2. 2
Agenda
Motor Types Overview
BLDC Motor Applications
Comparison of DC to Brushless DC Motors
Hall Sensors
Six-Step Commutation
Sensorless Commutation with Back-EMF
Vector Motor Control basics
Closed-Loop Speed Control
Summary
4. 4
Brushed DC Motors Review
A winding assembly (armature) within a stationary
magnetic field
Brushes and Commutators switch current to
different windings in correct relation to the outer
permanent magnet field.
Pros:
Electronic control is simple, no need to commutate
in controller
Requires only four power transistors
Cons:
A sensor is required for speed control
The brushes and commutator create sparks and
wear out
Sparks limit peak power
Heat in armature is difficult to remove
Low power density
5. 5
Brushless DC Motors
Permanent
Magnet
Rotor
Stator
windings
Permanent magnet rotor within
stationary windings
Pros:
No brushes or commutator to wear out
No sparks and no extra friction
More efficient than DC motor
Higher speed than DC motor
Higher power density than DC motor
Cons:
Rotor sensor OR sensorless methods
needed to commutate
Requires six power transistors
6. 6
Brushed DC Commutation
The windings in the armature
are switched to the DC power
by the brushes and armature
Each winding sees a positive
voltage, then a disconnect,
then a negative voltage
The field produced in the
armature interacts with the
stationary magnet, producing
torque and rotation
+
-
N S
+
-
U
7. 7
DC Motor Bridge
The DC motor needs four transistors
to operate the DC motor
The combination of transistor is
called an H-Bridge, due to the
obvious shape
Transistors are switched diagonally
to allow DC current to flow in the
motor in either direction
The transistors can be Pulse Width
Modulated to reduce the average
voltage at the motor, useful for
controlling current and speed
0
1
1
1
0
0
0
8. 8
Three-Phase Bridge to Drive BLDC
Motor
The Brushless DC motor is really a DC motor constructed inside-out,
but without the Brushes and Commutators
The mechanical switches are replaced with transistors
The windings are moved from the armature, to the stator
The magnet is moved from the outside to become the rotor
N S
N S
U
V
W
10. 10
Six-Step Current Waveform
Here we see the individual steps in a real trapezoidal
current waveform
The PWM ripple is visible when the phase is active
The rising and falling edges are sloped, giving the
trapezoidal shape
The amount of slope is a function of the winding inductance
11. 11
Hall Sensors
Hall Sensors detect magnetic fields, and can be
used to sense rotor angle
The output is a digital 1 or 0 for each sensor,
depending on the magnetic field nearby
Each is mounted 120-degrees apart on the back
of the motor
As the rotor turns, the Hall sensors output
logic bits which indicate the angle
H1
H2
H3
N
S
H1 H2
H3
12. 12
Hall Sensor Commutation
H1
H2
H3
STEP1 STEP2 STEP3 STEP4 STEP5 STEP6 STEP1 STEP2 STEP3
The combination of all
three sensors produce
six unique logic
combinations or steps
These three bits are
decoded into the motor
phase combinations
U
V
W
13. 13
Brushless DC Motor BEMF
The Back-EMF is the voltage generated in stator windings as the rotor moves
BEMF voltages are more or less sinusoidal (depending on the motor) and are
symmetrical from phase to phase
We detect the zero crossings of each phase to commutate
The motor MUST be moving to generate BEMF voltages
14. 14
180° Sinusoidal Commutation
Modulates sine waves in all three windings
Pros:
No square edges
Lower Torque Ripple then six-step
drive
Lower audible noise
Higher efficiency and torque
Stator angle is rotated smoothly
rather than in 60 degree jumps
Each phase is utilized all of the time
Cons:
Needs higher resolution feedback to calculate sine
waves with low distortion
Needs more sophisticated processing to calculate
sine PWM values on the fly
Bandwidth of currents are limited due to motor
impedance, this hurts high speed performance
15. 15
*
r
Speed Regulator
r
*
q
i
0
*
d
i
r
id PI
Regulator
iq PI
Regulator d,q
to
,
)
(
1
T
Motor Model
Based Flux and
Position Observer
q
i
d
i
*
q
U
*
d
U
*
U
*
U
Voltage
Source
3-phase
Inverter
SIN
PWM
PWM1~6
to
a, b, c
,
3-phase
PMSM
r
to
d,q
,
)
(
T
a
i
b
i
d
i
q
i
i
i
a,b,c
to
,
Speed Estimation
DC Bus
Vector (Field Oriented Control) Drive
This method mathematically converts the 3-phase voltage and
current into a simple DC motor representation
Uses this data to calculate the best angle for commutation
Creates new 3-phase sinusoidal PWM based on calculation
Repeats the calculations at PWM frequency
Pros:
Highest Torque efficiency
Highest Bandwidth
Widest Speed Range
Lowest Audible Noise
Cons:
Complicated Algorithm
Needs powerful processor
16. 16
BLDC Motor Speed Control
The goal of most Electronic Motor Control Systems is Speed
Control
Speed Control systems are more or less complicated,
depending on accuracy required
The simplest speed control is Open-Loop, that is, without
speed feedback
In this configuration, a speed command is converted to a
fixed voltage (PWM duty) which is sent to the motor
The motor may go the right speed, or it may not, it depends
on the load
Without feedback, there is no way to tell internally what the
real speed is and so may require outside adjustment
Speed
Command
Pulse Width
Modulator
Transistors Motor Load
17. 17
Closed-Loop Control
To get automatic speed control, feedback is needed
Feedback systems could be Hall Sensors, Encoders,
Resolvers, tachometers or other devices
Below is a block diagram of a simple control loop
Hall
Sensors
Speed
Calculation
Motor
PWM
Generation
PI
Controller
ω*
ω θ
