Beckhoff Drive Accessories Explained:

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 How Main Chokes and Filters Impact EMC Compliance, System Reliability, and Long-Term Cost**

When selecting servo drives for an industrial automation system, accessories such as main chokes and power filters are often treated as optional components.

In practice, these accessories play a critical role in:

  • EMC compliance (EN 61800-3)

  • System stability and uptime

  • Motor cable length limitations

  • Long-term maintenance and project cost

For Beckhoff AX5000 servo drives, incorrect selection of chokes or filters can lead to EMC failures, unexpected drive faults, or expensive redesigns late in the project lifecycle.

This article explains how to correctly select main chokes, main filters, and motor chokes, with a focus on system-level engineering decisions rather than component-level descriptions.

Main Power Choke: Reducing Harmonics and Protecting the Power Grid

A main power choke (line choke) is installed on the supply side of the drive (L1, L2, L3).

Its primary functions include:

  • Reducing current peaks caused by IGBT switching

  • Limiting harmonic feedback into the main power supply

  • Improving electrical stability in multi-drive systems

For AX5160 and higher AX5000 drives with rated current above 60 A, the use of a main power choke is mandatory.

From a system design perspective, a main choke not only improves EMC performance, but also reduces electrical stress on upstream components, which directly impacts reliability and long-term operating cost.

Main Power Filter: Meeting EMC Compliance Requirements

A main power filter is designed to suppress conducted electromagnetic interference and improve immunity to external disturbances.

According to EN 61800-3:

  • C3 compliance is required for industrial environments

  • C2 compliance is required for public buildings and residential environments

Beckhoff AX5000-specific considerations:



  • AX5160 and AX5172 drives include an integrated main power filter
    → An external filter is only required when the supply cable length exceeds 25 meters (C3 environment)

  • AX5190 to AX5193 drives do not include an integrated filter
    → An external main power filter is always required

Failing to consider this during the design phase often results in failed EMC testing or costly retrofits, especially in projects with strict compliance requirements.

Motor Choke: Enabling Longer Cable Lengths and Improving EMC

A motor choke is installed between the drive and the motor, typically 0.2–0.3 meters from the drive output.

Its main purposes are:

  • Reducing commutation current flowing back through the motor cable shield

  • Improving EMC performance

  • Allowing longer motor cable lengths

For AX5140 and smaller drives, a motor choke is required when:

  • Motor cable length exceeds 25 meters

  • The system must meet C2 EMC requirements

In many projects, installing a motor choke is more cost-effective than redesigning cable routing or upgrading the drive later.

AX5140 Special Case: Cable Length Limitations

The AX5140 series has specific limitations regarding accessory support:

  • No dedicated main choke, main filter, or motor choke models are available

  • Maximum motor cable length:

    • 35 meters (400 V, C3 industrial environment)

    • 50 meters with special version AX5140-0000-1202

These limitations should be evaluated during the system architecture and cabinet layout phase, not after commissioning.

Single Choke vs. Individual Chokes in Multi-Drive Cabinets

In control cabinets with multiple servo drives, two common approaches exist:

  1. Installing one large main choke at the cabinet power inlet

  2. Installing individual main chokes for each drive

Although a single choke may appear more economical, Beckhoff recommends individual chokes per drive, because:

  • Shared chokes increase harmonic interaction between drives

  • EMC performance is more difficult to control

  • Cross-interference can affect drive reliability and other cabinet components

In most industrial applications, individual chokes reduce system-level risk and improve long-term stability.

Key Takeaways for Electrical Engineers and System Integrators

  • Chokes and filters are system-level design decisions, not optional accessories

  • EMC compliance failures are expensive and time-consuming to fix late in a project

  • Proper selection improves uptime, scalability, and long-term cost control

  • Beckhoff AX5000 drives require different strategies depending on current rating and cable length

Making these decisions early helps avoid unexpected compliance issues and hidden project costs.