Speed Control of an Induction Motor Using Field

Oriented Control Technique

Conan Michael Reynolds

M.Tech. Power Electronics and Drives

VIT University, Chennai Campus

Chennai, India

conanreynolds@yahoo.co.in

AbstractThis paper shows the use of indirect field oriented

control for the speed control of an induction motor

drive. The control system is modeled and simulated

using a 3HP induction motor. The speed and torque

characteristics will be observed to assess its

performance.

Keywordsindirect field oriented control; induction motor

I. INTRODUCTION

The most widely used motor in the industry nowadays is the three phase induction motor. They have higher mechanical strength or robustness, lower prices, higher reliability and higher efficiency when compared to other motor types. However, their complex dynamic modeling, nonlinear characteristics at saturation and electrical parameter variation, which is dependent on the temperature, makes its use challenging. Such factors make the control of an induction motor difficult and complex. For this reason effective and efficient control techniques are required for motor drives.

There are generally two types of control for motor drives: (1) scalar control, and (2) vector control.

In scalar control of motor drives, good steady state performance can be achieved, but the dynamic performance is poor. This is due to the variation of the flux linkages in the air gap from their set values. This deviation takes place not only in magnitude, but in flux as well.

Vector control, also known as field oriented control (FOC), offers good steady as well as dynamic state performance. This is because, in scalar control, there is a coupling effect as flux and the torque of the machine are related to currents or voltages and the frequency. Field oriented control decouples this effect. This means that the torque control does not affect the flux parameters. Also, using FOC, four quadrant speed control of the motor can be achieved without additional control required.

There are two types of field oriented control: (1) Direct field oriented control (DFOC) In this type of FOC, the rotor flux vector is directly measured, either by using a flux sensor placed in the air gap, or by using the voltage equations of the motor electrical parameters.

(2) Indirect field oriented control (IFOC) Here, the rotor flux vector is calculated based on equations. For these calculations, the rotor speed is measured.

The method implemented in this paper will be the indirect field oriented control. This is due to the ease of speed measurement with the help of a shaft position sensor.

II. INDIRECT FIELD ORIENTED CONTROL METHODOLOGY

The IFOC requires the following parameters to be measured: (1) Instantaneous phase currents of stator - ia, ib, and ic. (2) Mechanical speed of the rotor. This can be achieved by use of appropriate current sensors and speed measurement devices such as tachometer or shaft position sensor.

This control strategy is based on the FOC algorithm, which is as given below:

Stator phase currents ia, ib, and ic , are measured. Or if only ia and ib are measured, then ic can be calculated as ia + ib + ic = 0 for balanced currents.

These three phase currents are then transformed to the two axis system to give id and iq.

The rotor flux and its orientation are then calculated.

This estimated flux value, along with a reference flux is used to calculate the flux error signal. Using this error value, a PI controller will calculate i

*d.

i*q is calculated from the estimated flux value and a reference torque value.

i*d and i*q are then transformed back to a set of

three phase currents t produce i*a, i

* b, and i

* c.

i*a, i* b, i

* c and ia, ib, ic are compared using a

hysteresis comparator and the corresponding inverter gate signals are generated.

III. SYSTEM MODELING

The indirect field oriented controlled induction motor drive system comprises of four main parts: (1) the three bridge current controlled IGBT inverter, (2) the field oriented

controller block, (3) a speed controller, and (4) the induction motor.

The inverter gate pulses are generated from the FOC based on the algorithm. Its output is used to supply the induction motor. The speed controller is used to deliver the reference values of flux and torque required by the FOC for various calculations. The abbreviations and symbols used are explained in the appendix section of this paper.

A. FOC Modeling

The block diagram of the field oriented controller is as

shown below:

Fig. 1. Block diagram of field oriented controller

(1) r calculation

The motors rotor flux is estimated in this block. The equation used for calculation is given by:

r = Lm id / (1+Tr.s) (1)

(2) e calculation

The phase angle of the rotor flux is calculated here using the equation:

e = m + r (2)

Where, r = Lm id / Trr

(3) i*q calculation

This block uses the torque reference and estimated rotor flux to calculate the stator current q component using the following formula:

i*q = Te / (Kte* r) (3)

Where, Kte = (2/3)*P Lm / Lr (4)

(4) abc dq block

This block performs the Park transformation for the abc to dq two axis reference frame

(5) dq abc block

This block performs the inverse Park transformation to convert the i

*d and i

*q to three phase i

*a, i

* b, and i

* c.

(6) i*d calculation

Here, the error signal between the reference flux and the estimated flux are given to a PI controller and the output of the PI controller is used to calculate the stator current d component with the formula:

i*

d = (1 / Lm) * error (5)

(7) Current regulator

The current regulator is a hysteresis controller used to calculate the difference between i

*a, i

* b, i

* c and ia, ib, ic and

the output of this regulator is fed as the gate pulses to the inverter.

B. Speed Controller Model

The block diagram of the speed controller is as shown below:

Fig. 2. Block diagram of speed controller

Here the measured speed and the reference speed are taken as inputs. The difference is fed to a PI controller and torque limiter, which is used to obtain a reference torque used as an input to the FOC block.

The measured speed is passed through a low pass filter and fed to a flux table or flux function. This flux function calculates a reference flux from the measured speed based on the formula:

flux* = bs*nf / |N| (6)

Where, bs = 2*30*fn / P (7)

C. Induction Motor

The induction motor used is a 3HP, 380V, 1425rpm three

phase motor. Its ratings and various parameters are listed in

the table below.

TABLE 1. INDUCTION MOTOR PARAMETERS

Parameter Value

Phase 3

No. of poles 4

Nominal frequency 50Hz

Line to line voltage 380V

Stator resistance 3.5

Rotor resistance 3.16

Core loss resistance 701

Mutual inductance 0.26674H

Stator magnetizing leakage

inductance 6.9mH

Rotor magnetizing leakage inductance

6.8mH

IV. MATLAB SIMULATION OF FOC SYSTEM

The system was designed and simulated using MATLAB Simulink. A variable torque load was applied and the results were obtained for rotor speed(m), electromagnetic torque (Te), and stator currents isa, isb, and isc. The complete Simulink model of the IFOC based induction motor drive system is as shown below:

Fig. 3. MATLAB Field Oriented Controlled Induction Motor Drive Model

The FOC block design in MATLAB can be seen as shown

below:

Fig. 4. MATLAB Field Oriented Controlled Block

Fig. 5. Simulation Output

The speed control block diagram can be seen below:

Fig. 6. MATLAB Speed Controller Model

The result for simulating the designed system for a time period of 4s was observed and recorded as shown in Fig. 5.

V. CONCLUSION

The developed model was successfully simulated and

results obtained. Various methods of induction motor drive

control were studied. Future works will include study and

implementation of more control strategies and a fault tolerant

system will be developed to control the drive, which can be

implemented in applications such as hybrid electric vehicles.

VI. APPENDIX

Abbreviations and symbols used, their significance and units: 1) r = estimated rotor flux (Wb)

2) Lm = Mutual inductance of motor (H)

3) Lr = Rotor inductance (H)

Lr = Llr + Lm

Where, Llr = rotor magnetizing leakage inductance (H)

4) Tr = Rotor time constant (s)

Tr = Lr / Rr

Where Rr = rotor resistance()

5) e = Rotor flux phase angle ()

6) m = Measured rotor speed (rad/s)

7) r = estimated speed (rad/s)

8) Te = Reference torque (N-m)

9) Kte = Torque constant

10) P = No. of pole pairs of machine

11) fn = Nominal frequency of machine(Hz)

12) nf = Nominal flux of machine(Wb)

13) N = Speed of machine(rpm)

REFERENCES

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