Performance of Induction Motor driven by Three Level Neutral Clamped Inverter using Space Vector Modulation as Voltage Control Technique

Performance of Induction Motor driven by Three Level Neutral Clamped Inverter using Space Vector Modulation as Voltage Control Technique

P G K Aruna1

1Assistant Professor, Department of EEE, Malla Reddy University,

Hyderabad, Telangana, India 500075.

Banda Sai Kishore2

2UG Scholar, Department of EEE, Mahatma Gandhi Institute of Technology, Hyderabad,

Telangana, India 500043.

Abstract – This paper deals with the performance of Induction Motor which is driven by a three-level neutral clamped inverter using space vector modulation technique as voltage control strategy. SVM has increasing applications in power converters and motor control. Compared to other modulation techniques, space vector modulation technique has the advantages like higher modulation index, utilizing DC bus voltage more effectively, generating less THD and thus less harmonics and low ripple content. In this paper, DC voltage is given as input to the three-level neutral clamped Inverter. This inverter has been designed and implemented in MATLAB/SIMULINK software. IGBTs are used as active switches. The triggering pulses to these IGBTs are given by using space vector modulation technique. By selecting proper switching states and by calculating the time-period in particular switching state AC voltage with less THD, low dV/dt is obtained as output. AC output of this inverter is given as input to the Induction Motor. After simulating, the output parameters of induction motor like torque, speed, load current is studied.

Keywords: Induction Motor (IM), Neutral Clamped Inverter (NCI), Space Vector Modulation Technique (SVM), Total Hormonic Distortion (THD).

1. INTRODUCTION

   Multilevel inverters are gaining importance in high power and high voltage applications like manufacturing industry, transportation, renewable energy and in reactive power compensation due to their superior performance compared to two-level inverters. If we use two-level inverters in high voltage and high-power applications, the rate of rise of voltage (dV/dt) is significantly high, the switching losses will be more and also there will be constraints of device rating [1]. Hence, Multilevel Inverters are becoming popular which produce high power, high voltages without requiring transformers or higher rating individual devices. Multilevel Inverters produce higher level voltages with low dV/dt, low hormonic content and low ripple content [3]. The only disadvantage of this technique is that by increasing the levels, the number of switches will increase, and the switching combination will become complex and control will become complicated and expensive [9]. Its various applications are in Renewable DC source utilization, Uninterruptible Power Supplies (UPS), Power Transmission through high voltage DC (HVDC), Variable frequency drives and electrical vehicle drives [1]. The multilevel inverters are classified into three types:

   1. Neutral clamped multilevel inverter

   2. Flying-capacitors multilevel inverter

   3. Cascade H-Bridge multilevel inverter.

2. NEUTRAL CLAMPED MULTILEVEL

   INVERTER

   Neutral clamped multilevel inverter is also known as Diode clamped multilevel inverter. Circuit diagram of a three-level neutral clamped inverter is shown in the Fig.1. In m level inverter with one leg consists of (m-1) dc bus capacitors, (m-1) (m-2) clamping diodes and 2(m-1) main switching devices. Thus, a 3-level neutral clamped inverter with one leg consists of 2 dc bus capacitors, 2 clamping diodes and 4 main switching devices. There fore, for a 3- phase, 3-level neutral clamped inverters there are 2 dc bus capacitors, 6 midpoint clamping diodes, 12 main switching devices [1], [4], [5]. Table.I shows the switching states of three-level inverter. Each phase has three output states, P, O, and N, corresponding to positive voltage (+), zero voltage (0), and negative voltage (-). Take phase a as an example, if switch Sa1 and Sa2 are turned on, then phase a is on P state; if switch Sa2 and Sa3 are turned on, then phase a is on O state. Similarly, when switch Sa3 and Sa4 are turned on, then phase a is on N state.

   Fig.1. Circuit diagram of a three-level neutral clamped inverter

   Switches S1x and S3x are complementary and switches S2x and S4x are complimentary. If one of the complementary switch pairs is turned ON, the other of the same pair must be OFF. Two switches are always turned ON at the same time [3], [4], [5].

   Table I: Switching states and terminal voltages of a three- level inverter

   symbols	Switching States				Output Voltage
   	S1x	S2x	S3x	S4x
   P	ON	ON	OFF	OFF	Vdc/2
   O	OFF	ON	ON	OFF	0
   N	OFF	OFF	ON	ON	-Vdc/2

3. SPACE VECTOR PULSE WIDTH MODULATION TECHNIQUE

   Space Vector Pulse Width Modulation Technique is a voltage control technique used in inverters which is used to regulate voltage of inverters, cope up with variations of DC input voltage and maintain constant voltage and frequency. Voltage is controlled in each sampling period by Space Vector Transformation concept [1] by

   1. Properly selecting the switching states for the inverter and

   2. Calculation of appropriate time-period for each state [4], [7], [8].

   Since three kinds of switching states exist in each phase, three-level inverter has (33=) 27 switching states, out of which 3 are null states, and remaining are active states [3], [7]. But we consider only 19 states to avoid redundancy and memory wastage in MATLAB [10].

   From Fig.1. The resultant output load phase voltage of inverter is

   Table II. All Switching states with their corresponding voltage vectors: magnitude and angle

   VR(t) =

   Here,

   2(VAN(t)+VBN(t)

   3

   2

   3 +VCN(t)

   4

   Switching States			Corresponding Voltage Vectors
   Va	Vb	Vc	Vector	Magnitude	Angle
   P	P	P	V0	0	0
   O	O	O
   N	N	N
   P	O	O	V1	2Vdc 3	0
   O	N	N
   P	N	N	V2	4Vdc 3	0
   P	O	N	V3	2 Vdc 3	6
   P	P	O	V4	2Vdc 3	3
   O	O	N
   P	P	N	V5	4Vdc 3	3
   O	P		V6	2 Vdc 3	2
   O	P	O	V7	2Vdc 3	2 3
   N	O	N
   N	P	N	V8	4Vdc 3	2 3
   Switching States			Corresponding Voltage Vectors
   Va	Vb	Vc	Vector	Magnitude	Angle
   N	P	O	V9	2 Vdc 3	5 6
   O	P	P	V10	2Vdc 3
   N	O	O
   N	P	P	V11	4Vdc 3
   N	O	P	V12	2 Vdc 3	5 6
   O	O	P	V13	2Vdc 3	2 3
   N	N	O
   N	N	P	V14	4Vdc 3	2 3
   O	N	P	V15	2 Vdc 3	2
   P	O	P	V16	2Vdc 3	3
   O	N	O
   P	N	P	V17	4Vdc 3	3
   P	N	O	V18	2 Vdc 3	6

   Switching States			Corresponding Voltage Vectors
   Va	Vb	Vc	Vector	Magnitude	Angle
   P	P	P	V0	0	0
   O	O	O
   N	N	N
   P	O	O	V1	2Vdc 3	0
   O	N	N
   P	N	N	V2	4Vdc 3	0
   P	O	N	V3	2 Vdc 3	6
   P	P	O	V4	2Vdc 3	3
   O	O	N
   P	P	N	V5	4Vdc 3	3
   O	P	N	V6	2 Vdc 3	2
   O	P	O	V7	2Vdc 3	2 3
   N	O	N
   N	P	N	V8	4Vdc 3	2 3
   Switching States			Corresponding Voltage Vectors
   Va	Vb	Vc	Vector	Magnitude	Angle
   N	P	O	V9	2 Vdc 3	5 6
   O	P	P	V10	2Vdc 3
   N	O	O
   N	P	P	V11	4Vdc 3
   N	O	P	V12	2 Vdc 3	5 6
   O	O	P	V13	2Vdc 3	2 3
   N	N	O
   N	N	P	V14	4Vdc 3	2 3
   O	N	P	V15	2 Vdc 3	2
   P	O	P	V16	2Vdc 3	3
   O	N	O
   P	N	P	V17	4Vdc 3	3
   P	N	O	V18	2 Vdc 3	6

   3 (1)

   V (t) = 2V

   -1V

   -1V

   (2)

   AN 3

   AO 3

   BO 3 CO

   VBN(t) = 2VBO-1VAO-1VCO (3)

   3 3 3

   VCN(t) = 2VCO-1VBO-1VAO (4)

   When all these switching states are represented in space by using space vectors then we get a hexagon with 6 sectors [3],

   [6] and with 4 subsectors in each sector as shown in Fig.2.

   3 3 3

   By substituting the values of VAO, VBO, VCO we can find the voltage vector magnitudes and phase angles of all switching states [1], [10] as shown in Table II.

   Fig.2. Space Vector representation of 3-level Inverter

   A. Space Vector Modulation Algorithm:

   Steps to execute SVM logic in MATLAB/SIMULINK are as follow:

   Step I: Give three-phase sinusoidal input voltage Va, Vb, Vc.

   Va = Vm sinwt (5)

   Vb = Vm sin(wt-120o) (6)

   Vc = Vm sin(wt-240o) (7)

   Step II: The coordinate transformation from the abc- axis to the axis.

   The transformation from the ab-c axis to the – axis, which is rotating with an angular velocity of , can be obtained by clerk and perk transformations as shown below

   Fig.3. Transformation from ab-c axis to the – axis

   Step IV: Selection of sector in which Vref is considered

   Let n be the sector. As there are 6 sectors, n can be from 1 to 6. n is the function of deta () [9]. The sector in which the command vector Vref is located, is determined as follow:

   • If is between 0° and 60°, then Vref is located in Sector 1, n=1.

   • If is between 60° and 120°, then Vref is located in Sector 2, n=2.

   • If is between 120° and 180°, then Vref is located in Sector 3, n=3.

   • If is between 180° and 240°, then Vref is located in Sector 4, n=4.

   • If is between 240° and 300°, then Vref is located in Sector 5, n=5.

   • If is between 300° and 360°, then Vref is located in Sector 6, n=6.

     Step V: Selection of Sector angle

     Let be the sector angle. sector angle is the function of deta and n. The value of sector angleis given as follow:

   • If n=1 i.e., in sector 1: =

   • If n=2 i.e., in sector 2: = -60o

   • If n=3 i.e., in sector 3: = -120o

   • If n=4 i.e., in sector 4: = +180o

   • If n=5 i.e., in sector 5: = +120o

   • If n=6 i.e., in sector 6: = + 60o

     1 (2) ( )

     2

     2

     [ ] = 2 [ 3

     3 ] []

     Step VI: Selection of Subsector

     3

     3

     2 2

     0 ( ) ( )

     Let y be the subsector. y is the function of Vdc,

     3 3

     1 1 1

     Vref and sector angle (). In each sector there are 4 subsectors, y can be from 1 to 4. Subsector is selected by

     [] = 2 [

     2 2 ] []

     using Modulation Index (MI). Modulation Index differs

     3 0 3

     2

     3 2

     from modulation to modulation and from inverter to inverter. For space vector modulation technique, the

     Which can also be written as

     V = 2 ( 1 3 ) (8)

     modulation index is given by

     M =

     2

     3

     (12)

     3 2 2

     V = 2 (3 3 ) (9)

     3 2 2

     Here -axis is direct axis (real axis) and -axis is quadrature axis (imaginary axis).

     Step III: Calculate the value of deta () and Reference voltage (Vref)

     Which is given by,

     deta = = tan-1() (10)

     Reference voltage = Vref = 2 + 2 (11)

     This Reference voltage is selected as control instruction, which rotates in the space with an angular frequency . When we consider the reference vector in one of the 6 sectors, two active space vectors nearest to Vref is selected along with one of the three null vectors by means of their corresponding operating times [7], [8], [10].

     Fig.4. Calculation of M1 and M2

     From above figure, by using trigonometric ratios we get

     M1 = M 2 sin( ) (13)

     3 3

     M2= M 2 sin (14)

     3

     Table III. Conditions for Subsector selection

     y =1	M1<0.5, M2<0.5 and (M1+M2) <0.5
     y = 2	M1<0.5, M2<0.5 and (M1+M2) >0.5
     y = 3	M2>0.5
     y = 4	M1>0.5

     Step VII: Calculation of Switching Timings

     Vref Ts = V1TA+V7 TB+V2 TC (15)

     Consider a resultant vector in sector 1. Choose any three vectors nearest to the resultant vector.

     Fig.5. calculation of switching timings

     Table IV. Calculated values of Switching Timings

     Step VIII: Subsector Switching

     Now time period for each switching device is calculated by using the switching timings calculated in each subsector in the above step and tabulated below.

     Table V. Subsector Switching Timings

     V1 = 0 V2 =

     V7 =

     3 3 3 3 3

     2

     2

     Substituting V1, V2 and V7 in (15) we get

     3

     = 1TA + 3 TC + 1 TC (16)

     Ts

     2 2 6 2 3

     M Ts = 1 TA + 3 TB + 1 TC (17) M Ts

   • To reduce the harmonics, S1c is kept zero

   2 2 6 2 3

   [cos + jsin ] =1 TA + 3 [cos + jsin ] TB + 1[cos

   Step IX: All Sector Switching

   2 2 6 6 2

   + jsin ] TC (18)

   Reference voltage in sector A is given by

   3 3

   Vref

   (A) =

   2

   2

   2

   (Va + Vb 3 + Vc

   2

   3 ) (24)

   Equating real and imaginary terms we get,

   When V

   ref

   3

   (A) is

   rotated in anticlockwise direction at an angle

   Mn Ts cos = 1 TA + 3 TB + 1 cos TC (19)

   of 60 degree.

   2 2 6 2 3

   Mn Ts sin = 3sin TB + 1 sin TC (20)

   2 6 2 3

   Ts = TA + TB + TC (21)

   By solving equations (19), (20) and (21) we get

   TA = Ts 2 k sin (22) TB = 2 k sin ( + ) – Ts (23)

   3

   TC = Ts 2 k sin ( ) (24)

   3

   Similarly, we can calculate the switching timings for all

   subsectors. The calculated values are tabulated below

   Fig.6. All sector switching

   Vref(B) = Vref (A) * 3 (25)

   V (B) = 2

   (-v

   2 2

   + -v ) + (-v )

   (26)

   ref

   3 b C 3 a 3

   Similarly, reference voltages are calculated in all sectors. Now, the relationship between phase voltages and the reference voltages in the subsectors is:

   Table VI. Relation between phase voltages and the reference voltages in the subsectors

   Sector	Phase Voltage A	Phase Voltage B	Phase Voltage C
   A	Va	Vb	Vc
   B	-Vb	-Vc	-Va
   C	Vc	Va	Vb
   D	-Va	-Vb	-Vc
   E	Vb	Vc	Va
   F	-Vc	-Va	-Vb

4. SIMULATION

   The MATLAB\SIMULINK diagram of 3-phase, 3-level neutral clamped inverter connected to an induction motor is shown in Fig.6. The input given to the inverter is DC voltage and the output voltage of inverter is given to the 3-phase induction motor. The Output voltage of inverter is observed in the scope using voltage measurement blocks. Output parameters of induction motor i.e., torque, speed, and load current are observed in the scope.

   Fig.6. Simulation model of 3-phase, 3-level neutral clamped inverter fed Induction Motor

   In MATLAB software 3-phase induction motor is available as Asynchronous machine (Implements a three-phase asynchronous machine modelled in a selectable dq reference frame). Stator and rotor windings are connected in wye to an internal neutral point. In SVM block 3-phase sinusoidal voltage is given as input. The output pulses obtained from all sector switching block are compared with the triangular pulses, and the resultant pulses generated by SVM block are given to the IGBTs in the inverter by using GOTO and FROM blocks. These IGBT switches are used to control the output voltage.

   Parameters:

   • DC input voltage of inverter = 400V

   • Output frequency = 50Hz

   • Induction motor Rotor type = Squirrel cage

   • Number of poles in the Induction motor = 4

   • Inertia of Induction motor = 0.0131 J(kg.m2)

   • Rated Power of Induction motor = 4KW (5.4 HP)

   • Rated Speed of Induction motor = 1430 rpm

   • Stator Resistance of Induction motor = 1.405

   • Rotor Resistance of Induction motor = 1.395

   • Stator Inductance of Induction motor = 5.839 mH

   • Rotor Inductance of Induction motor = 5.839 mH

   • Mutual Inductance of Induction motor = 0.1722 mH

5. SIMULATION RESULTS

   The output voltage of three phases of 3-level neutral clamped inverter by using space vector modulation as voltage control strategy is shown in Fig.7.

   Fig.7: Output voltage of 3-phase, 3-level neutral clamped inverter.

   The FFT (Fast Fourier Transform) analysis is used to calculate the total harmonic distortion present in the output voltage and output current of inverter. The FFT analysis of output current of 3-phase, neutral clamped 3-level inverter for Space Vector Modulation is shown in the Fig.8.

   Fig.8: FFT analysis of output current of inverter for Space Vector Pulse Width Modulation.

   The waveforms of electromagnetic torque (Te), speed (w) and three-phase current (Iabc) of induction motor are shown below in Fig.9. In MATLAB/SIMLINK a step input (magnitude of 10 from 0 sec to 1 sec and magnitude of 20 after one second) is given to the torque terminal of Induction motor. Initially the magnitde of torque is 10 N-m and at 1 second the torque magnitude is increased from10 N-m to 20 N-m as shown in Fig.9. As a result, the magnitude of speed decreases and current increases at 1 second [10].

   Fig.9: Waveforms of Speed, Torque and 3-phase output current of Induction Motor

6. CONCLUSION

In this paper, the performance of induction motor which is driven by a three-phase, three-level neutral clamped Inverter using space vector modulation technique as voltage control strategy has been studied. A MATLAB/Simulink model for implementation of SVM is presented. The step- by-step algorithm of SVM is reported. The Speed, Torque

and Output current of the Induction Motor, the output voltage of the inverter have been plotted. From the above results we observe that the THD value of output current of 3-phase, 3-level neutral clamped inverter with space vector pulse width modulation technique is less than 5% i.e., 2.01%.

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