A servo shaft may exhibit adequate positioning accuracy, dynamic response, and torque during commissioning, but lose performance in the field due to a less obvious cause: incorrect cable specification. Knowing how to size cable for servodrive It requires evaluating more than just the conductor cross-section. The drive's PWM output, RMS current, overload peaks, circuit length, electromagnetic compatibility, and mechanical movement are all part of the same engineering decision.

In critical industrial applications, an undersized cable can increase losses, cause excessive heating, reduce the voltage margin available to the motor, and accelerate insulation failures. On the other hand, increasing the cross-section without evaluating capacitance, shielding, and the drive manufacturer's limits will not solve the system problem. Sizing should be based on the servo drive, servo motor, installation, and operating cycle.

What changes in the cable of a servo system?

The cable between the servodrive and servomotor carries a pulse-width modulated voltage. Unlike a conventional sinusoidal power supply, high-frequency pulses exhibit fast voltage fronts, known as high dv/dt. This behavior increases the demands on the insulation, shielding, and construction geometry of the cable.

Over longer runs, the interaction between the inverter, cable, and motor can generate wave reflections. In practice, voltage peaks at the motor terminals can significantly exceed the nominal bus voltage. This effect depends on the drive technology, cable length, switching frequency, characteristic impedance, and motor type. Therefore, the maximum permissible length specified by the servodrive manufacturer is a design parameter, not just a recommendation.

It is also necessary to separate functions. In general, there is a power cord For powering the servomotor, a dedicated signal cable is used for the encoder, resolver, or feedback. Mixing these circuits in the same path without the appropriate architecture increases the possibility of interference, feedback errors, and control instability. In some systems, brakes, thermal sensors, and auxiliary signals have specific requirements and must be considered when selecting the assembly.

How to size cable for servodrive in practice.

The starting point is the technical documentation for the servo drive and the motor. The motor's rated current should not be analyzed in isolation. The cable needs to withstand the expected effective current for the duty cycle, as well as the overload events permitted by the drive. In rapidly accelerating shafts, the peak current can be high, albeit for a limited time.

The engineering sequence must consider, at a minimum, the following data:

• Nominal current and peak current of the servomotor;
• Power supply voltage and servodrive topology;
• Actual length between drive and engine, including routes, curves and technical overhangs;
• Installation method, ambient temperature, and cable management;
• movement system, such as fixed installation, cable carrier or robotics;
• Length limit and filter requirements defined by the drive manufacturer.

With this information, the conductor cross-section is determined by the current-carrying capacity corrected for installation conditions. High temperature, cables bundled in conduits, poor ventilation, and the presence of other energized circuits reduce the thermal capacity of the assembly. A cable that meets the current requirements on a workbench may not meet them inside a panel, in an occupied tray, or in an industrial route with high temperatures.

The applicable normative reference varies according to the machine, the market served, and the contractual specification. In industrial installations, requirements associated with NBR 5410, IEC 60204-1, and product standards may guide the analysis. However, actual compliance depends on the complete system, including protection, installation, OEM documentation, and conditions of use. The cable datasheet should clearly state the nominal voltage, flexibility class, conductor composition, thermal range, and shielding characteristics.

Voltage drop: calculate it, but don't stop there.

Voltage drop remains a necessary criterion, especially in long circuits and those with higher current. For a three-phase circuit, an initial approximation can be expressed as:

ΔV = √3 × I × L × R

In this expression, ΔV is the voltage drop, I is the current, L is the length of the circuit, and R is the resistance of the conductor per unit length. In more complete analyses, reactance, power factor, and thermal conditions that alter electrical resistance are also included.

The goal is to maintain sufficient voltage at the motor terminals during the most demanding cycle conditions. However, this calculation does not replace verifying the maximum length for the PWM output. A cable may exhibit an acceptable voltage drop and still exceed the limit recommended by the drive manufacturer due to distributed capacitance and reflection effects.

When the required length exceeds this limit, the solution may involve a dv/dt filter, a sinusoidal filter, an output reactor, or a different architecture for the installation. The choice should be validated based on the specific drive. Applying a filter generically can affect losses, system response, panel space, and project cost.

Armor and EMC are not optional items.

Servo drives operate with high-frequency switching and can introduce conducted or radiated noise into the industrial environment. The power cable must be selected with shielding appropriate to the application and installed in a manner consistent with the machine's grounding strategy. A copper shield with good coverage, combined with 360-degree terminations on compatible connectors or cable glands, offers significantly superior performance compared to a long pigtail connection.

The continuity of the shielding must be preserved throughout the entire route. Splices, inadequate connectors, passages near instrumentation cables, and poorly executed grounding compromise electromagnetic compatibility. Simply choosing a shielded cable from a catalog is not enough: the assembly determines a significant part of the result.

The encoder cable requires even greater attention, as it carries low-amplitude, high-sensitivity signals. Physical separation between power and signal, the use of properly shielded pairs, and correct loop connections reduce the risk of intermittent failures, loss of reference, and alarms that are difficult to reproduce. In high-precision axes, these precautions directly influence the stability of the control.

The mechanical application defines the cable construction.

For fixed installations, priority may be given to electrical capacity, EMC, thermal resistance, and resistance to oil or chemical agents. In cable carrier systems, other factors come into play. minimum radius of curvaturebending cycle, speed, acceleration, track filling, and eventual torsional resistance. A cable designed for fixed use should not be assumed to be suitable for continuous motion.

In robotics, torsion is often as critical as bending. The internal construction, conductor stranding, pair arrangement, shielding covering, and sheath material must withstand repetitive stress without electrical alteration or premature failure. The correct section with an inadequate mechanical construction remains an inadequate specification.

It is also necessary to assess the environment. Oil, grease, abrasive particles, humidity, UV radiation, industrial washing, heat, and corrosive atmospheres all alter the choice of insulation and external covering. In naval, mining, oil and gas, or outdoor projects, requirements may include fire behavior, low smoke emission, chemical resistance, or compliance with specific customer standards.

Errors that compromise shaft performance

A common mistake is specifying the cable based solely on the motor's rated current. Another is using standard control cables for servo feedback or unshielded power cables for the drive output. Both may work under simple conditions, but will fail when the machine operates under load, when there are other inverters on the same panel, or when the shaft enters continuous cycling.

The choice of connectors also deserves attention. The permissible current, shielding, sealing, strain relief, and compatibility with the motor standard must match the cable. A high-performance system loses reliability when the transition between cable and connector creates hot spots, shielding discontinuity, or mechanical stress on the terminals.

The documentation must record the section, number of ways, shielding, voltage, pair identification, installation type, and application limit. This traceability facilitates maintenance, replacement, and expansion of the machine without improvisations that alter the drive's behavior.

For projects requiring continuous performance, Innovcable develops special cables considering the electrical, mechanical, and environmental requirements of the application. Custom manufacturing allows the solution to be tailored to the route, movement, standard, and reliability level required by the operation.

The best cable sizing isn't necessarily the one with the largest cross-section or the lowest initial cost. It's the one that keeps the servo drive within its limits, preserves signal integrity, and withstands the real-world operating environment. Before authorizing the purchase, validate the assembly with the drive, motor, and installation data: this is the stage where engineering prevents downtime that would be much more expensive in the field.