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Research of the ship and marine engineering structure induction heating flattening machine

Xiang He, Chenfu Fang, Yong Chen, Zhidong Yang,

Feifei Han, Ruixiang Ma, Yu Gao, Lianbin Li

( School of Materials Science and Engineering, Research

Institute of Marine Equipments, Jiangsu University of Science

and Technology, Zhenjiang 212003, China)

Abstract: The ship and marine engineering structure

induction heating flattening machine uses high frequency AC

outputted by the induction heating power source, generating

induced eddy current in the distorted hull steel structure

by special sensor, and then the eddy current produces

resistance heat，which can play the role of deformation

correction in the heating process. This new heating method

has advantages of high efficiency, short heating time,

mechanization, automation, safety, reliability and non-

pollution, which has great economic and social benefits and

broad market prospect. The induction heating flattening

machine in this paper is mainly composed of a power

regulator, an inverter and a frequency tracker. The power

regulator adopts a DC chopper topology, and the inverter

adopts a full bridge circuit topology. The frequency tracker

adopts a control system based on PLL(phase locked loop)

and UC2825. The results show that the induction heating

flattening machine can adjust the power stably and output

maximum power at the switching frequency of 30~35kHz.

Key words: induction heating; power regulator; inverter;

frequency tracker

DOI: 10.7512/ j.issn.1001-2303.2017.13.22

0 IntroductionIn the process of shipbuilding engineering, each section of the hull

structure is mainly connected by welding method. During the welding

process, due to the uneven heating of the arc, it will inevitably cause

partial or integral welding deformation. The large volume and quality

of the hull structure limits the method of flattening by mechanical

means. Therefore, flame heating is the main method of hull structure

flattening in China at present. Flame heating flattening method has

many disadvantages, such as long heating time, over-burning, high

requirements of the operator's experience and skill, low flattening

stability and uniformity, etc. On the other hand, the flattening process

will produce toxic gases and dust pollution, which is harmful to the

environment, and the acetylene and liquefied gas exist security threat as

well. In this paper, the ship and marine engineering structure induction

heating flattening machine, by using electromagnetic induction and

eddy current thermal effect principles, levels hull structure deformation.

Chenfu Fang, the corresponding author, was born in 1954. He is Doctor of Engineering ( D.Eng ), Professor, Ph.D supervisor, and a specialist who enjoys the State Council Special Allowance. At present, he is the deputy director of The National Welding Standardization Technical Committee (SAC/TC 70 ), Fusion Welding Technology and Equipment Professional Committee of China Welding Association, and Welding Technology Professional Committee of China Electrical Society. Professor Fang has completed more than 30 national, provincial, ministry and enterprise major scientific research projects, in which he obtained first prize once and second prize four times of the provincial and ministry science and technology award. He has also been authorized 10 national patents and has published more than 60 papers in both domestic and international academic journals, teaching 10 Undergraduate, Master and Doctor courses and instructing more than 60 masters and Ph.Ds. He guided the students to participate in the national college students' science and technology competition and won the national first prize and Jiangsu Province awarded grand prize and he obtained the National Excellence Guide Teacher Award.

Prof. Chenfu FangEmail: 13952846832@163.com

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This new method has advantages of high efficiency, short heating time,

mechanization and automation, safety, reliability and non-pollution,

which has great economic and social benefits and broad market

prospect. The induction heating flattening machine in this paper is

mainly composed of a power regulator, an inverter and a frequency

tracker.

1 Design of the power regulatorThe power regulator has the function of energy transfer and power

conversion, which is an important component of the induction heating

flattening machine. The power regulator adopts DC chopper converter

and the control circuit uses industrial grade UC2846 PWM chip,

constituting the closed loop control system, which has fast response

speed and good adaptability to nonlinear load. The designed power

regulating system can be effectively applied to high frequency induction

heating occasion. The principle block diagram of circuit structure is

shown in Fig.1.

Fig.1 The principle block diagram of power regulator

The UC2846 peripheral circuit includes soft start circuit, current

feedback circuit, voltage feedback circuit, slope compensation circuit

and fixed frequency circuit, constituting the closed-loop control system.

The system has high control precision and fast response speed, which is

suitable for high frequency, high power applied occasions.

2 Design of the inverter and frequency trackerThe function of the inverter is to convert the DC outputted by

the power regulator to high frequency AC, which is required in the

induction heating process. In this paper, the designed outputted power

of induction heating flattening machine is 30 kW, which is suitable

for the full bridge inverter main circuit topology. The principle block

diagram of circuit structure is shown in Fig.2.

The voltage type series resonant inverter can output stable square

wave voltage and sine wave current, working in the partial inductive

state, which has simple protection circuit and stable output power.

As a result, it is suitable for the occasions of frequent starting. The

outputted signal of the inverter passes through sampling circuit and

zero comparison circuit, and then inputs the frequency tracker based on

PLL and UC2825, following inherent frequency of the resonant circuit.

In the heating process of induction heating flattening machine, due

to the influence of the temperature, heating time, current, etc. The

inherent frequency of the resonant circuit appears nonlinear variation,

and the switching frequency deviates from the inherent frequency, so

that the inverter cannot work at resonance state. In the high frequency

and power occasion, the loss of switch tube and electrical stress increase

greatly, reducing the overall quality and reliability. At the same time,

the hard switch working state also reduces the output power factor, so

that the design capacity cannot be effectively utilized. In this case, the

application of frequency tracking technology is particularly important.

Through appropriate closed-loop feedback control circuit, the switching

frequency of always follows the change of inherent frequency, so that

the circuit can work at resonant or quasi resonant state. The phases of

voltage and current are the same and the power factor of the circuit is

close to 1, obtaining the maximum power output.

The frequency tracking function can be realized in two steps: PLL

completes frequency locking and then UC2825 outputs two switch

signals with the complementary phase and same frequency. The

frequency of the UC2825 outputted signal can be determined by

external timing capacitor and resistor, and can also be determined

by the external signal frequency. This paper adopts the second way,

regarding the PLL output signal as the UC2825 external signal, as

a result, the UC2825 outputs two switching signals with the same

frequency of PLL.

3 The waveform and process testsThe outputted waveforms of the power regulator, inverter and

frequency tracker are tested by Tektronix TPS2024 oscilloscope.

Fig.3 shows the test results, among which Fig.3(a) and 3(b) are the

switching tube waveforms of power regulator. Channel 1 is the current

waveform passed through switching tube, and channel 2 is the voltage

waveform of the collector electrode, while channel 3 is the voltage

waveform of the driving signal.

Fig.3(c) is the inverter's output voltage and current waveform

in the maximum condition, among which Channel1 is the voltage

waveform, while Channel 2 is the current waveform. It can be seen

that the output voltage waveform of the inverter is square wave,

and the current waveform is sine wave, which is the typical output

waveform of the series resonant induction heating power source. The Fig.2 The principle block diagram of inverter and frequency tracker

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phases of the output voltage and current are the same, and the circuit

reaches resonant or quasi resonant state. The power factor is close to 1,

obtaining the maximum power output.

Fig.4 shows the heating process of the induction heating flattening

machine. It can be seen that the machine can heat steel plate through

the electromagnetic induction principle. Before heating process, the

steel plate has a bit bending deformation, and the steel plate reaches

flat state after deformation correction by the induction heating

flattening machine, which indicates that the designed machine can

achieve the function of steel structure flattening.

4 Conclusion(1) The power regulator of induction heating flattening machine is

designed, which adopts the DC chopper circuit topology, realizing the

function of stable and adjustable outputted voltage;

(2) The inverter of induction heating flattening machine is designed,

which adopts the full bridge inverter circuit topology, realizing the

function of electrical energy conversion, and the high frequency AC is

used to induction heating;

(3) The frequency tracker of induction heating flattening machine is

designed, which adopts the control system based on PLL and UC2825,

realizing the function of switching frequency tracking the inherent

frequency at any moment;

(4) The ship and marine engineering structure induction heating

flattening machine can adjust the power stably and output maximum

power at the switching frequency of 30~35kHz.

AcknowledgmentsThe authors are grateful to the financial support for this research

from the National Natural Science Foundation of China (grant

NO.51575250 and NO.51275224), Prospective Joint Research

Project of Jiangsu Province of China ( grant NO.BY2015065-06),

Collaborative Innovation Center of Ship Technology of China (grant

NO.HZ20170004).

(a) In the medium power condition

(b) In the maximum power condition

(c) The waveform of inverter

Fig.3 The tested waveform of power source system

(a) The induction heating flattening machine

(b) Process test

Fig.4 Process test of the induction heating flattening machine

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