Auxiliary power supply for single-phase and phase-to-phase applications

RECOM Power GmbH

Thursday, 01 November, 2018


Auxiliary power supply for single-phase and phase-to-phase applications

There are many wide input voltage AC/DC power supplies on the market. An input range of 90–305 VAC will cover 100, 115, 230 and 277 VAC nominal supply voltages to make a universal worldwide power supply. However, in many industrial three-phase installations, a neutral wire is not available and an even wider input voltage range to cover phase-to-phase inputs is needed. This article will explain why a new low-power AC/DC converter with a 6:1 input voltage range was developed.

Firstly, a short technical briefing. In a three-phase mains supply, the three phases are 120° from each other. The phase-to-phase voltage is shown as a vector diagram in Figure 1. VAB has a magnitude that is √3 higher than the single-phase voltage, VA.

VA = single-phase voltage (rms) VAB = phase-to-phase voltage (rms)
115 VAC 200 VAC
230 VAC 400 VAC
277 VAC 480 VAC

Figure 1: Vector diagram of a three-phase supply.

In some industrial applications, the AC mains input is rectified to create an unsmoothed DC supply. Any standard AC/DC converter can operate from a DC supply if it is within their input voltage range. The root mean square (rms) AC voltage is defined as the equivalent DC voltage that would cause the same heating effect as the AC supply in a purely resistive load. The peak-to-peak AC voltage is actually much higher than the rms voltage (Figure 2). If the mains supply is full-wave rectified, then the resulting DC voltage will be higher than the nominal rms voltage by a factor of √2. Thus, to cover all of the possible rectified three-phase AC input voltages, the converter DC input voltage range must go up to at least 678 VDC.

Nominal single-phase voltage (rms) Peak rectified voltage (DC) Nominal three-phase voltage (rms) Peak rectified voltage (DC)
115 VAC 163 V 200 VAC 283 V
120 VAC 170 V 208 VAC 285 V
220 VAC 311 V 380 VAC 538 V
230 VAC 325 V 400 VAC 566 V
240 VAC 340 V 416 VAC 588 V
277 VAC 391 V 480 VAC 678 V

Figure 2: Full-wave rectified AC waveforms.

To meet the requirements of both single-phase and phase-to-phase applications both AC and rectified AC, RECOM Power has introduced the RAC05-K/480 series with an ultrawide-input AC voltage range from 85–528 VAC and a DC input voltage range of 120 V to 745 VDC, so it can also be used with any AC or rectified AC mains supply from 100 up to 480 VAC.

There are three main application areas for such an ultrawide input voltage AC/DC converter: condition-based maintenance (CBM), smart street lighting and phase-loss tolerant industrial systems.

Condition-based maintenance

CBM is a posh way of saying “if it ain’t broke, don’t fix it!” The concept is that if machines and systems are continually monitored and if all readings are within tolerance, then there is no need to do any preventive maintenance. Only if the indicators start to show a new trend (eg, unexpected temperature rise, change in the vibration signature of a motor or an unusual pattern in the data) will an investigation and, if necessary, a service or repair be carried out.

CBM is used in intelligent maintenance systems (‘predict and prevent’ rather than ‘fail and fix’) and is especially useful for critical systems where downtime could cause serious problems or in very large industrial plants where the use of CBM can significantly reduce the number of spare parts on stock and the cost of running a maintenance department (fewer engineers can keep more machines running).

As with all information-based systems, the CBM concept requires regular data communication from the equipment being monitored. If we take the example of a motor health monitoring system, the sensors might measure temperature, vibration, acoustic noise, rotation speed and the supply current to build up an accurate picture of the motor performance. This information can be sent continuously over a wired fieldbus system or pre-processed and sent irregularly over a wireless IoT link to reduce the data traffic. In either case, a stable and reliable low-voltage DC power supply for the CBM monitoring system is required. Figure 3 shows such a system powered from two phases of the three-phase supply as there is no neutral wire available.

Figure 3: Motor health monitoring system powered from two phases of a three-phase motor supply.

Similar local phase-to-phase AC/DC power supplies are also needed in three-phase power metering, electric vehicle charging stations, and sustainable energy monitoring and control systems.

Smart street lighting

The second most common application for low-power 380–480 V AC/DC converters is smart street lighting. It is common to connect up streetlights with each lamp-post or sections of lighting across two out of three phases. The load can therefore be evenly matched across all three phases while at the same time reducing the amount of cabling required. Each lamp-post is separately earth-spike grounded so a separate earth or neutral wire is not required.

Figure 4: Smart street lighting typical wiring diagram. Each lamp is connected across two of the phases.

The lamp control can be centrally coordinated with wireless or powerline communication or the street lamps can be autonomous (a GPS receiver can be built into every lamp-post so each lamp knows its location, time and the sunset and sunrise — civil twilight — times which are transmitted as part of the GPS astronomical data). In some lighting schemes such as multistorey carparks, active street lighting and large area lighting, movement and vehicle detectors can also be included to turn on groups of lamp-posts only in the area where light is needed.

Whether the control is local or centralised, the power supply for the sensors and communication transceivers has to be supplied from the phase-to-phase supply voltage. The power requirement is not great; 3–4 W is usually sufficient to power an IR detector, GPS module or mesh radio. The main issue with powering sensitive sensor or radio equipment from high-voltage mains is coping with the surges and transients. An outdoor smart light installation is usually classified as Overvoltage Category (OVC) III or IV, meaning that voltage surges can be very energetic as the higher OVC categories have much lower source impedances.

Working voltage
(AC rms)
Transient voltage withstand level (1.2/50 µs)
OVC I (30Ω) OVC II (12Ω) OVC III (2Ω) OVC IV (2Ω)
150 800 1500 2500 4000
300 1500 2500 4000 6000
600 2500 4000 6000 8000

Figure 5: Overvoltage categories.

The RAC05-K/480 has OVC III compliance built-in (gas-discharge tube, fuses and MOVs are fitted internally). An OVC IV installation requires a pre-filter consisting of an external, sacrificial lightning arrestor that can be replaced after multiple surge voltage/surge current events, as required by the regulations.

Fault-tolerant installations

It is estimated that nearly 90% of mains fault conditions are caused by a short across a single phase. Three-phase faults are extremely rare. Therefore, in industrial applications where power continuity is essential for continuous processes or safety, ungrounded installations may be used.

This is particularly valid for Delta power distribution systems, rather than the Wye configuration shown in Figure 1. A Delta configuration may be left ungrounded, corner-grounded or centre-tap grounded, either with a high-impedance ground connection or a solid ground. The reasoning behind such arrangements is that a ground fault may cause unwanted currents to occur in some parts of the system, but the rest of the system still runs as normal. So, although Wye-connections are the most common, Delta connections are still used in many process critical installations such as data centres and continuous production plants such as automotive plants or paper mills.

The RAC05-xxk/480 does not need a ground connection as it is a Class II device with 4 kVAC double isolation, so it is compatible with either Delta or Wye supplies. It can also be used with a three-phase rectifier as shown in Figure 2 to create a phase-redundant power supply. If any single phase fails, the power supply is still supplied by the other two phases.

Figure 6: Connection possibilities for the RAC05-xxK/480.

Summary

The RAC05-K/480 series is a versatile AC/DC power supply that can be used with single-phase or phase-to-phase applications from 85 up to 528 VAC. The built-in OVC III surge withstand capability means that it can be installed in many industrial applications without any external components. It is finding many applications in smart industry, smart lighting and fault-tolerant industrial applications.

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