Sine modified trapezoidal drive for brushless DC motors
A pulse width modulated (PWM) trapezoidal commutation drive to a brushless direct current (BLDC) motor is sine modified so that the applied drive voltage substantially matches the induced voltage generated in the BLDC motor. The values of the cosine of the angles between −30 degrees and +30 degrees are used to modify the duty cycle of the PWM drive signal dependent upon the rotor angular positions determined from the times between the zero crossing BEMF voltages measured at the unconnected motor terminals.
1. A method for modifying trapezoidal drive to a brushless direct current (BLDC) motor, said method comprising the steps of:
starting a commutation drive period;
dividing the commutation drive period into N time segments;
associating the N time segments into N degree segments, wherein a one of the N degree segments in the middle of the N time segments is substantially zero (0) degrees;
determining cosine values for each of the N degree segments; and
modifying a duty cycle of a pulse width modulation (PWM) drive to the BLDC motor with the cosine values at appropriate times during the commutation drive period.
2. The method according to claim 1 , wherein the BLDC motor is three phase and has first, second and third motor terminals.
3. The method according to claim 2 , wherein the step of starting the commutation drive period comprises the steps of:
coupling the first motor terminal to a power supply common;
coupling the second motor terminal to a power supply voltage controlled by the PWM drive;
determining when a back electromotive force (BEMF) voltage is substantially zero at the third motor terminal, wherein the third motor terminal is not connected to either the power supply common or voltage; and
determine the commutation drive period from a time between the BEMF voltage at substantially zero volts and a previous BEMF voltage at substantially zero volts.
4. The method according to claim 2 , wherein there are six commutation drive periods of sixty (60) electrical degrees rotation each, and each one of the six commutation drive periods having a different connection configuration of motor terminals.
5. The method according to claim 2 , wherein the N degree segments are from about minus thirty (−30) degrees to about plus thirty (+30) degrees.
6. The method according to claim 5 , wherein the duty cycle of the PWM drive is greatest at a degree segment of zero (0) degrees.
7. The method according to claim 2 , wherein the three phase BLDC motor has three stator pole pair windings.
8. The method according to claim 7 , wherein the three stator pole pair windings are WYE connected.
9. The method according to claim 7 , wherein the three stator pole pair windings are Delta connected.
10. The method according to claim 2 , wherein the three phase BLDC motor has one pole pair winding per phase.
11. The method according to claim 2 , wherein the three phase BLDC motor has at least two pole pair windings per phase.
12. The method according to claim 1 , wherein the step of determining cosine values is done with a microcontroller and a cosine look-up table.
13. The method according to claim 1 , wherein the steps of starting a commutation drive period, dividing the commutation drive period into N time segments, associating the N time segments into N degree segments, determining cosine values, and modifying the duty cycle of the PWM drive are done with a microcontroller.
14. A system for driving a brushless direct current (BLDC) motor with sine modified pulse width modulation (PWM), said system comprising:
a three phase BLDC motor having first, second and third motor terminals;
high and low side power switches coupled to each of the motor terminals, wherein the high and low side power switches are further coupled between a power supply common and a power supply voltage controlled by the PWM drive, wherein each of the high and low side power switches will connected the respective one of the motor terminals to either the power supply common, voltage or neither;
zero crossing voltage detectors, wherein each one of the motor terminals is connected to a respective one of the zero crossing voltage detectors;
a PWM master time base;
PWM generators coupled to the PWM master time base and respective ones of the motor terminals; and
a microcontroller comprising a cosine lookup table and at least one timer/counter for storing time periods between back electromotive force (BEMF) zero voltage crossings at ones of the motor terminals not connected to either the power supply common or voltage, wherein the zero crossing voltage detectors provide when the BEMF zero voltage crossings occur;
wherein the microcontroller generates a sine modified duty cycle PWM control signal to the PWM generators for controlling a duty cycle of the PWM drive therefrom based upon cosine values of rotational degrees associated with the stored time periods between the BEMF zero voltage crossings.
15. The system according to claim 14 , further comprising high and low driver pairs coupled between the PWM generators and respective ones of the high and low side power switches.
16. The system according to claim 14 , wherein the high and low side power switches comprise high and low side power metal oxide semiconductor field effect transistors (MOSFETs).
17. The system according to claim 14 , wherein the duty cycle of the PWM control signal is greatest at a one-half time of each stored time period.
18. A motor controller for providing sine modified pulse width modulation (PWM) drive to a brushless direct current (BLDC) motor, comprising:
high and low side power switches coupled to each motor terminal, wherein the high and low side power switches are further coupled between a power supply common and a power supply voltage controlled by the PWM drive, wherein each of the high and low side power switches will connected the respective one of the motor terminals to either the power supply common, voltage or neither;
zero crossing voltage detectors, wherein each one of the motor terminals is connected to a respective one of the zero crossing voltage detectors;
a PWM master time base;
PWM generators coupled to the PWM master time base and respective ones of the motor terminals; and
a microcontroller comprising a cosine lookup table and at least one timer/counter for storing time periods between back electromotive force (BEMF) zero voltage crossings at ones of the motor terminals not connected to either the power supply common or voltage, wherein the zero crossing voltage detectors provide when the BEMF zero voltage crossings occur;
wherein the microcontroller generates a sine modified duty cycle PWM control signal to the PWM generators for controlling a duty cycle of the PWM drive therefrom based upon cosine values of rotational degrees associated with the stored time periods between the BEMF zero voltage crossings.
19. The motor controller according to claim 18 , wherein the duty cycle of the PWM control signal is greatest at a one-half time of each stored time period.