Back to Basics: Induction Furnace Daily Maintenance
Note: This article is based on a portion of the AFS Institute course “Introduction to Coreless Induction Furnace Operations.” Keep the learning going by taking the upcoming Iron Melting 201 course offered live, online September 6-8. This course provides a detailed coverage of iron melting and related processes.
An induction furnace is an alternating current (AC) electric furnace in which the primary conductor is coiled and generates a secondary current by electromagnetic induction that heats the metal charge.
- The benefits of using an induction furnace include:
- Melting process can easily be controlled.
- Offers versatile solutions for dealing with many different types of metals.
- Geared toward energy efficiency.
- Offers a “green” approach.
- High melt quality.
- No combustion products are created.
- Lower metal loss.
A coreless induction melting furnace consists of a refractory lining to hold the material to be melted that is surrounded by a helical coil made of hollow electrolytic copper tubing. For protection, the coil is usually housed in a box made of high-strength thermal board or a fabricated steel shell, sometimes with electromagnetic shunts for shielding to prevent stray heating. The furnace may be stationary such that the crucible is picked up and carried away for pouring the molten metal or may be supported on trunnions for tilt pouring.
The refractory lining has the ability to retain its strength and shape at high temperatures and may consist of a rammed lining or a crucible. A rammed lining is made up of a mixture of course- and fine-grained oxide aggregates, with fluxes added for bonding. It is installed using a melt-out or removable form to create the cavity to hold the molten metal. A crucible is a preformed containment vessel for molten metal and may be made of similar materials as rammed linings, or conductive materials like silicon carbide or machine graphite.
A power supply converts the main’s voltage and frequency to that which is required for proper operation of the furnace. Typical output frequencies range from 50 Hz to 10 kHz, at power levels from 5 kW to 16.5 MW. Today, most induction power supplies use solid-state technology, based on thyristors (SCRs) or IGBTs. The power supplies are usually water-cooled, as are the furnace coils. Cooling water is typically recirculated between the equipment and an open or closed cooling tower, to remove the heat.
The current in the induction coil creates the magnetic field, which in turn induces a current in the charge. The interaction of the magnetic field and the current in the charges creates the electromagnetic forces that create the stirring. The stirring has a “figure eight” pattern with metal flowing up the center and down the sides on the top half of the bath (opposite directions on the bottom half of the bath) with the characteristic inverse meniscus. The stirring action within the bath is important as it helps with mixing of alloys and melting of turnings as well as homogenizing of temperature throughout the furnace. However, excessive stirring can increase gas pick up, lining wear, and oxidation of alloys.
Most modern-day induction power supplies are frequency converters, consisting of a rectifier and an inverter. The rectifier converts AC to DC and the inverter converts DC back to AC. The purpose of doing this is to be able to control the power level and in most cases, operate the furnace at a frequency that’s different from the line frequency.
Most induction power supplies have a single output, sometimes with switches to be able to connect multiple furnaces, but not at the same time.
A multiple or dual output power supply allows two furnaces to be powered at the same time, so you could melt at high power on one furnace while you’re holding at low power on the other one.
Many ancillary systems come into play with an induction furnace.
The first is the water pumps. There are many different types of configurations for pumps, inside and outside of the unit. The idea is to pump enough water to cool the equipment. The power supply and furnace are water cooled.
Furnace systems also incorporate cooling towers. The three most common types are:
- Open evaporative tower that provides a large temperature drop. However, the water is exposed to the elements (dust, dirt, etc.).
- Dry air cooler, which is closed and maintains the integrity of the cooling water but provides a smaller temperature drop.An industrial cooling tower is a combination of the other two, combining the benefits of both. An industrial cooling tower maintains the integrity of the cooling water and provides a large temperature drop.
The third ancillary system is the hydraulic system. Furnaces can have either a single or dual hydraulic power unit. They are primarily used to tilt the furnace. Some furnaces also have a hydraulically-operated lid. Others may have a hydraulic push-out or ram to push the lining out.
Providing the most efficient operation of induction melting and holding equipment and maximizing its useful service life are worthy goals for a foundry maintenance program. The most important goal, however, is the safe operation of equipment and the protection of workers and visitors. Poor, improper, or delayed maintenance is a major contributor to accidents involving induction equipment in foundries.
Following are daily furnace maintenance tasks.
1) Check refractory lining before melting. When checking the refractory linings, you are looking for signs of finning, spalling, erosion, or build-up. Linings can fail for several reasons, including:
- Wrong refractory material was used for a particular application.
- Improper installation.
- Lining allowed to wear too thin.
- Physical shocks or mechanical stress damage the lining.
- Excessive temperatures or thermal shock (damage).
- Slag/dross build-up.
The furnace’s refractory lining is all that stands between a worker and the molten metal in the furnace. Therefore, it is important that the lining is selected, installed, and sintered properly. The lining should be inspected frequently and removed immediately when worn out or damaged. The safety of those operating the furnace and who are in the vicinity depends on how well every lining is installed and maintained.
2) Check fault/limit indicating lights. The circuit monitor display will tell you that the system is ready for operation (no lamps lit). First, set the power control knob to Zero. Then press the green “ON” button to start the inverter. The green LED will illuminate indicating “INVTR ON.” Check the circuit monitor to make sure that no new alarms have appeared and to confirm the inverter is running. Turn the power control knob to the desired kilowatts.
3) Check hydraulic fluid level. During daily maintenance, the operator is looking for the level and temperature of the hydraulic fluid.
4) Check internal and external water levels and look for leaks. Power lead and hose connections need to be checked for leaks during the preventive maintenance procedure. Water leaks can cause coil arcing, resulting in short coil life.
5) Check operation of ground/leak detector. The ground/leak detector comes on in the tripped mode. It must be reset so it can be tested, and the operator will need to reset it again after testing. First, press “reset” to ready the ground leak detector (GLD) for operation. Then test the GLD to ensure it is functioning properly. Hold the TEST button until the GLD Current Trip Red LED illuminates (7-12 seconds).
6) Keep work area around furnace and power unit clean. On a daily basis, make sure the work area is clean—both around the furnace and the power unit.