Understanding cable grounding, shielding and interface signaling is critical to successfully installing servo drives and servo motor systems. In our new blog series, we review the different types of grounding typical to a servo motor system. We will also provide an overview of signaling, transmission lines, termination, shielding and installation practices. Kicking off the series, let’s dive into grounding.

Grounding

Typical servo motor systems feature earth, chassis and signal grounding. Their symbols are depicted in Figure 1. The protective earth symbol denotes a specific chassis connection point and fault current return path.

Figure 1

Before we discuss each type of grounding, it is important to keep these considerations in mind:

Voltage has no absolute value. In an electrical circuit, voltage is the potential difference between two points, and ground is often referred to as zero volts because it is the reference point for all other voltages.
The individual grounds within an installation may have a potential difference.
Electric current can only flow in a completed circuit. Current returns to the voltage source via all available paths. Higher current flows through the path of least impedance.

Earth Ground

The planet’s ground is conductive and a stable reference point of zero volts (V) with an unlimited capacity to absorb electric charge and remain neutral. Most AC power distribution transformers are earth-grounded sources featuring a ground wire connecting the neutral to a rod embedded in the earth.

This ground rod protects the transformer from lightning surges by bleeding off charge into earth. Grounding this way also eliminates voltage imbalances between transformers. With earth-grounded sources, it is important to remember that when standing on the ground, you are standing on one terminal of a potentially large power supply.

A modelf of a three-phase distribution transformer with a short between the L2 phase and the metal case of the power supply.

Figure 2

Figure 2 shows a three-phase distribution transformer with a short between the L2 phase and the metal case of the power supply. A person touching the metal case would complete a circuit between the person, the earth and the transformer neutral — resulting in a potentially fatal electric shock.

While there are varying techniques for mitigating this issue in three-phase power distribution, the basic concept is to bypass the human by placing a low-impedance path from the case to the transformer neutral. This low-impedance path is the protective earth (PE) path, and its impedance is low enough so that if a short occurs, it generates a high enough current to trip a circuit breaker and disconnect power.

Chassis Ground

Chassis grounds are different depending on the application. In the distribution transformer example of Figure 2, the chassis ground is the metallic enclosure. The metal enclosure protects against dangerous voltages and mitigates electromagnetic interference (EMI). Cable shields and a single PE are typically connected to the enclosure as well.

However, in the case of an automobile, the chassis ground is the return current path to the battery’s negative terminal. All conductive components in the automobile are connected to both the positive terminal and the chassis. The chassis is in turn connected to the negative battery terminal. In portable devices, a ground plane is sometimes referred to as chassis ground.

A servo motor and drive installation featuring multiple PE connections to the chassis ground.

Figure 3

Figure 3 shows a servo motor and drive installation featuring multiple PE connections to the chassis ground. The encompassing building features a ground rod. Because the ground rod’s impedance may be too high to trip a circuit breaker, it usually isn’t used as a fault current return to the transformer neutral. Instead, the ground rod’s primary function is to protect against lightning.

Signal Ground

The signal ground is an arbitrary reference point for voltages in a circuit, typically chosen as the negative side of a power supply or signal source. Circuits operating from different power supply voltages will often feature multiple grounds, and there may also be separate grounds for analog and digital circuits. These various grounds are usually connected at one single point in a star configuration.

For analog and digital grounds, the central point is adjacent to a hybrid component such as an analog-to-digital converter. To mitigate radiated emissions, the signal ground may be tied to the chassis ground via a capacitor because of its low impedance to high frequencies.

Signal grounds in servo drives are typically connected to the chassis ground. Daisy chaining the grounds can allow for fault currents from one drive to raise the potential of the signal ground on another drive. To protect against high-voltage shorts to the motor case, the motor PE may be routed through the motor cable and then to the chassis ground. In Figure 3, the PE wires are connected in a star configuration, which is universally considered to be good practice.

Stay tuned for our next installment — we’ll explore signaling, transmissions and terminations.

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Better Servo Systems — Part I: Grounding Types

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