Erasmus LLP Intensive Programme
Powering the Future
With Zero Emission and
Human Powered
Vehicles
Antoni Garcia Espinosa
UPC
Erasmus LLP Intensive Programme
CONTROL OF
BRUSHLESS
PERMANENT
MAGNET MACHINES
(BLDC)
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DC motors
 In
I DC motors,
t
th brushes
the
b h off this
thi motor
t are an obvious
b i
problem; there will be friction between the brushes and
the commutator,
commutator and both will gradually wear away.
away
However, a more serious problem with this type of
motor: This is that the heat associated with the losses
is generated in the middle of the motor, in the rotor.
 If the motor could be so arranged that the heat was
generated in the outer stator, that would allow the heat
to be removed much more easily,
y and allow smaller
motors.
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BLDC->Permanent magnet rotor within
stationary windings
 PROS
 No brushes or commutator to wear out




N sparks
No
k and
d no extra
t friction
f i ti
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
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 The
Th rotor
t is
i made
d off permanentt magnett and
d can vary
from two to eight pole pairs with alternate North (N) and
South (S) poles
poles.
 Ferrite magnets are traditionally used to make
permanentt magnets.
t
 Neodymium
y
((Nd),
) Samarium Cobalt ((SmCo)) and the
alloy of Neodymium, Ferrite and Boron (NdFeB) are
some examples of rare earth alloy magnets.
C ti
Continuous
research
h iis going
i on tto iimprove th
the flflux
density to compress the rotor further
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Demagnetisation curves for a
NdFeB material
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WHAT IS BACK EMF?
 When
Wh a BLDC motor
t rotates,
t t
each
h winding
i di generates
t a
voltage known as back Electromotive Force or back
EMF which opposes the main voltage supplied to the
EMF,
windings according to Lenz’s Law. The polarity of this
back EMF is in opposite direction of the energized
g
voltage.
 Back EMF depends mainly on three factors:
 Angular velocity of the rotor
 Magnetic field generated by rotor magnets
 The number of turns in the stator windings
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Trapezoidal Back EMF
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Flux-linkage of coil u1u1’
as the rotor rotates
1max  N  B r l d  N1 Bg r l 
and the var iation with  as the rotor rotates
from 0 to 180º is given by



 1max
1    1 



2

  
d   d
d  
 1
  1
e1   1
dt
d dt
d
e1  2 N1 Bg lr [V ]
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e1  2 N1 Bg l r [V ]
N ph  2 N1
e  eu  ev  ew  2 N ph Bg l r  [V ]
P  T    2  e  i [W ]
2  e  i 2  2 N ph Bg l r 
T

 4 N ph Bg l r i [ Nm]


E  k T  k i
k
4 N ph

and   Bg r  l
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in DC motor V  E  R i
in BLDC motor V  ea  eb   Ra  Rb  i

  0 1 


Tstall torque 
T

The no  load speed
p
is
0 
V
k
Tstall torque
istall 
 rad 
 s 


 k istall This is the torque at zero speed
V
R
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a
b
c
 In Figure (a) the current flows in the direction that
magnetises the stator so that the rotor is turned
clockwise, as shown. In (b) the rotor passes between
the poles of the stator, and the stator current is
switched off. Momentum carries the rotor on, and in the
stator coil is re-energised, but the current and hence
the magnetic field, are reversed. So the rotor is pulled
on round in a clockwise direction. The process
continues with the current in the stator coil alternating
continues,
alternating.
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a
b
c
 Obviously, the switching of the current must be
synchronized with the position of the rotor. This is done
using
sing sensors
sensors. These are often Hall effect sensors that
use the magnetism of the rotor to sense its position, but
optical sensors are also used
used.
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 The
Th Brushless
B hl
DC motor
t iis really
ll a DC motor
t
constructed insideout, but without the Brushes and
Commutators.
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
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Q1
Q3
Q4
Q6
Q5
Q2
 Basically are two modes of operation:
 Six step commutation
 PWM mode to control the voltage and current
 Six step commutation
 PWM mode to control the voltage and current
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 Unlike
U lik a brushed
b
h d DC motor,
t the
th commutation
t ti off a
BLDC motor is controlled electronically.
 To rotate the BLDC motor, the stator windings
should be energized in a sequence. It is important to
k
know
th rotor
the
t position
iti in
i order
d tto understand
d t d which
hi h
winding will be energized following the energizing
sequence Rotor position is sensed using Hall effect
sequence.
sensors embedded into the stator.
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 Most
M t BLDC motors
t
h
have th
three H
Hall
ll sensors
embedded into the stator on the non-driving end of
the motor
motor. Each is mounted 120-degrees
120 degrees or 6060
degrees apart on the back of the motor.
 Whenever
Wh
th
the rotor
t magnetic
ti poles
l pass near the
th Hall
H ll
sensors, they give a high or low signal, indicating the
N or S pole is passing near the sensors.
sensors
 Based on the combination of these three Hall
sensor signals,
i
l the
th exactt sequence off commutation
t ti
can be determined.
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 Note:
N t Hall
H ll Eff
Effectt Theory:
Th
If an electric
l t i currentt carrying
i
conductor is kept in a magnetic field, the magnetic field
exerts a transverse force on the moving charge carriers
which tends to push them to one side of the conductor.
This is most evident in a thin flat conductor. A buildup of
charge at the sides of the conductors will balance this
magnetic influence, producing a measurable voltage
b t
between
th ttwo sides
the
id off th
the conductor.
d t Th
The presence
of this measurable transverse voltage is called the Hall
effect after E.
E H.
H Hall who discovered it in 1879
1879.
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Theory of Operation
 Each commutation sequence has one of the windings
energized to positive power (current enters into the
winding), the second winding is negative (current exits
the winding) and the third is in a non-energized
condition.
 Torque
q is p
produced because of the interaction between the
magnetic field generated by the stator coils and the
permanent magnets. Ideally, the peak torque occurs
when these two fields are at 90º to each other and falls
off as the fields move together. In order to keep the motor
running, the magnetic field produced by the windings should
shift position, as the rotor moves to catch up with the stator
field. What is known as “Six-Step Commutation” defines the
sequence of energizing the windings
windings.
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Rotor position is 000
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Rotor position is 001
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Rotor position is 011
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Rotor position is 111
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Rotor position is 110
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Six step commutation
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Six step commutation
 At any instant,
i t t two
t
switches
it h are on, one in
i th
the upper
group and another in the lower group. For example,
from instant t1
t1, Q1 and Q6 are one when the supply
voltage Vd and line current Id are placed across line ab
((phase a and phase b in series)) so that Id is positive in
phase a but negative in phase b. Then after 60º, the
middle of phase a, Q6 is turned off and Q2 is turned on,
b t Q1 continues
but
ti
conduction
d ti ffor th
the ffullll 120º angle.
l
This switching conmutates –Id from phase b to phase c
while phase a continues to carry +Id
+Id.
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Six step commutation
 The
Th switching
it hi pattern
tt
changes
h
every 60º
60º, iindicating
di ti 6
switching modes in a full electrical cycle.
 The sensors dictate the switching at the precise
instants of the waves.
 It can be seen that any instant, two phase Back EMF
appear
pp
in series across the inverter input,
p neglecting
g
g
the resistance and inductance drops.
 The power flow of the machine at any instant is
P=2Vc Id
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Brushless DC Motor Drive with
6-pulse Operation
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 This example shows the similarity in characteristics between the
brushless dc motor and the conventional dc motor. By changing
the
h d
dc b
bus voltage,
l
the
h motor speed
d can b
be controlled.
ll d
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PWM control
 It is possible to control the switches in PWM chopping mode for
controller the voltage and current continuously at the machine
terminal.
 The devices are turned on and turned off to control the average
current and the average voltage
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PWM control
 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.
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Torque/Speed Characteristics
 It is in the parallel hybrid that there is scope for some novelty in
machine design.
 One example is the crankshaft mounted electrical machine that is
used in a number of designs, including the groundbreaking Honda
Insight Here the electrical machine
Insight.
machine, which can work as either a
motor or generator, is mounted directly in line with the engine
crankcase. Such machines are in most cases a type of brushless
DC.
DC
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E  B  l  vel  B  l  n º r  
B  l  n º r  k
E  k 
Vd  2  RS  I  2  E  2  RS  I  2  k  

Vd  2  RS  I
2k
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F  B  l  I  nº
M b  F  r  B  l  I  n º r
Mb  k  I
M  2k  I
Vd  2  RS  I  2  k  
I
Vd  2  k  
2  RS
M  2k 
Vd  2  k  
2  RS
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 100 kW,
kW oilil cooled
l d BLDC motor
t ffor automotive
t
ti
application. This unit weighs just 21 kg.
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 It is
i in
i the
th parallel
ll l h
hybrid
b id th
thatt th
there iis scope ffor some
novelty in machine design.
 One example is the crankshaft mounted electrical machine
that is used in a number of designs, including the
groundbreaking Honda Insight. Here the electrical machine,
which can work as either a motor or generator, is mounted
directly in line with the engine crankcase. Such machines
are in most cases a type of brushless DC
DC.
 They are usually ‘turned inside out’, with the stationary coils
being on the inside, and the rotor being a band of magnets
moving outside the coil.
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Diagram of inside out electric motor
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BLDC Motor for Electric/ Hybrid
Automobiles
HEV Motor
42V ISG
(Starter/
Generator)
Scroll
Compressor
for HEV
Diesel HEV
Motor
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Comparing a BLDC motor to a
Brushed DC motor
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Comparing a BLDC motor to an
inductor motor
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