EuroWire January 2007

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overlapping of the opposing currents flowing in the opposite surfaces of the conductor should be avoided in order to prevent current cancellation. Typically, ‘p’ should be less than half the conductor radius, although this rule is not always applied. Different current penetration depths also apply for different materials and temperatures at various frequencies. In the induction heating process, a metal component placed within or adjacent to an induction coil is heated by passing an induction current through that coil, which in turn, introduces another current within the component. Heat is produced by resistance to that induced current, according to the I²R law (where I = Current and R = Resistance) and also by hysteresis loss in magnetic materials – an effect that disappears at the Curie temperature (approximately 760°C/1400°F). Power selection (relative to through-heated wire) With correct frequency determined, and suitable power units selected, the next step to consider is power requirement, with the first stage being the determination of the heat content of the conductor. The heat content of a moving wire is purely a function of mass throughput, specific heat and temperature rise. However, this apparently straightforward calculation is complicated by the fact that specific heat varies as temperature rises. Taking a medium of carbon steel as an example, the specific heat varies by a factor of 1.3 between 68°F (20°C) and 1,022°F (550°C) and 1.5 between 68°F (20°C) and 1,652°F (900°C).

Therefore, in determining heat content to heat carbon steel 1,022°F (550°C) and 652°F (900°C), as a rough rule of thumb, specific heats of 0.58 and 0.63 may be used. Taking this rule, the heat content of wire heated to 1,022°F (550°C) is 2.31 x lb/minute (1.05 x kg/minute) and to 1,652°F (900°C) 4.27 x lb/minute (1.94 x kg/minute) with the result expressed in KW. Having determined the heat content of the product, the next step is to determine the power output of the power unit by establishing a heating efficiency relative to the power unit output. Heating efficiency The typical induction system comprises a power unit, a heating coil system and facilities to ‘match’ the heating coil (and processed wire) to the power unit. The power unit is also known as a converter, inverter, or generator. This unit converts a 3 phase supply of 50 or 60 Hz to a nominal output frequency in the range of 250Hz to 800Khz at a single phase with power outputs from 1KW to 4MW in a wide range of power frequency combinations. Some dual frequency combinations are also available. These power units are either thyristor or transistor based. The heating coil system, as applied to wire heating applications, consists of a copper tube wound into a spiral. The tube may be round, square or rectangular and often has additional copper strip brazed on the internal diameter of the spiral. The coil length, internal diameter, number of turns and percentage copper to free space along the inside diameter of the spiral are all relevant to efficiency. All power units will run within a frequency band eg 7-11kHz, 20-25kHz, and 40-50kHz for nominal output frequencies of 10kHz,

25kHz and50kHz units respectively. Inorder to achieve operation within this band, the coil inductance, coil operational voltage and amount of capacity (KVAR) in the power unit tank circuit are all varied to suit specific wire sizes, materials, throughput rates and temperatures. Considering efficiency, we must first look at the coil system. The internal diameter of the copper spiral is the most significant aspect determining the efficiency. In turn, this diameter is dependant on mainly mechanical aspects relative to wire guidance, wire vibration and wire contamination in addition to wire size and method of joining wire reel to wire reel. In general, the closer the coil to the material the higher the efficiency. In many instances, itmay be required to rundifferent wire sizes through a single coil. The smaller sizes will be produced at a lower efficiency, but the compromise may be justified due to lower capital cost for fewer coil sizes and less down time due to reduction in coil changeovers for different sizes. The second aspect of coil design is coil length. In theory, to evenly through- heat a diameter to a given temperature, a time approximately equivalent to D²/25 (where D = wire diameter in mm) seconds is required. The minimum coil length in metres is therefore D²M/25 (where M = wire speed in metres/second). In practice, particularly for small wire diameters, this minimum length would result in an excessive power density due to very short coil length with a consequent poor efficiency. Coil lengths are extended to improve efficiency. An experienced judgment is made on coil length (with coil diameter fixed due to wire dimensions) and several computations made with respect to coil voltage, number of turns percentage of copper to free space with a view to obtaining optimum efficiency. Within these computations the initial judgement on coil length may be varied to improve efficiency. Wire heating applications Nowadays, induction heating is applied to a widevarietyofwireprocessestreatingeither single wires, multi wires running in parallel or wires stranded into ropes. Applications for wire heating include: heating prior to drawing – generally by heating the drawing dies; heating prior to encapsulation – for example in themanufacture of PVC covered electrical cables; heat treatment of wire – typically hardening, followed sometimes by tempering; annealing of single strand andmulti strand wires; heating of wire prior to coating – either with a metallic coating or insulative compounds; relaxation as performed on prestressed concrete wires, and preheating prior to a conventional heating process.

Harden and Temper process wire line

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EuroWire – January 2007