Electricity: electrical systems and devices – Control circuits for electromagnetic devices – Systems for magnetizing – demagnetizing – or controlling the...
Reexamination Certificate
2000-11-15
2003-02-18
Jackson, Stephen W. (Department: 2836)
Electricity: electrical systems and devices
Control circuits for electromagnetic devices
Systems for magnetizing, demagnetizing, or controlling the...
C361S146000, C361S149000
Reexamination Certificate
active
06522517
ABSTRACT:
BACKGROUND ART
This invention relates to the art of controlling the magnetic flux density within magnetic bodies. A varying voltage applied to a conductive winding causes the voltage induced in the winding to have such waveform and magnitude that the integral over time of the induced voltage correlates to desired changes of the magnetic induction level.
Prior art methods, that utilize a conductive winding to control magnetization, generally focus on controlling winding current to control the induction level of a magnetic body. The winding current directly correlates with magnetic field intensity. Changes in magnetic field intensity are coordinated with hysteresis-curve properties of the magnetic material to control the induction level. (As used throughout this disclosure, the following terms are synonymous. “magnetization,” “magnetic induction,” “flux density” and “induction level”).
Many applications are particularly concerned with how a magnetic body may be demagnetized. Most prior-art demagnetizing methods (that effectively reduce the induction level of a magnetic body to near zero) utilize a declining alternating magnetic field. The alternating magnetic field is usually produced by a conductive winding conducting a declining alternating current. Many different embodiments of this demagnetizing method are well established in the prior art.
Other applications are concerned with how a magnetic body may be magnetized to a preferred induction level. To establish a preferred induction level, prior art methods generally apply a current with controlled magnitude to a winding (usually with the magnetic body initially in a demagnetized state). The current is understood to produce a magnetic field in the magnetic body that is proportional to current magnitude. Current magnitude (and magnetic field strength) is coordinated with hysteresis curve properties of the magnetic body to produce a preferred induction level. Many different embodiments of this magnetizing method are also well established in the prior art.
Of particular relevance to the present invention is the relationship between voltage induced in a winding and changing magnetic flux. The relationship between induced voltage and changing magnetic flux was originally stated in a general way by Michael Faraday, and is widely known as Faraday's Law. Faraday's Law basically states that the voltage induced around a closed path is proportional to the rate of change of magnetic flux within the closed path. It is well established in prior art how Faraday's Law can be used along with a conductive winding to measure changes of induction level in a magnetic body. However, the prior art does not appear to address how the induction level of a magnetic body can be controlled by controlling the voltage induced in a winding.
The initial motivation for developing the present invention was to provide a way to demagnetize current transformers while the current transformers remain in service. The prior art does not address this problem very well.
Most current monitoring systems for a-c (alternating-current) electric power systems utilize current transformers to provide input currents that are isolated from the electric power system conductors. A primary winding of a current transformer is connected in series with a current-carrying conductor while a secondary winding is magnetically coupled to the primary winding by a suitable magnetic core. A current is induced in the secondary winding that is proportional to the primary current. The secondary current is isolated from the primary current and is smaller than the primary current by the turns ratio of the primary and secondary windings. The primary winding frequently consists of only one turn, which is often just the current-carrying conductor installed through an opening in the middle of the current transformer magnetic core. The secondary winding usually consists of multiple turns wrapped around the magnetic core.
The accuracy of a current transformer is usually related to the coercive force of the magnetic core material (less is better), the cross sectional area of the magnetic core (bigger is better), the effective magnetic length of the magnetic core (shorter is better), any air gap in the magnetic core (less or none is better), and the “squareness” of the magnetic core material hysteresis curve (squarer is usually preferred if not operating near saturation, otherwise characteristics that are not square may be preferred). In the case of high-quality current transformers with very little air gap (usually tape-wound construction, not split-core construction), acceptable accuracy is often achieved as long as operation near saturation is avoided. Split-core current transformer cores generally have hysteresis curves that are less square than standard current transformer cores due to the small air gaps inherent in the design of split-core current transformers.
In order for the secondary current generated by a current transformer to be an accurate representation of the primary current, the impedance of the circuit connected to the secondary winding must be kept low so that current can flow freely. The impedance of the secondary circuit is often called the “burden.” The burden generally includes all impedances in the loop through which the secondary current flows, including stray winding impedances, stray impedances of connecting conductors, and the impedances of any devices connected in the loop (such as current-sensing resistors and relay operating coils). In order for a current transformer to drive a secondary current through a non-zero burden, a voltage must be induced in the secondary winding. The induced voltage is proportional to secondary current and is proportional to the burden, in accordance with Ohm's law. The induced voltage is induced in the secondary winding by a fluctuating induction level in the magnetic core. The fluctuating induction level is associated with a magnetizing current in accordance with well-known electromagnetic principles. The magnetizing current accounts for most of the error in the secondary current. Generally speaking, the accuracy of a current transformer is inversely related to the burden of the secondary circuit. A higher burden causes the secondary current to be a less accurate representation of the primary current.
The accuracy of the secondary current may also be adversely affected by either of the following:
(a) A primary current that is not symmetrical. “Symmetrical” is intended to mean that the waveform has positive and negative half-cycles with the same waveform and magnitude. An alternating current that has a d-c (direct-current) component is a common example of a primary current that is not symmetrical. Also, transient a-c fault currents are often not symmetrical. D-c currents are, by definition, not symmetrical.
(b) A burden that is not a linear impedance. Nonlinear burdens are common in applications that derive power from the secondary current.
In either of these cases, the current transformer core may become magnetized. This magnetization may cause significant error in the secondary current. This error may include distortion of the secondary current, including the loss of any d-c component that is present in the primary current.
BRIEF SUMMARY OF THE INVENTION
In accordance with the present invention, a varying voltage is applied to a conductive winding that magnetically interacts with a magnetic body. The varying voltage controls the voltage induced in the winding in such a way that the integral over time of the induced voltage correlates to desired changes of the induction level (magnetization) of the magnetic body. The invention may be used to control the induction level of a magnetic body in several ways:
(a) The induction level of a magnetic body may be caused to transition from a known induction level to a preferred induction level. (A preferred induction level of zero may be chosen to demagnetize a magnetic body).
(b) When the induction level is not known, a preferred induction level may be established by changing the induct
LandOfFree
Method and apparatus for controlling the magnetization of... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method and apparatus for controlling the magnetization of..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method and apparatus for controlling the magnetization of... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3178417