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Brushless DC (BLDC) Motor Control
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This block diagram shows a system that could be used to drive a 3-phase brushless DC motor. Rotor feedback is required and is
usually implemented as 3 hall-effect sensors that detect the position of the rotor magnets. A ‘sensorless’ method can be used to
remove mechanical sensors by measuring voltage /current feedback.
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Application below
BLDC Motor Tutorial
Overview:
Brushless DC motors are referred to by many aliases: Brushless Permanent Magnet, Permanent Magnet AC Motors,
Permanent Magnet Synchronous Motors etc. The confusion arises because a brushless dc motor does not directly
operate off a dc voltage source. However, the basic principle of operation is similar to a dc motor.
A brushless dc motor has a rotor with permanent magnets and a stator with windings. It is essentially a dc motor turned
inside out. The brushes and commutator have been eliminated and the windings are connected to the control
electronics. The control electronics replace the function of the commutator and energize the proper winding.The
windings are energized in a pattern which rotates around the stator. The energized stator winding leads the rotor
magnet, and switches just as the rotor aligns with the stator.
There are no sparks, which is one advantage of the brushless DC motor. The brushes of a dc motor have several
limitations; brush life, brush residue, maximum speed, and electrical noise. BLDC motors are potentially cleaner, faster,
more efficient, less noisy and more reliable. However, BLDC motors require electronic control.
Key characteristics of the BLDC Motor:
 Heat is generated in the stator: Easier to remove
and maintain.
 Rotor has permanent magnets Vs. coils thus
lighter less inertia: Easier to Start/ Stop
 Linear torque/current relationship smooth
acceleration or constant torque
 Higher torque ripple due to lack of information
between sectors
 Low Cost to manufacture
 Simple, low-cost design for fixed-speed
applications
 Clean, Fast and Efficient
 Speed proportionate to line frequency (50 or 60
Hz)
 Complex control for variable speed and torque
How it works:
The Brushless DC motor does not operate directly off a DC voltage source. The Brushless DC motor has a rotor with
permanent magnets, a stator with windings and commutation that is performed electronically. Typically three Hall
sensors are used to detect the rotor position and commutation is performed based on Hall sensor inputs.
The motor is driven by rectangular or trapezoidal voltage strokes coupled with the given rotor position. The voltage
strokes must be properly applied between the phases, so that the angle between the stator flux and the rotor flux is
kept close to 90° to get the maximum generated torque. The position sensor required for the commutation can be very
simple, since only six pulses per revolution (in a three-phase machine) are required. Typically, the position feedback is
comprised using three Hall effect sensors aligned with the back-EMF of the motor. In sensorless control, back EMF
zero crossing is used for commutation.
BLDC Control
BLDC Motor Six Step Control: Back EMF method
Input:


Typically torque, speed, position, and/or
direction
Inputs can be analog voltage,
potentiometer, switches, or digital
communications
Control:




Basic I/O for firmware bit-bang for 6-step
3 phase PWMs for hardware PWM
Comparators for speed sensing in
sensorless control, over-current detection
Capture/Compare/PWM or input captures
for speed sensing
Feedback: Hall-effect sensors, optical encoder, or
back-EMF voltage
BLDC Design Flowchart:
BLDC Motor Applications:
BLDC Motor Applications include:
 Anti-lock Braking System
 Disk Drive Servo
 Throttle control
 Fuel pump
 Oil pump
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BLDC Motor Application Example: Sensorless BLDC
Sensorless motors are lower cost due to the lack of the sensors, but they are more complicated to drive. A sensorless
motor performs very well in applications that don’t require the motor to start and stop. A sensor motor would be a
better choice in applications that must periodically stop the motor.
Want to eliminate your Hall-Effect sensors and cabling cost by going sensorless? Take a look at Microchip’s PIC18F
The PIC18 MCU’s or dsPIC DSC’s A/D samples the motor phase voltages. From the voltages, the CPU determines
the rotor position and drives the motor control PWM module to generate trapezoidal output signals for the 3-phase
inverter circuit.
Brushless DC Motor Application: Brushless Fan Control
Need a highly integrated fan controller with a customizable speed/temperature profile? Take a look at Microchip’s
PIC12HV and PIC16HV devices. The PIC12HV and PIC16HV devices have a built-in 5V regulator and on-chip
comparator to save system cost. The rotor position is determined by a Hall-Effect sensor connected to the on-chip
comparator. The Enhanced Capture Compare PWM (ECCP) Module uses this feedback information to drive the
motor by steering the PWM signal to the appropriate motor phase. Temperature sensor inputs can be used to create
a unique fan speed profi le and the application can provide digital status information to a host device.
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