Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – With coupling means
Reexamination Certificate
1999-10-18
2001-11-27
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
With coupling means
C324S126000, C324S11700H, C324S076750, C324S253000, C323S358000
Reexamination Certificate
active
06323635
ABSTRACT:
MEC and MER are described in copending application Ser. No. 08/579,395.
FEDERAL SPONSORSHIP
None.
REFERENCE TO A “MICROFICHE APPENDIX”
None.
BACKGROUND OF THE INVENTION
1) Field of the Invention
This invention is an improvement on the inventor's prior work. Swain's U.S. Pat. No. 3,768,011 includes teaching of a clamp-on DC ammeter. Swain's application Ser. No. 08/579,395 includes teaching of a better clamp-on DC Ammeter. This MER2 invention teaches how to build still better sensors for clamp-on DC Ammeters for measuring continuous electrical current. MEC and MER are described in copending application Ser. No. 08/579,395.
2) Description of the Related Art
We build clamp-on DC Ammeters for measuring direct current as small as 5 milliamperes. Measurement accuracy can be damaged in many ways. One is zero offset error. This can be due to several things, one of which is the non-uniform magnetic field set up by magnetized material relatively close to the sensor. Copending application Ser. No. 08/579,395 reduced this error, but generally does not eliminate it. This invention teaches a counter intuitive way to still further reduce error due to nearby magnets so as to get even greater accuracy. A by-product is that the sensor's zero offset error due to the generally uniform magnetic field of the Earth is also reduced.
BRIEF SUMMARY OF THE INVENTION
The standard sensors for clamp-on DC Ammeters, called Swain Meters®, have for many years been constructed to resemble, in so far as practical, a split core toroid. This is because a toroid is widely accepted as the form of the highest quality transformers. Core symmetry and winding uniformity were accepted as keys to avoiding zero offset error due to a magnet acting on the sensor. Swain's U.S. Pat. No. 3,768,011 has numerous references to toroid core symmetry and winding uniformity. See columns 7, 8, 15, and 16. The “magnet” can be the Earth's uniform magnetic field, or the non-uniform field of a nearby magnet. Nearby magnets can be found in steel pipe used for transporting natural gas, or in an alternator in a car. Zero offset error is a problem, especially when the current to be measured is small and continuous.
Accuracy was substantially improved using the teaching of copending patent application Ser. No. 08/579,395 which includes the MER Meter™. Zero offset error due to local magnets was reduced by 2 or 3 to one. We used 2 or 3 times as much ampere turn magnetomotive force H
sm
to switch the flux density &bgr; in the steel core of the sensor. But still the core and winding structure were as uniform as practicable. We all thought this was best.
While evaluating a series of sensors, by the grace of God I realized that the lips were the big problem and that I could improve the lips by using extra MER teaching close to the lips. This idea was different from prior art—use a non-uniform winding—a higher ampere turn concentration H
sm
to switch the flux density in the core near the lips. We call this the MER2 structure.
It worked. With the MER2, the zero offset error due to local magnets was still further reduced to ½ or ⅓ that of the MER Meter usage. The new non-uniform sensor is at least twice as resistant to local magnets.
A pleasing byproduct appears to be less zero offset error due to the uniform Earth's magnetic field.
Now that a non-uniform winding is proven beneficial, I expect we will get superior results when we make non-uniform cores and lips.
REFERENCES:
patent: 3629693 (1971-12-01), Anderson
patent: 3768011 (1973-10-01), Swain
patent: 5053695 (1991-10-01), Canter
Deb Anjan K
Metjahic Safet
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