Tag Archives: Current Sensors

Coreless Magnetic Current Sensors

Modern industrial drives require accurate current measurement for effectively regulating the torque and ensuring maximization of operational efficiency levels. For achieving necessary efficiency levels along with the safety requirements, the measurement methodology must achieve a high degree of linearity and respond rapidly. This is especially true for detecting conditions such as short-circuit and over-current. For instance, it is necessary to arrest the fault condition from an over-current situation within 3us or less. The detection, evaluation, and triggering process must occur within 1 us or less. Therefore, it makes tremendous sense to include this capability within the current sensor.

A popular current measuring scheme involves using a shunt resistor in series with the current under measurement. However, this involves insertion loss, with the resistance of the PCB track, solder joints, and wiring contributing to the loss in addition to that from the shunt resistance. The design becomes more complex if the shunt resistor requires galvanic isolation between control electronics and power output stages.

A better alternative is the magnetic current sensor, primarily based on Hall effect and using core-based or core-less sensing. Being non-resistive, magnetic current sensors involve an insertion loss of a far lower amount. Moreover, magnetic current sensors are contact-less, thereby providing inherent isolation between low voltage and high voltage circuits.

A current flowing through a conductor generates a magnetic flux. A core-based sensor typically concentrates the flux in its ferromagnetic core. The open-loop configuration of the sensor typically uses a sensing element within the air-gap, where the flux concentration is the maximum. This arrangement can have hysteresis and temperature drift errors.

The closed-loop configuration has a compensation winding with current flowing in the opposite direction to minimize the hysteresis and temperature drift errors. Although providing very precise current measurements, the approach is complex and the introduction of the compensation winding generates additional power losses.

In contrast, a core-less sensor does not use a ferromagnetic core, thereby avoiding the hysteresis and temperature drift errors altogether. Current measurement now depends totally on the magnetic field that the current-carrying conductor generates. Although the flux density that the wire generates is much lower, modern electronics design easily compensates for this.

Like the core-based sensor, the core-less sensor also has an open-loop and a closed-loop design. In closed-loop sensing, compensatory windings equalize the flux density and use Hall element sensing. The open-loop sensing uses highly linear Hall elements. Therefore, closed loop sensing does not depend on the linearity of its Hall elements.

With core-less sensors using very low levels of flux density, industrial environments with EMI often makes it difficult to measure the current accurately. Shielding improves the situation to a certain extent, but may not be totally adequate.

A differential measurement approach resolves the situation. This requires a suitable conductor structure along with the presence of at least two sensor elements arranged with their sensitivities in perpendicular. If the electrical connection has the polarities of the sensors opposing each other, and the positioning of the elements above the conductor is symmetrical, they effectively cancel the common-mode component of any external stray fields that may disturb the current measurement.

How to Select Current Sensors?

To select an appropriate AC current sensor for an application, you must know the operational frequency range and the current rating the sensor will encounter. Additional considerations that you will need to decide are the type of the sensor, its mounting (through-hole or surface mount), turns ratio, and the overall dimensions.

Sensor type refers to a sensor only configuration, where a conductor integral to the application forms the primary. Another type could be a complete current transformer where the primary is included as a winding. Engineers typically use current sensors to measure and control the load current in control circuits, safety circuits, and power supplies. Power supplies usually require accurate control of current, and this requires sensing the magnitude of the current accurately.

Irrespective of whether you are using the sensor or transformer, the highest flux density handled is dependent on the worst-case current and frequency faced by the device. However, note that exceeding 2000 Gauss will mean most AC current sensors output will be non-linear. Therefore, the current through the sensor and its output voltage will no longer remain proportional, as the magnetic core of the sensor saturates at very high flux densities. To keep the flux density below the saturation limit, it is necessary to use higher secondary turns.

For instance, in wire-through-the-hole style of current transformers, looping additional primary turns through the hole can dramatically reduce the turns ratio, provided the wire diameter and the hole size permit. Increasing the primary turns allows the use of a higher input current transformer to provide higher output voltage across the terminating resistor on the secondary.

Manufacturers of current transformers offer online tools to help designers select the right current sensor or current transformer for specific application conditions. Initially, the user has to select the type of sensor—a transformer or a sensor only. The next selection is the preferred mounting style—SMT or Through-hole. The online tool also requires other parameters such as the maximum sensed current expected in amperes, the input frequency in kHz, the duty cycle of the primary current waveform as a percentage, and the desired output voltage corresponding to the expected maximum input current.

The tool then calculates the required terminating resistance based on the maximum input current, the number of secondary turns and the output voltage—basing the calculations on a single-turn primary. Next, the tool calculates the maximum flux density of the secondary, making sure it does not exceed 2000 Gauss. It does this by taking into account the output voltage, the duty cycle, secondary turns, and the frequency of operation.

The result lists all part numbers of the manufacturer that meet these input conditions, typically including a graph of the output voltage versus the sensed current for the calculated terminating resistance.

To select an appropriate current sense transformer for your application, you require knowledge of the maximum current, frequency, and duty cycle of the sensed current, including the output voltage you require. Using this information, the online selector tools will provide you with the appropriate terminating resistor value and a list of current sensors that meet the conditions of the application.