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Brushless Motor Circuit
EE452: Electric Drive Design
Brushless Motor
Wiring
Switching Table for 120o Hall Spacing
Clockwise Motion
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Counter-clockwise Motion
4
c
c
a
ic
b
b
ia
a
ib
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5
c
Step 1
a
b
a
-c
g
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6
c
Step 2
a
b
b
-c
g
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7
c
Step 3
a
b
-a
g
b
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8
Step 1: With Rotor Magnet
a
-c
m
g
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9
Step 2: With Rotor Magnet
-c
b
m
g
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10
Step 3: With Rotor Magnet
-a
b
g
m
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11
Power Circuit
Q1
Vdc
Q5
Q3
a
Q4
c
b
Q2
Q6
o
Motor windings
n
Step 1: Activated Transistors
Q1
Vdc
a
c
b
Q2
o
Motor windings
n
Step 2: Activated Transistors
Q3
Vdc
a
c
b
Q2
o
Motor windings
n
Step 6: Activated Transistors
Q1
Vdc
a
c
b
Q6
o
Motor windings
n
Switching Signals
Phase a
t
Time
Phase b
tstep
Phase c
Time
Time
Voltage Regulation - Fixed Width Modulation
Maximum Voltage
Vdc
Without modullation, the voltage during 𝑡𝑠𝑡𝑒𝑝 is 𝑉𝑑𝑐
tstep
Time
Reduced Voltage
Vdc
With Modulation, 𝑉𝑎𝑣𝑒 = 𝑉𝑑𝑐 𝑑
𝑑=
ton
toff
Time
𝑡𝑜𝑛
𝑡𝑠𝑡𝑒𝑝
Voltage Regulation - Fixed Width Modulation
Maximum Voltage
Vdc
Without modullation, the voltage during 𝑡𝑠𝑡𝑒𝑝 is 𝑉𝑑𝑐
tstep
Time
Reduced Voltage
Vdc
With Modulation, phase voltage 𝑉𝑟𝑚𝑠
ton
toff
Time
𝑑=
𝑡𝑜𝑛
𝑡𝑠𝑡𝑒𝑝
2𝑑
=
𝑉𝑑𝑐
3
Encoder output
Problem
Facts:
• Transistors do not
immediately commutate
(open)
• Transistors do not
immediately close
Problem:
• There could be periods
𝑑𝑖
where high transient
𝑑𝑡
current exist.
a
Time
b
Time
c
Time
Encoder output
Commutation
Solution
• Make before break
a
Time
b
Time
c
Time
Dynamic Braking
Method 1:
Q1
Vdc
Q5
Q3
a
Q4
Open all top transistors
Close and regulate all lower transistors
c
b
Q2
Q6
o
Motor windings
n
Dynamic Braking
Method 2:
Q1
Vdc
Q5
Q3
a
Q4
Open all transistors
Close and regulate the triacs
c
b
Q2
Q6
o
Motor windings
n
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