Why a Mag-Gap?


Metal cores used for Inductive Sensors are needed for high gain but will saturate. A mag-gap can control saturation. New AC sensor technology, using a combination of a metal core plus a mag-gap and almost unlimited turns, achieves high efficiency and wide-range current/voltage sensing.

SRT has designed an alternating current (AC) sensing device that is more efficient, and will adapt to automatic manufacturing methods better than do conventional toroids or current transformers (CT's). These new sensors differ from the conventional toroid or CT in that, although it does use an inductive technique, it uses an mag-gapped high-permeability core in such a manner that increases its linear response range and minimizes nonlinear saturation effects. The result is a superior AC current sensing device (although not intended to be used as a power transfer device). A particular adaptation of this new current sensor technology was used to make an effective AC voltage sensor.

The technology of current sensing with an inductive type sensor started many years ago in the power industry, where the amount of power used to make the measurement (i.e. current with a current transformer) was incidental. However, the instrumentation channels of the future for microprocessor and computer data collection and control of processes, must make measurements with a minimum expenditure of energy.

Toroids and current transformers are excellent for power conversion. But how well do they operate as an AC instrumentation current sensor, or meet the demand for data acquisition for computer control of systems? This new current sensor technology described is an inductive current-to-voltage converter designed to use a minimum of power. (The hall-effect inductive sensor was not addressed here because it takes auxiliary power to measure current).

Our High Linearity Current Sensors are self powered by the line being monitored and they will not saturate. This series of high linearity sensors are available in either AC or DC output voltage models, as well as a 4-20 mA driver.

Linearity of better than 1% is available on all DC output models from 2 to 100 amps, linearity then goes from 1% at 100 amperes to 2% at 600 amperes. Most of our DC output AC current sensors give 5 Vdc output at full scale current, but some models allow 10 Vdc output and fast response for SPECIAL FUNCTIONS. The input current range for 5 Vdc output can be as low as 100 mA with additional primary turns.

Frequency response of our DC output sensors is usually in the 50 Hz to 20 kHz range, however SRT Current Actuated Switches (which control the switching process with DC voltage) can work easily at 10 Hz.

All AC output current sensors have 10V peak-to-peak or 3.5 Vrms output at full scale from 2 to 600 amps.

Other features in the AC output current sensor series are fast response for statistically reliable sampling rates, isolated analog signal output, and frequency response allowing operation from variable frequency drive frequencies to a maximum frequency of 14 MHz. Advantages are small size, light weight, no need for an external power source, and they always run cool.

Applications include process control, data acquisition, power measurement, low level current sensing, motor protection, building control, and factory automation.

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