Analytical Method of a Torque Ripple Calculation for Two-Phase IM Supplied by Three-Leg SPWM Inverter
Pavel Záskalický Department of Electrotechnics and Mechatronics Technical University (TUKE) Košice, Slovak Republic pavel.zaskalicky@tuke.sk
Branislav Dobrucký Department of Mechatronics and Electronics
, Slovak Republic dobrucky@fel.uniza.sk
Abstract—The contribution deals with steady state estimation of a electromagnetic torque ripples and a current waveform of a two-phase induction motor, which is supplied by an three-leg IGBT bridge connected inverter. The inverter’s output voltage is controlled by a modified sinusoidal SPWM of the input DC voltage. The complex Fourier series analysis of the inverter’s output voltage was made, to obtain a spectrum of the harmonic supply voltage. The different voltage harmonics have been applied to the two-phase induction machine model to obtain electromagnetic torque and supply current waveforms for various operation states.
Keywords—two-phase induction motor, torque ripple estimation, bridge invertor, Fourier series, sinusoidal PWM control I. INTRODUCTION The electrical low-power drives which are supplied by a single phase voltage used in different industrial and domestic devices are presently increasingly deployed by two-phase motors. A two-phase motor by their characteristics no differs from the three-phase ones. Their advantage is simpler winding layout, which is of great importance for automated motor production [1]-[2].
The two-phase motors are at present manufactured as either squirrel cage induction or permanents magnets synchronous motors. They are very often deployed as a pumps drives in a washing machines and dishwashers, but also in a circulating pumps for central domestic heating. A permanent magnet in this case is water and lye resistant, which allows making a pump with an absolute waterproof.
The stator winding can be configured in either a serial or parallel two-phase system. Normally, the winding are identical. The windings which form one phase are connected to induce opposite magnetic polarity. II. MATHEMATICAL MODEL OF A SUPPLY CONVERTER To build a mathematical model of a two-phase inverter’s a complex Fourier series approach was used. In the model we consider following idealized conditions: Power switch can handle unlimited current and blocks unlimited voltage.
The voltage drop and leakage current across the switches are zero. The switches are turned on and off with no rise and fall times. Sufficiently good size capacity of the inverter’s input voltage capacitors, to can suppose constant converter input DC voltage for any load.
These assumptions help us simpler to analyze a motor power supply circuit and help us to build a mathematical model of invertor at steady state. Fig. 1 shows a two-phase convertor circuit layout, supplied by a single-phase network. The inverter of the converter consists of three transistors branches (b-, a-, c- ones). The first branch (b) is common branch for both other phases. Contrary to common analysis method the reference voltage potential is not created by centre tap of DC link as [4] but with its negative potential for Fourier series analysis used in the next part of paper.
Fig. 1. Supply convertor circuit layout.
978-1-4799-4749-2/14/$31.00 ©2014 IEEE
2014 International Symposium on Power Electronics, Electrical Drives, Automation and Motion
731
A. Two-Phase ‘Modified’ Sinusoidal PWM Technique This form of voltage control is the classical sinusoidal pulse-width-modulation (SPWM) but modified for two-phase system [5]. The pulse-width-modulated wave has much lower order harmonics than the other waveforms. If the desired inverter’s output voltage is sine-wave, two parameters define the control [3]-[4]: Coefficient of the modulation m - equal to the ratio of the modulation and reference frequency. Voltage control coefficient r - equal to the ratio of the desired voltage amplitude and DC supply voltage level.
Generally the synchronous modulation is used. In synchronic modulation a modulation frequency m f is an integer multiple of the reference frequency f .
Output voltage vectors VAS and VBS
are line to line voltages between the middle leg b and another leg a and leg c, Fig. 2. From Fig. 2 we can see that the amplitudes of output voltages are equal, but the complexity of control system SVPWM is much harder like using the control scheme with two-phase sinusoidal SPWM [5]-[6].
Fig. 2. Referenced voltage for each leg of VSI inverter [5].
From the figure also can see that the amplitude of voltage vectors is VDC . Referenced voltages for two-phase SPWM for each leg of VSI inverter are depicted on Fig. 3.
Fig. 3. Referenced voltages of the two-phase sinusoidal PWM technique.
To inverter output voltage analysis the number of math methods can be used [7]-[13], one of them based on Fourier analysis is used. Then, the turn on angle and turn off angle for any modulation interval are calculated, based on the reference sine-wave.
The desired output voltage of the branches has a form:
0
01 02
22
2 2 2 2
ee
e e e e
UU u r sin ;
U U U U u r cos ; u r cos
(1)
To calculate a turn on (
) and turn off (
) angles we compare the DC pulse area with the requested voltage area, as depicted on the Fig. 4 [7].
Fig. 4. Comparison of the voltages area.
For the left and right crosshatched areas of the first output transistors branch the following equations are valid [10]-[13]:
2
01
21 2
21 2
01
2
2
22
2
22
n
m
ee
en
n
m
n
m
ee
en
n
m
UU r sin d U n m
UU r sin d U n m
(2)
After the calculus we obtain for the turn-on and turn-off angles of the first transistors branch the following expressions:
0
0
12 22 1 22 12 22 1 22
n
n
r n cosn cos n m m m r n cosn cos n m m m
(3)
It will be similarly for the second transistor branch.
01
01
12 2 2 1 22 12 2 2 1 22
n
n
rn n sin sin n m m m rn n sin sin n m m m
(4)
So does for third transistor branch.
02
02
12 22 1 22 12 22 1 22
n
n
rn n sin sin n m m m rn n sin sin n m m m
(5) Then we can write the output voltage of the first branch in the form of a complex Fourier series [7], [10]:
Sine referenced voltage for Leg b
Minus cosine referenced voltage for Leg a
Cosine referenced voltage for Leg c
VAS
VBS
0 0.005 0.01 0.015 0.02 0
0.2
0.4
0.6
0.8
1
Leg A Leg B Leg C
732
00
0
00 1
00
0
1 2
0
0
2
nnjk jk
n
m
jk en kn
nn
n
c e e jk u U c e ; for k
c for k
(6) Similarly for the output voltage of the second transistor branch: 01 01 01
01 01 1
01 01
01
1 2
0
0
2
nnjk jk
n
m
jk en kn
nn
n
c e e jk u U c e ; for k
c for k
(7) So does for third transistor branch. 02 02 02
02 02 1
02 02
02
1 2
0
0
2
nnjk jk
n
m
jk en kn
nn
n
c e e jk u U c e ; for k
c for k
(8) The phase voltages are given by a difference between branch voltages as following.
1 0 01 0 01 1
2 0 02 0 02 1
m
jk S e n n kn
m
jk S e n n kn
u u u U c c e ;
u u u U c c e ;
(9)
Where e U -DC inverter input voltage In the Fig. 5 are shown the phase voltages waveforms, which were calculated on the base on equations (9).
Fig. 5. Waveforms of the phase voltages.
The waveforms were calculated for modulation frequency of 2 kHz ( ).
III. HARMONIC ANALYSIS OF THE SUPPLY VOLTAGES On the base of Fourier series formulas of the phase supply voltages, can be made a harmonic analysis of the supply waveforms. Phasor of each of voltage harmonics is given by a product of sum of complex Fourier’s coefficient and DC input voltage 10 0 1 1 20 0 2 1 2 2 m k k k s e n n n m k k k s e n n n U c c U c c U U (10)
Fig. 6. Harmonic analysis of the PWM controlled output voltage.
With amplitude and phase k k k k A abs ; P angle ; UU (11) The Fig.6 depicts a harmonic analysis of the PWM controlled output voltage for desired frequency of 50 Hz and modulation frequency of 2 kHz ( 40 m ).
0 2 4 6 8 10 12 14 16 18
-300 -200 -100 0 100 200 300 u1S (V)
t (ms)
0 2 4 6 8 10 12 14 16 18
-300 -200 -100 0 100 200 300 u2S (V)
t (ms)
0 10 20 30 40 50 60 70 80 90 100
0
50
100
150
200
250
A1 k (V)
harmonics
0 10 20 30 40 50 60 70 80 90 100
-3
-2
-1
0
1
2
3
P1 k (rad)
harmonics
0 10 20 30 40 50 60 70 80 90 100
0
50
100
150
200
250
A2 k (V)
harmonics
0 10 20 30 40 50 60 70 80 90 100
-3
-2
-1
0
1
2
3
P2 k (rad)
harmonics
733
For practical application we are interested only for significant harmonics. Therefore we neglect the harmonics with amplitude less then5V.
IV. MATHEMATICAL MODEL OF A TWO-PHASE INDUCTION MOTOR For the system which is associated with the rotating
magnetic field, following equation of a two-phase asynchronous machine are valid [14].
1
2
1
2
ds s s ds s qs
qs s s qs s ds
dr r r dr s m qr
qr r r qr s m dr
d u R i dt d u R i dt d u R i dt d u R i dt !
"!
!
"!
!
" " !
!
" " !
(13)
Since homopolar components of the system are zero, the equation of the machine can be
transformed into single axis.
ds qs
r dr qr
s ds qs
r dr qr
s ds qs
r dr qr
u ju
u ju
j
j
i ji
i ji
!! !!
su u
i
i
Equations (11) take a form.
s s s s s s
r r r r s m r
d Rj dt d Rj dt " "" ui ui
(14)
With the flux linking components defined as.
s s s m r
r m s r r
LL LL (15) After the solving (12) and (13), we obtain for stator and rotor current phasors.
22
22
/ ./
./
s r r
s
s s
Analytical Method of a Torque Ripple Calculation for Two-Phase IM Supplied by Three-Leg SPWM Inverter
Pavel Záskalický Department of Electrotechnics and Mechatronics Technical University (TUKE) Košice, Slovak Republic pavel.zaskalicky@tuke.sk
Branislav Dobrucký Department of Mechatronics and Electronics
, Slovak Republic dobrucky@fel.uniza.sk
Abstract—The contribution deals with steady state estimation of a electromagnetic torque ripples and a current waveform of a two-phase induction motor, which is supplied by an three-leg IGBT bridge connected inverter. The inverter’s output voltage is controlled by a modified sinusoidal SPWM of the input DC voltage. The complex Fourier series analysis of the inverter’s output voltage was made, to obtain a spectrum of the harmonic supply voltage. The different voltage harmonics have been applied to the two-phase induction machine model to obtain electromagnetic torque and supply current waveforms for various operation states.
Keywords—two-phase induction motor, torque ripple estimation, bridge invertor, Fourier series, sinusoidal PWM control I. INTRODUCTION The electrical low-power drives which are supplied by a single phase voltage used in different industrial and domestic devices are presently increasingly deployed by two-phase motors. A two-phase motor by their characteristics no differs from the three-phase ones. Their advantage is simpler winding layout, which is of great importance for automated motor production [1]-[2].
The two-phase motors are at present manufactured as either squirrel cage induction or permanents magnets synchronous motors. They are very often deployed as a pumps drives in a washing machines and dishwashers, but also in a circulating pumps for central domestic heating. A permanent magnet in this case is water and lye resistant, which allows making a pump with an absolute waterproof.
The stator winding can be configured in either a serial or parallel two-phase system. Normally, the winding are identical. The windings which form one phase are connected to induce opposite magnetic polarity. II. MATHEMATICAL MODEL OF A SUPPLY CONVERTER To build a mathematical model of a two-phase inverter’s a complex Fourier series approach was used. In the model we consider following idealized conditions: Power switch can handle unlimited current and blocks unlimited voltage.
The voltage drop and leakage current across the switches are zero. The switches are turned on and off with no rise and fall times. Sufficiently good size capacity of the inverter’s input voltage capacitors, to can suppose constant converter input DC voltage for any load.
These assumptions help us simpler to analyze a motor power supply circuit and help us to build a mathematical model of invertor at steady state. Fig. 1 shows a two-phase convertor circuit layout, supplied by a single-phase network. The inverter of the converter consists of three transistors branches (b-, a-, c- ones). The first branch (b) is common branch for both other phases. Contrary to common analysis method the reference voltage potential is not created by centre tap of DC link as [4] but with its negative potential for Fourier series analysis used in the next part of paper.
Fig. 1. Supply convertor circuit layout.
978-1-4799-4749-2/14/$31.00 ©2014 IEEE
2014 International Symposium on Power Electronics, Electrical Drives, Automation and Motion
731
A. Two-Phase ‘Modified’ Sinusoidal PWM Technique This form of voltage control is the classical sinusoidal pulse-width-modulation (SPWM) but modified for two-phase system [5]. The pulse-width-modulated wave has much lower order harmonics than the other waveforms. If the desired inverter’s output voltage is sine-wave, two parameters define the control [3]-[4]: Coefficient of the modulation m - equal to the ratio of the modulation and reference frequency. Voltage control coefficient r - equal to the ratio of the desired voltage amplitude and DC supply voltage level.
Generally the synchronous modulation is used. In synchronic modulation a modulation frequency m f is an integer multiple of the reference frequency f .
Output voltage vectors VAS and VBS
are line to line voltages between the middle leg b and another leg a and leg c, Fig. 2. From Fig. 2 we can see that the amplitudes of output voltages are equal, but the complexity of control system SVPWM is much harder like using the control scheme with two-phase sinusoidal SPWM [5]-[6].
Fig. 2. Referenced voltage for each leg of VSI inverter [5].
From the figure also can see that the amplitude of voltage vectors is VDC . Referenced voltages for two-phase SPWM for each leg of VSI inverter are depicted on Fig. 3.
Fig. 3. Referenced voltages of the two-phase sinusoidal PWM technique.
To inverter output voltage analysis the number of math methods can be used [7]-[13], one of them based on Fourier analysis is used. Then, the turn on angle and turn off angle for any modulation interval are calculated, based on the reference sine-wave.
The desired output voltage of the branches has a form:
0
01 02
22
2 2 2 2
ee
e e e e
UU u r sin ;
U U U U u r cos ; u r cos
(1)
To calculate a turn on (
) and turn off (
) angles we compare the DC pulse area with the requested voltage area, as depicted on the Fig. 4 [7].
Fig. 4. Comparison of the voltages area.
For the left and right crosshatched areas of the first output transistors branch the following equations are valid [10]-[13]:
2
01
21 2
21 2
01
2
2
22
2
22
n
m
ee
en
n
m
n
m
ee
en
n
m
UU r sin d U n m
UU r sin d U n m
(2)
After the calculus we obtain for the turn-on and turn-off angles of the first transistors branch the following expressions:
0
0
12 22 1 22 12 22 1 22
n
n
r n cosn cos n m m m r n cosn cos n m m m
(3)
It will be similarly for the second transistor branch.
01
01
12 2 2 1 22 12 2 2 1 22
n
n
rn n sin sin n m m m rn n sin sin n m m m
(4)
So does for third transistor branch.
02
02
12 22 1 22 12 22 1 22
n
n
rn n sin sin n m m m rn n sin sin n m m m
(5) Then we can write the output voltage of the first branch in the form of a complex Fourier series [7], [10]:
Sine referenced voltage for Leg b
Minus cosine referenced voltage for Leg a
Cosine referenced voltage for Leg c
VAS
VBS
0 0.005 0.01 0.015 0.02 0
0.2
0.4
0.6
0.8
1
Leg A Leg B Leg C
732
00
0
00 1
00
0
1 2
0
0
2
nnjk jk
n
m
jk en kn
nn
n
c e e jk u U c e ; for k
c for k
(6) Similarly for the output voltage of the second transistor branch: 01 01 01
01 01 1
01 01
01
1 2
0
0
2
nnjk jk
n
m
jk en kn
nn
n
c e e jk u U c e ; for k
c for k
(7) So does for third transistor branch. 02 02 02
02 02 1
02 02
02
1 2
0
0
2
nnjk jk
n
m
jk en kn
nn
n
c e e jk u U c e ; for k
c for k
(8) The phase voltages are given by a difference between branch voltages as following.
1 0 01 0 01 1
2 0 02 0 02 1
m
jk S e n n kn
m
jk S e n n kn
u u u U c c e ;
u u u U c c e ;
(9)
Where e U -DC inverter input voltage In the Fig. 5 are shown the phase voltages waveforms, which were calculated on the base on equations (9).
Fig. 5. Waveforms of the phase voltages.
The waveforms were calculated for modulation frequency of 2 kHz ( ).
III. HARMONIC ANALYSIS OF THE SUPPLY VOLTAGES On the base of Fourier series formulas of the phase supply voltages, can be made a harmonic analysis of the supply waveforms. Phasor of each of voltage harmonics is given by a product of sum of complex Fourier’s coefficient and DC input voltage 10 0 1 1 20 0 2 1 2 2 m k k k s e n n n m k k k s e n n n U c c U c c U U (10)
Fig. 6. Harmonic analysis of the PWM controlled output voltage.
With amplitude and phase k k k k A abs ; P angle ; UU (11) The Fig.6 depicts a harmonic analysis of the PWM controlled output voltage for desired frequency of 50 Hz and modulation frequency of 2 kHz ( 40 m ).
0 2 4 6 8 10 12 14 16 18
-300 -200 -100 0 100 200 300 u1S (V)
t (ms)
0 2 4 6 8 10 12 14 16 18
-300 -200 -100 0 100 200 300 u2S (V)
t (ms)
0 10 20 30 40 50 60 70 80 90 100
0
50
100
150
200
250
A1 k (V)
harmonics
0 10 20 30 40 50 60 70 80 90 100
-3
-2
-1
0
1
2
3
P1 k (rad)
harmonics
0 10 20 30 40 50 60 70 80 90 100
0
50
100
150
200
250
A2 k (V)
harmonics
0 10 20 30 40 50 60 70 80 90 100
-3
-2
-1
0
1
2
3
P2 k (rad)
harmonics
733
For practical application we are interested only for significant harmonics. Therefore we neglect the harmonics with amplitude less then5V.
IV. MATHEMATICAL MODEL OF A TWO-PHASE INDUCTION MOTOR For the system which is associated with the rotating
magnetic field, following equation of a two-phase asynchronous machine are valid [14].
1
2
1
2
ds s s ds s qs
qs s s qs s ds
dr r r dr s m qr
qr r r qr s m dr
d u R i dt d u R i dt d u R i dt d u R i dt !
"!
!
"!
!
" " !
!
" " !
(13)
Since homopolar components of the system are zero, the equation of the machine can be
transformed into single axis.
ds qs
r dr qr
s ds qs
r dr qr
s ds qs
r dr qr
u ju
u ju
j
j
i ji
i ji
!! !!
su u
i
i
Equations (11) take a form.
s s s s s s
r r r r s m r
d Rj dt d Rj dt " "" ui ui
(14)
With the flux linking components defined as.
s s s m r
r m s r r
LL LL (15) After the solving (12) and (13), we obtain for stator and rotor current phasors.
22
22
/ ./
./
s r r
s
s s
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