The proper installation of switch-mode power supplies in control systems is crucial to guarantee 24/7 availability and reduce problems associated with grounding faults, harmonic distortion, and other electrical disturbances. Today's high-power drives and inverters now share their 3-phase power sources with a new type of switch-mode power supply — the 3-phase power supply. Designed to accept 3-phase, 480VAC power, this device isolates and steps the voltage down to 24VDC for use with the majority of sensors, actuators and controllers. In this article, I'll discuss grounding requirements for switch-mode power supplies and offer wiring recommendations.
Today, 80% of all I/O in automation applications are supplied with 24VDC power. The second most important voltage is 120VAC, which is used on 15% of all I/Os. Power supply development has transitioned from linear regulated designs to primary switch-mode designs over the past decade, and the corresponding installation and grounding requirements are worthy of review.
Most power supplies come with an AC power input connection that incorporates line (L), neutral (N), and earth (E) connections. The E connection (also referred to as ground (GND) or protective earth (PE) connections) normally ties into the frame (in an open-frame design) or the housing (if enclosed). Voltage conversion in the power supply occurs through a step-down transformer with a 5:1 ratio, specifically 120:24. The DC output terminations become isolated from the input because of transformer isolation.
The basic internal layout of a power supply is shown in Fig. 1, below. Note that the grounding of the iron-core transformer normally feeds into the incoming AC voltage. The corresponding step-down voltage DC (after full-wave rectification) is isolated from the input terminations.
In 3-phase, 480V applications, large inverters, drives, and motors can introduce significant distortion and interference on the power lines. Using a single grounding system can, and will, introduce unwanted problems. Depending on the application, system integrators choose between a single ground point that ties all AC and DC voltages together, or separate grounds for the AC and DC voltages. In creating a common ground, engineers typically connect the incoming AC voltage ground with the negative leg of the DC voltage (see GND in Fig. 2, on page 33).
Of course, this common ground compromises the isolation between the AC and regulated DC and ultimately negates the requirement of an isolating transformer. Because a power supply that uses transformer isolation achieves single (and even double) isolation, the common-ground approach comes further into question.
By using the right circuit layout and components that feature touch-proof connections, system designers can create insulating boundaries between different voltages. These boundaries maintain the integrity of the power supply and all of the devices connected to its DC voltage output. In short, it's possible to design a safe system with true isolation between AC and DC and two separate grounding systems.
All DC power supplies offer a plus (+) and minus (-) output connection that is isolated from the AC input. With the DC circuit, a common plus or minus connection is possible using touch-proof, DIN-rail terminal blocks that allow voltage isolation from the DIN-rail itself. By keeping different voltages physically and visually separated, the need to keep a common ground is no longer important. The physical distance between the various voltages negates the possibility of creating a dangerous situation, unless it's intentional.
Because 24VDC would normally power analog 4mA to 20mA loops, or 24VDC relays, contactors, and proximity sensors, it's important to make sure that all metal casings and shields are tied to ground (unless otherwise specified). However, the AC ground should in no way be connected to either the positive or negative connection of the 24VDC power supply.
The switch-mode power supply relies on a high-switching frequency typically in the single kilohertz to hundred kilohertz range. This is far higher than the typical 60 Hz found in North America and the 50 Hz found in the rest of the world. Switch-mode power supplies operate with a higher efficiency (>85%) than its linear regulated counterpart (40% to 60%). In addition, they are half the physical size and usually 80% lighter.
While the standard power supply has always converted 120VAC to 24VDC, the newest power supplies incorporate a 3-phase, 480VAC input (see Fig. 2). The direct conversion of 3-phase, 480VAC to 24VDC is an improvement in power supply applications. Eliminating a 3-phase, 480VAC/120VAC step-down transformer not only allows for reductions in space and weight, it helps to simplify control-system assembly.
Unfortunately, the high-switching frequencies associated with switch-mode power supplies introduces harmonics on incoming AC lines. These harmonics, which are normally the even multiples of the switching frequencies, create a host of problems within AC power networks.
Using 120VAC power (or an alternating current) is a simple proposition when harmonics are involved. This also is true of 3-phase, 480VAC and 24VDC power conversion applications with common-ground ties.
Consider the implication of having harmonics present on the commoned DC leg of a power supply. The most severe problem is the overload experienced on the neutral and ground wires. In a 3-phase, 4-wire layout, the sum total current (including all harmonics) flows through the neutral (and sometimes the ground) wire. The National Electric Code requires that the dimension and gauge size of the neutral are twice the gauge of the current carrying live wires. Harmonic current flow in the ground wire is another reason to keep the AC input separate and isolated from the DC output.
Switch-mode power supplies normally feature power factor correction to filter out higher-order harmonics and meet international standards such as those required in IEC 61000-3-2.
Higher Common-Ground Currents
The step-down ratio from 480VDC to 24VDC is 20:1. Unlike 120VAC, where a combined current total of 15A to 20A can occur, the possible maximum current can be considerably higher because of other larger loads connected to the 480VAC power network. These larger loads could include 3-phase motors with related control gear, drives, and inverters, plus other devices powered with 480VAC.
Imagine the implications when 24VDC analog and digital control signals are referenced with heavy-duty 480VAC loads. Again, the best installation for 3-phase, 480V power supplies calls for separate grounds — one for AC and one for DC.
As more and more tasks become automated, the use and importance of 3-phase, 480VAC power supplies will steadily increase. As the technology's visibility grows over the next decade, system integrators will debate the pros and cons and hash over the grounding rules, correct layouts, and appropriate insulations for different voltages. Look for some lively discussions in the not-too-distant future.
Arnold Offner is the product marketing manager for Phoenix Contact in Harrisburg, Penn. He can be reached at 717-948-3469 or [email protected].