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Precision Current Measurement in Industrial Drives

Fri, 07/29/2011 - 8:20am
Arek Kacprzak and Marc Royer, Texas Instruments

Government regulations around the world are calling for higher efficiency of industrial motor drives because they account for a considerable amount of our total energy consumption. Higher efficiency requires optimization of the torque and rotor speed, depending on the actual demand of the system, which can be achieved in part by increasing the accuracy of the motor current measurement.

The industrial drives market includes five segments: DC, servo, compact AC, standard AC, and premium AC. For the premium and servo drives segments, designers tend to use discrete ADCs for current and position signal information conversion, as shown in the block diagram below. Historically, the ADC architecture used for the discrete ADCs has been SAR based.

ADS1200_dig1

This requires the use of expensive components such as Hall-effect magnetic sensors for current sensing, and very precise, discrete, simultaneous-sampling SAR converters for measurement of the load position and/or speed.

One industry trend that is gaining more and more traction is to use a different ADC architecture, delta-sigma modulation, to perform current and position measurement. The primary reason for this shift is that delta-sigma modulation reduces overall system cost while providing high performance.

Although the primary function of the magnetic sensors is to measure current, they also provide important electrical isolation. The magnetic sensor can be replaced with a current shunt resistor and isolated delta-sigma modulator, a more accurate and less expensive option, and provide direct measurement of current. TI’s latest isolated delta-sigma modulator, the AMC1204, is designed specifically for direct connection to current shunt resistors and has a galvanic isolation barrier. A block diagram of the new approach is shown below.

ADS1200_Dig2

In the compact drives segment, cost is an even stronger consideration. This results in designers using ADCs integrated in the host controller rather than a discrete ADC solution. To further reduce costs, designers are able to use current shunt resistors connected to isolated amplifiers, which are then connected directly to the integrated ADCs on the host controller as an alternative to more costly magnetic sensor architecture. TI’s latest isolated amplifier, the AMC1200, is designed specifically for direct connection to current shunt resistors and has a galvanic isolation barrier. A block diagram of this solution is shown below.

ADS1200_Dig3

Regardless of the class of drive, designers are always looking for ways to reduce the overall cost of their system while maintaining performance. Switching from a magnetic sensing to a current-sensing based system using an isolated delta-sigma modulator or isolated amplifier can help achieve this.

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