MATLAB based Performance Prediction of a Three-Phase Induction Motor Utilising its Test Results

MATLAB based Performance Prediction of a Three-Phase Induction Motor Utilising its Test Results

Jayanta Bhusan Basu

Assistant Professor ,Department of EE Siliguri Institute of Technology Siliguri, West Bengal, India

Abstract The most common type of A.C motor being used throughout the world today is the induction motor. Induction motors are singly excited motor which drive loads at fairly constant speed This present study discusses about the development of a Simulink model of a three phase induction motor block which takes the inputs in form of the test results obtained by the No-load , Blocked rotor & DC resistance tests and simulates to give the equivalent circuit parameters as well as various operating characteristics.

Keywords Induction motor, No-load test, Blocked rotor test, DC resistance test, MATLAB, Simulink

1. INTRODUCTION

   In 1893 Dolivo Dobrowolsky developed an induction motor, with a distributed stator winding and a cage rotor [1]. The motor was developed based on the principle of rotating magnetic field, produced by using poly phase currents, as explained by Tesla and Ferraris in 1889. The advantages of induction motor over other types of motors are,

   It requires less maintenance

   These are more rugged & less expensive in comparison to the d.c machine of equal kilowatt & speed ratings.

   The induction motor derives its name from the fact that it operates on the principle of induction. The rotating magnetic field generated in the stator induces a magnetic field in the rotor. The two fields interact and cause the rotor to turn in the same direction as the rotating stator field. . The rotor receives power from the stator due to Induction. The rotor is not connected to an external source of voltage. To obtain maximum interaction between the fields, the air gap between the rotor and stator should be very small.

   A three-phase squirrel-cage induction motor is one of the more readily used motors because it operates with its best efficiency and highest power factor at rated load. The motor is also more reliable since there are no brushes or slip rings to wear out. Consequently, there are no sparks produced, so this motor can be used in areas where volatiles are present.

   Squirrel cage induction motors are generally used in low- speed applications. In fact, the speed of the rotor decreases as the load applied to the motor increases. For any given induction motor, the parameters of rotor speed, power factor, efficiency, and torque plotted versus output power provide all of the necessary information to determine the operating points

   for that motor. The operating characteristics of a squirrel cage induction motor are determined experimentally by the blocked rotor test, the no-load test, and the load test..

2. INDUCTION MOTOR TESTS

   While selecting a proper motor it is necessary to know the various operating characteristics as well as the equivalent circuit parameters of the motor. For this purpose following tests are conducted on a three phase induction motor. No-Load Test Balanced voltages are applied to the stator terminals at the rated frequency with the rotor uncoupled from any mechanical load. Current, voltage and power are measured at the motor input. The losses in the no-load test are those due to core losses, winding losses, windage and friction.

   Blocked Rotor Test The rotor is blocked to prevent rotation and balanced voltages are applied to the stator terminals at a frequency of 25 percent of the rated frequency at a voltage where the rated current is achieved. Current, voltage and power are measured at the motor input.

   In addition to these tests, the DC resistance of the stator winding should be measured in order to determine the complete equivalent circuit.

3. SIMULINK IMPLEMENTATION

   The induction motor model developed is shown in Figure 1.

   Figure 1: The Induction motor block

   The Thevenin voltage (open-circuit voltage) for the stator portion of the equivalent circuit (to the left of the air gap) is

   V V

   jxm

   1 1

   TH 1 r j(x x )

   m

   The Thevenin impedance (impedance seen after shorting V1 ) is

   Figure 2 : Different blocks of the developed model

   It consists of three major blocks:.

   1. The Circuit parameter estimation Block

      This block calculates the equivalent circuit parameters

   2. The Torque estimation Block

      This block calculates the shaft torque of the motor.

   3. Stator Current-Power-Efficiency estimation Block

   This block calculates the shaft power, stator current & efficiency of the motor. While calculating the above parameters the equivalent circuit model is approximated to as shown in Figure 3.

   Figure 3 : Equivalent circuit diagram of the Induction motor

   Here the Core loss resistance value is neglected, as it is very high. In order to simplify the determination of torque and power equations from the induction machine equivalent circuit, the network to the left of the reflected components is replaced by a Thevenin equivalent source.

   ZTH jxm || (r1 jx1 ) RTH jXTH

   Inserting the Thevenin equivalent source into the induction machine equivalent circuit yields the approximated equivalent circuit as shown in Figure 4.

   Figure 4 : Approximate equivalent circuit with Thevenins equivalent source & impedance

4. DIALOG BOX AND INPUT PARAMETERS

   To use the model data is fed through a dialog box as shown in Figure 5. The input parameters are,

   • Nominal Power power rating of the motor in Kw

   • Rated Voltage rated voltage of the motor

   • Frequency – operating frequency

   • No. of poles – no. of poles in motor

   • Stator Connection – stator connection type (e.g. Delta or Star)

   • Class of Motor – motor class (e.g. Squirel cage Class A, B, C, D)

   • No Load Line Current, Voltage & Power No-load test results

   • Blocked Rotor Line Current, Voltage & Power Blocked rotor test results

   • Stator Resistance – measured stator resistance by DC test.

   Figure 5 : Input Dialog Box

5. SIMULATION AND RESULTS

The model was used to simulate the performance of a three phase, 4 pole, 3 hp, 50 Hz, 5 A, 415 Volt , class A squirrel cage induction motor whose Test results are

No-Load Test

Stator Line to line voltage = 415 V Stator Line to Line Current = 3.04 A Power input to Stator = 232 watts Speed = 1486 rpm

Blocked Rotor Test

Stator Line to line voltage = 84 V Stator Line to Line Current = 5 A Power input to Stator = 433 watts

DC resistance Test

Stator Resistance = 7.5 ohms

On conducting the simulation using the test results various operating characteristics obtained are shown in Figure 5. These characteristics will predict the operational behaviour of the motor.

Figure 6 : Various performance characteristics

V. CONCLUSIONS

The developed Simulink Model is modular and easy to use. The model can be a very handy tool in conducting undergraduate laboratory. The block developed can be used as a library block for further usage. The future scope of work includes developing similar models for other machines and improvement in parameter calculations by removing the approximations made during creation of the model. The developed model file can be downloaded from http://www.mathworks.com/matlabcentral/.

REFERENCES

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3. Erxuan Liu – GUI Development for a Harmonic Measurement Technique

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