Current measuring device

• US 5,345,169 A
• Filed: 05/24/1993
• Issued: 09/06/1994
• Est. Priority Date: 02/15/1991
• Status: Expired due to Term

• Current Assignee: Liaisons Electroniques-Mecaniques LEM SA
• Original Assignee: Liaisons Electroniques-Mecaniques LEM SA
• Inventors: Etter, Marcel
• Primary Examiner(s): NGUYEN, VINH P
• Application Number:
  US08/067,483
• Time in Patent Office: 470 Days
• Field of Search: 324/117 R, 324/117 H, 324/126, 324/127, 324/250, 324/253, 336/212, 336/215, 323/356, 323/357
• US Class Current: 324/117.00R
• CPC Class Codes: G01R 15/185 with compensation or feedback windings or interacting coils, e.g. 0-flux sensors using galvano-magnetic field sensors G01R15/20; conversion of DC into AC using transductors G01R19/20
• CPC Class Codes: G01R 19/20 using transductors , i.e. a magnetic core transducer the saturation of which is cyclically reversed by an AC source on the secondary side
• CPC Class Codes: H01F 27/427 for current transformers

Abstract

A current measuring device includes first and second current transformers, having each a magnetic core, a primary conductor or winding and a secondary winding coupled with the magnetic core, the primary windings or conductors being connected to carry the current to be measured. The secondary windings of the first and second transformers have an equal number of turns and are connected in series with each other, and arranged so that the currents induced in the secondary windings by the current to be measured flow in the same direction. A current generator has an output connected to the secondary winding of the first current transformer for feeding to the same a current comprising a magnetizing current component capable of driving the corresponding core, alternately, in opposite states of saturation. A device is provided for controlling the generator by the magnetizing current component, and measuring device are provided for measuring the current flowing in the secondary winding of the second current transformer, which is proportional to the current to be measured.

Claims

1. A current measuring device comprising first and second current transformers having each a magnetic core, a primary conductor or winding and a secondary winding coupled with said magnetic core, said primary windings or conductors being connected to carry a current to be measured, said secondary windings of said first and second transformers having an equal number of turns and being connected in series with each other and arranged so that the currents induced in said secondary windings by the current to be measured flow in the same direction, a current generator having an output connected to said secondary winding of said first current transformer for feeding to the same a current comprising a magnetizing current component capable of driving the corresponding core alternately into opposite states of saturation, a device for controlling said generator by the said magnetizing current component, current measuring means connected to said secondary winding of said second current transformer for measuring the current flowing in said secondary winding, compensating means comprising an operational amplifier a first input of which is connected to a common connection point of said series-connected secondary windings, a biasing potential being applied to a second input of said amplifier by coupling said second input through a negative feed-back loop including the series connection of a low-pass filter and an inverter with the output of said amplifier, the output of said amplifier being further coupled with said first input thereof through a series connection of a condenser and a resistor, so as to maintain the potential of said common connection point substantially constant with respect to variations of said magnetizing current component.

2. A current measuring device according to claim 1, wherein the common connection point of said resistor and said condenser is coupled with the input of said device for controlling said current generator.

3. A current measuring device according to claim 1, wherein said second input of said amplifier of said compensating means is further coupled with said output of said current measuring means.