One of the most significant problems associated with PWM motor drives is common-mode current. The high-frequency common-mode voltage generated by the inverter can couple through parasitic capacitance between the motor windings, motor frame, cable shield, machine chassis and earth.

This can produce unwanted current circulating through the motor and machine structure. In severe cases, common-mode current can contribute to motor bearing currents, shaft voltage, encoder interference, leakage-current problems and interference with neighbouring electronic equipment.

BLA can control this phenomenon using common-mode chokes, common-mode inductors, three-phase common-mode filters, ferrite cores and custom common-mode suppression assemblies. For high-current VFD applications, the magnetic design can be optimized for saturation, copper loss, core loss, thermal rise and high-frequency impedance.

VFDs also generate differential-mode noise, which appears between phases or between positive and negative conductors. The switching current produced by the inverter can contain significant high-frequency components that propagate back toward the mains or through the DC link.

BLA can address differential-mode interference using differential-mode chokes, line inductors, DC chokes, X capacitors and LC filtering networks. These components can be combined with common-mode filtering to create a multi-stage EMI filter capable of addressing both noise mechanisms simultaneously.

The filter must be engineered carefully because the interaction between converter input impedance, filter impedance and load impedance can create resonances. BLA's customized approach allows the filtering network to be developed around the actual electrical characteristics of the drive rather than relying solely on a generic catalogue solution.

Most industrial VFDs are supplied from three-phase power, making three-phase EMI filters an important part of the EMC architecture. BLA can engineer three-phase filters for different voltage, current and frequency requirements, including solutions for high-power drives and machine tools.

The filter can be configured to provide both common-mode and differential-mode attenuation, with the magnetic components selected according to continuous RMS current, overload conditions, switching frequency, core saturation and required insertion loss.

Where necessary, BLA can also provide three-phase + neutral filtering for systems where the neutral conductor forms part of the noise propagation path.

AC line reactors and input chokes are particularly valuable in VFD installations because they introduce controlled impedance between the drive and the supply. They can help reduce current ripple, limit certain transient currents and provide additional impedance to unwanted high-frequency components.

BLA can develop single-phase reactors, three-phase line reactors, input chokes and custom high-current inductors for drive applications. The reactor can be used independently or coordinated with an EMI filter to create a more comprehensive input-side noise-control architecture.

For high-power drives, magnetic design becomes critical because the component must handle continuous current without excessive temperature rise or saturation while maintaining the desired impedance.

The VFD's DC link is another important noise-control location. The rectifier and inverter stages create ripple and switching-current components on the DC bus. A DC link choke/DC reactor introduces additional impedance into this path and can reduce the propagation of certain unwanted current components.

BLA can manufacture DC reactors, DC link chokes and high-current inductive components designed around the DC bus voltage, current waveform, ripple current and required thermal performance.

For demanding applications, the DC choke can be combined with an input EMI filter and output-side filtering to create a coordinated three-stage noise-control architecture.

The cable between the VFD and motor is frequently one of the most important parts of the EMI problem. A PWM inverter does not produce a smooth sinusoidal voltage directly; instead, the motor sees rapidly switched voltage pulses. The cable's distributed inductance and capacitance can produce reflections and high-frequency current paths.

Long motor cables can therefore increase common-mode current, reflected-wave voltage, motor insulation stress and radiated emissions. BLA can address these issues using output reactors, motor chokes, common-mode chokes, dv/dt filters and sine-wave filters, depending on the application.

Where a much cleaner motor waveform is required, BLA can provide sine-wave filters. Instead of simply reducing dv/dt, a properly designed sine-wave filter significantly attenuates the carrier-frequency components of the PWM waveform and produces a much more sinusoidal voltage at the motor.

This can be valuable for applications involving long motor cables, sensitive motors, older motors, acoustic-noise-sensitive machines and demanding industrial drive installations.

BLA's published product categories include sine-wave filters and VFD-related filtering solutions, allowing the output stage to be engineered according to the motor and drive combination.

Servo systems operate with rapid acceleration and deceleration and high-bandwidth current and position-control loops. Their switching frequencies and fast dynamic response can produce significant high-frequency electrical activity.

Multiple servo axes can also interact through the common DC bus or machine chassis. Noise generated by one axis can therefore couple into another axis or into sensitive feedback electronics.

BLA can develop servo-drive input EMI filters, common-mode chokes, DC-link chokes, output reactors, ferrite suppression and signal-line filters for multi-axis CNC and robotic systems.

Feedback systems are particularly vulnerable because encoder and resolver signals can be low-level compared with the high-voltage PWM switching waveform in the same machine.

Common-mode currents can couple through cable shields and machine structures, while capacitive and inductive coupling can introduce high-frequency components into the feedback signal.

BLA can provide signal-line EMI filters, common-mode signal chokes, ferrite beads, feedthrough filters and customized encoder-line suppression solutions while taking into account signal bandwidth, impedance and pulse integrity.

The objective is to suppress unwanted high-frequency noise without filtering out the actual encoder or resolver information.

Modern machine tools increasingly use Industrial Ethernet, EtherCAT, PROFINET, CAN, RS-485 and other high-speed communication networks. These interfaces can be affected by common-mode noise generated by VFDs and servo drives.

BLA can develop data-line common-mode chokes, signal filters, ferrite suppression components and feedthrough filters for communication interfaces. High-speed data filtering requires careful control of parasitic capacitance and differential-mode insertion loss because excessive filtering can distort the communication waveform.

VFDs do not only create high-frequency EMI. Their front-end rectifier can also draw non-sinusoidal current from the supply, producing harmonic currents. These lower-frequency disturbances are different from high-frequency EMI and require different mitigation strategies.

BLA can complement EMI filtering with line reactors, harmonic filters, DC chokes and power-quality filtering solutions. The appropriate combination depends on the drive architecture, load profile, power system impedance and applicable power-quality requirements.

This distinction is important: an EMI filter is not automatically a harmonic filter, and a harmonic filter is not automatically an effective high-frequency EMI filter. BLA can engineer the two functions separately or integrate them into a coordinated system.

For modern adjustable-speed drives and machine tools, IEC 61800-3:2022 is a key EMC standard. The IEC states that it specifies EMC requirements and specific test methods for adjustable-speed power drive systems and machine tools, covering AC and DC PDS equipment and machine tools with input/output voltages up to 35 kV AC RMS. It applies across power ratings and includes both emission and immunity requirements.

The latest IEC edition also expanded the scope to machine tools with embedded power drive systems and extended radiated-immunity testing to 6 GHz. A 2025 corrigendum is incorporated into the current edition.

For BLA, this is highly relevant because the filter should be selected according to the actual PDS/machine-tool EMC environment, rather than simply selecting a filter based on voltage and current.

IEC 61800-3 recognizes different installation environments and drive categories because the EMC requirements depend strongly on where the equipment operates. A small drive installed close to residential equipment presents a different electromagnetic challenge from a high-power industrial drive installed inside a factory.

BLA can therefore engineer the filtering architecture around the intended application environment, including industrial machinery, CNC equipment, machine tools, pumps, compressors, conveyors, fans, HVAC drives, robotics and automated production lines. The objective is to achieve the required emission and immunity performance without unnecessarily increasing filter size, leakage current or power loss.

One of the most overlooked aspects of VFD EMC is that the filter's laboratory insertion-loss performance does not automatically translate into identical performance after installation. At high frequencies, cable inductance, chassis impedance, parasitic capacitance and grounding arrangements become significant.

A filter connected to the drive with long conductors can have a considerably different high-frequency behaviour from the same filter installed directly at the equipment entry point. BLA's customized approach therefore considers filter placement, chassis bonding, cable routing, motor-cable shielding, PE connections and separation between dirty and clean wiring.

The fundamental EMC approach is:

Noise Source → Coupling Path → Sensitive/Victim Circuit.

The most effective solution is normally the one that interrupts this chain at the most practical point.

Ferrite cores, ferrite rings, ferrite beads and cable-mounted ferrites can provide additional impedance to high-frequency common-mode currents. BLA can select ferrite materials according to the required frequency range, current level and cable geometry.

Ferrite suppression can be applied to motor cables, control wiring, encoder cables, communication lines and DC power connections. The ferrite's complex impedance versus frequency is important because the objective is to suppress the unwanted high-frequency spectrum while avoiding unnecessary losses at the normal operating frequency.

For CNC control cabinets and industrial machine enclosures, feedthrough filters and chassis-mounted EMI filters can provide filtering directly at the point where cables cross the enclosure boundary.

This minimizes the length of unfiltered conductors inside the cabinet and helps establish a controlled high-frequency return path. BLA can customize feedthrough capacitors, filtered terminals and chassis-mounted filter assemblies according to the machine's mechanical configuration.

BLA's major advantage is its ability to move beyond a standard "one filter fits all" approach. Its Machines & CNC offering specifically highlights customized solutions and collaboration with manufacturers to analyze requirements and implement tailored solutions.

For a VFD manufacturer, BLA can develop the input EMI filter around the drive's switching spectrum. For a CNC OEM, BLA can develop a complete machine-level EMC architecture incorporating power filters, reactors, servo filtering and signal-line suppression. For a motor manufacturer, BLA can develop common-mode chokes, motor chokes and output filtering. For a machine builder experiencing EMC test failures, BLA can engineer a targeted suppression network around the actual problematic frequency.

BLA's solution set can include single-phase EMI filters, three-phase EMI filters, three-phase + neutral filters, VFD input filters, AC line reactors, input chokes, DC reactors, DC link chokes, common-mode chokes, differential-mode chokes, motor chokes, output reactors, dv/dt filters, sine-wave filters, harmonic filters, LC filters, LCL filters, ferrite cores, ferrite beads, X/Y capacitors, DC EMI filters, signal-line filters, encoder filters, data-line common-mode chokes, feedthrough filters and custom inductive components.

This allows BLA to address the complete drive architecture from the mains input → rectifier → DC link → inverter → motor cable → motor, while also addressing the secondary paths involving encoder, PLC, CNC controller, communication network and machine chassis.