# US 5,708,677 A · Arc voltage distribution skewness as an indicator of electrode gap during vacuum arc remelting

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- Number: US 5,708,677 A
- Document ID: US5708677A
- Status: Granted
- Kind: A
- Application number: 08/426,545
- Filed: 1995-04-21
- Granted: 1998-01-13
- Earliest claimed: 1995-04-21
- Assignees: Sandia Corporation
- Inventors: Rodney L. Williamson; Frank J. Zanner; Stephen M. Grose
- Web page: https://patentgrep.com/patent/US5708677A

## CPC

- H05B 7/152: by electromechanical means for positioning of electrodes
- F27B 3/28: Arrangement of controlling, monitoring, alarm or the like devices
- F27D 11/08: Heating by electric discharge, e.g. arc discharge
- Y02P 10/25: Process efficiency
- H05B: ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
- F27B: FURNACES, KILNS, OVENS OR RETORTS IN GENERAL; OPEN SINTERING OR LIKE APPARATUS
- F27D: DETAILS OR ACCESSORIES OF FURNACES, KILNS, OVENS OR RETORTS, IN SO FAR AS THEY ARE OF KINDS OCCURRING IN MORE THAN ONE KIND OF FURNACE
- Y02P: CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS

## Abstract

The electrode gap of a VAR is monitored by determining the skewness of a distribution of gap voltage measurements. A decrease in skewness indicates an increase in gap and may be used to control the gap.

## Claims

### Claim 1

A method for controlling electrode gap width in a vacuum arc remelting furnace having an electrode spaced across an adjustable gap from an electrically conductive surface, and a direct current power source causing an electric arc across the gap, an electrode gap voltage being formed between the electrode and the surface, the method comprising the steps of:
a) determining an arc voltage distribution skewness of the electrode gap voltage; and
b) controlling the gap based on changes in the skewness.

### Claim 2 (depends on 1)

The method of claim 1 wherein step a) comprises:
making n measurements of electrode gap voltage over a period of time;
determining the mean, .mu., and standard deviation, .sigma., of the n voltage measurements; and
calculating skewness, ##EQU5##

### Claim 3 (depends on 2)

The method of claim 2 wherein the controlling step b) is performed at least approximately every 10 seconds.

### Claim 4 (depends on 2)

The method of claim 2 wherein the n measurements of voltage data are acquired at a rate greater than twice a voltage ripple frequency of 360 Hz.

### Claim 5 (depends on 1)

The method of claim 1 additionally comprising the step of determining an approximate value of the electrode gap width from the skewness.

### Claim 6 (depends on 1)

The method of claim 1 wherein the controlling step b) comprises providing the skewness to a display device viewable by a furnace operator.

### Claim 7 (depends on 1)

The method of claim 1 additionally comprising the step of adjusting the electrode gap width based upon the skewness.

### Claim 8 (depends on 7)

The method of claim 7 wherein the adjusting step comprises altering an electrode feed velocity according to the following formula: ##EQU6## where
v.sub.o (t+.tau.)=v.sub.o (t)+.gamma..upsilon.{sign›g.sub.e.sup.o -g.sub.e (t)!}
and v.sub.o is a base feed velocity, v is an electrode feed (ram) velocity, g.sub.e.sup.o is a set-point electrode gap, g.sub.e (t) is the electrode gap, .gamma. is a gain, .upsilon. is a maximum allowable velocity change, t is time, and .tau. is a time increment between velocity adjustments.

### Claim 9

An apparatus for controlling electrode gap width in a vacuum arc remelting furnace having an electrode spaced across an adjustable gap from an electrically conductive surface, and a direct current power source causing an electric arc across the gap, an electrode gap voltage being formed between the electrode and the surface, the apparatus comprising:
means for determining an arc voltage distribution skewness of the electrode gap voltage; and
means for controlling the gap based on changes in the skewness.

### Claim 10 (depends on 9)

The apparatus of claim 9 wherein said determining means comprises: comprises:
means for making n measurements of electrode gap voltage over a period of time;
means for determining the mean, .mu., and standard deviation, .sigma., of the n voltage measurements; and
means for calculating skewness, ##EQU7##

### Claim 11 (depends on 10)

The apparatus of claim 10 wherein said controlling means operates at least every 10 seconds.

### Claim 12 (depends on 10)

The apparatus of claim 10 wherein the n measurements of voltage data are acquired at a rate greater than twice a voltage ripple frequency of 360 Hz.

### Claim 13 (depends on 9)

The apparatus of claim 9 additionally comprising means for determining an approximate value of the electrode gap width from said skewness.

### Claim 14 (depends on 9)

The apparatus of claim 9 wherein said controlling means comprises means for providing said skewness to a display device viewable by a furnace operator.

### Claim 15 (depends on 9)

The apparatus of claim 9 additionally comprising means for adjusting the electrode gap width based upon the skewness.

### Claim 16 (depends on 15)

The apparatus of claim 15 wherein said adjusting means comprises means for altering an electrode feed velocity according to the following formula: ##EQU8## where
v.sub.o (t+.tau.)=v.sub.o (t)+.gamma..upsilon.{sign›g.sub.e.sup.o -g.sub.e (t)!}
and v.sub.o is a base feed velocity, v is an electrode feed (ram) velocity, g.sub.e.sup.o is a set-point electrode gap, g.sub.e (t) is the electrode gap, .gamma. is a gain, .upsilon. is a maximum allowable velocity change, t is time, and .tau. is a time increment between velocity adjustments.