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Sensorless Commutation of Brushless Dc Motors

[Category : - ELECTRONICS]
[Viewed 3793 times]

A method and an apparatus for determining rotor position information of a Brushless DC motor using the resultant voltage vector produced by addition of phase voltage vectors of the energised windings and the BEMF vector of the unenergised winding. This resultant voltage vector called the BEMF Space Vector rotates at same speed as the rotor and possesses rotor position information used to commutate phase windings. Phase voltage vectors for the computation of the BEMF Space Vector are obtained by referring scalar phase voltages in the electrical circuits to the magnetic axes of the magnetic circuits, since scalar and vector current magnitudes are equal. Angles that the BEMF Space Vector makes with the real axis are measured to commutate the phase windings.
This technique is referred to as the De Four BEMF Space Vector Resolver, used to efficiently start the motor from rest and commutate phase windings during normal operation.

Brushless DC (BLDC) motors have been gaining popularity in the Appliance, Automotive, Aerospace, Consumer, Medical, Computer and Industrial Automation industries and possess many advantages over other motors. Some of these advantages are:
(a) High power density, low inertia and high torque to inertia ratio and high dynamic
response due to the small size, low weight and high flux density neodymium-iron-
boron permanent magnet rotor.
(b) High efficiency due to the low rotor losses as a result of the absence of current
carrying conductors on the rotor and reduced friction and windage losses in the rotor.
(c) Long operating life and high reliability due to the absence of brushes and metallic
(d) Clean operation due to the absence of brushes, resulting in no brush dust during
operation and allowing for clean room applications.
(e) Low audible noise operation due to the absence of brushes, commutators and smooth low air resistance rotor.
(f) High speed operation in excess of 80,000 rpm is possible, since these motors are
electronically commutated and are not subjected to the limitations of conventional
(g) Low thermal resistance since most of the machine losses occur in the stationary
stator, thereby allowing heat dissipation by the process of direct conduction. In
addition, since the rotor losses are small, heat transfer to machine tools and work
pieces when these motors are utilized in machine tools is minimal, thereby reducing the effects of heat on the machining operation.
(h) Low EMI/RFI due to the absence of brushes and metallic commutators.

The advantages of BLDC motors stated above, provides an attractive solution to energy efficiency and environmental pollution, resulting in the motor doing more work per KWh of electricity. However, BLDC motors have some disadvantages over the traditional AC Induction and Brush DC motors. These disadvantages are the need for an inverter switch unit, a Digital Signal Processor (DSP) or Microcontroller and rotor position sensors to provide rotor position information to commutate and drive the motor, thereby adding increased cost and complexity, and resulting in decreased overall system reliability.

The rapid advancements in the power semiconductor, DSP and Microcontroller technologies over the years have reduced the cost, complexity and size of electronic drives for BLDC motors, thereby making them competitive with the traditional motors. This leaves the BLDC motor with one disadvantage, the need for rotor position sensors to provide rotor position information to commutate and drive the motor. BLDC motors have been traditionally commutated with Hall sensor devices. This sensored commutation technique employs a sensing magnet of same polarity as that of the rotor magnet and is attached to the rotor in close proximity to the Hall sensors. Three-phase BLDC motors require three Hall sensors and an external dc power supply, resulting in five motor connected wires for the purpose of commutation, together with the three power wires that are necessary for the electromechanical energy conversion process.

Research in the area of sensorless commutation and control of BLDC motors to reduce motor drive complexity, and hence increase the reliability of the system, provide further cost reduction of the motor drive and maintain system efficiency, has been taking place for many years. The Self Starting Method and an Apparatus for Sensorless Commutation of Brushless DC Motors presented in U.S. Pat. No. 7,737,651 presents a revolutionary method for starting and operating BLDC motors over their entire speed and torque ranges.

Continuous torque production and energy efficient operation of BLDC motors require knowledge of the rotor position to commutate phase windings and introduce a new winding pair in the production of electromagnetic torque. This is accomplished with the development of the BEMF Space Vector and using its resolver properties to measure the angle this space vector makes with the real axis in the complex plane to commutate motor phase windings. The BEMF Space Vector is produced by the vector addition of the three motor line voltages that are computed from the two constant phase voltages of the energized phase windings and the phase voltage of the unenergized phase winding. The varying BEMF of the unenergized winding results in rotation of the BEMF Space Vector in the same direction and with the same speed of the rotor. Since each pair of phase winding is energized for 60 electrical degrees, then the angle that the BEMF Space Vector makes with the real axis in the complex plane is utilize to commutate phase windings every 60 electrical degrees.

The BEMF Space Vector Resolver commutation method has many advantages over other sensorless methods including:
(a) Its self-starting ability and does not require a ramp-up procedure on starting.
(b) Acquires near zero speed operation over the entire torque range of the motor.
(c) Energy efficient operation over entire speed and torque range of the motor without stalling.
(d) Smooth speed transitions in speed and torque control systems.
(e) Ability to advance or retard the commutation of windings.
(f) Computes motor supply voltage at each step of motor operation.
(g) Displays the physical phenomena in torque production during motor operation.
The BEMF Space Vector Resolver commutation method as presented in U.S. Pat. No. 7,737,651 is implemented on microcontrollers using a Commutation Trigger C Macro program having the three motor phase voltages as inputs and a commutation trigger output for selecting the pair of phase winding that must be energized as determined by the angle made by the BEMF Space Vector and the real axis in the complex plane. This commutation method produces energy efficient operation of the BLDC motor over the entire torque and speed range of the motor and ensures that all the positive attributes of BLDC motors are realized.

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