closed loop – pi control of a single phase induction motor · pdf file ·...

International Journals of Advanced Research in Computer Science and Software Engineering ISSN: 2277-128X (Volume-7, Issue-6)

Research Article

June 2017

© www.ijarcsse.com, All Rights Reserved Page | 123

Closed Loop – PI Control of a Single Phase Induction Motor

Using SPWM Kuheli Ghosh Goswami

EE Department, UCSTA

Assistant Professor, BGI-SDET

Amit Ghosh

Technology Analyst,

Infosys Limited, India

DOI: 10.23956/ijarcsse/V7I6/0169

Abstract— In this paper we discuss the speed control of a single phase induction motor by proportional integral

control. The PI controller fuses the properties of the P and I controllers. It shows a maximum overshoot and settling

time similar to the P controller but no steady-state error. The output of the inverter has been smoothen by using LC

filter. THIPWM techniques have lower total harmonic distortion than the SPWM technique and SVPWM have much

lower THD than that of THIPWM. The THIPWM can increase the fundamental output voltage by 15.5% over the

SPWM technique and SVPWM more than that. SPWM Simulation results are obtained using MATLAB / Simulink

environment for effectiveness of the study.

Keywords— SPWM, ASD, INVERTER DESIGN, PI CONTROL STRATEGIES, MATLAB.

I. INTRODUCTION

Variable Speed Drives (VSDs), also known as adjustable speed drives, are large industrial electric motors whose speed

can be adjusted by means of an external controller. They are used in process control and help saving energy in plants that

use many powerful electric motors.The use of adjustable speed in process control matches the motor speed to the

required tasks and may compensate for changes in the process's variables. The use of adjustable speed for saving energy

is exemplified by the adjusting the speed of a cooling fan motor to match the temperature of the machinery parts it is

cooling.Adjustable frequency drives are a specific type of VSDs; they are controlling the rotational speed of an

alternating current (AC) electric motor by controlling the frequency of the electrical power supplied to the

motor.Sinusoidal PWM is a type of "carrier-based" pulse width modulation. Carrier based PWM uses pre-defined

modulation signals to determine output voltages. In sinusoidal PWM, the modulation signal is sinusoidal, with the peak

of the modulating signal ways less than the peak of the carrier signal.Here a SPWM inverter is designed and DC voltage

is given to it, DC is converted to ac and a single phase induction motor is driven by this ac o/p and speed of this motor is

adjusted by closed loop speed control method. Here PI control strategies has been adopted.

II. THEORITICAL BACKGROUND

Sinusoidal Pulse width modulation (SPWM) generated by comparing amplitude of triangular wave (carrier) and

sinusoidal reference wave (modulating) signal. SPWM switching for three phase inverter is similar to that of a single

phase inverter. Basically, each switch is controlled by comparing a sinusoidal reference wave with a triangular carrier

wave. The fundamental frequency of the output is the same as the reference wave, and the amplitude of the output is

determined by relative amplitudes of the reference and carrier waves. Each pair of switches requires a separate sinusoidal

reference wave. The three phase reference sinusoidal is 120° apart to produce balanced three phase output.

Figure 1: Sinusoidal Pulse Width Modulation

dx.doi.org/10.23956/ijarcsse/V7I6/0169

Goswami et al., International Journals of Advanced Research in Computer Science and Software Engineering

ISSN: 2277-128X (Volume-7, Issue-6)


Figure 2: Three Phase Voltage Source Inverter

Figure 3: pulse width modulation technique.

Inverter output voltage:

When V control>V tri, V dc/ 2

When V control<V tri,-V dc/ 2

Control of inverter output voltage:

SPWM frequency is the same as the frequency of V tri

Amplitude is controlled by the peak value of Vcontrol

Fundamental frequency is controlled by the frequency of Vcontrol.

Modulation Index (m),

Figure 4: waveforms of three phase spwm inverters




BENEFITS OF VVFD

1. Controlled starting current. When an ac motor is started “across the line,” it takes as much as seven-to-eight

times the motor full-load current to start the motor and load. But when using an adjustable speed drive, the

substantially reduced starting current extends the life of an ac motor. The payback is less wear and tear on the

motor, which translates to extended motor life and less motor rewinds.

2. Reduced power line disturbances. Starting an ac motor across the line and the demand for starting current places

an enormous drain on the power distribution system connected to the motor. Typically, the supply voltage sags,

which can cause sensitive equipment connected on the same distribution system to trip. Items such as computers,

sensors, proximity switches and contactors are voltage sensitive and can drop out when a large ac motor is started

nearby. Using an adjustable speed drive eliminates this voltage sag.

3. Lower power demand on start. The power needed to start an ac motor across the line is significantly higher than

with an adjustable speed drive. This is true only at start, since the power to run the motor at load would be equal

regardless if it were fixed speed or variable speed

4. Adjustable operating speed. Using an adjustable speed drive enables process optimization and making changes in

a process. It also allows starting at reduced speed and allows remote adjustment of speed by programmable

controller or process controller.

5. Controlled stopping. Controlled stopping helps reduce product loss due to breakage or mechanical wear and tear

attributed to shocks to the process.

6. Energy savings. Centrifugal fan and pump loads operated with adjustable speed drives reduce energy

consumption. If the speed of a fan is cut in half, the horsepower required to run the fan is cut by a factor of eight.

The energy savings could relate to a return on investment of less than two years.

ADVANTAGES OF SPWM

The main advantage of SPWM is that power loss in the switching devices is very low. When a switch is off there is

practically no current, and when it is on,there is almost no voltage drop across the switch. Power loss, being the product

of voltage and current, is thus in both cases close to zero. SPWM also works well with digital controls, which, because of

their on/off nature, can easily set the needed duty cycle.

DISADVANTAGES OF SPWM

There are two significant drawbacks with sinusoidal PWM.

(i) Available output voltage

Assuming that the DC voltage is created using a diode rectifier and capacitor dc link , the maximum available DC

voltage is given by

where VLLS is the line-line supply voltage. The maximum output using sinusoidal PWM (ma=1) is

a sinusoidal PWM drive cannot produce a line-line output voltage as high as the line supply. One option to mitigate this

discrepancy is to use higher supply voltages (e.g. 480V supply, 460V motor; 600V supply, 575V motor).

(ii) Short Pulses If the output is to be truly sinusoidal PWM, it is important to include very small pulses when the peak modulation signal

is close to the peak carrier voltage. These small pulses can contribute significantly to inverter losses, while not

significantly effecting the output voltage. In addition, small pulses may be impractical due to the time required to switch

one device off and another device on. Most industrial drives "drop" small pulses to improve efficiency.

III. SOFTWARE IMPLEMENTATION & EXPERIMENTAL RESULTS

The main aim of any modulation technique is to obtain variable output having maximum fundamental component with

minimum harmonics. The objective of Pulse Width Modulation techniques is enhancement of fundamental output

voltage and reduction of harmonic content in Three Phase Voltage Source Inverters. In this paper different PWM

techniques are compared in terms of Total Harmonic Distortion (THD). Simulink Models has been developed for

Sinusoidal PWM (SPWM), Space vector PWM (SVPWM), and Space vector PWM switching Patterns. Simulation work

is carried in MATLAB 13.0/Simulink. The simulation parameters used are: Fundamental frequency 50 Hz

Switching frequency 2 kHz

Amplitude modulation index 1.0

DC voltage 600 Volt

ph. Asynchronous machine: 415 V, 0.5 HP.

ODE Solver ode23tb




Figure 5: Circuit diagram for PI Control of a single phase induction motor

Figure 6: Unipolar SPWM output without Filter

Figure 7: Unipolar SPWM output with LC Filter




Figure 8: SPWM Technique

Figure 9: Motor speed and Torque

IV. CONCLUSIONS This report introduced adjustable speed drive system of a single phase induction motor. We presented SPWM inverter

and control its speed. When we are changing the load, the voltage and frequency are changed, but voltage upon

frequency ratio is constant so speed is remaining constant. Thus we have adjusted the speed and we have designed

adjustable speed drive system incorporating SPWM Technique.

But, while doing the project we got some error, LOSS OF VOLTAGE- 17.26%

Actual voltage- 230V Practical value- 190.3V Harmonics are present.

Harmonic voltages produce magnetic fields that rotate at speeds corresponding to the harmonic frequencies, resulting in

increased losses, motor heating, mechanical vibrations and noise, pulsating torques, increased eddy current and hysteresis

losses in stator and rotor windings, reduced efficiency, reduced life, and voltage stress on motor winding insulation.

Proper speed control means that we need to vary both the motor voltage and frequency to control slip and keep it in a

sweet zone in relation to the actual rotation speed. This is what Variable Speed Drives do, for single phase induction

motors this idea does not really work.With just one phase, we get a pulsing voltage only. For this reason single phase

motors artificially create a second phase to create the rotation. This second phase can be created in many ways using

auxiliary windings and phase shift capacitors or coils.All proper variable speed drive controlled induction motors are 3

phase, as are the outputs of the variable speed drives. 3 phase shifted voltages is the minimum required to create a

rotating field.

But still for harmonic reduction tuned banks will be needed to totally divert all harmonics away from the system.

For better approach PLC, fuzzy logic can be used.




ACKNOWLEDGMENT

I would like to express my gratitude to my professors for their guidance, support and suggestion. I am extremely grateful

to the “Applied Physics Dept., University of Calcutta” and EE department of Brainware Group of Institutions. I humbly

express my sense of gratitude to the faculty members, Laboratory staff, Library staff and administration of the institute

for providing me a congenial working environment.

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Induction Motors (IEEE)

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