Brushless DC Motor Drive
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Brushless DC Motor Drive
Why Brushless DC motor?
• The maximum speed and torque of the BrushedDC Motor is limited by the carbon brushcommutation.
• However, the control of speed and torque of a DCmotor is the simplest as the torque characteristicis linear with ia and if.
• The brushless structure has all the linearcharacteristics of the brushed DC motor butwithout the complexity and maintenancerequirement of the brush‐commutator.
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Construction of brushed and brushless DC machines
ELEC4613 - Electric Drive Systems
Brushed DC Motor Brushless DC Motor
Stator - Field
Rotor- Armature
Stator- Armature
Rotor- Field
2-poles 4-poles
4-poles
3
Permanent magnet Brushed and Brushless DC motor
ELEC4613 - Electric Drive Systems
The brush‐commutator is replaced by a 3‐phase electroniccommutator, consisting of three Hall sensors for rotor position anda 3‐phase inverter.
Permanent magnets for BLDC motor
ELEC4613 - Electric Drive Systems
NdFeB: Br = 0.8 – 1.4 T; Hc > 1MA/m; recoil 1; TCurie = 120 C
Neodymium Iron Boron and Samarium Cobalt
5
Three phase stator
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A-axis
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A
B
C
=150o
Rotor
Stator
The Back EMF waveform
EELEC4613 - Electric Drive Systems
fs m
de N Blv Blr
dt
8
Phase & line‐line voltages
9
Machine back emf
ELEC4613 - Electric Drive Systems
max s s me 2N Blv 2N lrB
where l is the active length of the conductors in metersr is the radius of the stator in metersm is the rotational speed of the rotor in rad/sec.B is the air-gap field in TeslaNs is the number of conductors per pole
'max s m e f me 2N plrB k
For the two-pole (p = 1) machine,
For a star-connected machine with p pole-pairs (i.e., 2p poles),
V/phase
V
'abmax a b s m E f me max e e 4N plrB k
10
s mde N Blv Blrdt
Vline-line
Inverter drive BLDC machine
ELEC4613 - Electric Drive Systems
bT1
D4
Vs
T4 T6 D6
T3 D3D1
T2 D2
T5 D5
c
HS
Inverter Switching
Logic
a
SA
SC
11
Ideal motor current waveforms
a a b b c c
m
e i e i e iT
12
Inverter switching table
ELEC4613 - Electric Drive Systems
Switches ON Current flow
-30º - 30º T5, T6 C+ and B30º - 90º T6, T1 B and A+
90º - 150º T1, T2 A+ and C150º - 210º T2, T3 C and B+210º - 270º T3, T4 B+ and A270º - 330º T4, T5 A and C+
Inverter switching table for ω > 0, or CCW motion
13
Inverter switching table
ELEC4613 - Electric Drive Systems
Switches ON Current flow
-30º - 30º T5, T6 C+ and B30º - 90º T6, T1 B and A+
90º - 150º T1, T2 A+ and C150º - 210º T2, T3 C and B+210º - 270º T3, T4 B+ and A270º - 330º T4, T5 A and C+
Inverter switching table for < 0, or CW motion
14
&T1 SA SB
&T 3 SB SC
&T 5 SC SA
&T 4 SA SB
&T6 SB SC
&T 2 SC SA 15
The developed torque
ELEC4613 - Electric Drive Systems
s
s
T 2N rlBI4N rlBI
For a 2-pole machine (p = 1)
Nm/phase
For a machine with p pole-pairs (i.e., 2p poles)'
s T fT 4N prlBI k I Nm
Note that the back emf and torque equations of this AC machineare exactly the same as for the brushed DC machine. There areno brushes or commutator. Hence the name Brushless DCMachine.
Nm
16
ELEC4613 - Electric Drive Systems
bT1
D4
Vs
T4 T6 D6
T3 D3D1
T2 D2
T5 D5
c
HS
Inverter Switching
Logic
a
SA
SC
17
Torque control via DC link current control
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Ideally, the DC-link currents (+ve and –ve) should remain constant,at all times. This can be achieved by one DC current regulator, orthree separate phase current regulators.
DC link voltage versus speed
ELEC4613 - Electric Drive Systems
3 cosd dV I VI
Assuming quasi-square phase currents in each winding, via DC-link or phase current controls
19
Fourier analysis of phase current
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Phase current harmonics
ELEC4613 - Electric Drive Systems
/3 2 /3
/3 2 /3
1 cos
1 1 1cos cos
2 2sin sin3 3
4 cos ( =1,3,5)6
n
d d d
d
d
a i n t d t
I n t d t I n t d t I cos n t d t
I n nnI n n
n
1 14 6/ 2 cos
62d dI II a
6dI I
21
where I is the RMS value of the fundamental phase current. Note, I is also the peak value.
Phasor diagram
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Relationship between Vd and mcos cos ;fV E IR 2
d dV I 3VI cos 3IE cos 3I R
0.427 cosdV E IR
'd d0.427V E IR V When = 0,
'd 1V
'd 2V
'd 3V
'd 4V
'd 3V
'dV
is the per phase RMS voltage supplied to the motor by the inverter. 'dV
23
6dI INoting that and cancelling I,
Thus, for small IR, m Vd
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