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LEM Hall Current Transducers HTR, LF, LT
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LEM high accuracy Hall effect based Current Transducers enable an electrically isolated measurement of the total instantaneous electric current in conductors through the Transducer aperture by effectively integrating the magnetic field produced around the conductors. All of the LEM Hall Transducers have a frequency response from dc to over 10kHz and for most models to over 100kHz. These Transducers are very suitable for power efficiency measurements in dc, 50/60/400Hz, as well as switch mode or variable speed systems where accurate amplitude and phase response is required.
GMW is stocking a selection of the LEM Hall Current Transducers to cover the current range from ±200A to ±15kA with accuracy in the 0.1 to 1% range, voltage isolation, common mode rejection, good thermal and long-term stability and excellent reliability. Compared to the LEM IT Transducers series of Fluxgate based Current Transducers, the Hall Transducers have a reduced dynamic range, lesser accuracy, reduced temperature stability and lesser rejection of external magnetic fields. But the voltage isolation and common mode rejection are similar, and the frequency response is from dc to equal or higher frequencies than the IT series. The LEM Hall effect Transducers are particularly suitable for production line testing or process monitoring where long-term stability and reliability are critical and the very high performance of the IT series is not required.
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 When it is not practicable to disconnect the current conductor to install a fixed core Current Transducer, the LEM HTR series have a magnetic core that can be split and then closed around the conductor. Current ranges of ±100A and ±1000A are normally carried in stock. These Transducers are particularly useful in diagnostic studies lasting a few days on equipment or electric vehicles. They are of the "open loop", or uncompensated type. The magnetic field from the conductor is essentially amplified by the magnetic core and measured by the Hall field sensor. This type of open loop sensor does not have as good dynamic range, linearity or frequency response as the "closed loop" type.
For the highest accuracy and highest upper frequency current measurements a fixed core, "closed loop" or "zero flux" Hall Transducer is the most appropriate choice. In a closed loop Current Transducer, the Hall magnetic field sensor is used in a circuit to drive the current in a compensating winding around the magnetic core to a value which maintains the magnetic core at zero field or "zero flux". If the net primary current through the Current Transducer aperture is I, the number of turns in the compensation winding is N, and the current in the compensation winding is i, then at the zero flux condition
I = Ni.
N is called the" turns ratio". For a LEM Hall Transducer a typical value is N = 1000. Then a 1000A primary current can be "compensated" by a 1000mA current. This compensation current is the output from the Current Transducer. Model LF Closed Loop Current Transducers are carried in stock with full scale primary current ranges of ±200A to ±3000A.
For higher primary currents the LT Current Transducers are carried with full scale primary current ranges of ±6kA to ±15kA.
The compensation current from closed-loop Current Transducers is converted to a voltage output at the ADC or DVM by using an appropriate value "burden resistor." For a ±1000mA compensation current a 1.000 ohm burden resistor will give a ±1.000V signal. The burden resistor value should be of 0.1% or better accuracy and stability to maintain the overall primary current to signal voltage conversion accuracy. To avoid high open-circuit voltage which may damage the ADC or the current transducer, the burden resistor should always be connected.
All of the LEM Hall Current Transducers stocked by GMW measure the total current through the Transducer aperture, enabling by measurement of the total net current through a bundle of conductors. For example, by feeding all the conductors through the Transducer aperture, the total current to two or three phase loads can be measured to check the net leakage. It is also possible to increase the sensitivity of the current measurement by wrapping the primary current conductors through the Transducer aperture. For example if the primary is wrapped 10 times through the aperture, the effective sensitivity is increase ten times.
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