Servo Motor(5)

7/30/2019 Servo Motor(5)

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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


2/15

Torque production of induction motor


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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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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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


5/15

Selection Criteria for induction motor and servo motor


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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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


7/15



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


8/15

Selection Criteria for servo motors : BLDC and PMSM


[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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* 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.



10/15



[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.



11/15



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



12/15

Vector control of PMSM


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



13/15



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



14/15


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



15/15


f


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


a

bc

Vas+ -A+

C-A-

C+

B-

B+

Vdc

+

-

Vbs+ -

Vcs+ -

P M S M

0asV 0bsV 0csV

0sV

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Servo Motor(5)