings W2 and W4 corresponding to stator salient poles P2 and P4 wound in same direction and connected in series, the winding direction being such that when a positive puise is given to set of stator pole windings a set of north and south magnetic poles are created on the corresponding rotor poles and the reverse effect is created when a negative pulse is given to the set of stator pole windings, an electronic sequential pulse generating power supply being provided to generate sequential pulses to create a rotating field in the stator winding characterized in that the rotor of the motor has unmagnettzed poles, said rotor being made of high saturation flux density, low coercivity, high retentivrty soft magnetic material and provided with a rotor speed sensor.|
|Full Text||Field of Invention:
The present invention relates to a salient pole brushless DC motor. Background of the Inventions
Wound field DC motors arc generally fitted with commutator and brushes. Due to abrasive action between commutator and brushes the brushes become the wearable parts of a D.C. motor and have to be replaced periodically. The same problem is faced by the D.C. motors using permanent magnets instead of wound field.
In order to overcome the problem, brushless permanent magnet type D.C. motors have been developed. In these motors a permanent magnet rotor and a wound stator is used.An electronic generator is used to give sequential pulses to the stator windings to create a rotating magnetic field which in turn rotates the permanent magnet rotor. Whenever there is a sudden change in the load driven by such a motor, like change in required torque or speed, the permanent magnet rotor tends to get out of step with the rotating magnetic field. This can result in demagnetization of the permanent magnet rotor, loss of power and eventual loss of life of the rotor. To avoid this from happening very high coercivity permanent magnets are required to be used which are costly.
Permanent magnets being hard and brittle, also have poor machinabi1ity. Fixing the permanent magnets on shaft is therefore, a difficult process. A rotor position sensor can be used for adjusting the speed of the rotating magnetic field in line with the rotor. This will prevent the rotor from getting out of step with the rotating magnetic field. However, this involves additional complexity in making the rotor as well as the electronic pulse generator and the resultant increase in cost of manufacture.
Further in the case of change in loads the hunting or vibration of the permanent magnet rotors can be considerable which can result in damage to the associated parts such as shaft, bearing, coupling, etc.
Thus there is scope for improvements in the design for overcoming the above mentioned drawbacks in a brushless DC motor.
Summary of the Invention:
One object of the present invention is to provide a rotor design for a brushless DC motor which is simple and cost effective.
Another object of the invention is to provide smoother operation of the motor, with considerable reduction in hunting/ vibration of the rotor while operating with fast changing loads.
One more object of the invention is to provide a simplified design of the rotor and the electronic pulse generator of a hrushless D.C. motor by eliminating the need for rotor position sensor, at least in the case of mildly fluctuating loads.
Yet aother object of the present invention is to provide a brushless DC motor with a longer expected life, eliminating the need for permanent magnets which can get demagnetised over a period of time.
These objects are achieved by providing a rotor made from hiyh saturation flux density, low coercivity (low He), high retentivity ( hiqh Br), low hystensis loss, high resistivity, soft magnetic material such as mild steel or stampings of suitable magnetic material.
Thus the present invention provides a salient pole brushless DC motor comprising pairs of salient poles with corresponding pairs of windings wound in the same direction and connected in series; a control pulse generating circuit for generating positive pulses and negative pulses and sequentially providing said pulses to said pairs of windings; for creating a rotating magnetic f ie Id; charat: t*ri zed in that said motor is provided with a rotor comprising a. high saturation flux density, low coercivitv
In case of sudden change in load if the rotor falls back momentarily it gets demagnetised and remtqnttised in thi eppailii direction to be again in step with respect to the rotating magnetic field. This allows for some slippage in the rotor speed with respect to the rotating magnetic field. During this time the pulsed power to the stator windings can get adjusted to meet the requirement of the changed load. In case very large and fast load changes are expected then the power and frequency of pulses can be automatically adjusted using feedback from rotor speed sensor which is expected to be simpler and easier as compared to the rotor nositiion sensor used in case of permanent magnet rotor. Even a rotor with stampings of suitable magnetic material can be used for this proposed rotor design.
Detailed Description of the Invention
The present invention will now be described in detail with the help of the accompanying drawings, where
Fig. 1 schematically shows the brushless DC motor of the present invention.
Fig. 2 shows the pulsed power supply for the brushless DC
motor of Fig. 1.
Fig. 3 schematically shows the circuit for pulsed power supply for brushless DC motor of Fig. 1
The configuration of the salient pole brushless D.C. motor is as shown in Fig. 1. The stator windings Ml and W3 for stator salient poles PI and P3 are wound in the same direction and *re connected in series. Similarly another set of windings W2 and W4 far poles P2 and P4 are wound in the same direction and are connected in series. The winding direction is such that when a postive voltage/current pulse is given to windings Wl and W3 a magnetic north pole is induced at PI and a magnetic south pole is induced at P3. Similarly when a positive voltage/current pulse is given to windings W2 and W4 a north and a south pole are induced respectively at P2 and P4. For negative pulses the opposite poles are induced.
The rotor is made of high saturation flux density, low coercivity ( low He) high retentivity (high Br), soft magnetic material such as mild steel. It can also be made from stamping of suitable magnetic material.
An electronic sequential pulse generating power supply (D.C) is used far driving the motor as illustrated in Fig. 2 and Fig. 3. First a postive pulse 1 is given to windings Wl and W3. Since
the rotor is of easily magnetisable soft magnetic material it gets aligned ( and magnetized) as shown in Fig.l. Next positive pulse 2 is given to windings W2 and W4. Due to this second pulse the rotor now moves 90 degrees clockwise and gets aligned vertically. After this a negative pulse 3 is given to the windings Wl and W3. Due to this the rotor now moves further 90 degrees clockwise. After this a negative pulse 4 is given to the windings W2 and W4. Due to this rotor now moves further 90 degrees clockwise. Again when a postive pulse 1 is given to the windings Wl and W3 the rotor moves 90 degrees clockwise and attains the start position thus completing one cycle or one revolution. By gradually increasing the frequency of pulses (by reducing time interval t2 the speed (r.p.m.) of the rotor can be increased. A rotor speed sensor ( not shown) can be provided and the feedback from this sensor can be used for automatically adjusting the power and frequency of the pulses whenever very large 'and fast load changes are expected. The power to the windings can be increased by increasing pulse height i.e. voltage/current and to some extent by increasing pulse - width tl.
It can be seen that pulses 1-3-1-3 form one A.C. wave and the pulses 2-4-2-4 form another A.C wave with a phase difference of 90 degrees. Thus a rotating magnetic field is created, similar to the one in the split - phase single phase induction motor, which in turn rotates the rotor.
Although the invention of the present application has been described in detail, it should be understood that variations and modifications can be made by one skilled in the art without departing from the spirit and scope of the invention as hereinafter claimed. For example, more number of poles and more number of windings can be provided to enhance the power and performance of the motor.
1. A salient pole brushless D.C. motor comprising pairs of salient poles with corresponding pairs of windings wound in the same direction and connected in series - a. control pulse generating circuit for generating positive pulses and negative pulses and sequentially providing said pulses to said pairs of windings for creating a rotating magnetic field; characterized in that said motor is provided with a rotor comprising a high saturation flux density, low coercivity high retentivity, soft magnetic material, said rotor getting magnetized by the rotating magnetic field created by the pairs of windings.
2. The salient pole brushless O.C. motor as claimed in claim
I, wherein said rotor is made from mild steel.
3. The salient pole brushless D.C. motor as claimed in claim
1, wherein said rotor is made from stampings of a magnetic
4. The salient pole brushless D.C. motor as claimed in claim
1, wherein said rotor is provided with a rotor speed sensor for
automatic adjustment of power and frequency of the pulses in case
of fast load changes.
5. . A salient pole brushiess D.C. motor substantially as herein described and illustrated in the accompnying drawings.
|Indian Patent Application Number||1256/DEL/2002|
|PG Journal Number||44/2008|
|Date of Filing||13-Dec-2002|
|Name of Patentee||BHARAT HEAVY ELECRICALS LTD|
|Applicant Address||BHEL HOUSE, SIRI FORT, NEW DELHI-110 049, INDIA.|
|PCT International Classification Number||H02K 23/04|
|PCT International Application Number||N/A|
|PCT International Filing date|