Overview
Motor drives are among the highest-volume applications for current transducers, because an accurate phase-current signal is the foundation of modern motor control. A variable-speed drive converts a DC bus into three-phase AC and regulates torque and speed by controlling the current in each phase, so the quality of the current feedback directly limits the achievable control bandwidth and efficiency. LEM current transducers provide that feedback with galvanic isolation, and BeiLuo supplies them with genuine traceability and FAE support.
Why Isolation Matters in a Drive
A drive's power stage sits on a high-voltage DC bus, often several hundred volts, while the controller runs at low voltage. The phase conductor of the motor carries a switching voltage with a large common-mode component, so measuring it without isolation would expose the controller to dangerous voltage and destroy it. Isolation breaks that path while still passing an accurate representation of the current. It also allows the sensor and its associated circuitry to float with the power stage, which avoids ground loops and simplifies the grounding scheme of the drive. Every LEM current transducer in this solution provides this isolation as a core function, not an accessory.
Closed-Loop Transducers for High-Performance Control
Closed-loop, or compensated, transducers such as the LA55-P and the LTS25-NP drive a secondary current that exactly balances the primary ampere-turns. Because the magnetic core is held at zero flux, the output tracks the primary current with high accuracy, low nonlinearity and very low offset drift over temperature. The fast response preserves the phase information that field-oriented control depends on, so the current loop can run at higher bandwidth and the drive can produce smoother torque with less ripple. The LA55-P uses a bipolar analog supply and suits conventional control boards, while the LTS25-NP runs from a single +5 V rail for modern digital designs. Both cover the rated and overload current of a compact to medium-power drive.
Field-Oriented Control
Field-oriented control transforms the measured phase currents into a rotating reference frame so that torque and flux can be controlled independently. The accuracy of that transformation depends on the current signal: any offset or gain error becomes a torque error, and any phase lag reduces the achievable bandwidth. A closed-loop transducer minimizes all three, which is why it is the default choice for servo and high-performance induction-motor drives.
Open-Loop Transducers for Cost-Sensitive Drives
Open-loop transducers such as the HAS50-S measure the magnetic field produced by the primary current directly, so they are smaller, lower cost and need less power than closed-loop parts. They suit general-purpose inverters and auxiliary current measurement where moderate accuracy and a wide measuring range are more important than the last point of precision. The HAS50-S delivers a direct voltage output with generous overload headroom from a simple bipolar supply, which simplifies design-in. For a compact board, the HLSR20-P open-loop sensor with a single +5 V supply is a good fit for auxiliary rails and lower currents.
Designing the Sensing Stage
Getting the best accuracy from a current transducer is as much about layout as about the part itself. Keep the primary conductor centered in the aperture, route it symmetrically, and follow the recommended copper width and thickness, because the conductor geometry is part of the factory calibration. Keep the secondary output away from switching nodes, use a short, screened connection to the ADC, and place the transducer so that external magnetic fields do not disturb the measurement. Choose the measuring resistor to match the ADC input range and respect the maximum measuring resistance at your supply. For DC-link monitoring, add an isolated voltage transducer such as the LV25-P on the same board so the control loop sees both current and voltage.
Selection and Supply
Selection starts with the rated phase current and the overload profile, then narrows by closed-loop or open-loop, supply, mounting and accuracy budget. Confirm the measuring range at the operating supply voltage, because the usable span depends on it, and check the temperature range against the drive enclosure. BeiLuo keeps the mainstream LEM current-transducer models in regional stock, ships them with import declaration, certificate of origin and RoHS documents, and supports every design from sample evaluation to volume production. Our FAE team can review the sensing stage, size the measuring resistor and help you match the transducer to your control bandwidth and accuracy target.