1 Step motor: load angle B s θ N S S N I 1 B r The load angle θ ... is defined as the angle with which the rotor magnetic field lags behind the stator magnetic field while rotating synchronously with it. ...
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1
Step motor: load angle
Bs
θN
S
S
N
I1
Br
The load angle θ
... is defined as the angle with which the rotor magnetic field lags behind the stator magnetic field while rotating synchronously with it. ...
2
Ph1+
Ph1–
Ph2+
Ph2–
I1
I2
N
S
S
N
SN
S
N
S NS N
S
NS
N
SN
Two-phase PM step motor: one phase on drive (1)
• Rotor and stator magnetic fields
are synchronous
• Phase turn-on time corresponds to 90° rotation of the rotor
• No-load condition: - Rotor magnetic field first lags
then leads the stator magnetic field
3
Ph1+
Ph1–
Ph2+
Ph2–
I1
I2
N
S
S
N
SN
S
N
S N
Θ=45°
Θ=45°
Θ=45°
Θ=-45°Θ=45°
Θ=-45°Θ=-45°
Θ=-45°
+ 45° (motor) > Θ > - 45° (brake)
• Phase turn-on: Θ=45°
• Phase turn-off:Θ=-45°
Θ=0, i.e. Tav=0
Two-phase PM step motor: one phase on drive (2)
4
• Zero average torque, but positive and negative istantaneous torques
• Motor runs jerkily, with high torque ripple
Two-phase PM step motor: one phase on drive (3)
5
Ph1+
Ph1–
Ph2+
Ph2–
I1
I2
N S
S
N
SN
S
N
S NS N
S
NS
N
SN
Load angle is always positive
Torque is always positive
Two-phase PM step motor: one phase on drive (4)
6
Ph1+
Ph1–
Ph2+
Ph2–
I1
I2N S
S
N
SN
S
N
S N
Θ=0°
Θ=0°
Θ=90°Θ=0°
Θ=90°Θ=0°
Θ=90°Θ=90°
+ 90 > Θ > 0
• Phase turn-on: Θ=90°
• Phase turn-off:Θ=0°
Θ = 45° ; Tav= 2 TPK
Two-phase PM step motor: one phase on drive (5)
7
• Positive (motor) average torque
• Still high torque ripple
Two-phase PM step motor: one phase on drive (6)
8
Ph1+
Ph1–
Ph2+
Ph2–
I1
I2
N
S
S
N
SN
S
N
S NS N
S
NS
N
SN
Load angle is always positive
Torque is always positive
Two-phase PM step motor: one phase on drive (7)
9
Ph1+
Ph1–
Ph2+
Ph2–
I1
I2
N
S
S
N
SN
S
N
S NΘ=135°
Θ=45°Θ=135°
Θ=45°
Θ=135°
Θ=45°
Θ=135°Θ=45°
+ 135° > Θ > 45°
• Phase turn-on: Θ=135°
• Phase turn-off:Θ=45°
Θ = 90° ; Tav= 0.9 TPK
Two-phase PM step motor: one phase on drive (8)
10
With Θ = 90° :
• average torque is maximum
• torque ripple is minimum
(30% of the peak torque)
Two-phase PM step motor: one phase on drive (9)
11
Step motor: load angle (conclusion)
Bs
θN
S
S
N
I1
Br
At steady-state, the average load angle self-regulates to ensure
Tav = TLOAD
12Two-phase PM step motor: two phase on drive (1)
Total torque
Ph1 + Ph2
• Zero average torque, but positive and negative istantaneous torques
• Motor runs jerkily, with high torque ripple
+ 45° (motor) > Θ > - 45° (brake)
Θ=0, i.e. Tav=0
T
ωt
i1
t
i2
t
T1 T2 T1' T2' T2T1
i1+, i2
+
i1-, i2
+i1
-, i2
-
i1+, i2
-
Hm
13Two-phase PM step motor: two phase on drive (3)
Total torque
Ph1 + Ph2
+ 90 > Θ > 0
Θ = 45° ; Tav= 2 TPK
TPK(TPO) = 1.4 TPK (OPO)
T
ωt
T1 T2 T1' T2' T2T1
i1
t
i2
t
i1+, i2
+
i1-, i2
+i1
-, i2
-
i1+, i2
-
Hm
14Two-phase PM step motor: two phase on drive (3)
Total torque
Ph1 + Ph2
135° > Θ > 45°
Θ = 90° ; Tav= 0.9 TPK
TPK(TPO) = 1.4 TPK (OPO)
T
ωt
T1 T2 T1' T2' T2
TTOT
i2
t
i1
t
T1
i1+, i2
+
i1-, i2
+i1
-, i2
-
i1+, i2
-
Hm
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