Induction Melting of Metals

S. A. Mansoor

Batch melting of metals using an induction furnace provides better operating flexibility, better yield and overall cost improvement. Further the cost of environment control decreases. The chief overall benefit is cost reduction compared to melting in an electric arc furnace. Arc melting temperatures are significantly higher than induction melting. Arc melting temperatures are around 3000°C compared to around 1500°C for induction melting. However the localized high temperature in arc furnace melting enables it to melt dirtier scrap than in induction melting. However dust, fume, slag and refractory losses in arc melting are far higher.


Furnace Power Supply
Power supply package of an induction furnace provides power and the necessary control required for melting. Early induction furnaces functioned at line frequency with power from a special transformer and tuning circuits. Switching capacitors regulated power factor adjustments and transformer tap change controlled the power level. For maximum power in melting the resonant frequency of a tuned LC circuit had to match with line frequency. This limits coil current and hence the furnace efficiency. Early induction furnaces were usually single-phase loads on a three phase utility power supply limiting the supply capability.

Presently power supply through three phase converters operating at high power factors provides increased power ratings. These also precisely control frequencies and depth of penetration to effectively melt metal without over-stirring the melt. Also the variable frequency power supply available can match the varying electrical characteristics of different charged materials. Electronic power supply to modern induction furnaces eliminates the need to maintain a molten heel between charges; a significant advantage!


Power Supply Circuit
Most electronic power supplies to induction furnaces rectify AC line current to provide a DC source. This DC is inverted at a frequency to obtain desired induction from the furnace resonant circuit. Two main types of power supplies are voltage fed power supply and current fed power supply shown in Figure 1 and 2.
 

 Fig 1. Voltage Fed Power Supply (6 Pulse Bridge)

Fig 2. Current Fed Power Supply (6 Pulse Bridge)

Both types of power supply are used in medium frequency induction furnaces. The impact of these two power supply system and their performances is tabulated below:
 

Characteristics	Current Fed Inverter	Voltage Fed Inverter
1. Melt Controllability	Poor	Excellent
2. Efficiency of Melt	70 ~ 80%	75 ~ 85%
3. Power-Line Interface	Phase Controlled Rectifier	Diode Rectifier
4. DC Energy Storage	Inductive, Dynamic	Capacitive, Static

A disadvantage of current fed power supply is voltage notching as seen in Fig 2. Notch propagating in a plant electrical system causes equipment operating problem. Most common is tripping other power supplies and DC drives.

Large three phase power supplies for induction furnaces are also responsibility for changing load currents which can mitigate utility line voltage regulation and quality. Effect of Single Rectifier Bridge power supply (Fig 1 & Fig 2) is tabulated below:

Both the current and voltage fed inverter generate harmonics back into grid supply, when rectifying AC to DC. Large furnaces are provided with more than one rectifier bridge along with phase shifting transformers. This brings down the current per bridge and the level of harmonics in the current drawn from the utility. A two bridge circuit is shown in Fig 3.

 

Fig 3. 12 Pulse Bridge Rectifier


Increasing number of rectifier bridges makes the waveform of line current more sinusoidal. Power factor improves as numbers of pulses are increased. Expected power factor for full wave rectifier with different number of pulses is tabulated as under:

 

Conclusion

Better control of induction furnace frequency and power is needed to avoid adverse effect to the utility supply. Impact of utility voltage sags, momentary interruptions and switching transients needs to be better understood and prevented to operate melting processes with sensitive process control systems incorporated. Effect of inter-harmonics to the power supply needs to be measured and better elimination methods required as more induction melting of metal becomes prevalent in engineering industries.

S. A. Mansoor : Director, Engineering, Partex Group



China to Build New Generation Nuclear Reactor

China will begin building a revolutionary "pebble-bed" nuclear reactor this year with the aim of making the technology commercially viable by 2020, state press reported.

Construction of the 190 megawatt reactor will begin near Weihai city in eastern China's Shandong province with the production of electricity slated for 2010, the China Daily reported.

The cost of the reactor, which the Beijing Institute of Nuclear Engineering is developing, will be 370 mln usd, the paper said.

The plant will be the first radically new reactor designed globally in decades, previous reports said.

It will put China at the forefront in nuclear energy research that offers a "meltdown-proof" alternative to conventional nuclear power stations.

"Pebble bed" reactors are fueled by thousands of small graphite balls with minute uranium cores, which provide the fuel for the nuclear reaction.

The technology is said to be proliferation proof, meaning that spent fuel cannot be reprocessed to make weapons-grade uranium.

The new technology is still not commercially viable as costs remain much higher than conventional pressurized water nuclear power technology, Liu Wei, vice president of the Beijing institute, told the paper.

"As the research evolves, the new technology could be competitive in 2020 or 2030," he was cited as saying.

China Huaneng Group, parent of the Hong Kong-listed Huaneng Power International Inc, will own half of the project, while the China Nuclear Engineering and Construction Corp will take a 35 pct stake and Beijing's Tsinghua University will take five pct, it said.

The owner of the remaining 10 pct has yet to be determined, it said.

The modular design of the reactor means that future 190 to 200 megawatt pebble bed reactors could be built in factories, then transported to sites and assembled together to make a power plant of up to five reactor modules.

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