Start-up procedure for a brushless, sensorless motor

A method for operating a motor having multiple stator windings and a rotor, comprising the steps of:

• (a.) providing current to said windings, for a first maximum time duration, to excite an initial phase thereof;
• wherein Said first duration is sufficiently short that the rotor travels, during said first duration, through no more than one-half Of the angular separation between two adjacent ones of the equilibrium positions, stable or unstable, of all of said phases;
• (b.) monitoring back electromotive force (back EMF) on multiple ones of said windings, and generating digital signals accordingly;
• (c.) IF a zero-crossing of back EMF occurs on any one of said windings within at most a second time duration, and if the signs of said back EMF values after said zero-crossing are consistent with the signs of said back EMF values before said zero-crossing, then using said digital signals to determine an optimal phase for excitation, and exciting said optimal phase;
• and
• (d.) if NO zero-crossing of back EMF occurs on any one of said windings within said second time duration, then repeating said step
• (a) with a different initial phase which is not adjacent to said first initial phase in said predetermined sequence of excitation phases;
• whereby said motor is started and operated without requiring sensor inputs to ascertain rotor position, and with minimal backward rotation at the time of startup.

A method for starting-up a brushless and sensorless DC motor having multiple stator windings and a rotor, without causing significant backward rotation, which comprises the steps of:

• (a) temporarily exciting the windings, in a first initial phase thereof, for a first interval of time;
• wherein said first interval has a maximum duration which is sufficiently short that the rotor travels, during said first interval, through no more than one-half of the angular separation between two adjacent ones of the equilibrium positions, stable or unstable, of all of the phases of said windings;
• (b) after said excitation step (a), digitally reading the signs of respective back electromotive forces (back EMF)on multiple ones of the windings;
• (c) detecting the occurrence of a first zero-crossing event in any one of said induced back electromotive forces within a preset second interval of time after said first excitation step, and accordingly:
• if such a zero-crossing event occurs within said second interval, again digitally reading the signs of said induced back electromotive forces, and utilizing said readings for decoding an optimal phase for excitation;
• if such occurrence is not detected before the elapsing of said second interval of time subsequent to the instant of interruption of said first excitation step, repeating said steps (b) and (c) with a second initial phase which is functionally shifted by at least two phase positions in respect to said first initial phase;
• and
• (d) starting-up the motor by sequentially exciting phases of said windings, initiating from said decoded optimal phase.

A circuit for starting-up in a desired sense of rotation a multiphase brushless and sensorless DC motor having multiple stator windings and a rotor, which comprises

• first means capable of producing a logic signal representative of the sign of back electromotive forces induced in the motor's windings by the rotation of the rotor;
• second means capable of generating a signal representative of the occurrence of an inversion of sign of any one of said induced back electromotive forces;
• means for decoding through a look-up table of codes of excitable phases of the motor capable of identifying an optimal phase to be excited for accelerating the rotor in a desired direction in function of said logic signals produced by said first means before and after said first zero-crossing event as detected by said second means;
• at least a first timer started by a start signal and capable of causing the excitation, for a preset first interval of time, of a predefined phase or of a phase functionally shifted by two phase positions from said predefined phase;
• wherein said first duration is sufficiently short that the rotor travels, during said first duration, through no more than one-half of the angular separation between two adjacent ones of the equilibrium positions, stable or unstable, of all of said phases;
• a second timer started by the signal produced by said second means and capable of enabling said first timer to excite said phase shifted by two phase positions from said predefined phase in the case said second means fail to generate said signal before the elapsing of a second interval of time.