Controller for brushless DC motor with flexible startup and method therefor

#Related subject matter is contained in copending patent application Ser. No. 14/146,096, filed Jan. 2, 2014, entitled “Controller for Brushless DC Motor with Low Torque Ripple and Method Therefor,” invented by the inventor hereof and assigned to the assignee hereof.

#The present disclosure relates generally to controllers for motors, and more particularly to controllers for brushless direct current motors and associated methods.

#A brushless direct current (BLDC) motor typically includes a rotor containing one or more permanent magnets and a stator with electromagnets that are dynamically switched. A controller controls the motor speed and commutation by modulating the width of pulses applied to switches in each motor phase driver. In a typical BLDC motor startup sequence, the controller drives the motor like a stepper motor because there is no significant back EMF at zero or low speed to use in making zero-crossing commutation decisions. Thus the motor is initially commutated very slowly, and then its velocity is increased by reducing the commutation time while the PWM duty cycle is increased over time to boost the applied motor voltage and to keep the current constant. This startup sequence is typically implemented using a table of duty cycles and another table timing delays for each commutation for a typical configuration and load.

#The present disclosure may be better understood, and its numerous features and advantages made apparent to those skilled in the art by referencing the accompanying drawings, in which:

#FIG. 1 illustrates in partial block diagram and partial schematic form a brushless direct current (BLDC) motor system according to one embodiment; and

#FIG. 2 illustrates a timing diagram useful in understanding the operation of the BLDC motor system of FIG. 1.

#The use of the same reference symbols in different drawings indicates similar or identical items. Unless otherwise noted, the word “coupled” and its associated verb forms include both direct connection and indirect electrical connection by means known in the art, and unless otherwise noted any description of direct connection implies alternate embodiments using suitable forms of indirect electrical connection as well.

#A motor controller for a brushless direct current (BLDC) motor as disclosed herein includes a pulse width modulator and a control circuit. The pulse width modulator provides a plurality of phase control signals to control corresponding ones of a plurality of phases of the BLDC motor. The control circuit controls the pulse width modulator to provide pulses to the plurality of phases to control a speed of the BLDC motor by causing the pulse width modulator to adjust widths of the pulses when a measured current in an active one of a corresponding phase exceeds a threshold in a startup mode. By using a small amount of added hardware, the motor controller can eliminate the table of duty cycles, reducing the size of startup tables in memory by a few hundred bytes and allowing the use of a lower cost microcontroller unit (MCU). In one form, the control circuit can include a processor and a duty cycle adjustment circuit that responds to a measured current signal. The processor can include a central processing unit and a memory for storing motor control instructions that include the startup tables.

#The duty cycle adjustment circuit can take the form of a comparator and a switch. In this case, the comparator has a first input for receiving the measured current signal, a second input for receiving the reference voltage, and an output, and the switch has a first input connected to the output of the comparator, a second input for receiving the startup signal, and an output connected to the second input of the pulse width modulator. Thus the duty cycle adjustment circuit adds a small amount of circuitry and the comparator is frequently available as a peripheral in the MCU.