Pinpointing electric coil problems in the coreless furnace.

Pinpointing Electric Coil Problems in the Coreless Furnace

Coreless induction furnaces are most often used to heat and melt metals and other conductive materials. The most common metals heated are alloys of iron, steel, copper and aluminum. The major components of the furnace include a refractory lining which contains the molten metal bath; a power coil surrounding the refractory lining; and a number of magnetic yokes for concentrating the magnetic field established by the coil current. The power coil is surrounded by a number of vertically oriented silicon steel columns, called magnetic yokes, which are electrically grounded. The yokes support the power coil and provide a magnetic path for the alternating magnetic flux field.

For safety, the molten bath is electrically grounded with rods which extend through the bottom of the refractory lining. The ground rods are fabricated from highly conductive, high-temperature wire which ensure that the melt bath is always at ground potential. The power coil carries a large electric current which establishes a magnetic field as shown in Fig. 1. The field induces electric currents, called eddy currents, in the molten metal bath which cause heating of the bath by resistance.

Power coils can be configured in a variety of ways. Some furnaces have single coils with typically 10-40 turns. Other furnaces have more than one coil, and some have concurrently wound coils. The power coils are connected to power sources of various frequencies, and voltages. Electricity can be supplied in either single or multi-phases.

Ground Faults

Figure 2 shows a typical power supply connection to a coreless induction furnace coil. The cross section of the power coil is shown connected to the 60 hz AC supply with a ground detection device connected to the center of two power factor correction capacitors. Under normal conditions, the power coil is not grounded, so no electrical current can flow through the ground detection device. But under a ground condition, the coil is in electrical contact with the ground and a current flows. The electrical flow moves from the power supply through an isolation switch to the coil. From the coil it flows to a magnetic yoke, down the yoke to the frame, down the frame ground, through the building structure, up the detection device's ground connection, through the detection device, through a capacitor and back to the power supply. This flow is called a ground fault current.

The ground fault current activates the ground detection device and results in the automatic opening of the power supply's main circuit breaker. This shutdown action is known as a ground fault trip. The tripping or circuit opening action is very fast with typical opening times of one-tenth of a second. This speed is necessary to protect the coil from large ground fault current and from possible short circuiting of the coil. Large fault currents can break down insulation and severely damage the power coil.

Ground faults and short circuits are commonly misunderstood. Ground faults are usually limited to approximately 50 amperes of current flow. This amount of current can damage insulation such as the mica behind yokes, but will not typically damage the water-cooled copper power coil. On the other hand, short circuits are fault current paths normally between two adjacent coil turns. The short path can experience current flows in thousands of amperes and can be very destructive to the copper coil as well as the insulation and nearby yokes.

Where Is the Fault?

When an electrical path occurs between a coil turn and ground, a fault detection system turns off the power supply. Two types of ground will cause the power to trip off: external grounds or internal grounds.

The external ground condition is caused when an object outside the coil's perimeter touches both the coil and a ground member of the furnace such as a water hose, frame or yoke. (see inset "A" Fig. 2) Such an external ground fault is caused by a metal part wedged between a coil turn and one of the grounded magnetic yokes. A ground path may also develop when excessive moisture accumulates between the coil and a grounded element.

An internal ground fault, on the other hand, occurs when the melt bath penetrates the refractory lining and touches the coil. (see inset "B" Fig. 2) Because the bath is grounded, the coil is also grounded through the melt. This also causes a ground current to flow through the ground detection device. The ground fault path is shown to travel from the power supply, through the isolation switch, into the coil, through a melt sliver in the refractory lining, through the bath, down the ground rods to the detection device, and then back to the power supply through a capacitor.

When a ground fault is detected, it must be located and removed before the molten bath solidifies. Bath solidification typically occurs between 3-10 hours after power is lost. If the fault is not found quickly, the molten bath must be drained. Usually, ground faults must be located by visually inspecting the outside surface of the coil between the magnetic yokes. Visual inspection is inherently slow and can result in loss of production, heat-energy loss and possible refractory damage. In addition, visual inspection does not readily reveal ground faults located behind magnetic yokes or those resulting from melt bath penetration. Once located, the fault point can be removed and power restored.

Metal Penetration

When a weak spot in the refractory lining develops, the bath can penetrate the lining and leak out past the power coil by running between the coil turns. This condition, referred to as a run-out, is a damaging and dangerous condition as both the power coil and furnace structure can be severely impaired. The result is loss of production due to the time required for repair.

An internal ground fault typically precedes a run-out. However, visual inspection of the coil when a ground fault is detected may not readily reveal whether the fault is external or internal. Consequently, the operator detecting a ground fault only knows that a possibility exists that the fault is internal. Upon detecting the ground fault, the operator may either drain the melt bath to prevent a run-out, or retain the melt while hoping to clear the ground condition. Premature draining of the molten bath can result in excessive production losses.

A method that would allow furnace operators to quickly locate ground faults and determine if the refractory lining has been penetrated would provide an additional safety margin when operating the coreless furnace.

Locating a Fault

The process of locating a ground fault usually relies on a visual inspection and a test procedure called the Murray-Loop test. This method requires disconnecting the coil from the high-voltage power supply by opening the isolation switches. A low voltage DC supply is then connected across the coil and a very high current is passed through it. The DC current is typically supplied by an arc welder. A variable resistor, called a potentiometer, is connected across the coil with its adjustable slide wiper connected to ground through a sensitive ammeter. The slide wiper is moved upward until the meter current indicates zero amperes. By measuring the setting of the potentiometer and comparing it to its total travel setting, an approximation of the ground faults' position is possible. The higher the wiper, the higher the fault location. Because ground faults normally occur at higher voltages, the effectiveness of the Murray-Loop method is limited. With its lower applied voltage of about 10 volts, the test may not produce accurate results unless a very stable, solid ground is present. Also, physical calculations are required to estimate the fault's position. Generally, this method can locate a stable, solid ground fault condition to within two turns of a 14 turn coil. This can mean that nearly all of the magnetic yokes may have to be backed away from the coil to examine for a ground fault. This behind-the-yoke inspection can require nearly two hours per yoke on larger furnaces, and there may be 20 or more yokes in the furnace body.

A newer method of determining the ground's position uses the furnace's normal power supply voltage. A computer mounted in a two-wheel portable enclosure is connected to the coil through four cables. When the furnace power is applied, the computer quickly measures and calculates the position of the fault.

The computer does this by measuring the total coil voltage as well as the voltage from the coil's top connection to a ground reference. By using the ratio of these two measurements and comparing it with a predetermined correction table called a MAP, which is stored in the computer, the distance from the top coil connection to the ground fault is determined. The computer then generates a graphics picture of the coil on a video monitor as shown in Fig. 3.

The coil is drawn by the computer with its associated connection points marked and magnetic yokes numbered. The fault's position is shown on the coil picture by a flashing white cursor. In this example, the fault is located between yoke number 4 and 5, and nine turns up from the bottom. This furnace has two concurrently wound coils. One is labeled coil AB and the other is marked CD. In this example, the CD coil has the ground fault. It should be mentioned that this system does not determine which coil of the two has the ground.

It is important to realize that power coils do not have a linear impedance characteristic. In other words, voltages measured between equal physical distances are not necessarily equal in value. This can be seen in Fig. 4. The coil's top turn-to-turn voltage is 200 volts, while the turn-to-turn voltage near the center of the coil is 230 volts. Other nonlinear relationships exist and are corrected with a look-up table (MAP) stored in a computer. Because this method of locating ground faults uses operating power supply voltage, there is no need to isolate the coil from the power supply. The procedure usually takes less than five seconds to perform.

The advantage of displaying the power coil structure on a video monitor is that the fault is shown in relation to the coil turns, power and water terminals and the magnetic yokes. This allows maintenance personnel to readily see and identify the location of a fault.

Lining Penetration

Once the fault has been located, it is only shown with respect to its perimeter position on the coil. But is the fault due to metal penetrating the refractory lining or is it due to some ground condition outside the coil?

The traditional method used to determine if the ground fault is due to bath penetration requires the opening of isolation switches and disconnecting the ground rod leads under the furnace. An ohmmeter is then connected to the power coil and the ground rods. If current flows, the meter reads low resistance and it is assumed that the bath is touching the coil. Because ground faults are initiated only after a minimum breakdown voltage is applied across a weakened area, the ohmmeter procedure is limited because it relies on low voltage which is often not high enough to initiate the current track of the ground fault.

Another disadvantage of the ohmmeter method is that normally there is splatter of melt material at the top of the furnace lining. This metal build-up grounds the bath through the furnace frame. The ohmmeter can indicate a bath fault even though the fault is external to the coil. The ohmmeter current travels into the coil, out an external fault point, down a yoke, up the furnace frame, over the splatter build-up, down the bath, down the ground rods, and back to the meter. This test, therefore, does not produce conclusive results.

During normal operation AC current flows down the ground rod leads and back into the furnace bath through the top melt splatter paths. This current is called circulating bath current. Because of this, detecting current at the ground rods does not mean the bath is touching the coil. The detected current may only be the circulating bath current.

A newly developed method uses a computer to measure the coil fault current at the detection device and compares it to the current flowing down the ground rods at the furnace bottom. The computer compares the time relationships of the two currents to determine if the rod current is the fault current. Only by measuring the instantaneous values of ground rod current and fault current and then by comparing their relative values (phase angles) can a determination be made as to bath refractory penetration. With this system, furnace power is not locked open and normal operating voltages are used. The test is completed within five seconds and the results are conclusive.

PHOTO : Fig. 1. In the coreless induction furnace, the power coil carries an electric current which establishes a magnetic field. The field induces eddy currents in the molten metal bath which results in heating by resistance.

PHOTO : Fig. 2. This schematic drawing shows a typical power supply connection to a coreless induction furnace coil. The cross section of the power coil is shown connected to the 60 hz AC supply with a ground detection device connected to the center of two power factor correction capacitors. Inset "A" illustrates an external ground fault caused by a metal part wedged between a coil turn and one of the grounded magnetic yokes. Insert "B" shows an internal ground fault which occurs when molten metal penetrates the refractory lining and touches the coil.

PHOTO : Fig. 3. One system for determining the location of a ground generates a graphic picture of the furnace coil. The fault's position is shown by a flashing cursor. In this case, the fault is located between yoke number 4 and 5.

PHOTO : Fig. 4. As illustrated here, power coils do not have a linear impedance characteristic which means voltages measured between equal physical distances are not necessarily equal in value. In this case, the coil's turn-to-turn voltage 200 volts, while the turn-to-turn voltage near the center of the coil is 230 volts.