SIMULATION OF INVERTED FER INDUCTION MOTOR DRIVE WITH LABVIEW

SIMULATION OF INVERTER FED INDUCTION MOTOR DRIVE WITH LABVIEW

Electrical and Electronics Project by Ravi Devani

ABSTRACT

This paper describes a software approach for modeling inverted fed induction motor drive using Laboratory Virtual Instrument Engineering Workbench (LabVIEW). The reason behind the selection of LabVIEW software is because of its strong graphical interface, flexibility of its programming language combined with built-in tools designed specifically for test, measurement and control. LabVIEW is generally used in most of the applications for data acquisition, test and control. In this paper, inverter and induction motor are modeled using LabVIEW toolkits. Simulation results are presented and are validated.

Keywords—Induction motor, LabVIEW, State model.

INTRODUCTION

MATHEMATICAL models of electrical machines are used to learn about their dynamic characteristics using simulation tools. With the advancement of computer technology, the drawbacks of conventional simulation methods become more and more obvious. With the rapid growth in computer hardware and software techniques, new user friendly simulation software packages are now available. Putting virtual instruments software into mathematical modeling and analysis of electrical machines will result in not only the cost reduction of the hardware but also the enhancement of the teaching or training effect. The direct application of this sort of software drastically simplifies simulation procedures for several practicing engineers. LabVIEW programs are called Virtual Instruments (VI) because their appearance and operation imitate physical instruments, such as oscilloscopes and multi-meters. LabVIEW is a graphical programming language for data acquisition, analysis and presentation. The components of LabVIEW are front panel and block diagram. The front panel is built with controls and indicators and the block diagram contains the pictorial representation of source code. At present, LabVIEW simplifies the scientific computation, process control, research, industrial applications and measurement applications. MATLAB is a high performance language for technical computing. It integrates computation, visualization and programming in an easy-to-use environment where problems and solutions are expressed in familiar mathematical notation. MATLAB simulink block sets are widely used in simulation of all kind of systems. MATLAB was used for simulation of vector control of induction motor and different electric motors. But Simulink lack the imitation of physical instruments or equipment in appearance and operation. Considering the above, LabVIEW is used as a simulation tool to model inverter fed induction motor drive. Field oriented control of induction motor drive using LabVIEW was discussed in but simulation results were not explained clearly. LabVIEW was used to simulate and find the characteristics of synchronous generator but the speed-torque characteristics were not explained. LabVIEW was used to identify the induction motor parameter, automatic testing for DC motor of portable washing machine, harmonic measurement, speed control of stepper motor, fault diagnosis of induction motor, etc. modeling of inverter and induction motor using LabVIEW is not discussed in literature. In this paper, LABVIEW is used as a simulation tool to model the inverter and induction motor. The induction motor state equations are constructed in LabVIEW environment using control design and simulation toolkit, math script node and simple VIs. Inverter is constructed using simple VIs.

 Electrical and Electronics Project by Ravi Devani

INVERTER MODEL

Voltage fed inverter receives DC voltage at one side and convert it to AC voltage. The AC voltage and frequency may be variable or constant depending on the application. Voltage fed inverter is used extensively for AC motor drive, induction heating, UPS etc. In voltage fed inverters, transistor family devices are generally preferred because of self-commutation and high switching frequency. When a new converter circuit is developed, or a control strategy of a converter or drive system is formulated, it is often convenient to study the system performance by simulation before building the breadboard or prototype. The simulation not only validates the system’s operation, but also permits optimization of the system’s performance. Besides control and circuit parameters, the plant parameter variation effect can be studied. Fig. 1 shows the simple simulation block diagram for a three-phase, two-level PWM inverter. Each leg of the inverter is represented by a “switch” which has three input terminals and one output terminal. The output of a switch (Va0, Vb0 or Vc0) is connected to the upper input terminal (+0.5 Vd ) if the PWM control signal (middle input) is positive. Otherwise, the output is connected to the lower input terminal (-0.5 Vd). The output or Va0 voltage thus oscillates between +0.5Vd and -0.5Vd, which is characteristics of a pole of an inverter. The output phase voltages are constructed by the following equations:

 

Fig. 1 Block diagram codes for modeling of inverter

INDUCTION MOTOR MODEL

State space equations are used to solve linear and nonlinear system equations. In state space representations, any nth order differential equations describing a control system could be reduced to n 1st order equations. These equations are arranged in the form of matrix equations to find the solution easily using software tools. This method is named as the state variable method. The selection of state variables depends upon the system structure and the goals of study. The state variables chosen for induction motor are stator currents and rotor fluxes. Torque is calculated from the state variables and motor parameters. The continuous state space representation of the induction motor is as follows:

 

The state variables are d and q-axes stator currents and rotor fluxes.

 

Speed is calculated from the resultant stator currents and rotor fluxes. The equation for rotor speed is given by

 

The above state equations are constructed in LabVIEW using math script node inside the simulation loop. Steps:

1. Control and simulation loop is created

2. Math script node is constructed inside the simulation loop.

3. First order differential equations are written inside the math script node.

4. Integration is performed to find the different state variables.

5. By using simple arithmetic VIs, the speed and torque are calculated from the stator voltages and currents.

6. Three phase stator voltages are converted into two phase quantities in formula node.

A part of the block diagram codes framed in LabVIEW is shown in Fig. 2. The math script node, simple arithmetic Vis and scope for measurement are illustrated. Fig. 3 shows the speed and torque waveforms created in the front panel using graphical display. Initially the motor runs at no load speed. At t=2.5s, a load of 5 Nm is applied to the motor. As the motor runs at open loop condition, speed of the motor decreases from 1497 rpm to 1480 rpm. The dq-axes stator currents and rotor fluxes are considered as state variables. Fig. 4 shows the dqaxes stator phase currents. The dq-axes rotor fluxes are shown in Fig. 5. The output of the inverter contains harmonics. The dq-axes stator voltages are depicted in Fig. 6.

 

Fig. 2 Block diagram codes for modeling of induction motor

Electrical and Electronics Project by Ravi Devani

    

Fig. 3 Speed and torque waveforms of induction motor

    

Fig. 4 dq axes stator phase currents

Fig. 5 dq axes rotor fluxes

Fig. 6 dq axes stator voltages

CONCLUSION

In this work, the inverter and induction motor are modeled and simulated using LabVIEW. The models are constructed using simple VIs and mathscript node. To prove the effectiveness of the given method, simulation results are provided.

REFERENCES

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Electrical and Electronics Project by Ravi Devani