MATLAB Simulation of a Variable Speed Controller for a Three Phase Induction Motor

26th IEEEP Students’ Seminar 2011

Pakistan Navy Engineering College

National University of Sciences & Technology

Shezana Zulfiqar Ali, Azka Khalil, Sumayyah Waheed,and Arshad Aziz

Department of Electronics and Power Engineering

National University of Sciences and Technology (NUST) H-12, Islamabad

Pakistan Navy Engineering College(PNEC), Karachi-75350, Pakistan

[email protected], [email protected], [email protected], [email protected]

Abstract— Speed control for induction motor is an essential

part of today’s industry. Conventionally we use mechanical

methods for speed control like gear box which are getting

obsolete. Now a day’s digital approach is used because of its

higher reliability and energy conservation. This paper

describes the development and simulation of a speed

controller for a three phase squirrel cage induction motor

using MATLAB/ SimulinkTM

. In this work we use Direct

Torque Control (DTC) strategy, which is known to produce

quick and robust response in speed controllers. The AC4

Simulink blockset is selected from SimPowerSystemTM

library

and is used to develop our controller. Internal parameters of

the system are controlled to obtain the required output. The

AC4 block set integrates all the required subsystems including

rectifier, inverter, speed and Direct Torque Controller. The

system is simulated and the torque and speed outputs of the

controller are obtained.

Keywords- MATLAB; Simulink; three phase power; squirrel

cage induction motor; Direct Torque Control; speed controller;

inverter; rectifier

I. INTRODUCTION

Three phase induction motor is the prime mover for all

the major industrial applications, covering each stage of

manufacturing and processing. These motors are popular

due to their simplicity, reliability and low cost.

The squirrel cage motors are robust because the only parts

of the motor that can wear are the bearings. Unlike DC

motors slip rings and brushes are not required for such

motors. Furthermore, their high power capability gives them

an edge over Single-phase AC motors.[1][2]

When 3-phase AC power is supplied to stator terminals of

an induction motor, 3-phase alternating current flows in the

stator windings. These currents set up a rotating magnetic

field (flux pattern) inside the stator, known as stator

magnetic field Bs. this magnetic field rotates at synchronous

speed, ns. [1][3]

(1)

Where:

f is the system frequency

p is the number of poles

The rotating magnetic field Bs induces a voltage in the

rotor. The voltage induced is given by:

(2)

Hence there is flow of a lagging rotor current due to the

inductive element present in the rotor. And this rotor current

produces a magnetic field at the rotor, Br. The interaction

between both magnetic fields produces torque: [3]

(3)

Industrial systems require variable speed for different processes. Thus efficient and accurate methods of speed

controlling are required. Variable speed controllers are used

to control and/or adjust the speed of AC induction motor in

short time and conserving energy too.

Previously, speed had been controlled using various

methods like throttling valves and gearbox but now the new

approach is speed controlling along with energy

conservation.

The mathematical relationship of power and speed depends upon the type of load. For example in variable

torque loads such as centrifugal fans, centrifugal pumps,

HVAC systems etc the horsepower varies as the cube of

speed thus speed controlling results in energy

conservation.[1]

II. MATLAB/SIMULINK MODEL DEVELOPED

The model is developed using the AC4 block of

SimPowerSystems™ library of MATLAB as shown in fig.

1. Fig.2 illustrates its connections with a 3 hp three phase

induction motor (detailed parameters specified in table.1)

using direct torque control (DTC) technique. A three phase

power is supplied to the controller along with speed and

torque reference values. The induction motor is fed by an

inverter which is built using Universal Bridge Block. The

speed control loop uses a proportional-integral controller to

produce the flux and torque references for the DTC block.

Parameters for controller and associated power electronic

devices are specified in fig.3 and fig.4 respectively. The

DTC block computes the motor torque and flux estimates

and compares them to their respective reference. The

comparators’ outputs are then used by an optimal switching

table which generates the inverter switching pulses. The

output of the block displays motor current, speed, and torque

signals.[4]

MATLAB Simulation of a Variable Speed

Controller for a Three Phase Induction Motor




A. Power Electronics Blocks

Following blocks from the power electronics library of

SimulinkTM are used:

1. Three phase diode rectifier

2. Braking chopper

3. Three phase inverter

Three phase AC power is rectified and the DC power

obtained is inverted to get an AC power output of the

desired frequency.

B. Speed Controller

Inputs to speed controller block are the actual motor speed

i.e. N and the speed set point i.e. N* given by the user.

Speed controller compares both these speeds i.e. N and N*

to give flux reference flux* and torque reference torque* as outputs. The outputs of speed controller, flux* and torque*,

are applied to the DTC controller block.

Figure1. Block diagram of MATLAB model for speed controller.

Figure 2. Subsystem of DTC induction motor drive (AC4).

Figure 3. Parameters of Power Electronics Blocks.




C. Direct Torque Control (DTC)

The fundamental principle of DTC is that it uses inverter

switching to directly control the motor flux and torque. DTC

uses motor’s torque and stator flux as primary control

variables, obtained directly from the motor. DTC can

provide accurate control at a very low speed. It eliminates

the need to use a separate sensor for feedback. [5][6]

The measured input variables to DTC block are the line

current, I_ab, and the DC bus voltage, V_abc, which

calculates the exact values of flux and torque by the

following formulae:[4]

(4)

(5)

(6)

(7)

Where:

is stator flux in direct frame

is stator flux in quadrature frame

Rs is stator resistance

is direct axis component of stator current

is quadrature axis component of stator current

is direct axis component of stator voltage

is quadrature axis component of stator voltage

Te is electromagnetic torque

The reference values of flux and torque are then compared

with the calculated values of flux and torque. The Switching

table inside the DTC block contains two lookup tables that select a specific voltage vector in accordance with the output

of the Flux & Torque comparators. The switching table is

made up of two lookup tables responsible for inverter

switching thereby providing new current values to the

motor. This is achieved by the gate pulses produced by the

DTC block.

III. CALCULATIONS

Braking Torque:

(8)

Where:

Power is in hp

Speed is in rpm

Braking torque is in lb-ft

Hence

lb-ft

Nm

TABLE I. PARAMETERS OF INDUCTION MOTOR

Quantity Value

Pole pairs 2

Power 2238 VA

Frequency 50 Hz

Voltage 400 V

Phase 3

Speed 1705 rpm

Current 5.6 A

Stator leakage inductance 0.002 H

rotor leakage inductance 0.002 H

Stator mutual inductance 0.0693 H

Rotor inertia 0.089 (kgm2)

Rotor friction 0.005 (Nms)

Stator resistance 0.435 Ω

Rotor resistance 0.816 Ω

Figure 4. Controller Parameters.




IV. SIMULATION AND RESULTS

In the model shown in Fig. 1 the stator current, the rotor

speed, the electromagnetic torque and the DC bus voltage of

the motor are observed on the scope. The speed set point

and the torque set point blocks are used to define the speed

and torque reference values at specified time.

As the simulation begins, at time t = 0 s, the speed set

point is 500 rpm. The speed controller alters the required

speed by precisely following the acceleration and deceleration ramps. Ramping speed is specified here as 1800

rpm/s. At t = 0.4 s, the full load torque is applied to the

motor shaft while the motor speed is still ramping to its final

value. This forces the electromagnetic torque to increase to

the user-defined maximum value 17.8 Nm and then to

stabilize at 13 Nm once the speed ramping is completed and

the motor has reached 500 rpm.

The speed of motor remains at 500rpm until at time t = 2s,

the controller attains the new set point of 800 rpm.

Accelerating at the specified ramping speed that is 1800rpm/s the motor attains the required speed of 800 rpm.

At t = 4 s, the speed set point is changed to 0 rpm. The

speed decreases down to 0 rpm by following precisely the

deceleration ramp even though the mechanical load is

inverted abruptly, passing from 12.53 Nm to –12.53 Nm, at t

= 1.5 s.

Shortly after, the motor speed stabilizes at 0 rpm.

In the course of the whole simulation the regulation of DC

bus voltage is notable.

V. CONCLUSIONS

The proposed Direct Torque Control strategy is used

because of its higher reliability and quicker response. This

strategy is used to control the speed of a three phase squirrel

cage induction motor. MATLAB/SimulinkTM is used for the

development and simulation of the controller designed. The

simulation results show that the model developed has the

capability of controlling the speed according to the user

defined reference values. This method, especially suitable

for sophisticated industrial applications like centrifugal fans and pumps etc, can conserve energy as well as stabilise the

torque.

VI. ACKNOWLEDGEMENT

The authors of this paper would like to thank the entire

faculty of Electrical Power Engineering at Pakistan Navy

Engineering College, National University of Sciences and

Technology. Special thanks to Mr. Ashraf Yahya, Mr. Nusrat Hussain and Cdr (R) Riaz Mehmood for providing us

with their guidance and valuable time during the entire

research process.

VII. REFERENCES

[1] Malcolm Barnes, Practical Variable Speed Drives and Power Electronics.

[2] A.K. Theraja and B.L. Theraja, Electrical Technology, S Chand.

[3] Stephen J Chapman, Electrical Machinery Fundamentals.

[4] MATLAB help The MathWorks, Inc. Published with MATLAB® 7.11.

[5] R.Rajendran and Dr.N.Devarajan, “FPGA Based Implementation of Space Vector Modulated Direct Torque Control for Induction Motor

Drive,” International Journal of Computer and Electrical Engineering, Vol. 2, No. 3, June, 2010 1793-8163.

[6] ABB technical guide ―Direct torque control.

Figure 5. Simulation results viewed on the scope.

MATLAB Simulation of a Variable Speed Controller for a Three Phase Induction Motor

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