System and method for commutating a motor

1. A method for adjusting a rotational speed of a rotor in a polyphase motor from a first speed towards a second, faster speed when the motor is commutating according to a commutation scheme at a first commutation phase angle, the method comprising:

determining said first commutation angle utilizing a first back EMF voltage detected in said motor;

commutating said motor when the first back EMF voltage is detected in said motor;

selecting an advanced phase angle relative to the first commutation phase angle for commutating the motor; and

adjusting the commutation scheme to be commutating at the advanced phase angle by

calculating a rate of change of back EMF in the motor at the first speed using a back EMF constant of the motor;

determining a second back EMF voltage according to the rate of change in respect of the advance phase angle; and

adjusting the commutation scheme to commutate when the second back EMF voltage is observed,

whereby the rotational speed of the rotor increases from the first speed towards the second speed as the motor is commutating at the advanced phase angle.

2. The method as claimed in claim 1 , wherein the steps of selecting the advanced phase angle and adjusting the commutation scheme are steps of a feedback control loop for adjusting the rotational speed of the rotor towards the second speed, the feedback control loop further comprising the steps of:

evaluating a present speed of rotation of the rotor; and

if the present speed is less than the second speed, then:

setting the present speed as the first speed; and

repeating the steps of selecting the advanced phase angle, adjusting the commutation scheme, and evaluating the present speed of rotation of the rotor.

3. The method as claimed in claim 2 , wherein the motor is a three-phase motor.

4. The method as claimed in claim 3 , wherein said commutation scheme utilizes trapezoidal current commutation.

5. The method as claimed in claim 3 , wherein the first back EMF value is not taken after the zero cross of the back EMF.

6. The method as claimed in claim 5 , wherein the second back EMF value is not taken after the zero cross of the back EMF.

7. The method as claimed in claim 6 , wherein the motor is connected in either a wye or delta configuration.

8. A system for adjusting a rotational speed of a rotor in a polyphase motor from a first speed towards a second, faster speed when the motor is commutating according to a commutation scheme at a first commutation phase angle, the system comprising:

the motor; and

a motor controller

for selecting an advanced phase angle relative to the first commutation phase angle for commutating the motor;

for calculating a rate of change of back EMF in the motor at the first speed using a back EMF constant of the motor;

for determining a second back EMF voltage for the motor according to the rate of change in respect of the advanced phase angle.

for adjusting the commutation scheme to commutate the motor when the second back EMF voltage is observed at the advanced phase angle so that the rotational speed of the rotor increases from the first speed towards the second speed as the motor commutates to the advanced phase angle; and

wherein

the first commutation phase angle is determined with respect to a first back EMF voltage; and

the commutation scheme commutates when the first back EMF voltage is detected in the motor.

9. The system as claimed in claim 6 , further comprising a feedback control system for said motor controller, the feedback control system comprising:

a module to evaluate a present speed of rotation of the rotor; and

a module to set the present speed at the first speed and to initiate and control repeating the steps of selecting the advanced phase angle, adjusting the commutation scheme, and evaluating the present speed of rotation of the rotor, if the present speed is less than the second speed.

10. The system as claimed in claim 9 , wherein the motor is a three-phase motor.

11. The system as claimed in claim 10 , wherein the motor controller is a digital signal processor.

12. The system as claimed in claim 11 , wherein said commutation scheme utilizes trapezoidal current commutation.

13. The system as claimed in claim 12 , wherein said motor is battery powered.

14. A motor controller for adjusting a rotational speed of a rotor in a polyphase motor from a first speed towards a second, faster speed when the motor is commutating according to a commutation scheme at a first commutation phase angle, the controller comprising:

a first module to select an advanced phase angle relative to the first commutation phase angle for commutating the motor;

a second module to calculate a rate of change of back EMF in the motor at the first speed using a back EMF constant of the motor;

a third module to determine a second back EMF voltage for the motor according to the rate of change in respect of the advanced phase angle; and

a fourth module to adjust the commutation scheme to commutate the motor when the second back EMF voltage is observed at the advanced phase angle so that the rotational speed of the rotor increases from the first speed towards the second speed as the motor commutates to the advanced phase angle,

wherein

the first commutation phase angle is determined with respect to a first back EMF voltage; and

the commutation scheme commutates when the first back EMF voltage is detected in the motor.

15. The motor controller for adjusting a rotational speed of a rotor in a polyphase motor as claimed in claim 14 , further comprising a feedback control system to:

evaluate a present speed of rotation of the rotor; and

set the present speed at the first speed and initiate and control repeating the steps of selecting the advanced phase angle, adjusting the commutation scheme, and evaluating the present speed of rotation of the rotor, if the present speed is less than the second speed.

16. The motor controller for adjusting a rotational speed of a rotor in a polyphase motor as claimed in claim 15 , wherein the motor controller is a digital signal processor.

17. The motor controller for adjusting a rotational speed of a rotor in a polyphase motor as claimed in claim 16 , wherein said commutation scheme utilizes trapezoidal current commutation.