24 September 2001
Current-measurement technology continues its advance
Hall-effect sensors are gaining in popularity.
Process engineers have used a wide variety of methods to measure and monitor current through the years, and there have been several advances in the methods used to sense current. But often the measured signal requires further conditioning in order to make it valuable to the controller. Simply connecting an ammeter and reading out a value is no longer enough. The current signal, whether AC or DC, must be electrically isolated, filtered, and converted in an analog process signal or alarm.
The shunt method is the oldest method used for current measurement; it works by measuring the voltage drop across a resistor placed in the current path. The resistance used is very small; therefore, it is large in size. An external signal conditioner then amplifies, filters, and isolates the resulting output. This method increases the overall installation and system cost without providing adequate accuracy.
The current transformer (CT), or "doughnut," is a very simple, popular, widely used device based on the transformer principle. One common design consists of a soft iron core and two windings. The high current on the input side steps down to a lower level in specified ratiose.g., 10:1 = 100 amps (A) on the primary to 1 A on the secondary). Other designs feature a wire-wound core, which includes some signal conditioning circuitry. The current-carrying conductor is the primary, and the core winding is the secondary. The current on the secondary provides a process signal such as 4-20 mA.
Hall-effect sensors are gaining popularity due to their unique characteristics. These sensors measure the magnetic flux density around a current-carrying conductor and, unlike a CT, measure DC currents. The sensor converts the magnetic flux into a voltage signal that is proportional to the current. The sensor assembly consists of the magnetic core, the Hall-effect device, and the signal conditioning circuitry. The magnetic core surrounds the current-carrying conductor to concentrate the magnetic field. The Hall element is in a small air gap in the core perpendicular to the concentrated magnetic field.
Hall devices are three- or four-wire sensors requiring a power source to energize itself. When the energized Hall device detects a constant current flow, it produces a potential difference, or voltage, that is proportional to the measured current. Further, the sensor provides galvanic isolation because it is isolated from the monitored voltage. The sensor circuitry must amplify and compensate the very small voltage output of the Hall sensor for inaccuracies arising from the sensitivity of the device. Hall sensors measure both AC and DC, which serves as an advantage over current transformers.
Current transducers convert a current measurement into a DC voltage or current (mA) signal and are available in transformer or Hall-based designs. They are ideal for applications that require an analog signal proportional to the measured input current.
Designers use current transducers in applications such as motor current monitoring or pump status control in the wastewater, semiconductor, and food and beverage industries. Preventing damage of sludge motor windings in a wastewater application, for example, is critical because it could cause downtime and system failure; installing a current transducer on one leg of the motor lets the user monitor the analog output for overload and underload conditions. Devices with alarm contacts and analog output can shut off motor control circuits or switch to a backup control. Alternatively, the alarms can give a warning signal before a catastrophic failure.
Advances in transducer designs offer compact DIN-rail mounted devices in AC- or DC-powered or input loop-powered designs. Some designs feature analog outputs and relay alarm set points with time delays, combining several functions into one space-saving, economical package. IT
Davis Mathews is an instrumentation product specialist at Phoenix Contact Inc. in Harrisburg, Pa.