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 Authors : Manish Kumar Sahu, Satyadharma Bharti
 Paper ID : IJERTV2IS100503
 Volume & Issue : Volume 02, Issue 10 (October 2013)
 Published (First Online): 19102013
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
Implementation of Three Phase Three Leg AC/AC Converter Drives for BLDC Motor
Manish Kumar Sahu
Department of Electrical Engineering Rungta College of Engg. and Technology Bhilai, India
Satyadharma Bharti
Department of Electrical Engineering Rungta College of Engg. and Technology Bhilai, India
AbstractThis paper presents the implementation of a novel three phase three leg AC/AC converter for BLDC motors. Since the number of switches required in this converter is only nine which is much less than that of the number of switches required in other direct AC/AC converter, the gate drivers used in this converter is reduced drastically which in turn reduces the cost and the losses of the drives used for motors by more than thirty percent. Apart from those only six pulses is required to drive the circuit with the help of space vector modulation (SVPWM) which is used to control the input current and hence unity power factor is maintained even for BLDC motors. The duty cycle calculation for the input current control of BLDC motor is also shown. Simulation results show the effectiveness and efficiency of the proposed technique.
KeywordsAC/AC Converter; BLDC motor drives; Space Vector Modulation.

Introduction
The threephase acac power conversion can be achieved with the fullbridge configuration in which twelve semiconductor switches (six legs) are employed [1]; such a topology requires a relatively large number of power switches. In general, the use of too many power switches increases the cost and at the same time it reduces the reliability of the power conversion
controllable power factor, and is capable of bidirectional energy transfer from the supply to the load or vice versa. Since there is no dclink circuit, the dc capacitor in the VSC is not necessary here, leading to cost reduction as well as improved reliability and longevity.
There are several reasons why AC/AC converters remain very attractive for some applications and a very promising technology that contribute to the development of power electronics. Few of these are [7]:

Energy storage elements like inductors and capacitors are absent.

The bidirectional power flow capability and input displacement factor control of these converters make them an ideal solution for same application.

Large Power Density factors are achievable.
P
S1 S3
IA IX
A X
I
I
I
B B Vd Cd Y Y
IC C I Z
system. Thus, the study of topologies with a reduced number Z
of power switches constitutes an important topic in power S S4
electronics [2]. 2
Threephase acdcac and acac converters with variable frequency (VF) and variable voltage operation have been
found wide applications in the industry. The most popular N
configuration uses the voltage source inverter (VSI) with a diode rectifier as the front end for adjustable speed drives (ASDs), uninterruptible power supplies (UPS), and other industrial applications [3]. This configuration features low cost and reliable operation due to the use of a diode rectifier, but it generates highly distorted input line currents and does not have regenerative or dynamic braking capability. These problems can be mitigated by using a backtoback twolevel voltage source converter (B2B 2LVSC), shown in Fig. 1, where a pulse width modulation (PWM) voltage source rectifier is used to replace the diode rectifier [4][5]. Unlike VSCs that inevitably require the dclink stage, the matrix converter [6] presents a radical change in topology and directly converts a fixed ac input voltage to an adjustable ac output voltage. It features sinusoidal inputoutput,
Fig. 1. BacktoBack Two Level Voltage Source Converter


Three Phase Three Leg AC/AC Converter
Fig. 2 shows the proposed threephase nineswitch converter topology [8]. This converter has only three legs with three switches installed on each of them. The novelty herein is that the middle switch in each individual leg is shared by both the rectifier and the inverter, thereby reducing the switch count by 33% and 50% in comparison to the B2B 2LVSC and CMC, respectively. The input power is delivered to the output partially through the middle three switches and partially through a quasidclink circuit. For the convenience of discussion, we can consider that the rectifier of the nine switch converter is composed of the top three and middle three
switches, whereas the inverter consists of the middle three and bottom three switches.
The converter has two modes of operation: 1) constant frequency (CF) mode, where the output frequency of the inverter is constant and also the same as that of the utility supply, while the inverter output voltage is adjustable; and
2) VF mode, where both magnitude and frequency of the inverter output voltage are adjustable. The CFmode operation is particularly suitable for applications in UPS, whereas the VF mode can be applied to variablespeed drives.
P
(THIPWM), are well established in the literature [9]. The principles of these methods can all be applied to the nine
TABLE 1: SWITCHING STATES OF BACKTOBACK CONVERTER
S3 S2 S1
a b c
iA
A
LS R S
vAS
B
B
Vdc Cd C
iB
iC
a b c
S6 S5
S4 iX
Backtoback converter
Switching State
S1
S2
S3
S4
VAN
VXN
1
On
Off
On
Off
Vd
Vd
2
Off
On
Off
On
0
0
3
On
Off
Off
On
Vd
0
4
Off
On
On
Off
0
Vd
Backtoback converter
Switching State
S1
S2
S3
S4
VAN
VXN
1
On
Off
On
Off
Vd
Vd
2
Off
On
Off
On
0
0
3
On
Off
Off
On
Vd
0
4
Off
On
On
Off
0
Vd
X i
LL R L
Y
Z
a b c
Y o
iZ
TABLE 2: SWITCHING STATES OF PROPOSED CONVERTER
Proposed nine switch converter
Switching State
S1
S4
S7
VAN
VXN
1
On
Off
On
Vd
Vd
2
Off
On
Off
0
0
3
On
Off
Off
Vd
<>0 Proposed nine switch converter
Switching State
S1
S4
S7
VAN
VXN
1
On
Off
On
Vd
Vd
2
Off
On
Off
0
0
3
On
Off
Off
Vd
0
S9 S8 S7
N
Fig. 2. Proposed Nine Switch AC/AC Converter with a Quasi DCLink

Modulation Schemes

Switching Constraint
The reduction of the number of switches in the proposed converter topology imposes certain switching constraints for the switching pattern design [8]. In the B2B 2LVSC shown in Fig. 1, the rectifier leg voltage vAN , which is the voltage at node A with respect to the negative dc bus N, can be controlled by switches S1 and S2 in the rectifier, whereas the inverter leg voltage vXN can be controlled by S3 and S4 in the inverter. This means that the rectifier and inverter leg voltages can be controlled independently. The B2B 2LVSC has four switching states per phase, as defined in Table I.
For the nineswitch topology, the control of the input and output voltages has to be accomplished through the three switches on each leg. Because the middle switches are shared by the rectifier and inverter, the proposed converter has only three switching states per phase, as listed in Table I. It can be observed that switching state 4 for the B2B 2LVSC does not exist in the nineswitch converter, which implies that the inverter leg voltage vXN cannot be higher than the rectifier leg voltage vAN at any instant. This is, in fact, the main constraint for the switching scheme design of the nineswitch converter. Carrierbased continuous PWM schemes for modulating the 2LVSC, such as sinusoidal PWM (SPWM), space vector PWM (SVPWM), and thirdharmonic injection PWM
switch converter but a little modification would be necessary, because when designing the switching pattern for the nine switch converter, the switching constraint discussed earlier must be satisfied. Fig. 3 illustrates the generalized carrier based modulation scheme in a single switching period for the nineswitch converter. The rectifier modulating wave vmr and the inverter modulating wave vmi are arranged such that vmr is not lower than vmi at any instant of time. These two modulating waveforms are compared with a common triangular carrier vc . The generated rectifier and inverter leg voltages vAN and vXN are also shown in the figure. This arrangement guarantees that switch state 4 in the B2B 2LVSC is eliminated here for the nineswitch converter.

Space Vector Pulse Width Modulation(SVPWM) for input current control and output voltage control
The rectifier stage is similar to the traditional one except that all the six switches are bidirectional. The purpose of the rectifier stage is to generate sinusoidal input currents a well as to maintain the positive dclink output voltage.
The space vector of the rectifier stage is composed of six
active current vectors with fixed directions and two zero
stage corresponding to the phase of the input voltage. Space vector modulation techniques are widely used in inverter control because they reduce the harmonic components of output voltages easily. Given a sampled reference vector Vref and angle out in sector 1, as shown in Fig. 4(b), the duty cycles of two active vectors V1 and V2 and zero vectors V0 and V7 are
Vref
vectors, as shown in Fig. 3(a) [10]. The reference current vector is generated from two active current vectors. For example, in sector 1, the reference current vector Iref is synthesized from two vectors Iab and Iac. Current vectors Iab and Iac represent the connection of the input phase a to the positive pole and the input phases b and c to the negative pole
d1
d2
3 sin / 3 out
Vdc
out
out
3 Vref sin
Vdc
(5)
(6)
of dclink bus, respectively. For the sake of explaining the indirect SVPWM method in MC without missing the generality of the analysis, it is assumed that the rectifier stage
d0 d7 0.5(1 d1 d2 )
(7)
operates in sector 1 and the reference output voltage vector Vref is located in sector 1 at the inverter stage, as shown in Fig. 3(a) and (b), respectively. The duty cycles d and d for the active vectors in the rectifier stage are given by [10]
where out is the angle of reference output voltage vector Vref
and d1, d2, d0, and d7 are the duty cycles of output voltage space vectors V1, V2, V0, and V7, respectively.
To obtain balanced input currents and output voltages in the same sampling period, the PWM pattern should produce all
d m1sin / 3 in
d m1 sinin
(1)
(2)
combinations of the rectifier and the inverter switching states. As there are two switching states for the rectifier stage, the switching states at the inverter stage should be divided into two groups, as shown in Fig. 4. As shown in Fig. 4, the
Where m1 is the rectifier stage modulation index and in is the
angle between the reference vector Iref and the right neighbor
inverter switching frequency is twice the rectifier switching frequency, where Ts is the sampling period. The duty cycles of
V
V
Ibc
b
I
V
010 3
2 V
2110
active vectors and zero vectors of the output voltage space
vectors in dab and dac are calculated by:
Iba
3 2 ac
d 3
I I
V 1
d ref
d0ab d0 .dab
d7ab d7 .dab
(8)
cc
4 Iaa
ref
1 Va
V4 V
2
0000
111
out V1
d1ab d1.dab
d2ab d2 .dab
d
d
Ibb in
5
6
Iab
011 V 7
4
d1 100
6
d0ac d0 .dac
d1ac d1.dac
d7ac d7 .dac
d2ac d2 .dac
(9)
Ica
Vc
Icb
V
5001
V6
101
5
The voltage transfer ratio of MC is defined as follows:
m Vref (10)
Fig. 3. Space Vector Diagram of (a) Rectifier stage and (b) Inverter Stage
active current vector Iab. In the modulation of the rectifier stage, the zero vectors are not considered, and the modulation index is unity. Hence, the switching sequence only consists of the two active current vectors Iab and Iac, whose duty cycles are
V1
According to (5)(7), the voltage transfer ratio m should be smaller than 0.866 in orders to maintain all duty cycles positive.
dclink voltage
given by:
d
dab
(3)
Rectifier stage
vab
vac
d d
0 dab
dac
Ts/2
d
d
dac d d
(4)
Inverter stage
V0
V1
V2
V7
V7
V2
V1
V0
By utilizing the same approach, the duty cycles and the switching states for all sectors can be obtained. Table I
d0ab
d1ab
d2ab
d7 ab
d7 ac
d2ac
d1ac
d0ac
summarizes the sectors and the switching states at the rectifier
Fig. 4. Synthesis of switching states for rectifier and inverter stages in MC


Modelling of BLDC Motor

Voltage Controller
The BLDC Motor requires a power electronic drive circuit and a commutation system for its operation [12]. The Fig.5 describes the functional units present in the drive circuit and the associated commutation controller or the BLDC Motor. A 4pole BLDC motor is driven by the inverter for 1200 commutation. The rotor position can be sensed by a halleffect sensor or slotted optical disk, providing three square wave signals with phase shift of 1200. These signals are decoded by a combinational logic to provide the firing signals for 1200 conduction on each of the three phases. The inverter voltage for the motor is filtered by the filter circuit provided, which minimizes the high frequency switching voltage ripple
component. The LC filter for the proposed work is connected
by the shaft position sensor. The torque is directly commanded by Iref . The larger the reference Iref , the higher the torque produced. The strategy becomes simple, because the control only needs to be in command of one dc current instead of
three alternating waveforms. Another advantage of this
strategy is that the modulation of the currents can be done using one of the simplest control strategies available: the Triangular carrier modulation strategy which offers the following additional advantages: 1) the switching frequency becomes defined by the triangular carrier Fig.7. Stator and Rotors MMF during step change from motor to brake operation. 2) The ability to follow the template with the proposed method becomes quite accurate when triangular carrier is used 3) the hardware implementation is very simple.
3phase
3phase supply
ACAC
Converter
FILTER
BLDC
speed
Supply
ACAC Converter
3phase BLDC
motor
LOGIC CIRCUIT
LOGIC CIRCUIT
FROM HALL SENSORS
GATE DRIVE
CURRENT SENSER
POSITION SENSER
Nref
RECTIFIER
RECTIFIER
PI CONROLLER
COMPARATOR
N
CONTORLLER
TRIANGULER WAVE
Fig. 5. Voltage Controller for BLDC Motor With Filter
in the interface of the drive and the motor. The LC filter in this system acts as a low pass filtering circuit which offer high impedance to high frequency component of the voltage and very minimum impedance to the power frequency voltage components and thereby minimizes harmonics in the supply voltage to the motor and the series inductance opposes the sudden changes in the current due to electronic commutation and thereby reduces the torque ripple.

Current Controller
Fig.4 shows the simple block diagram of the proposed method. The current controller block is shown in the Fig.6. The operation of the system is as follows: as the motor is of The brushless dc type, the waveforms of the armature currents are quasi square. These currents are sensed through current sensors, and converted to voltage signals. These signals are
Fig. 6. Current Controller of BLDC Motor
The control strategy also allows regenerative braking, which is very important in many applications, like electric vehicles, where energy can be returned to the battery pack. To brake the motor (regenerative braking) the stator magnetic field is reversed. This action is accomplished through the inversion of the signals given by the position sensor. The position sensor discriminates six positions each 360o electric degrees. During motor operation, the rotor moves clockwise. When the brake signal is applied, the stator field is reversed 180o electric degrees. This action produces an instantaneous change in the direction of the torque, making a fast reduction of the speed of the machine, which begins to return its energy to the dc link. The same strategy can be used for reversal of rotation of the machine.
Iref
then rectified, and a dc component, with the value of the
Idc
+Ierr
ceiling of the currents, Imax is obtained as shown in Fig.5 this
– PI –
dc signal is compared with a desired reference Iref
, and from
this comparison, and error signal
Iref is obtained. This error is +
then passed through a PI control to generate the PWM for all the six valves of the inverter, which are sequentially activated
Traingular Carrier
Fig.7. Current Controller Block
The tuning of the current controller starts with the determination of the amplitude and frequency of the triangular carrier, and the gains of the PI control. To get a PWM signal operating at the carrier frequency, the control should be adjusted to keep the reference current moving around the reference.


Proposed Technique
The duty cycles for the rectifier stage and the inverter stage are calculated using the equations (1) to (10) according to the switching restrictions explained in section III. Thus the six pulses (P1 to P6)are obtained with the help of space vector modulation as explained in above section and the combination of these pulses are applied to the nine switches of the proposed converter as shown in Table 3. And the same combination of pulses are applied to the ACAC converters as shown in the fig. 5 and fig. 6 in BLDC modeling for the operation of BLDC motor at unity power factor.
TABLE 3: COMBINATION OF PULSES APPLIED TO VARIOUS SWITCHES
Pulse
Switches
P1
S4
P2
S1
S7
P3
S5
P4
S2
S8
P5
S6
P6
S3
S9

Simulation Results
Simulations are carried out for a three phase RL load using MATLAB software. The simulation parameters are as follows:

Power Supply (Phase peak Voltage) is 230V/50Hz.

BLDC Motor with circuit parameters R=0.2, L=8.5mH.

Output frequency fout=50Hz.

All switches in the converter are ideal.

Switching Frequency used=6 KHz.

Modulation Index mr=0.9 and mi=0.9.
Fig. 8 shows the input voltage applied to the proposed converter and the input current obtained at the converter side. As shown in the result the input current reaches to the steady state after few cycles because of the transients obtained. Fig. 9 shows the Three Phase Output Line Voltages of the converter which is fed to the BLDC Motor and acts as input voltage source for the same. In Fig. 10 the variation of stator current is shown which changes in accordance with the reference rotor speed and reaches to the steady state at 0.5 sec i.e. only after when the actual rotor speed of the motor comes to steady state as shown in Fig. 11. Now the variation of actual electromagnetic torque and the reference torque is shown in Fig. 12 which is varying according to the speed variation and
load demand. Fig. 14 shows the input power factor which is settled to unity after transient period while the Fig. 13 shows the fictitious dc voltage obtained at the converter side.
Input Voltage(Volts)
Input Voltage(Volts)
400
200
0
200
4000 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4
Time(sec)
Input Current
Input Current
500
0
5000 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4
Time(sec)
Fig. 8. Three Phase Input Voltage and Input Current at the Converter Side
500
Vab
Vab
0
0 0.05 
0.1 
0.15 
0.2 
0.25 
0.3 
0.35 
0.4 
0 0.05 
0.1 
0.15 
0.2 
0.25 
0.3 
0.35 
0.4 
0 0.05 
0.1 
0.15 
0.2 
0.25 
0.35 
0.4 

0 0.05 
0.1 
0.15 
0.2 
0.25 
0.3 
0.35 
0.4 
500
500
Vbc
Vbc
0
500
500
Vca
Vca
0
5000 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4
Time
Fig. 9. Three Phase Output Voltages of the Converter fed to the BLDC motor
15
10
Stator Current(Amp)
Stator Current(Amp)
5
0
5
100 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Time(sec)
Fig. 10. Stator Current
350
300
Rotor Speed(rad/sec)
Rotor Speed(rad/sec)
250
200
150
100
50
0
500 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Time(sec)
Fig. 11 Rotor Speed Variation of BLDC Motor
16
14
Electromagnetic Torque(Nm)
Electromagnetic Torque(Nm)
12
10
8
6
4
2
0
20 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Time(sec)
Fig. 12. Electromagnetic Torque Variation of BLDC Motor
paper. The advantages of this strategy are a) very simple control scheme; b) phase currents are kept balanced; c) current is controlled through a quasi dc component, and hence phase over currents are eliminated. So, this technique provides the desired result which can be used in various direct ACAC conversion applications with minimum number of gate driver circuits as the number of pulses used is only six as compared to other AC/AC converters.
8. References

H. Kohlmeier, O. Niermeyer, and D. F. Schroder, Highly dynamic fourquadrant ac motor drive with improved power factor and online optimized pulse pattern with PROMC, IEEE Trans. Ind. Appl., vol. IA23, no. 6, pp. 10011009, Nov./Dec. 1987.

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350
300
DC Bus Voltage(V)
DC Bus Voltage(V)
250
200
150
100
50
0
0 0.05 0.1 0.15 0.2 0.25
Time(sec)
Fig. 13. DC Bus Voltage
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1.005
1
Power Factor
Power Factor
0.995
0.99
0.985
0.98
0.9750 0.01 0.02 0.03 0.04 0.05 0.06 0.07
Time(sec)
Fig. 14. Input Power Factor
7. Conclusion
In this paper, the effectiveness of the proposed technique for BLDC motor has been clearly justified. For controlling the input current and to increase the input power factor, space vector modulation technique is used which is clearly shown in the results. It has also been shown that how the only six pulses can be used to control the nine switches of the proposed converter whose duty cycle calculation is also shown in this
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