Brushless DC Motors Introduction

Last Updated on Sunday, 15 August 2010 14:00 Written by Administrator Wednesday, 06 August 2008 21:27

Introduction

• Brushless DC (BLDC) motors are the ideal choice for applications that require high reliability, high efficiency, and high power to volume ratio. Generally speaking, BLDC motors are considered to be high performance motors that are capable of providing large amounts of torque over a vast speed range.

• BLDC motors are a derivative of the most commonly used DC motor, the brushed DC motor, and they share the same torque and speed performance curve characteristics. The major difference between BLDC and brushed DC motors is the use of brushes. BLDC motors do not have brushes, hence the name “brushless DC”, and must be electronically commutated.
• Commutation is the act of changing the motor phase current at the appropriate times to produce rotational torque.
• BLDC motors are highly reliable since they do not have any brushes to wear out and replace.

BLDC Motor Advantages

• High Speed Operation - BLDC motors can operate at speeds above 10,000 rpm under loaded and unloaded conditions.
• Responsiveness and Quick Acceleration - Inner rotor BLDC motors have low rotor inertia, allowing them to accelerate, decelerate, and reverse direction quickly.
• High Reliability - BLDC motors do not have brushes, have life expectancies over 10,000 hours.
• High Power Density - A good weight/size to power ratio.

Theory of Operation

• The coils are attached to the stator. The commutation is controlled by electronics. Commutation times are provided either by position sensors or by coils Back Electromotive Force measurements.
• Brushless DC motors usually consist of three main parts: a Stator, a Rotor and Hall Sensors.

Stator

• A basic three phases BLDC motor Stator has three coils. In many motors the number of coils is replicated to have a smaller torque ripple.
• It consists of three coils each including three elements in series, an inductance, a resistance and one back electromotive force.

Rotor

• The rotor in a BLDC motor consists of an even number of permanent magnets. The number of magnetic poles in the rotor also affects the step size and torque ripple of the motor. More poles give smaller steps and less torque ripple. The permanent magnets go from 1 to 5 pairs of poles

Hall Sensor

• For the estimation of the rotor position, the motor is equipped with three hall sensors. These hall sensors are placed every 120°. With these sensors, 6 different commutations are possible. Phase commutation depends on hall sensor values.
• Power supply to the coils changes when hall sensor values change. With right synchronized commutations, the torque remains nearly constant and high.

Phase Commutations

• To simplify the explanation of how to operate a three phase BLDC motor, a typical BLDC motor with only three coils is considered. As previously shown, phases commutation depends on the hall sensor values. When motor coils are correctly supplied, a magnetic field is created and the rotor moves. The most elementary commutation driving method used for BLDC motors is an on-off scheme: a coil is either conducting or not conducting. Only two windings are supplied at the
• same time and the third winding is floating. Connecting the coils to the power and neutral bus induces the current flow. This is referred to as trapezoidal commutation or block commutation
• To command brushless DC motors, a power stage made of 3 half bridges is used

• For motors with multiple poles the electrical rotation does not correspond to a mechanical rotation. A four pole BLDC motor uses four electrical rotation cycles to have one mechanical rotation.
• The strength of the magnetic field determines the force and speed of the motor. By varying the current flow through the coils, the speed and torque of the motor can be adjusted. The most common way to control the current flow is to control the average current flow through the coils. PWM (Pulse Width Modulation) is used to adjust the average voltage and thereby the average current, inducing the speed.

Power-Stage Board

• Hardware designing must ensure some functions: run a motor, a variety of feedback signals that facilitate control algorithm development are provided.
• Power-Stage needs the following modules:

+ 3-Phase Bridge

+ Bus Voltage and Current Feedback

+ Over-current and Undervoltage Functions

+ Temperature Sensing

+ Back EMF Signals

+ Phase Current Sensing

+ Brake

+ Test Points and LED Indication

+ Power Supplies and Voltage Reference

3-Phase Bridge

IR2133 Functional Block Diagram

Phase Current Sensing

Bus Voltage and Current Feedback

Over-current and Undervoltage Functions

Back EMF Signals

• What’s the Back EMF Signals ?
• Within each coil, the rotating permanent magnet induces an alternating voltage with an amplitude proportional to its angular speed. As a result, the shape of this voltage is assumed to be a sine wave. A sine wave shape is valid for many motors and is a good approximation.

• A zero crossing of the voltage induced within a coil occurs when the permanent magnet rotor –here modeled as a bar magnet is perpendicularly orientated to the axis of the coil.

BEMF and Hall Sensors

• For most BLDC motors, the Hall sensors are mounted within the 120° scheme that gives an information which is in phase with the step changes. While the zero crossing of the BEMF appears with a shift of 30°.

Back EMF Signals