FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
As amended by the Patents (Amendment) Act, 2005
& The Patents Rules, 2003 As amended by the Patents (Amendment) Rules, 2006
PROVISIONAL SPECIFICATION
(See section 10 and rule 13)
TITLE OF THE INVENTION
A method to design the single phase BLDC motor with reduced ripple torque
APPLICANTS
Crompton Greaves Limited, CG House, Dr Annie Besant Road, Worli,
Mumbai 400 030, Maharashtra, India, an Indian Company
Inventor
Mohammed Fazil of Crompton Greaves Ltd, Analystics Department, Global R&D, Kanjur Marg, Mumbai, India, an Indian National
PREAMBLE TO THE DESCRIPTION
The following specification describes the invention


This invention relates to a method design a new tooth profile of stator of Single Phase BLDC (SPBLDC) motor which reduce torque ripple.
Conventionally various methods are known to design tooth profile of single-phase permanent magnet brushless DC motor by using topologies such as tapered airgap, stepped airgap, Asymmetric airgap and slotted teeth.
One of the methods used the topology of tapered air gap to design tooth profile. This method do not have control over the peak to peak cogging torque or starting torque. In this method, non-uniform air gap is achieved by either shining the centre of tooth edge along X axis or by shining the end point downwards.
A brushless dc (BLDC) motor has a permanent magnet on rotor and electronic commutating switches with winding on stator. It has a many performance benefits over conventional motors such as induction motors and dc mechanical commutating motor, such as higher efficiency better speed control etc. Single phase BLDC (SPBLDC) motor is widely used in low-cost and power applications such as exhaust fans, cooling fans, CD-ROM spindle motors etc.
The SPBLDC motors have dead point, where the excitation net torque is zero, hence this motors are not self starting. [See Figure 1]. The Dead point is location where the rotor rests when no power is supplied to motor

Figurel: Dead point in a uniform air gap SPBLDC motor The asymmetric air gap is thus designed, so that the starting point of motor can avoid dead position, but the asymmetric air gap introduces torque ripples and deteriorates the motor characteristics. It will result in to vibrations and torque oscillations. Total harmonic distortion (THD) is a measure of harmonic or ripple present in a wave
form and is given by THD where V1 is fundamental
waveform V2 to n is higher order harmonics. Tooth profile is called a shape of tooth tip exposed to air gap of motor. Different tooth profiles existing and it total harmonic distortion is shown in table 1 and 2.

Table: 2
Existing technologies, as in analysis results shown above have a THD of more than 3%. This will make single phase brushless motor non-usable in areas, which are highly sensible to torque ripples.

The conventional methods lead to some disadvantages such as vibration and torque oscillation.
In prior art some of the patents such as US6744171 discloses modification of air gap through the introduction of unique surface configurations on stator teeth for reducing the cogging torque and US4940912 discloses the notches of stator inclined by predetermined angle to reduce the effect of harmonics.
One of US patent US7345440 discloses the different structures of asymmetrical air gap methods which is applicable to present invention. But this patent focusing more towards control aspects and also its used conventional methods.
An object of the invention is to provide a novel methods to reduce torque ripple by the new tooth profile of stator of Single Phase BLDC (SPBLDC) motor.
An objective of the invention is to identifying design parameters of tooth profile, which will influence cogging torque and starting torque in a single phase BLDC motor
Another objective of the invention is to reduce the torque ripple below THD 1%
Another objective of the invention is to enhance the life of the motor by reducing the vibration and also enhanced efficiency.
30 MAR 2009
5

The invention is related to a method of designing a stator tooth shape of single phase BLDC motor, which will result a reduced torque ripple to less than a THD of 1%. According to invention, a dip point is identified by parametric analysis of finite element model of single phase BLDC motor. The objective of finite element analysis is to identity a dip point on the periphery of tooth of single phase BLDC motor, which satisfies required starting torque arid peak to peak value of cogging torque.
The next step is to connect the start point, dip and end point in such a way that it will result in least possible torque ripple. In current invention a polynomial curve is used as connecting line. In specific example used in invention, the polynomial used is

Which is a forth order polynomial. Where 'R' is the radius and '0' is the angle from
reference line. Constants a, b, c, d and e are found out by using the constraints equations-2.
At start point At dip point At end point
In accordance to the present invention, a novel method comprises three step processes which is magnitude of dip, angle of dip and a profile shape connecting start point, dip and end point, to reduce torque ripple by the new tooth profile of stator of Single Phase BLDC (SPBLDC) motor.
6 3 0 MAR 2009

In accordance to the present invention, said magnitude of dip determine or control peak to peak value of ripple torque or cogging torque.
In accordance to the present invention, said angle of dip determine or control the dead point and further determine starting torque.
In accordance to the present invention, said a polynomial fit profile shape connecting start point, dip and end point determine reduction in the torque ripple.
In accordance to the present invention, is to design a new tooth profile with very low torque ripple content compared to existing. The subsequent analyses identified that magnitude of cogging torque and position of dead point and hence staring torque can be controlled by introducing a dip in tooth edge exposed to air gap. Also lower torque ripple is achieved by a tooth profile with polynomial fit of start point, dip and end point.
In accordance to the present invention, said peak value of cogging torque depends on the magnitude of dip and starting torque depends on position of dip. Based on these observations a profile is decided by connecting start point, dip and end point, with a polynomial fit.
In accordance to the present invention, said polynomial fit used the order of four to achieve a torque ripple as low as a THD of 0.3%
3 0 MAR 2009
7

In accordance to the present invention, said polynomial order is depending upon the dimensions of the motor and this polynomial may be in any order.
In accordance with the invention, a dip in periphery of tooth is found out in such a way that radial distance of dip point from rotor bore is based on the required peak to peak cogging torque and the position or angle of dip from a reference point is based on the required starting torque.
In accordance with the invention, position of dip is finalized based on the peak to peak cogging torque and starting torque through a parametric analysis of finite element model of single phase BLDC motor.
In accordance with the invention, a start point is decided by the minimum mechanical clearance need for stator teeth from bore of rotor; the minimum slot opening needed and number of poles.
In accordance with the invention, the end point radius is calculated as a mean of start point radius and dip radius and end point angle from reference is decided by the minimum slot opening needed and number of poles.
In accordance with the invention, the start point, dip and end points are connected by a polynomial curve, resulting in gradual decrease in radius until dip point and increase in radius after that up to end point.


In accordance with the invention, polynomial curves rate of change of variation with angle, at start point and dip point is zero.