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7/30/2019 Servo Motor(6)

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Current control strategy 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

d-axis

q-axis

E

Vsiqs is=

d-axis

q-axis

E

e Lsidse Lsiqs-

Vs

ids

iqsis

PMSM

* Normal operation * Flux weakening control


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Hysteresis Current control of PMSM


Block diagram of hysteresis current control

d-axis current ids = 0

d-q axis current in synchronous reference framed-q axis current in stationary reference frame d-q axis current in stationary reference frame3-phase reference current

Hysteresis control

[1] Hysteresis current control

- 3-phase motor current : control within the hysteresis band at a centered reference currents

2

3

a*

ia ib ic

eje

e

ib*c*Iqs*

Ids*=0HysteresisController

E

idss

iqss


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Operations of hysteresis current control

t

t1 t2

Ts

t

A

0.5E

-0.5E

Va

ia* ia+ ia

-

Ba

+

A-

Rs

Va

Vb

Vc

ia

ib

ic

Rs

Rs

L s

L

L

ea+ -

eb+ -

ec+ -

Vo

+-

+-0.5E

A+

s

s

B+ C+

A- B C--

0.5 E

Current controlled PWM Inverter

)(*

BHii aa

)(*

BHii aa

Upper switching device A+ is conducting Current is increased

Lower switching device A- is conducting Current is decreased


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Advantage of hysteresis current control

Simple implementation

- a-phase voltage equation

* Inductance Ls a variation of current switching frequency

Hysteresis band switching frequency current ripple

Hysteresis band switching frequency current ripple

Fast current response & Inherent peak current limiting

Disadvantage of hysteresis current control

Switching frequency of PWM inverter is widely varied with motor speed & load

Problems to design the filter and select switching frequency (turn-off time)

aa

sasa edtdiLiRV

- Neglecting resistance voltage drop, a variation of current

s

aaa

L

eV

dt

di

* Motor speed motor EMF ea a variation of current switching frequency * Motor speed motor EMF ea a variation of current switching frequency


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Simulation results

Motor current, reference current

with HB, and gating signal

Current ripple and switching frequency

with a variations of HB

(1) HB = 0.2A


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Simulation results

Current ripple and switching frequency with a variations of HB

(2) HB = 0.3A (3) HB = 0.4A

Hysteresis band switching frequency current ripple Switching frequency is widely varied at one period

The current is mostly controlled within the hystersis band

The Hysteresis current control method is not applied to industrial applications


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Ramp Comparison Method of PMSM


Ramp comparison method

PWM pulses : Three-phase reference voltages arecompared with the triangular wave withconstant frequency and magnitude

Both the frequency and magnitude of reference three-phase voltages is adjusted for

controlling the output voltage of PWM inverter

LOCKOUT A

AVas

* ComparatorCIRCUIT

LOCKOUT B

BVbs

* ComparatorCIRCUIT

LOCKOUT C

C

Vcs* Comparator

CIRCUIT

* Vas* > Triangular waveform : Upper switching device is conducting

* Vas* < Triangular waveform : Lower switching device is conducting

* Dead time is required to prevent the arm short

C l f S M


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a-phase reference voltage, triangular waveform, and switching signal with a variation of motor speed

(1) Speed = 400 rpm (2) Speed = 800 rpm

(3) Speed = 1600 rpm

C t l f S M t


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a-phase reference voltage, triangular waveform, and stator current with a variation of switching frequency

(1) switching frequency= 1500Hz (2) switching frequency= 3000Hz

- Motor speed = 1500 rpm, load torque = 0.8[N.m]




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a-phase reference voltage, triangular waveform, and stator current with a variation of switching frequency

(3) switching frequency= 4500Hz


Switch frequency of PWM inverter = frequency of triangular waveform

Constant switching frequency

Both the frequency and magnitude of reference voltages

the frequency and magnitude of output voltage of PWM inverter

Current control performance < Current control performance of hysteresis control

PWM pulses : Three-phase current errors arecompared with the triangular wave



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Block diagram of space voltage vector method

Space Voltage Vector Method of PMSM

ej e

Vds

Vqs

P W M

Vdss

Vqss

Space

Voltage

Vector

Controller

c

b

a

v( 2)

v( 1)

v(3 )

v( 4)

v(5 )

v(k )v(7 )

v( 6)

v( 0)

IIIII

IV IVV

Vdc3

vB

vA ds-axis

qs-axis

Space voltage vector

- Six sectors

- Reference voltage vector Vref is locate at sector 1

- Voltage vector is divided into vector V1 and V2

2211 TVTVVref

* Voltage vectorV1 is applied at time T1

& Voltage vectorV2 is applied at time T2

* Zero voltage vector Vo at T0

T0 = Ts (T1+T2)



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Three phase PWM signal at Sector 1

Ub

Ua

Uc

T T

Ts

1 2T02

T02

fs =12

Ts

T T

Ts

2 1T02

T02



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Features for space voltage method

Constant switching frequency

Good current control performance

Complex implementation

d-q axis reference voltage

d-q axis stator current

No. of sector

V

S e c t o r

ds

s

Vqss

idss

iqss



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f

Closed loop control of PMSM

Position P controller

Position control of PMSM using the hysteresis current control method

2

3

ai*

ia ib ic

e

PMSM

je

e

ib*

ci*Iqs

*

Ids*

=0

Hysteresis

Controller

E

*r

r

idss

iqss*e

e

p

)(**

eePr K

Speed PI controller )()(**

rrI

PqsS

KKI



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f


Position control of PMSM using the ramp comparison method

+

-

3

2

ej-

PWM

Inverter

PMSM

i b

i a

i c

Vdss

Vqss

Iqse

i qss

i dss

Iqse

Idse

+

-

Vqse

Iqse

= 0Idse

+

-

Vdse

Idse

Comparator

TrianglerWave

eje

e

e

e

r

r 3

2Vas

Vbs

Vcs

E

p

+

-

e

e

e



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Speed control of PMSM with the ramp comparison method using current errors



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Position control of PMSM using the space voltage vector

+

-

3

2

ej-

PWM

Inverter

PMSM

ib

ia

ic

Vdss

VqssIqs

e

iqss

idss

Iqse

Idse

+

-

Vqse

Iqse

= 0Idse

+

-

Vdse

Idse

eje

e

e

e

r

r

E

p

+

-

e

e

e

Space Voltag Vector

Controller