Friday, July 3, 2009

Brushless DC Motors Theory and Driver Circuit

Why use brushless DC motors (advantages/disadvantages)?
Brushless DC motors are synchronous motors suitable for use as a simple means of controlling permanent drives (e.g. ABS pumps, EHPS pumps, fuel pumps or cooling fans). This type of 3-, 4- or 5-phase brushless DC motor will increasingly replace brushed DC motors. Brushed DC motors require maintenance, e.g. to service coal brushes and commutator. Another major problem with a brushed DC machine is the possibility of brush burnout in the event of an overload or stall condition.

Functional principle of a brushless DC motor
Figure 1 shows a three-phase brushless DC motor with two pole pairs. The rotation of the electrical field (vector) has to be applied twice as fast as the desired mechanical speed of the brushless DC motor. The three coils of the stator are split into two groups of coils (A, B, C and A’, B’, C’). As you can see in Figure 1, coils A and C are energized and coil B is not energized. A 0° to 180° rotation will be shown in detail in section 2.1 to explain the setting of the appropriate switches of the B6 bridge pattern, the appropriate voltages relating to the coils, and the energized coils of the motor with the suitable rotor position between 0° and 180° mechanical.

Brushless DC Motors wiring diagrams
The wiring diagrams for a 3-pole armature (stator) Brushless DC Motors

The wiring diagrams for a 6-pole armature Brushless DC Motors


Brushless DC Motors Animation
Brushless DC motors are refered to by many aliases: brushless permanent magnet, permanent magnet ac motors, permanent magnet synchronous motors ect. The confusion arises because a brushless dc motor does not directly operate off a dc voltage source. However, as we shall see, the basic principle of operation is similar to a dc motor.


Introduction to Brushless DC Motors
Brushless Motor Construction
DC brushless motors are similar in performance and application to brush-type DC motors. Both have a speed vs. torque curve which is linear or nearly linear. The motors differ, however, in construction and method of commutation. A brush-type permanent magnet DC motor usually consists of an outer permanent magnet field and an inner rotating armature. A mechanical arrangement of commutator bars and brushes switches the current in the armature windings to maintain rotation. A DC brushless motor has a wound stator, a permanent magnet rotor assembly, and internal or external devices to sense rotor position. The sensing devices provide signals for electronically switching (commutating) the stator windings in the proper sequence to maintain rotation of the magnet assembly. The rotor assembly may be internal or external to the stator in a DC brushless motor. The combination of an inner permanent magnet rotor and outer windings offers the advantages of lower rotor inertia and more efficient heat dissipation than DC brush-type construction. The elimination of brushes reduces maintenance, increases life and reliability, and reduces noise and EMI generation.
DC Brushless Motor Control Block Diagram


Brushless DC Motor driver circuit

Closed Loop Brushless DC Motor Control With the MC33033 Using the MC33039 driver circuit

The MC33033 is a high performance second generation, limited
feature, monolithic brushless dc motor controller which has evolved
from ON Semiconductor's full featured MC33034 and MC33035
controllers. It contains all of the active functions required for the
implementation of open loop, three or four phase motor control. The
device consists of a rotor position decoder for proper commutation
sequencing, temperature compensated reference capable of supplying
sensor power, frequency programmable sawtooth oscillator, fully
accessible error amplifier, pulse width modulator comparator, three
open collector top drivers, and three high current totem pole bottom
drivers ideally suited for driving power MOSFETs. Unlike its
predecessors, it does not feature separate drive circuit supply and
ground pins, brake input, or fault output signal.

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The L6235 is a DMOS Fully Integrated Three-Phase
Motor Driver with Overcurrent Protection.
Realized in MultiPower-BCD technology, the device
combines isolated DMOS Power Transistors with
CMOS and bipolar circuits on the same chip.
The device includes all the circuitry needed to drive a
three-phase BLDC motor including: a three-phase
DMOS Bridge, a constant off time PWM Current Controller
and the decoding logic for single ended hall
sensors that generates the required sequence for the
power stage.

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3-Phase Full-Wave PWM Driver for Sensorless brushless Motors driver circuit
The TB6588FG is a three-phase full-wave PWM driver for
sensorless brushless DC (BLDC) motors. It controls rotation speed
by changing the PWM duty cycle, based on the voltage of an
analog control input.

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