We have reviewed grounding and terminations for servo drive and motor installations in our previous blog posts. Our final installment in this blog series wraps things up with an overview of cable shielding and installation best practices.

Cable Shielding

A Faraday cage is a metal enclosure that can shield its contents from electromagnetic interference (EMI). While Faraday cages do not require grounding to be effective, they usually are grounded to prevent charge from building up to unsafe levels. A cable’s shield acts as a Faraday cage by protecting the internal wires from EMI.

For cable shielding to be 100 percent effective, both ends should terminate 360 degrees around the chassis via the cable connectors. This is shown in Figure 1a. It is best to avoid flying lead connections; if unavoidable, keep them as short as possible.

If the cable carries analog signals like the sin/cos signals of an encoder, an internal shield may be used to cover individual twisted pairs in addition to an outer shield. The internal shield is typically tied to the servo drive’s signal ground while the outer shield is tied to the chassis. Figure 1b shows both shieldings.

Internal shielding is not ideal but does provide some crosstalk protection. Combining the two allows the outer shield to block a large percentage of external noise while the inner shield blocks any remaining noise.

A model showing effective and ineffective cable shielding.

Figure 1

Due to the enormous diversity of motion control installations, all of them won’t have perfect grounding schemes — and it may be necessary to bend the rules to achieve optimum performance. Figure 2 shows a shielding scenario that is often used when improvement of the motor case grounding does not eliminate encoder signal noise.

In this scenario, the motor shield is connected to both the motor and the drive, while the encoder shield is only connected to the drive. It is also important to point out that the PSU return is connected to the chassis ground, ensuring that noise generated by the drive has a low-impedance return path to the PSU.

A shielding model for when motor case grounding improvement doesn't eliminate encoder noise.

Figure 2

Ground Loops

Ground loops occur when multiple devices are connected to the same ground, but their potentials in reference to the ground differ and generate an unwanted current in the circuit. Figure 3 shows two possible scenarios that can generate a ground loop: connected signal grounds and a shield terminated at both ends.

In both cases, a closed circuit is created via the earth. The voltage VG may have a DC component from differing earth ground potentials. Also, VG may have an AC component as the ground loop acts as an antenna for EMI. It is common for these kinds of ground loops to pick up 50-60 Hz of AC power called a “hum”. The hum in the ground loop may then be coupled into the system. How severe of a problem this creates depends on the installation.

A model of two possible scenarios to generate a ground loop.

Figure 3

Here are some techniques to mitigate ground loops:

Shield termination. Break the ground loop by terminating a shield at one end. Unfortunately, this reduces the shield’s effectiveness.

Small capacitors. Instead of shield termination, another option is to insert a small capacitor between the shield and chassis. For example, inserting a capacitor of 100 picofarads (pF) blocks DC and attenuates lower-frequency AC humming currents. To implement this technique, look for special connectors that feature built-in capacitors.

Communication interfaces. Communication interfaces like RS-422, which have a shared ground to keep common-mode voltages within specification, may recommend including a resistor. Inserting a 100-ohm resistor between the signal ground and earth ground can limit ground loop currents.

Installation Good Practices

In addition to star grounding, differential signaling, line termination and shielding, we also recommend following these good practices for installation:

Follow any guidelines in the servo drive and servo motor installation manuals.
If connecting grounds to painted or anodized surfaces, scrape the connection point down to bare metal.
Use a line filter between the AC supply and the servo drive power supply input. This may be a requirement for systems to conform to CE marking.
Consider using an AC supply line reactor to protect against power surges.
Consider a choke on the drive 3-phase output to reduce PWM switching noise, which is particularly helpful for long motor cables.

It is also important to ensure power supply, motor and signal cables are routed separately. Keep in mind that the one-cable solutions of Hyperface DSL® and Endat 3 contradict this recommendation because these cables have a specific construction. Only use approved cable. If signal cables must cross power cables, try to cross them at right angles. Don’t coil cables together that are longer than required.

Another good practice is to follow a tree structure for ground cable cross-sections.

The trunk cable connecting the earth star to the transformer must be bigger than the branches.
The branches connecting the chassis to the earth star must be bigger than the small branches.
The small branches connect the signal ground to the chassis ground. Be aware that a thin cable near the root can create disturbances in all devices.

Contact us or read our technical note to learn more.

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Better Servo Systems — Part III: Shielding and Installation

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