Fundamental of Brushless DC Motor

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SOUTHERN TAIWAN UNIVERSITY
Presentation

• Fundamental of Brushless DC Motor
• and Control Principle

Professor Chen, Pei-Chung ? ?
? Student Thanh-Nhan Nguyen ? ? ?
2009, February, 26th
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Outline

• Introduction
• BLDC motor structure
• BLDC Motor Control

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1. Introduction

• Brushless dc motor is one kind of permanent
  magnet synchronous motor, having permanent
  magnets on the rotor and trapezoidal shape back
  EMF.
• BLDC motors combine high reliability with high
  efficiency, and for a lower cost in comparison
  with brush motors.
• The brushless characteristic can be apply to
  several kinds of motors AC synchronous motors,
  stepper motors, AC induction motors.

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2. BLDC motor structure
(B)
(A)

• Cross-section view of a brushless dc motor
• (B) A picture of a brushless dc motor
• Fig1. Structure of a brushless dc motor

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2. BLDC motor structure
Fig 2. BLDC motor rotation
In a brushless DC motor the position of the coils
(phases), with respect to the permanent magnet
field, are sensed and the current switched
electronically (commutated) to the appropriate
phases. Hall Effect sensors are typically used to
sense the rotor position. Otherwise, sensorless
techniques are used.
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2. BLDC motor structure
Fig 3. Typical brushless dc motor control system
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2. BLDC motor structure
Fig 4. BLDC motor structure and signals
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3. The BLDC Motor Control
Torque control
Simple torque and Back EMF equations
Speed control
Where E is The Back EMF magnitude T is torque,
l is the length of the rotor N is the number of
winding turns per phase r is the internal radius
of the rotor B is the rotor magnet flux density,
w is the motors angular velocity i is the phase
current , L is the phase inductance q is the
rotor position, R is the phase resistance
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3. The BLDC Motor Control

• Three Phase Inverter
• The Pulse Width Modulation (PWM) Mode
• The Hysteresis Mode
• Shaft Position Sensors
• Current Sensors
• Enhanced Sensorless Algorithms
• Direct Back EMF Measurement

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3. The BLDC Motor Control

• Three Phase Inverter

Fig 5. Three Phases Inverter

• The Pulse Width Modulation (PWM) Mode

To control the speed of the BLDC motor by PWM
duty cycle with the fixed frequency
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3. The BLDC Motor Control

• The Hysteresis Mode
• In the hysteresis-type current regulator, the
  power transistors are switched off and on
  according to whether the current is greater or
  less than a reference current.
• The error is used directly to control the states
  of the power transistors. As the supply voltage
  is fixed, the result is that the switching
  frequency varies as the current error varies.
• This method is more commonly implemented in
  drives where motor speed and load do not vary too
  much, so that the variation in switching
  frequency is small.

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3. The BLDC Motor Control

• Shaft Position Sensors
• The incremental sensors use optically coded disks
  with either single track or quadrature resolution
  to produce a series of square wave pulses. The
  position is determined by counting the number of
  pulses from a known reference position.
• The three Hall effect sensors provide three
  overlapping signals giving a 60 wide position
  range. The three signals can be wired to the DSP
  Input Capture pins, thus speed information is
  counted.
• The resolver is made up of three windings one
  linked to the rotor and supplied with a
  sinusoidal source and two other coils linked to
  the stator. A Back EMF is induced by the rotating
  coil in each of the two stator resolver windings.
  By decoding these two signals it is
• possible to get cos(q) and sin(q) where q is the
  rotor position. The resolver
• resolution depends only on the AD conversion.

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3. The BLDC Motor Control

• Current Sensors
• A characteristic of the BLDC control is to have
  only one current at a time in the motor (two
  phases ON).
• Consequently, it is not necessary to put a
  current sensor on each phase of the motor one
  sensor placed in the line inverter input makes it
  possible to control the current of each phase.
• Moreover, using this sensor on the ground line,
  insulated systems are not necessary, and a low
  cost resistor can be used.

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3. The BLDC Motor Control

• Enhanced Sensorless Algorithms
• Direct Back EMF measurement

For trapezoidal motors, the direct back-EMF
analogue measurement is the most popular method.
The motor is fed two phase ON, with 60º
commutation periods, and detection of the
commutation instant is performed by sensing the
back-EMF in the non-fed phase.
where u is the phase voltage, R is the phase
resistance, L the phase inductance and e the Back
EMF term.
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3. The BLDC Motor Control
Fig 5. Back EMF Measurement
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3. The BLDC Motor Control
Assuming that phase A is the non-fed phase it is
possible to write the following equations for the
three terminal voltages
When the Back EMF of the non-fed phase A is equal
to zero, the following relationship between the
terminal voltages arises
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Thank you very muchfor your listening!