Claims:WE CLAIM:
1. A system to adapt to variations in back EMF of BLDC motor [203], comprising:
permanent magnets;
microcontroller based control system;
firmware;
characterized in that the permanent magnets are magnetized to their maximum strength and assembled in the rotor of the motor [203], and on an initial selection of maximum speed after power ON [207], the microcontroller computes the instant speed value from the zero crossing values [210] of the back EMF and variation of back EMF is compensated by the preprogrammed firmware logic, and
wherein the firmware is preprogrammed to increase the Pulse width modulation (PWM) duty cycle gradually until desired speed is achieved.

2. The system to adapt to variations in back EMF of BLDC motor [203] as claimed in claim 1, wherein the microcontroller is 8-bit controller embedded in the motor body.

3. The system to adapt to variations in back EMF of BLDC motor [203] as claimed in claim 1, wherein microcontroller detects EMF values and identifies the maximum speed level.

4. The system to adapt to variations in back EMF of BLDC motor [203] as claimed in claim 1, wherein by increasing the ON state time of the PMW signal, the PMW duty is set by the microcontroller.

5. The system to adapt to variations in back EMF of BLDC motor [203] as claimed in claim 1, wherein the maximum speed value is identified by monitoring the zero crossing points [210] of the back EMF detected by using reference values of back EMF signals.

6. The system to adapt to variations in back EMF of BLDC motor [203] as claimed in claim 1, wherein the microcontroller monitors and compares the calculated speed value to the desired speed value [211] for a predetermined period of time.

7. The system to adapt to variations in back EMF of BLDC motor [203] as claimed in claim 1, wherein even after the predetermined period of time when exceeded and on mismatch of calculated speed value with desired speed value [211] the PMW duty is increased from the predetermined minimum duty [209] by increasing the ON time of the PMW duty cycle until the desired speed [211] is achieved.

8. The system to adapt to variations in back EMF of BLDC motor [203] as claimed in claim 1, wherein on reaching the desired speed [211] the duty cycle is fixed and is maintained constant until the subsequent power ON.
9. The system to adapt to variations in back EMF of BLDC motor [203] as claimed in claim 1, wherein the maximum limit for the duty is set in the firmware such that the fan operating on BLDC motor [203] does not exceed the maximum power at the highest speed.

10. A method to adapt to variations in back EMF of BLDC motor [203], comprises:
measuring the back EMF value by a microcontroller placed in an electronic controller embedded in a BLDC motor body,
setting a predefined-minimum duty value [209] by the microcontroller, on an initial selection of highest speed of the motor [203] on turning ON the power;
monitoring and calculating/ computing of the current/ instant speed of the BLDC motor and comparison with desired speed value [211] for a predefined time interval;
determining if the desired speed [211] matches the calculated/ computed speed,
increasing the PWM duty value on completion of the predefined time interval on mismatch of the calculated with desired speed [211] gradually until the calculated speed value equalizes with the desired speed value [211] for fixing the PWM duty value, by the microcontroller,
wherein PWM duty when fixed, the value remains constant until the subsequent power ON condition of the motor [203] is actuated, and
the firmware is configured to the maximum limit for the PWM duty value in the microcontroller such that the fan operating on the BLDC motor [203] does not exceed the maximum power at the highest speed.

11. The method to adapt to variations in back EMF of BLDC motor [203] as claimed in claim 10, wherein the speed is determined by the microcontroller based on back EMF voltage of the motor [203].

12. The method to adapt to variations in back EMF of BLDC motor [203] as claimed in claim 10, wherein the predefined time interval range is 0.2 to 3 second.

13. The method to adapt to variations in back EMF of BLDC motor [203] as claimed in claim 10, wherein the microcontroller calculates the speed of the BLDC motor [203] using the pre-defined constant value preferably in the range of 3000 to 5500 divided by the time difference between the rising zero crossing points and falling zero crossing points [210].

14. The method to adapt variations in back EMF of BLDC motor [203] as claimed in claim 10, wherein the predefined-minimum duty value [209] is 60 to 90% and the predefined-maximum duty value is 80 to 100%. , Description:FIELD OF INVENTION
The present invention relates to a speed control method for a sensor less BLDC motor-based ceiling fan. More particularly the present invention related to a duty adjustment method that adapts variations in the back EMF of the BLDC motor and achieves the desired speed of a sensor less BLDC motor-based ceiling fan.

BACKGROUND OF INVENTION
Three-phase brushless DC (BLDC) motors are widely used in fans for ventilating and cooling central processing units (CPUs), graphics processor, power supplies, and many other applications. The requirements for cooling fans are continuously increasing as the use of powerful cooling-requiring electronics are increasing. Further, as the number of fans in homes and offices is increased, the need to keep these fans as quiet and efficient as possible is also getting more pronounced. The characteristic of BLDC motors matches the requirements of fans very well.

The Back EMF of a BLDC motor has a significant impact on the speed of the motor for given applied voltage and the power consumed by the motor. In BLDC motor driven ceiling fan, variation in back EMF causes changes in the speed of the fan and the power consumed by the fan. Back EMF is the voltage generated in the stator when the flux linking with it changes. It is called as back EMF because it acts opposite to that of applied voltage. It is proportional to speed and flux. The following are the factors (variations in motor hardware) affecting the back EMF,
a) Strength of the magnet
b) Number of turns in the stator winding
c) Air gap
d) Stator core length
e) Rotor speed

In the BLDC motor based ceiling fan, the back EMF varies from one motor to other due to various factors (in manufacturing the lot/batch variation, skew), and this can lead to the variations in the performances namely the ‘speed’ and ‘power consumption’ of the ceiling fan. In order to avoid variations in the performances of the ceiling fan, the back EMF should be in narrow range. For instance, if the back EMF is above the specified threshold range, then the speed of the fan and power consumed by the fan decreases and if the back EMF is below the specified threshold range, then the speed of the fan and power consumed by the fan increases.

Figure 1 illustrates the method adopted to achieve the desired voltage of Back EMF according to the prior art. In BLDC motor, the rotor assembly essentially comprises the magnets made of ferrite material. Generally, the ferrites are magnetized using the magnetizer [201] for converting into permanent magnets. These permanent magnets are assembled in the rotor [202]. Then the stator and rotor [202] are assembled as a motor [203]. The assembled motor [203] is then subjected to back EMF testing.

The Back EMF testing is carried out by using the instrument called back EMF test jig [204]. During back EMF testing, the assembled motor [203] is coupled with the prime mover in the back EMF test jig [204] to run as a generator. Then the prime mover is made to run at the desired speed. While running, the RMS (Root mean square) voltage across the stator terminals and DC voltage across the rectifier output are measured. If the desired values of voltages are not obtained then the air gap and winding turns of the motor are verified, because the air gap and winding turns are also contributing for the back EMF voltage.

If the air gap and winding turns are within the predefined threshold value, then the rotor is disassembled to remove the magnets. These magnets are then demagnetized [205] and again magnetized using the magnetizer [201]. The re-magnetized [206] magnets are then assembled in the motor and the back EMF testing and magnetization processes are repeated until the desired voltages are obtained. If the air gap and winding turns are not within the predefined threshold value, then the air gap and number of turns in the stator winding is corrected accordingly. Then the stator and rotor [202] are assembled as a motor [203] and the back EMF is tested. These iteration processes are repeated until the desired voltages are obtained as shown in Figure 1. The above said iterative process is time consuming, hence slows down the production throughput and adds to the cost.

Accordingly, it will be advantageous to provide a method to adapt to the variations in back EMF, thereby eliminating the need of repeated testing of back EMF and re-magnetization process which in turn increases the production rate and reduces the cost.

OBJECTS OF INVENTION
One or more of the problems of the conventional prior art may be overcome by various embodiments of the present invention.

The primary object of the present invention is to provide a duty adjustment method to adapt variations in the back EMF value and to achieve the desired speed of a sensor less BLDC motor-based ceiling fan.

It is another object of the present invention, wherein the permanent magnets are used directly instead of ferrites which can eliminate the magnetization and demagnetization process which in turn increases the production rate and reduces the cost.
It is another object of the present invention, wherein the control logic is implemented by programming the microcontroller of the electronic controller embedded in the motor body.

It is another object of the present invention, whenever the 5th speed is selected for the first time after power ON, the microcontroller sets the PWM duty value to a predefined-minimum duty value.

It is another object of the present invention, wherein the microcontroller calculates the speed value by detecting the zero crossing points of the back EMF.

It is another object of the present invention, wherein the microcontroller compares the calculated speed value with the desired speed value.

It is another object of the present invention, wherein the microcontroller increases the PWM duty value gradually until the desired speed value is achieved and the PWM duty value is fixed.

It is another object of the present invention, wherein the maximum limit for the PWM duty value is set in the microcontroller such that the fan does not exceed the maximum power at the highest speed.

SUMMARY OF INVENTION
Thus according to the embodiment of the present invention there is provided a system to adapt to variations in back EMF of BLDC motor, comprising permanent magnets, microcontroller based control system and firmware.

It is another aspect of the present invention, wherein the permanent magnets are magnetized to their maximum strength and assembled in the rotor of the motor, and on an initial selection of maximum speed after power ON, the microcontroller computes the instant speed value from the zero crossing values of the back EMF and variation of back EMF is compensated by the preprogrammed firmware logic, and

It is another aspect of the present invention, wherein the firmware is preprogrammed to increase the Pulse width modulation (PWM) duty cycle gradually until desired speed is achieved.

It is another aspect of the present invention, wherein the microcontroller is 8-bit controller embedded in the motor body.

It is another aspect of the present invention, wherein the microcontroller detects EMF values and identifies the maximum speed level.
It is another aspect of the present invention, wherein by increasing the ON state time of the PMW signal, the Pulse width modulation (PWM) duty is set by the microcontroller.

It is another aspect of the present invention, wherein the maximum speed value is identified by monitoring the zero crossing points of the back EMF detected by using reference values of back EMF signals.

It is another aspect of the present invention, wherein the microcontroller monitors and compares the calculated speed value to the desired speed value for a predetermined period of time.

It is another aspect of the present invention, wherein even after the predetermined period of time when exceeded and on mismatch of calculated speed value with desired speed value the PMW duty is increased from the predetermined minimum duty by increasing the ON time of the PMW duty cycle until the desired speed is achieved.

It is another aspect of the present invention, wherein on reaching the desired speed the duty cycle is fixed and is maintained constant until the subsequent power ON.
It is another aspect of the present invention, wherein the maximum limit for the duty is set in the firmware such that the fan operating on BLDC motor does not exceed the maximum power at the highest speed.

According to another embodiment of the present invention there is provided method to adapt to variations in back EMF of BLDC motor, comprises: measuring the back EMF value by a Programmed microcontroller placed in an electronic controller embedded in a BLDC motor body, setting a predefined-minimum duty value (PWM) by the microcontroller, on an initial selection of highest speed of the motor on turning ON the power; monitoring and calculating/ computing of the current/ instant speed of the BLDC motor and comparison with desired speed value for a predefined time interval; determining if the desired speed matches the calculated/ computed speed, increasing the PWM duty value on completion of the predefined time interval on mismatch of the calculated with desired speed gradually until the calculated speed value equalizes with the desired speed value for fixing the PWM duty value, by the microcontroller.

It is another aspect of the present invention, wherein the speed is determined by the microcontroller based on back EMF voltage of the motor.

It is another aspect of the present invention, wherein the predefined time interval range is 0.2 to 3 second.
It is another aspect of the present invention, wherein the programmed microcontroller calculates the speed of the BLDC motor using the pre-defined constant value preferably in the range of 3000 to 5500 divided by the time difference between the rising zero crossing points and falling zero crossing points.

It is another aspect of the present invention, wherein the predefined-minimum duty value is 60 to 90% and the predefined-maximum duty value is 80 to 100%.

BRIEF DESCRIPTION OF THE DRAWINGS:
Figure 1: illustrates a flowchart of the method to achieve the desired voltage of Back EMF according to the prior art.
Figure 2: illustrates a flowchart of the method to achieve the desired speed of a sensor less BLDC motor-based ceiling fan according to the present invention.
Figure 3: illustrates a Zero crossing points of back EMF of the BLDC motor according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION WITH REFERENCE TO THE ACCOMPANYING FIGURES
The present invention provides a system and duty adjustment method that adapts variations in the back EMF value of the BLDC motor [203] and achieves the desired speed of a sensor less BLDC motor-based ceiling fan. According to the present invention, the system comprises permanent magnets, microcontroller placed in an electronic controller embedded in a BLDC motor body and firmware. The permanent magnets are used directly instead of ferrites which can eliminate the magnetization and demagnetization process which in turn increases the production rate and reduces the cost. The permanent magnets are magnetized to their maximum strength possible and assembled in the rotor. Then the stator and rotor [202] are assembled to form a motor [203]. The motor is then subjected to performance testing.

To achieve the desired performance by adjusting to the varying back EMF of the motor (which is in turn the wide range of magnet strength) the control is done in the firmware algorithm loaded in the microcontroller. This control logic is implemented by programming the 8-bit microcontroller of the electronic controller embedded in the motor [203] body. The ADC (Analog to Digital Converter) in the microcontroller coverts the inputted analog signal into the digital numbers to be processed by the microcontroller. The microcontroller monitors the ADC (Analog to Digital Converter) value and speed command for
1) Detection of back EMF value and
2) Identification of the speed level. (1st, 2nd, 3rd, 4th or 5th speed)

Refereeing to Figure 2, the microcontroller monitors the speed level of the fan. If the speed level other than highest speed level (5th) i.e.) 1st, 2nd, 3rd, 4th speed level is selected then the microcontroller runs the fan at set the speed level. When the 5th speed (maximum speed) is selected [208] for the first time after power ON [207], the microcontroller sets the Pulse width modulation (PWM) duty value to a predefined-minimum duty value [209]. According to the present invention, the predefined-minimum duty value [209] is 60 to 90%. The PWM duty value is the duration of time the PWM signal is in a high (ON) state as a percentage of the total time it takes to complete one cycle. The microcontroller sets PWM duty value to the predefined-minimum duty value [209] by increasing the ON state time of the PWM duty cycle.

After setting the PWM duty value, the microcontroller calculates the speed value by detecting the zero crossing points of the back EMF. The zero crossing points [210] are detected by using the reference value of back EMF signals. The reference value of back EMF signal is calculated as half of the DC bus voltage. If the back EMF value is greater than above mentioned reference value, then it is detected as rising zero crossing point [210]. If the back EMF value is less than the reference value then it is detected as falling zero crossing point [210]. The zero crossing points [210] of back EMF value are shown as graph in the figure 3.

The microcontroller calculates the speed of the BLDC motor [203] using the pre-defined constant value preferably in the range of 3000 to 5500 divided by the time difference between the rising zero crossing points and falling zero crossing points [210]. The microcontroller monitors the calculated speed for a predetermined time interval range is 0.2 to 3 second and then compares the calculated speed value with the desired speed value [211]. If the calculated speed value matches with the desired speed value [211] then the microcontroller runs the fan at this desired speed level [211].

If the calculated speed value not matches with the desired speed value [211] then the microcontroller increases the PWM duty value from the predefined-minimum duty [209] by increasing the ON state time of PWM duty cycle. Again the microcontroller calculates the speed for present increased PWM duty value and compares with the desired speed value [211]. The microcontroller increases the PWM duty value until the calculated speed value is matched with the desired speed value [211]. Once match is arrived, then the desired percentage of PWM duty value is fixed to achieve the desired speed value [211]. This fixed PWM duty value will not change until the next power ON.

The predefined-maximum limit for the PWM duty value is preset in the microcontroller such that the fan does not exceed the maximum power at the highest speed. In a preferred embodiment, the predefined-maximum duty value is 80 to 100%. At this percentage the fan runs at the desired speed [211] by adapting to the varying back EMF voltages. By doing this, the variation in the performances such as fan speed and power consumption is reduced, although there were variations in the back EMF. This eliminates the two steps Checking the back EMF of each motor and Re -magnetization of the ferrite which are used in the known manufacturing process, which are laborious very expensive. Also the above steps consume more time and impose restriction in the productivity.

Advantages of the present invention
The permanent magnets are used directly instead of ferrites which can eliminate the magnetization and demagnetization process which in turn increases the production rate and reduces the cost.
The present invention is a reliable and cost-effective duty adjustment method that adapts up to 9% variations in the back EMF value of the BLDC motor
Eliminates the time consuming back EMF testing process in the manufacturing of ceiling fan
The present invention extended to other types of domestic and industrial fans such as table fans, air circulator fans and other fans which are based on PM BLDC motors.

Although, the invention has been described with reference to specific embodiments, these descriptions are not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternative embodiments of the invention will become apparent to persons skilled in the art upon reference to the description of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. It is therefore, contemplated that the claims will cover any such modifications or embodiments that fall within the true scope of the invention.