Induction Furnace heating

INTRODUCTION

1.1 GENERAL

This manual is divided into vanious sections that will provide informaion on safety operation mainlenance and funclional descriptions of the systems. Itl is essential that the drawings and data packages included with each system be examined and understood prior to installation or operation of the unit Where a differenco exists between data presented in this manual and an installed unit, the drawings data package have prority over other .

The VIP melting systems desenbed in this manual provide infomation on systems in the 100 o 10000 Hz frequency range with power rating from 15 KW to over 6000 Kw.

Each meling system is composed of four main soctions

– the main power transformer and switch gear,
– a power unit,
– a water cooling system and
– a furnace.

These sections are descnbed separately.

One significant advantage of induction heating or meling aver other methods is that the heat is produced directy in the work pieoe. Most of the induced energy goes toward raising the temperature of the work piece or charge When gaseous, liquid or solid fuels are used to heat melal, the heat is applied to the surface of the charge (or the cruable surrounding the charge) Much of the heat produced escapes as stack or fuel losses without even coming in contact with the work piece or melt and makes no useful contribution to the process.

The following theory of operation and subsequent paragraphs in this section should clarity the principles of induction heating and melting.

1.2 THEORY OF OPERATION

Induction heating is the heating of an electrically conducting object immersed in a varylng magnetic field The object being heated need not be a magnetic material to heat efficiently All that is required is that it have reasonably good electrical conductivity Most ferrous and nonferrous motals can be hoated and melled Inductively Direct induction heating and melting is possible only with conducting materials. The eddy currents induced in the work piece or charge are primarly responsible for the heating.

EDDY CURRENTS

Eddy currents are electrical currents induced by transformer action in the malerial The term eddy ls derived from the action or now of current in swirls or eddies within a solid mass of material. Eddy current losses occur in any conducting material in a varying magnetic field This causes healing even if the materials do not have the magnetic properties associated with iron and steel.

HYSTERISIS

Hysterials is a discontinuity in the values of magnetization in a magnetic matenal due to changing magnalic feld A reversal of a magnetic fiold requires energy The energy converts to heat. The heal gonerated by hysterisis helps to increase the temperature of the magnetic material (ron, steel, nickel) The rate of expenditure of energy (power) increases with an increased rate of revernal (requency) Hysterisis losses disappear ater the metal temperature exceeds its “Curie temperature” .

HYSTERISIS AND EDDY CURRENT LOSSES

Eddy current losses are more important than hysterisis losses in induction funace. The induced flaw of current due to the changing magnetic field of the coll causes heating of the load .

POWER CONSUMPTION

Powar is consumed in a resistance when current flows through the conductor This power is to the square of the current, and does not depend upon the direction of flow Pal’R, where P the rate at which electrical energy is transformed into heat energy This power is proportional w is is the power in watts, I the current in amperes, and R the resistance in ohms Power dra y induced currents circulating in the work piece, melt or charge as described in the following parts of this section.

1.3 THE CORELESS INDUCTION FURNACE

The coreless induction furnace is composed of a refractory container, capable of holding the molten bath, which is surrounded by a water-cooled helical coil connected to a source of alternating current Figure 1-1 is a simplified cross section of a coreless induction funace.

The alternating current applied to the coil produces a varying magnetic field which is with in the helical coil. This magnetic field passing through the charge induces an alternating current in the charge. The current circulating in the charge produces I’R losses which heats the charge .

1.4 THE ELECTRICAL EQUIVALENT OF THE CORELESS FURNACE

Electrically, the charged furnace can be thought of as a transformer with single shorted turn for a secondary. Figure 1-2 shows the electrical equivalent of a charge. Table 1-1 defines symbols used in this section.


Applying a fixed alternating current to the load results in a poor ratio of active power (power which doos work in the load) to apparent power. Expressed another way, the power factor of the loaded coil is very poor

Power factor can be improved by tuning the circuit with capacitors to improve the ratio of active to apparent power. The VIP POWER-TRAK-R-SERIES contain tuning or “tank” capacitors for coil power factor correction. The circuit combination of load coil and tuning capacitors (illustrated in Figure 1-3 A and described in the following paragraphs) is known as the tank circuit.

1.5 TANK TUNING DURING THE PROGRESS OF A HEAT

furnace tank circuit can be likened to a radio receiver, which can be tuned to a given uency At any frequency, for a given set of furnace conditions, there is an optimum value tance that will tune the tank circuit to resonance At resonance, the capacitive reaclance e inductive reactance of the coil. This means that the power factor of the circuit is maximum Figure 1-3A shows a simplified freq of capaci unity and the real power delivered to the load is at circuit equivalent Rc, represents the resistance of the fumace coil Lc -Lr represents that the induclance of the loaded coll, which is almost always less than the inductance of the empty coil (Lc) Rl represents the resistance of the current paths in the load reflected into the coll circuit

 

Figure is an impedance diagram of the resonant circuit of Figure 1-3A Note that the value of the capacitance has been selected such that the capacitive reactance (Xc) is exactly equal to the inductive reactance (Xl). Their vector sum becomes zero To the power supply, the furnace circuit appears to be purely resistive as shown in figure 1- 3C Figure 1-3C, also illustrates that real power is consumed not only in the load (Rl) but also in the resistance of the coil (Rc) This coil loss, as well as the loss of heat conducted from the charge through the Fixare 13 Tank Ciret Equvalent refractory to the coil, makes water cooling of the coil necessary .

VIP “tank circuit” does not operate at unity power factor under any conditions The induclance of the load changes during the melt cycle as the charge melts With older power sources that operate at a fixed frequency, it was necessary to change the value of the power factor correction capacitors to compensate for changes in inductance These changes in capacitance, even if accomplished automatically without operator attention, were made in discreet steps Each step only approximated the value which would yield optimum operating efficiency Another approach to this problem suggests itself. I we could adjust the frequency of the transmitter (the power source) to follow changes in the tuning of the receiver (the furnace tank circuit), we could maintain the desired power factor without changing capacitors When we investigate lhe magnitude of the frequency change required during a melt to maintain optimum tuning, we find that generally it is less than 10 % of the operating frequency.
The VIP power Supply does this automalically . ahows the relative changes in furnace conditions during the course of a melt. Note that output power remains constant ot rated output while frequency increases steadily to compensate for the decrease in inductance that occurs as the bath goes from fresh change to fully molton at pouring temperature. In a VIP power unit, this precise tracking of furnace tuning le accompliahed smoothly withoul operators attention With fixed frequency systems, the required changes are made in discreet steps. Even if these adjustments are made automatically in small Inaremente, oach change yields only an approximation of the point of optimum efficiency which the VIP power supply follows precisely .

1.6 INVERTER BLOCK DIAGRAM ANALYSIS

shows a simple block diagram of a VIP power supply 50 Hz AC power is fed to a fant-acting three phase circult breaker which also serves as the primary means of energizing the equipment and shutting it down. This three phase circuit breaker is also connected to the cablnot door interlocks. Opening a cablnet doar with the circuit breaker ON will cause the breakor to trip to the OFF position The circuit breaker cannot be moved to the ON position unloss the cabinet doors are closed A three phase bridge-rectifer uses power semiconductors to convert the 50 Hz AC input to a DC voltage. This DC is smoothed by the action of the LC filter composed of the current limiting reactor and the filter capacitors. The current limiting reactor also serves to delay and limit any in rush of current caused by a tank circuit short or an inverter malfunction. This allows the fuses or circuit breaker to open before the current reaches a value that would damage diodes or silicon controlled rectifiers (SCRs).

The heart of the inverter section is the power SCRs They are high-speed electronic switches capable of controlling currents of many hundreds of amperes with relatively small input signals or gating pulses The frequency at the output of the inverter is determined by the rate at which these SCRs are fired The finng signals are generated by the control board.

The control board responds to Inputs representing coil voltage, coil current and coll powor to produce gating pulses for the SCRs. This results in either a power level selected by the operator or the maximum possible power input to the fumace tank circuit under a particular set of melting conditions The control board also places limits on inverter operatlon to prevent damage to the SCR dus to high current, or to prevent damage to the furnace power factor correction capactors due to excess voltage The control board also shuts down the unit if a SCR overvolage module (OVP) detects an instanianeous condition which could damage the semiconductors In Secondary Isolated units, the secondary capacitors are adjustable in steps to allow r tuning of the funace tank circult to a frequency within the VIP’s operating range This rough tuning provides the versaulity needed to match vanous coils, melt vanous alloys, and compensate for changes in lining charactenstics. particulany when sintering a new lining Transformer isolation o supplies It isolates the of Ground current during any groundinglearthing and protecls This unit also isolates the and operator from (Line Isol f the melling furnace from the power mains is a feature of VIP power power circuit and operator from ground faults and shocks With provision Leak Detector (GLD) and also GFL (Ground fault Limiter) which limits the fault round faults and shocks with the provision of GLD In primary Isolated units ated units) the supply to VIP unit ia isolated from Main Transformer with a electrostatic

shield between HT & LT winding and the supply voltage to ViP is higher (550 volt OR 460 volt) to reduce the I2R losses of unit and also rating of semi conductors.

1.7 Z-CONTROL

Z-Control is the latest innovalion in VIP control technology The system takes its name from the manner in which the primary control parameter is determined The timing, which sets the renetition rate of the SCR finng pulses, is derived from the signal analyais of inverter current and voltage This analysis is completed each half-cycle of current.

The advantage of this system is that it can respond almost instantly to rapid changes in furnace or lina conditions which can produce dramatic changes in the electrical load seen by the inverter. The ability of Z-control to control this basic inverter parameter on each cycle eliminates the problems that rapid changes in load or line could cause The Z system also provides for convenient Interface with microprocessor and computer-based control systems.

1.8 OVP

The new PoWER-TRAK-R-SERIES VIPs include the over voltage protection (OVP) circuit which eliminates tnps caused by low-level transient noise while retaining the protection af the SCRs from potonualy harmful high energy transients.

 

 

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