GB2034487A - Alternating current measuring devices - Google Patents

Alternating current measuring devices Download PDF

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Publication number
GB2034487A
GB2034487A GB7844410A GB7844410A GB2034487A GB 2034487 A GB2034487 A GB 2034487A GB 7844410 A GB7844410 A GB 7844410A GB 7844410 A GB7844410 A GB 7844410A GB 2034487 A GB2034487 A GB 2034487A
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United Kingdom
Prior art keywords
winding
former
resistance
coil
alternating current
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Granted
Application number
GB7844410A
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GB2034487B (en
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Central Electricity Generating Board
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Central Electricity Generating Board
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Priority to GB7844410A priority Critical patent/GB2034487B/en
Publication of GB2034487A publication Critical patent/GB2034487A/en
Application granted granted Critical
Publication of GB2034487B publication Critical patent/GB2034487B/en
Expired legal-status Critical Current

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Classifications

    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F38/00—Adaptations of transformers or inductances for specific applications or functions
    • H01F38/20—Instruments transformers
    • H01F38/22—Instruments transformers for single phase ac
    • H01F38/28—Current transformers
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01R—MEASURING ELECTRIC VARIABLES; MEASURING MAGNETIC VARIABLES
    • G01R15/00—Details of measuring arrangements of the types provided for in groups G01R17/00 - G01R29/00, G01R33/00 - G01R33/26 or G01R35/00
    • G01R15/14—Adaptations providing voltage or current isolation, e.g. for high-voltage or high-current networks
    • G01R15/18—Adaptations providing voltage or current isolation, e.g. for high-voltage or high-current networks using inductive devices, e.g. transformers
    • G01R15/181—Adaptations providing voltage or current isolation, e.g. for high-voltage or high-current networks using inductive devices, e.g. transformers using coils without a magnetic core, e.g. Rogowski coils
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01F—MAGNETS; INDUCTANCES; TRANSFORMERS; SELECTION OF MATERIALS FOR THEIR MAGNETIC PROPERTIES
    • H01F38/00—Adaptations of transformers or inductances for specific applications or functions
    • H01F38/20—Instruments transformers
    • H01F38/22—Instruments transformers for single phase ac
    • H01F38/28—Current transformers
    • H01F38/30—Constructions
    • H01F2038/305—Constructions with toroidal magnetic core

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Measuring Instrument Details And Bridges, And Automatic Balancing Devices (AREA)

Abstract

A toroidal winding 11 with uniformly spaced turns 12 is wound on a rigid non-magnetic former 10, the output voltage being applied to a resistive input 14 of an operational amplifier 15 having a capacitive feedback 16 to form an integrator. High accuracy is obtained. By making the ratio of the coil resistance to the sum of the coil resistance and integrating resistance equal to the ratio of the linear coefficient of expansion of the former to the temperature coefficient of resistance of the winding, temperature compensation is achieved over the range for which these coefficients are linear. <IMAGE>

Description

SPECIFICATION Improvements in or relating to alternating current measurement This invention relates to the measurement of alternating current and has particular application to the measurement of low frequency currents.
It is well known to measure an alternating current in a conductor by means of an iron-cored current transformer, usually in the form of a toroid arranged around the conductor in which the current is to be measured. Such devices are inherently large and heavy and cannot be used if there is a strong magnetic field due to any sources other than the current to be measured. The use of an air cored toroidal inductor, known as a Rogowski coil, has been proposed but has not found many practical applications. With an air cored current transformer the output from the coil essentially depends on the rate of change of the current to be measured and hence this output has to be applied to an integrator.Such a device is proposed in the paper "Method of Measuring Alternating Currents without Disturbing the Conducting Circuit" Proceedings IEE, Vol. 122, No. 10, October 1975, pages 1166-1167 by Dr R L Stoll, in which a flexible solenoid of fine wire is wound on a flexible non-magnetic tube; the tube is closed around the conductor carrying the current to be measured by bringing the two ends of the tube together to form a butt joint and the output from the coil is applied to an integrator.
Such prior devices however have had only limited accuracy. It is an object of the present invention to provide an improved form of current measuring device using a toroidal inductor with a non-magnetic core and which can be arranged to achieve a high accuracy over a wide frequency band extending in particular to very low frequencies.
According to this invention an alternating current measuring device comprises a rigid toroidal former of non-magnetic material carrying a winding with evenly-spaced turns around the toroid, said winding being connected to an operational amplifier having an input resistance and feedback capacitance to form an integrator, and means responsive to the output voltage of the operational amplifier.
Using a rigid core with evenly-spaced turns, minimal pick-up from adjacent conductors and radio frequency fields can be ensured. The exact path through the toroid of the current to be measured is not critical. The core of the toroid may be formed of plastics material and hence this device can be manufactured relatively cheaply and is much ligher in weight than a current transformer.
A particular advantage of this construction is that temperature compensation can readily be achieved. A change in temperature will change the resistance of the coil winding and will also cause thermal expansion of the former. Increase of temperature increases the coil resistance Rc and thereby decreases the sensitivity of the coil (the ratio of output voltage Vout to input current I) which may be expressed as
where ,uO is the permeability of air; n is the number of turns per unit length; A is the cross-sectional area of the coil; R1 is the input resistance of the amplifier; and C is the feedback capacitance.
The expansion of the former on the other hand will also change the sensitivity because of the increase in the cross-sectional area and reduction in the turns per unit length; the net effect of these is an increase in sensitivity proportional to the linear dimensions.
If a is the coefficient of thermal expansion of the former and ss is the temperature coefficient of resistance of the winding and t is the temperature change,
It may thus be seen that temperature compensation may be achieved over the range for which a and ss are linear by choosing the input resistance for the operational amplifier such that the ratio of the coil resistance to the sum of the coil resistance and input resistance is equal to the ratio of the linear coefficient of expansion of the former to the tempearture coefficient of the resistance of the winding.
Although such temperature compensation is only effective for one particular coil sensitivity, it is readily possible to provide a further stage of amplification after the integrator to adjust the sensitivity to any desired value. Alternatively the number of turns or the diameter of the winding wire may be chosen, when winding the coil, to obtain a required sensitivity.
The winding and former may be made in two parts which may be mechanically mounted to enable the former and winding to be put around a conductor.
The foilowing is a description of one embodiment of the invention, reference being made to the accompanying drawing which is a diagram illustrating an alternating current measuring apparatus.
Referring to the drawing there is shown diagrammatically a toroidal former 10 which is made of dielectric material. The former may be solid or may be hollow. Around the former is a coil 11 having uniformly spaced turns 12. One end of the coil is earthed as shown at 13 and the other end is connected to an input resistor 14 of a high gain amplifier 15. The amplifier has a feedback capacitor 16 to form an integrator. The output from the amplifier on lead 17 may be applied to a voltage measuring device 18 or recorder or utilised for other purposes for example as a control signal.
In use, the toroidal coil is put around a conductor 20 carrying the current to be measured. The voltage output from the coil is dependent on the rate of change of current in the conductor 20 and hence the integrated output on lead 17 is representative of the current.
As previously explained, the magnitude of the amplifier input resistance R1 is so chosen that the ratio of the coil resistance to the sum of the input resistance and coil resistance is equal to the ratio of the coefficient of thermal expansion of the former to the temperature coefficient of resistance of winding. By this choice of input resistance, temperature compensation is achieved.
If temperature compensation is not required, it would be possible to adjust the sensitivity of the instrument by adjusting the magnitude of the input resistance to the integrator. However if the temperature compensation is obtained as described above, a second operational amplifier having a resistive input and a resistive feedback may be provided, the output of the first amplifier being applied to this second amplifier.
Adjustment of both the resistances in the second amplifier enables the sensitivity of the device to be altered.
The voltage output depends on the number of turns per unit length around the former and it is generally desirable therefore to use closely spaced turns. The accuracy and the freedom from interference also depend on the uniformity of the winding. It is preferred to use a single-layer winding with the spacing between adjacent turns less than the diameter of the wire.
The former with its winding may be constructed in two parts, conveniently hinged or pivoted to one another, such that the former can be put around a current-carrying conductor. In this case, the windings must be suitably arranged to extend to the abutting ends of the parts of the former. if it is not possible to extend the winding to the abutting ends of the former, the winding with the uniformly spaced turns is taken to as near the ends of the former as is possible and extra turns are then put over this winding portion, as near to the ends as possible to compensate for those turns which are missing.
The above-described devices will readily withstand large overhead currents without damage. They can be constructed to measure currents to a high degree of accuracy and can operate over a wide frequency range.
In particular they can be used to measure low frequency currents such as rotor currents in slip ring induction motors.

Claims (6)

1. An alternating current measuring device comprises a rigid toroidal former of non-magnetic material carrying a winding with evently-spaced turns around the toroid, said winding being connected to an operational amplifier having an input resistance and feedback capacitance to form an integrator, and means responsive to the output voltage of the operational amplifier.
2. A measuring device as claimed in claim 1 wherein the core is formed of plastics material.
3. A device as claimed in either claim 1 or claim 2 wherein the input resistance for the operational amplifier is such that the ratio of the coil resistance to the sum of the coil resistance and input resistance is equal to the ratio of the linear coefficient of expansion of the former to the temperature coefficient of the resistance of the winding.
4. A device as claimed in claim 3 and having a further stage of amplification after the integrator.
5. A device as claimed in any of the preceding claims wherein the winding and former are made in two parts mechanically mounted to enable the former and winding to be put around a conductor.
6. An alternating current measuring device substantially as hereinbefore described with reference to the accompanying drawing.
GB7844410A 1978-11-14 1978-11-14 Alternating current measuring devices Expired GB2034487B (en)

Priority Applications (1)

Application Number Priority Date Filing Date Title
GB7844410A GB2034487B (en) 1978-11-14 1978-11-14 Alternating current measuring devices

Applications Claiming Priority (1)

Application Number Priority Date Filing Date Title
GB7844410A GB2034487B (en) 1978-11-14 1978-11-14 Alternating current measuring devices

Publications (2)

Publication Number Publication Date
GB2034487A true GB2034487A (en) 1980-06-04
GB2034487B GB2034487B (en) 1982-10-06

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Family Applications (1)

Application Number Title Priority Date Filing Date
GB7844410A Expired GB2034487B (en) 1978-11-14 1978-11-14 Alternating current measuring devices

Country Status (1)

Country Link
GB (1) GB2034487B (en)

Cited By (15)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2135786A (en) * 1983-02-25 1984-09-05 Clifford Yates Alternating current sensing device
GB2147708A (en) * 1983-09-22 1985-05-15 David William Knight Sensing apparatus
GB2152681A (en) * 1984-01-13 1985-08-07 Brush Switchgear Device for detecting current flowing in a conductor
DE3701779A1 (en) * 1987-02-13 1988-08-04 Budapesti Mueszaki Egyetem AS A CURRENT TRANSFORMER, LINEAR TRANSMITTER
US4894610A (en) * 1985-09-14 1990-01-16 LOZ Landis & Gyr Zug AG Current-transformer arrangement for an electrostatic meter
EP0409589A2 (en) * 1989-07-21 1991-01-23 Ngk Insulators, Ltd. Optical current transformer
GB2259150A (en) * 1991-08-28 1993-03-03 William Frederick Ray Current measurements
GB2272776A (en) * 1992-05-16 1994-05-25 William Rea Petticrew Sensing energisation of an electric device
US6064191A (en) * 1996-08-01 2000-05-16 Switched Reluctance Drives Limited Current transducer and current-monitoring method
US7227442B2 (en) 2005-04-01 2007-06-05 Schweitzer Engineering Laboratories, Inc. Precision printed circuit board based rogowski coil and method for manufacturing same
US7227441B2 (en) 2005-02-04 2007-06-05 Schweitzer Engineering Laboratories, Inc. Precision Rogowski coil and method for manufacturing same
GB2445571A (en) * 2007-01-10 2008-07-16 Vistec Lithography Ltd An apparatus support with a damping characteristic
WO2013114137A1 (en) 2012-02-03 2013-08-08 Power Electronic Measurements Limited Temperature compensated current measurement
US8928337B2 (en) 2012-01-27 2015-01-06 Schweitzer Engineering Laboratories, Inc. Device for measuring electrical current and method of manufacturing the same
CN104345203A (en) * 2013-07-31 2015-02-11 施耐德电器工业公司 Correction of the bandwidth of an air gap transformer

Cited By (20)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
GB2135786A (en) * 1983-02-25 1984-09-05 Clifford Yates Alternating current sensing device
GB2147708A (en) * 1983-09-22 1985-05-15 David William Knight Sensing apparatus
GB2152681A (en) * 1984-01-13 1985-08-07 Brush Switchgear Device for detecting current flowing in a conductor
US4894610A (en) * 1985-09-14 1990-01-16 LOZ Landis & Gyr Zug AG Current-transformer arrangement for an electrostatic meter
DE3701779A1 (en) * 1987-02-13 1988-08-04 Budapesti Mueszaki Egyetem AS A CURRENT TRANSFORMER, LINEAR TRANSMITTER
EP0409589A2 (en) * 1989-07-21 1991-01-23 Ngk Insulators, Ltd. Optical current transformer
EP0409589A3 (en) * 1989-07-21 1992-03-11 Ngk Insulators, Ltd. Optical current transformer
GB2259150A (en) * 1991-08-28 1993-03-03 William Frederick Ray Current measurements
GB2259150B (en) * 1991-08-28 1995-07-26 William Frederick Ray Improvements in current measurement
GB2272776A (en) * 1992-05-16 1994-05-25 William Rea Petticrew Sensing energisation of an electric device
US6064191A (en) * 1996-08-01 2000-05-16 Switched Reluctance Drives Limited Current transducer and current-monitoring method
US7227441B2 (en) 2005-02-04 2007-06-05 Schweitzer Engineering Laboratories, Inc. Precision Rogowski coil and method for manufacturing same
US7227442B2 (en) 2005-04-01 2007-06-05 Schweitzer Engineering Laboratories, Inc. Precision printed circuit board based rogowski coil and method for manufacturing same
GB2445571A (en) * 2007-01-10 2008-07-16 Vistec Lithography Ltd An apparatus support with a damping characteristic
GB2445571B (en) * 2007-01-10 2008-12-03 Vistec Lithography Ltd Apparatus support
US8006950B2 (en) 2007-01-10 2011-08-30 Vistec Lithography Inc. Apparatus support
US8928337B2 (en) 2012-01-27 2015-01-06 Schweitzer Engineering Laboratories, Inc. Device for measuring electrical current and method of manufacturing the same
WO2013114137A1 (en) 2012-02-03 2013-08-08 Power Electronic Measurements Limited Temperature compensated current measurement
US9970963B2 (en) 2012-02-03 2018-05-15 Power Electronic Measurements Limited Temperature compensated current measurement
CN104345203A (en) * 2013-07-31 2015-02-11 施耐德电器工业公司 Correction of the bandwidth of an air gap transformer

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Publication number Publication date
GB2034487B (en) 1982-10-06

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PCNP Patent ceased through non-payment of renewal fee

Effective date: 19971114