SVPWM Based Stator Oriented Non-linear Control of Induction Motor Drives

INTRODUCTION

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Induction motors are the workhorse of the industry since they are widely used in industrial drives. Induction motors are cheap, suitable for hazardous environment and easy to control (Abdalla et al., 2011). There are different types of induction motor drives which are reported in the literature and are classified based on the control techniques available for controlling the parameters like voltage, frequency and current fed to the motor (Dandil et al., 2005). Many researchers have done on Direct Torque Control (DTC) (Fakhfakh et al., 2006) and Flux Oriented Control of induction motors drive to operate in only one quadrant. There are so many intelligent controllers also used for control of inverters (Kang and Jin, 2010; Ustun, 2007). The major drawback of aforementioned techniques is occurrence of non-linearity during the change in flux linkage of the rotor present in the mechanical part which is not considered for the control purpose.

This study proposes a new method to overcome the aforementioned disadvantage. In this method, a small change is made in the variable transformation of rotor flux so that the flux linkage in the rotor can be changed without affecting the dynamics of the system. Space Vector Pulse Width Modulation (SVPWM) which is used widely for its superior performance is employed to eliminate the lower order harmonics. The drive is made to be operated in all the four quadrants such as forward motoring, forward braking, reverse motoring and plugging by stator oriented non-linear control. The prime benefit of this method is SVPWM control method which is used for non liner control. The proposed method is simulated using Simulink platform in the MATLAB software.

STATOR ORIENTED NON-LINEAR CONTROL SYSTEM

Figure 1 shows the block diagram of stator oriented non-linear control of induction motor drive system. This system consists of voltage fed induction motor drive in which the inverter switches are controlled by Space vector Pulse Width Modulation technique. The drive current and voltage is taken as reference and fed to the computational block in which the non-linear dynamic problems in the system are considered and rectified. The speed regulation, rotor current calculation and flux are processed in computational block. Then the processed values are compared with the actual values in the PI controller. The output of the PI controller is a two-phase quantity. The transformation of two-phase to three-phase quantities is done in variable transformation block. The output of the variable transformation is stator current (I_s) and speed (ω_i). The current controller compares the inverter current and feedback current to generate the Space Vector Pulses and control the switches with their modulation index.

In the literature survey, majority works deals the induction motor drive controllers for only single quadrant operation and control is linear but this proposed system in the induction motor is operated in all four quadrants and provides the stator oriented non linear control. The SVPWM technique which is suitable for variable frequency drive is employed in this study.

Fig. 1:	Stator oriented nonlinear control of four quadrant drive system using induction motor

In previous methods, the stator reference control and non-linear control is done separately leads to bulky size of the system (Luo et al., 2009). In the proposed system, non-linear control technique is implemented along with stator reference frame with the slight change in the variable transformation. Both the transformation and control are done on the stator simultaneously, so that cost and size of the system reduces.

In this control, the motor is operated in stationary reference frame and the current in the stator shown in X-axis is assumed not to be stationary (Benheniche and Bensaker, 2012). The following equations describe the steady state and transient electromagnetic behaviour of induction motor drive:

(1)

(2)

(3)

(4)

(5)

Where:

(6)

where, i_sxy is the shunt current, Ψ_sxy is the shunt flux, U_sxy is the stator voltage, ω_s is the rotor speed. L_s, L_r, L_m is the stator, rotor, mutual inductances. While R_s, R_r are the stator and rotor resistances.

State variable accessibility is very important when employing a non-linearity feedback to obtain linearity of the control system (Ershadi et al., 2010). It is presumed that all important variables are calculated directly and measured by a system of observer or model system. The non-linear model with variable transformation enables the control strategy to control efficiently. Hence, the new changes that are made in the variable transformation give a better response in the induction motor drive:

(7)

(8)

(9)

(10)

The drive system is converted into linear model by including the non-linear model variable transformation equations. The PI controller compares the actual and reference values of the current to employ the designed control. The kp and ki values of the PI controller are obtained by tuning the system by trial and error method.

Current controllers control the parameter stator current (I_s) that flows through the stator winding and generates the space vector pulses to the switches of the inverter fed induction motor drive. The control variables x₁₁, x₁₂, x₂₂ are the output signals of the speed controller, current controller’s rotor flux, speed and torque calculation using control feedback. The error signal from comparison generates the SVPWM pulses by comparing the reference and actual values.

Inverter model: A 3 phase, 6 pulse inverter has been modelled by assuming ideal switches in which the on state losses are neglected. The inverter is fed to an induction motor drive and switching pulses are given by the non-linear feedback of Space vector pulses:

SPACE VECTOR PULSE WIDTH MODULATION (SVPWM)

There are many PWM techniques which are available for inverter control (Balasubramanian et al., 2014) SVPWM is the advanced technique used in industrial drives to obtain high DC bus voltage (Bose, 2006). The space vector can be done both for transient and steady state conditions with phasor representation. Over modulation is possible in SVPWM so that the triple end harmonics gets eliminated and the power factor gets stabilized. It consists of 6 active vectors from v₁ to v₆ and two null vectors v_o and v₇ (Fig. 2). The size of active vectors depends upon the number of sectors. The DC link voltage appears as same as in the input. This method overcomes the disadvantage of other PWM techniques that are given in the literature (Habetler et al., 1992; Ravi Teja et al., 2012):

(11)

(12)

(13)

(14)

(15)