servo motor(5)
TRANSCRIPT
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Induction Motor
Control of Servo Motors
Induction motor (Rotor)
Structure of Squirrel-cageInduction motor
Squirrel-cage induction motor
Wound-rotor induction motor
Stator
3-phase winding : sinusoidal distribution of stator winding
Three-phase voltage is applied
Rotor
Conductor bars & End rings
No power supply
a
b c
Stator
Rotor
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Torque production of induction motor
Control of Servo Motors
A rotating field of constant amplitude is produced by three-phase ac current with synchronous angular speed e
Rotor :
- Rotor voltage at conductor bar is induced by Rotating field
- Rotor flux is generated by rotor current
- Rotor speed < synchronous speed
Stator : Three phase windings are excited by three-phase currentib
ia
ic
3-Phase
et
Current
e
reS
er S )1(
S = 1 : Standstill
S = 0 : Motor speed = synchronous
S motor speed
Slip:
Rotor speed :
er S )1(
eeeer SSS )1(
Rotor flux speed inside rotor : slip frequency =Rotor speed :Rotor flux speed at air-gap :
eS
a
b c
NS
N
S
e
e
Rotating field Axis
slr
+
Rotor flux Axis
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Torque production of induction motor
Control of Servo Motors
Torque production of induction motor
r
s
sinrseT
Torque production of PMSM
d-axis
q-axis
E
i qs i s=qse iKT
- Stator current = torque component + flux component - Stator current = torque component
Vector control of induction motor
Stator current is resolved into the d-axis current (Flux component) and q-axis current (Torque component)Control independently both currents Complex for implementation
d-axis
q-axis
iqs is
rids
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Torque production of induction motor
Control of Servo Motors
Flux model for indirect vector control
ej
r
es eP
1
Lm
Tr
e
e
-Tr
Iqse
Iqs
Idse
Ids
s
s
3
2
Ias
Ibs
Icsdr
Slip frequency
drr
qsm
eT
iLS
Where rotor time constant
r
rr
R
LT
Synchronous speed ere S
Synchronous angle dtee
Rotor resistanceRr is varied with temperatureRotor inductance Lr is changed at saturation situations due to high current
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Selection Criteria for induction motor and servo motor
Control of Servo Motors
Flux model for indirect vector control
rrss RiRi22
ss Ri2
[1] Position detector
PMSM : The absolute position of flux by permanent-magnet is detected IM : The rotor position is detected* Stator current
PMSM : Stator current = Torque component current IM : Stator current = Torque component current + Flux component current[2] Parameter sensitivity
- Induction motor : Rotor time constant (Rotor resistance) variation
[3] Thermal capability
Induction motor : Temperature Rotor resistance The Performance of vector control system
Servo motor : Temperature Flux loss (Br) Torque In order to keep torque constant, Stator current
[4] Loss
Copper loss :
* Copper of Induction motor =
* Copper of PMSM =
Core loss :Servo motor < IM
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Selection Criteria for induction motor and servo motor
Control of Servo Motors
d-axis
q-axis
E
Vsiqs is=
d-axis
q-axis
E
e Lsidse Lsiqs-
Vs
ids
iqsis
22
qsdss iii
[5] Flux weakening control
- Motor speed > base speed
Speed voltage is limited to rated value Flux Torque Power = constant
Constant power region
PMSM
* Normal operation * Flux weakening control
- Stator current of PMSM
Speed d-axis current stator current Limiting maximum stator current
Demagnetizing the Permanent Magnet
Copper loss is increased
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Selection Criteria for induction motor and servo motor
Control of Servo Motors
Induction motor
* Normal operation * Flux weakening control
d-axis
q-axis
iqs is
rids
d-axis
q-axis
iqs is
rids
Speed d-axis current stator current Copper loss is decreased
[6] Cost
Motor : Induction motor PMSM
C l f S M
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Selection Criteria for servo motors : BLDC and PMSM
Control of Servo Motors
[1] Power density
Output power to motor size The motor size of servo motor is nearly proportional to copper loss
Calculating the output power when copper losses (motor size) for both servo motors are the same
Current waveforms for servo motor
Current waveform of BLDC Current waveform of PMSM
IP2 IP1
- RMS of BLDC current =2
6
5
6
2
23
2)(
1PP ItdI
- Copper loss of BLDC = 22 )32(3 Ps IR
-RMS of PMSM current =2
1PI
- Copper loss of PMSM = 21 )2
(3 PsI
R
* Copper loss of BLDC = Copper loss of PMSM212
2 )2
(3)3
2(3 PsPs
IRIR
221 15.13
2PPP III
C t l f S M t
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Selection Criteria for servo motors : BLDC and PMSM
Control of Servo Motors
* Power density of BLDC is 1.15 times to power density of PMSM
- Output power of BLDC = 22 PpIE
- Output power of PMSM=
1
1
2
3)
22(3
Pp
PP IEIE
- Substituting into above equation21 15.1 PP II
- Output power of PMSM = 221 725.115.12
3
2
3PpPpPp IEIEIE
[2] Capacity of rectifier and inverter- Capacity of rectifier and inverter Maximum voltage and current
- Calculate output power when the maximum voltageEp and currentIp of BLDC and those of PMSM
Output power of BLDC = PpPp IEIE 22 2 Output power of PMSM = PpPp IEIE
2
3
2
31
- The ratio of output power of BLDC to output power of PMSM = 33.1
2
32
Pp
Pp
IE
IE
* Output power of BLDC = 1.33 Output power of PMSM at the same capacity of inverter.
Control of Servo Motors
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Selection Criteria for servo motors : BLDC and PMSM
Control of Servo Motors
[3] Torque per unit current
- Assume that maximum voltage and current of BLDC are identical with those of PMSM.
- Torque of BLDC =r
PpIE
2 - Torque of PMSM =r
PpIE
)2/3(
* Torque of BLDC = 1.33 torque of PMSM at the same maximum current
* Torque per unit current of BLDC is 1.3 times to PMSM
[4] Position detector
- BLDC : Detecting the position of rotor per 60 for one revolution.
- PMSM : Detecting continuously the position of rotor flux
* PMSM requires the more precise position detector
[5] Cogging torque and ripple torque
Cogging torque : generated by the rising and falling time of stator current at BLDC motor
- Frequency of cogging torque = 6 synchronous speed
- The cogging torque has more influent on motor torque at low speed.
Ripple torque : generated by switching at PWM inverter- The lowest harmonic frequency of ripple torque = switching frequency
- The ripple torque doesnt affect on motor torque because of high frequency of ripple torque.
Control of Servo Motors
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Selection Criteria for servo motors : BLDC and PMSM
Control of Servo Motors
PMSM : smooth torqueRipple torqueCogging torqueCogging and ripple
torque
BLDC : Detecting position per 60
PMSM : Detecting continuously
ComplexSimplePosition detector
The same maximum current and EMF33% HigherTorque/ unit current
Output power at the same capacity of
inverter
33% HigherConverter capacity
The same power density15% HigherTorque/
Moment of inertia
The size (Copper loss) is the same15% HigherPower density
DistributedConcentratedStator winding
SinusoidalTrapezoidalEMF Waveform
SinusoidalRectangularCurrent Waveform
ConditionsPMSMBLDCItems
Control of Servo Motors
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Vector control of PMSM
Control of Servo Motors
Current control strategy of vector controlled PMSM- Control both the magnitude and phasor of stator current for vector control of PMSM
The d-axis & q-axis current are controlled independently
Current control strategy Hysteresis current control
Ramp comparison method
Space vector control
Current controlled PWM inverter- Converting DC voltage into three phase ac voltage
- To control three phase currents to their reference current
a
bc
Vas+ -A+
C-A-
C+
B-
B+
Vdc
+
-
Vbs+ -
Vcs+ -
P M S M
Current controlled PWM inverter circuit
Control of Servo Motors
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Vector control of PMSM
Control of Servo Motors
Relationship between switching function and voltage vector
dcV32
3
1
3
2 jdceV
3
2
3
2 jdceV
j
dceV
3
2
3
4
3
2 jdceV
3
5
3
2 jdceV
01117
00000
1016
1005
1104
0103
0112
0011
Voltage vectorScSbSaMode
- Sa , Sb, Sc : Switching function of three phase leg
- 1 : Upper switching device is conducting
- 0 : Lower switching device is conducting
V(1)
V(2)(3)
V(4)
V(5) V(6)
V(7)(0)orRe Vds )
Vqs )m
Stator voltage vector for switching mode
Control of Servo Motors
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Vector control of PMSM
f
(1) Switching mode I (Sa=1, Sb=0. Sc=0)
a
b
c
V as+ -A+
C -A-
C +
B-
B+
Vdc
+
-
Vbs+ -
V cs+ -
PMSM
dcas VV 3
2
3
1
32 j
dcs eVV
dcs VV3
2
dcbs VV 3
1 dccs VV 3
1
(2) Switching mode 2 (Sa=1, Sb=1. Sc=0)
a
bc
Vas+ -A+
C-A-
C+
B-
B+
Vdc
+
-
Vbs+ -
Vcs+ -
P M S M
dcas VV3
1 dcbs VV
3
1
dccs VV3
2
Control of Servo Motors
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Vector control of PMSM
f
(3) Switching mode 3 (Sa=0, Sb=1. Sc=0)
dcas VV 3
1
3
2
3
2 jdcs eVV
dcbs VV 3
2 dccs VV 3
1
a
b
c
Vas+ -A+
C-A-
C+
B-
B+
Vdc
+
-
Vbs+ -
Vcs+ -
PMSM
(7) Switching mode 7 (Sa=1, Sb=1. Sc=1)
a
bc
Vas+ -A+
C-A-
C+
B-
B+
Vdc
+
-
Vbs+ -
Vcs+ -
P M S M
0asV 0bsV 0csV
0sV