EAF power control eases costs.

Productivity shortfalls in electric arc melting commonly are blamed on the electrical system. The truth in most cases, however, is that there is usually little relationship between productivity and electric power. Productivity is mostly a function of how the furnace is used. This has a greater effect on melting efficiency than electrical power input. But understanding and controlling electrical power to electric arc furnaces is a major contributor toward cost effective equipment utilization.

The total electrical system for an arc furnace consists of two major components: a power system and a control system. Both must perform to a set of ideal operational conditions to achieve maximum furnace productivity.

The Power System

The power system begins with the power company feeding electricity into the foundry's power system, and continues on to the furnace transformer concluding in the arc at the tips of the three electrodes. It is essential, however, to establish that the distribution transformers have enough capacity feeding into the furnace transformer to sustain maximum operating power for optimum melting conditions. This requires consideration not only of the size of the distribution transformer, but also its capacity to supply optimum power under adverse circumstances that might limit furnace efficiency, such as multiple furnaces, additional load distribution, transformer cooling capacity and the short circuit capacity of power company feeders.

Consideration also should be given to the capability of the electrical distribution system, its cables, breakers and safety devices. Once these have been determined as sufficient, focus should shift to the furnace transformer, initially considering the size of the transformer itself. At the transformer, the high-voltage, low-current power is converted into low-voltage, high-current power sufficient to melt the charge. The nameplate rating of the furnace transformer (KVA or MVA capacity) should, if possible, exceed the required furnace load. Some overload is acceptable, but the optimum electrical and melting conditions, as a result, may suffer.

An adequate cooling system for a furnace transformer is important to maintain optimum power to the furnace. Safety devices and instrumentation are required, of course, and a strong preventive maintenance program is an absolute necessity to maintain the electrical systems for maximum furnace production. The type, size and cooling capacity of the secondary power cables have a major effect on the power going into the arc furnace, as well as the electrical furnace balance.

The construction and cooling of the electrode holders are an essential consideration because the power system ends at the tip of the electrodes. The type of electrode is important, but the proper makeup of the electrode columns is vital to optimum power and maximum productivity. Plum electrode columns are required to avoid mechanical hot spots. The arc at the tip of the electrodes changes the electrical power into heat, converting the furnace charge into molten metal. Optimum performance of all power system components will provide maximum productivity at the lowest possible cost.

Arc Furnace Controls

The heart of an arc furnace control system is the furnace regulator and the tap changer. The tap changer is part of the transformer and is controlled by an operator or a computer that determines the amount of power entering the furnace and sets the arc length. Profitable furnaces, depending on the age and type of equipment, operate on a single tap. It uses the maximum furnace transformer capability, changing the arc length with the regulating system to conform with melting requirements. Overall operating procedures must be correlated for the system to work.

The regulator varies the preset power input per tap (depending on the conditions inside the arc furnace) by lowering and raising each electrode independently. This electrode control not only changes the arc lengths and melting characteristics, but also the power input.

All electrical and operating factors have to be considered when the arc furnace is tuned for optimum performance and should be performed only by qualified personnel. An understanding of electrical power and its control, foundry production requirements, charge material, the electrode movement, tip configuration and consumption, the recorded or indicated current per phase, power input |MW~ and the electrical energy |kWhr~ is essential. It may be advisable at times to replace some power system components, an old furnace regulator or an underpowered furnace transformer to maintain maximum productivity at the lowest cost.

Once electrical parameters have been set in a well-maintained environment, only minor changes are routinely required. Operating and maintenance delays usually have a far greater effect on operating costs than changes in electrical parameters.