Current Sensing Techniques for Motor Control

Understanding the Fundamentals of Current Sensing in Motor Control Systems

Current sensing forms the backbone of modern motor control systems, enabling precise regulation of torque, speed, and position while providing critical protection against fault conditions. For industries across Atlantic Canada—from the fishing and aquaculture operations along Nova Scotia's coastline to the manufacturing facilities throughout the Maritime provinces—reliable motor control is essential for maintaining operational efficiency and equipment longevity.

The selection of an appropriate current sensing technique directly impacts system performance, cost, and reliability. Engineers must carefully evaluate factors including bandwidth requirements, isolation needs, power dissipation, accuracy specifications, and environmental conditions when designing motor drive systems. This comprehensive guide explores the primary current sensing methodologies employed in contemporary motor control applications, providing practical insights for engineers and technical managers making critical design decisions.

Shunt Resistor Current Sensing: The Industry Workhorse

Shunt resistor current sensing remains the most widely implemented technique in motor control applications due to its simplicity, low cost, and excellent accuracy. This method measures current by detecting the voltage drop across a precision low-value resistor placed in the current path, following Ohm's law (V = IR).

Technical Specifications and Design Considerations

Modern current sense resistors typically range from 0.5 mΩ to 100 mΩ, with tolerances as tight as ±0.1% for precision applications. For a typical 10-amp motor drive application, a 10 mΩ shunt resistor produces a 100 mV signal at full scale, requiring careful signal conditioning to maintain measurement accuracy.

Key specifications to consider when selecting shunt resistors include:

• Temperature Coefficient of Resistance (TCR): Premium shunts offer TCR values below ±15 ppm/°C, essential for maintaining accuracy across the -40°C to +85°C operating range common in Maritime industrial environments

• Power Rating: Must accommodate I²R losses with adequate derating; a 10 mΩ resistor carrying 20 amps continuously dissipates 4 watts

• Inductance: Four-terminal (Kelvin) connections and specialised alloys minimise parasitic inductance to below 0.5 nH for high-frequency PWM applications

• Thermal EMF: Critical for low-current measurements; quality shunts exhibit thermal EMF below 3 μV/°C

Placement Strategies: High-Side vs. Low-Side Sensing

Low-side shunt placement (between the load and ground) simplifies the signal conditioning circuitry since the sense voltage is ground-referenced. However, this configuration cannot detect ground faults and may introduce ground bounce issues in high-current applications.

High-side sensing (between the power supply and load) detects all fault conditions including ground shorts but requires differential amplifiers or specialised current sense amplifiers capable of operating with high common-mode voltages. Modern integrated current sense amplifiers from manufacturers like Texas Instruments and Analog Devices support common-mode voltages exceeding 100 volts while providing gains from 20 V/V to 200 V/V with bandwidth specifications reaching 1 MHz.

Hall Effect Current Sensors: Galvanic Isolation Without Compromise

Hall effect sensors provide non-contact current measurement through magnetic field detection, offering inherent galvanic isolation between the power circuit and control electronics. This isolation proves particularly valuable in motor drives operating at elevated voltages or in applications requiring safety certification to standards such as CSA C22.2 for Canadian installations.

Operating Principles and Performance Characteristics

When current flows through a conductor, it generates a proportional magnetic field according to Ampère's law. Hall effect sensors detect this field by measuring the Hall voltage developed across a semiconductor element placed perpendicular to the magnetic flux. The relationship follows the equation V_H = (I × B × R_H) / t, where R_H is the Hall coefficient and t is the element thickness.

Open-loop Hall sensors provide cost-effective solutions for applications requiring ±1% to ±3% accuracy with bandwidth capabilities from DC to approximately 25 kHz. These devices are well-suited for general-purpose motor protection and monitoring applications common in Maritime processing facilities.

Closed-loop (compensated) Hall sensors achieve superior performance through magnetic feedback compensation, delivering accuracies of ±0.5% or better with bandwidth extending to 200 kHz. The compensation winding nulls the primary magnetic field, resulting in a current output proportional to the measured current with excellent linearity specifications below ±0.1%.

Integration Considerations for Maritime Applications

Nova Scotia's coastal industrial facilities face unique environmental challenges including salt air exposure, humidity levels frequently exceeding 80%, and temperature variations spanning -30°C winter conditions to +35°C summer peaks. Hall effect sensors with conformal coating and IP65 or higher ingress protection ratings provide reliable performance in these demanding conditions.

Leading manufacturers including LEM, Allegro MicroSystems, and Honeywell offer Hall effect current transducers rated for extended temperature ranges and featuring integrated signal conditioning. Devices such as the LEM HO series provide measurement ranges from 6 to 250 amps with response times under 3 microseconds, making them suitable for field-oriented control (FOC) implementations in industrial motor drives.

Current Transformers: High-Frequency AC Sensing Solutions

Current transformers (CTs) offer another galvanically isolated approach to current measurement, operating on the principle of electromagnetic induction. Unlike Hall effect sensors, CTs respond only to alternating currents, making them particularly suitable for AC motor applications and high-frequency sensing in PWM inverter systems.

Design Parameters and Selection Criteria

A current transformer consists of a magnetic core with a secondary winding, where the primary conductor (carrying the current to be measured) passes through the core aperture. The transformation ratio follows N_PI_P = N_SI_S, where a single-turn primary and 1000-turn secondary produces a 1000:1 current ratio.

Critical specifications for motor control applications include:

• Frequency Response: Wide-bandwidth CTs support frequencies from 1 kHz to over 1 MHz, essential for capturing PWM current ripple in drives operating at 10-20 kHz switching frequencies

• Phase Accuracy: Phase errors below ±0.5° ensure accurate power and power factor measurements

• Saturation Characteristics: The core must not saturate under maximum current and DC offset conditions; nanocrystalline and amorphous core materials offer superior saturation flux densities approaching 1.2 Tesla

• Burden Resistance: Output termination affects linearity and phase response; typical values range from 10 Ω to 200 Ω

Rogowski Coil Technology

Rogowski coils represent a specialised subset of current transformer technology using an air-cored design that eliminates magnetic saturation concerns. These flexible, lightweight sensors wrap around conductors of various sizes, providing exceptional linearity across wide dynamic ranges from milliamps to hundreds of kiloamps.

The integrating electronics required to convert the Rogowski coil's di/dt output signal to a proportional current measurement add complexity but enable frequency responses extending from below 1 Hz to several MHz. This bandwidth makes Rogowski coils ideal for analysing transient events and harmonics in variable frequency drive (VFD) applications powering pumps, compressors, and conveyor systems throughout Atlantic Canadian industrial facilities.

Integrated Current Sensing in Modern Motor Drives

Contemporary motor drive architectures increasingly incorporate current sensing within the power module itself, utilising the inherent resistance of MOSFET or IGBT devices for measurement. This approach, commonly termed "sensorless" current sensing, eliminates external components while maintaining adequate accuracy for many applications.

MOSFET RDS(on) Sensing

During conduction, the drain-source voltage of a MOSFET relates to current through its on-state resistance. For a typical power MOSFET with R_DS(on) of 10 mΩ, a 10-amp current produces a 100 mV sense signal identical to an equivalent shunt resistor.

The primary challenge with this technique involves the strong temperature dependence of R_DS(on), which can vary by a factor of 1.5 to 2.0 between 25°C and 125°C junction temperatures. Compensation algorithms using on-chip temperature sensors or thermal models are essential for maintaining accuracy across operating conditions.

Intelligent Power Modules with Integrated Sensing

Manufacturers including Infineon, ON Semiconductor, and Mitsubishi Electric offer intelligent power modules (IPMs) combining power switches, gate drivers, and current sensing in optimised packages. These modules often incorporate shunt resistors positioned for accurate phase current measurement, with specifications including:

• Current measurement accuracy of ±5% over temperature

• Bandwidth exceeding 100 kHz for FOC implementation

• Built-in overcurrent protection with response times under 1 microsecond

• Analogue or digital (SPI/SENT) output interfaces

Application-Specific Selection Guidelines

Selecting the optimal current sensing technique requires careful analysis of application requirements against the capabilities and limitations of each technology. The following guidelines address common motor control scenarios encountered across Maritime industries.

Three-Phase Induction Motor Drives

For vector-controlled induction motor drives powering industrial equipment—common in fish processing plants, pulp and paper operations, and manufacturing facilities throughout Nova Scotia—closed-loop Hall effect sensors or precision shunt resistors provide the accuracy and bandwidth necessary for dynamic torque control. A typical implementation senses two of the three phase currents, calculating the third using Kirchhoff's current law (I_A + I_B + I_C = 0).

Brushless DC (BLDC) Motor Control

BLDC motors driving compressors, pumps, and fans in commercial and industrial HVAC systems require current sensing for commutation timing and overcurrent protection. Low-cost low-side shunt sensing often suffices for six-step commutation, while sinusoidal control demands higher bandwidth sensors with better than ±1% accuracy.

Servo Motor Systems

High-performance servo applications in automated manufacturing require current sensing with bandwidth exceeding 200 kHz and accuracy below ±0.5% to achieve precise torque control during rapid acceleration and deceleration profiles. Closed-loop Hall sensors or precision isolated amplifiers with high-grade shunt resistors meet these demanding specifications.

Emerging Technologies and Future Directions

The evolution of current sensing technology continues to address the demands of next-generation motor control systems, including those supporting Nova Scotia's growing renewable energy sector and electric vehicle charging infrastructure.

Tunnel Magnetoresistance (TMR) Sensors

TMR sensors offer significant advantages over traditional Hall effect devices, including sensitivity improvements of 10× or greater and reduced temperature drift. These characteristics enable smaller, more accurate current sensors suitable for high-density power electronics designs.

Fluxgate Technology

Fluxgate current sensors provide exceptional DC accuracy with offset errors below 100 μA and drift specifications under 1 ppm/°C. These devices excel in renewable energy applications where precise DC current measurement is critical for maximum power point tracking and energy metering.

Digital Isolation Integration

Modern current sense amplifiers increasingly integrate digital isolation using capacitive or magnetic coupling, providing CMOS-compatible digital outputs that interface directly with microcontrollers and digital signal processors. This integration simplifies system design while improving noise immunity in electrically harsh industrial environments.

Partner with Atlantic Canada's Engineering Experts

Implementing effective current sensing solutions for motor control systems requires expertise spanning power electronics, signal conditioning, and control theory. Whether you're upgrading legacy motor drives, designing new automated systems, or troubleshooting existing installations, proper current sensing implementation is fundamental to achieving reliable, efficient operation.

Sangster Engineering Ltd. in Amherst, Nova Scotia, provides comprehensive engineering services to industrial clients throughout Atlantic Canada and beyond. Our team combines deep technical knowledge with practical experience in motor control systems, power electronics design, and industrial automation. From initial concept development through detailed design, prototyping, and production support, we deliver engineering solutions tailored to your specific requirements and operating environment.

Contact Sangster Engineering Ltd. today to discuss your motor control challenges and discover how our engineering expertise can help optimise your operations for improved performance, reliability, and energy efficiency.

Partner with Sangster Engineering

At Sangster Engineering Ltd. in Amherst, Nova Scotia, we bring decades of engineering experience to every project. Serving clients across Atlantic Canada and beyond.

Contact us today to discuss your engineering needs.