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Active power-factor correc­tion mini­mizes circuit feed­back

For the first time, Green­Tech EC fans with inte­grated, active power-factor correc­tion (PFC) and a three-phase supply are now able to fulfill the high require­ments imposed on data centers with regard to circuit feed­back and the harmonic distor­tion of the input current.

The world badly needs energy-saving solu­tions; from lighting right through to elec­trical drive systems. Even simply adapting the speed of drive systems to the actual torque/power require­ments saves on unnec­es­sary power consump­tion. The use of effi­cient EC motors also taps into addi­tional poten­tial for savings, which bene­fits the envi­ron­ment and the operator’s running costs in equal measure. However, this very welcome devel­op­ment has a catch: The input currents of the new energy-saving devices – which are gener­ally phase-shifted and pulsed as opposed to sinu­soidal – incur addi­tional losses in the gener­a­tors, cables and trans­formers of the power supply. The reac­tive power caused by the phase shift and harmonic currents must be made avail­able. This becomes an unwanted cost factor, partic­u­larly in such isolated networks as the ones found in data centers. A solu­tion needs to be found for this issue.

Figure 1: An example of a power supply system in a data center.

Circuit feed­back caused by current harmonics

The power supply of a data center essen­tially consists of a grid infeed trans­former, UPS system and a standby gener­ator (Fig. 1). Together, these compo­nents must ensure a reli­able power supply (through the prin­ciple of redun­dancy). On the other hand, the provi­sion of addi­tional reac­tive power requires all compo­nents involved in the power supply to deliver higher perfor­mance. Unnec­es­sary over-dimen­sioning of this kind is unde­sir­able because of the asso­ci­ated costs. However, as is the case for asyn­chro­nous motors with vari­able-frequency drives, power-consump­tion-opti­mized EC fans now have a pulsed – as opposed to sinu­soidal – current draw due to their circuitry, yet without needing addi­tional measures. Every depar­ture from a sinu­soidal current draw brings about current harmonics, which largely lead to what is known as “distor­tion reac­tive power”. This puts addi­tional strain on the supply network and leads to increased losses in all compo­nents involved in the power supply.

Figure 2: Circuit feed­back caused by harmonics in two fans with a rating of 3 kW.

By way of an example, Fig. 2 shows the input current of a server cooling system with two fans arranged in parallel, each of which has a power consump­tion of approx. 3 kW. At 6.69 kVA, the reac­tive power is above the effec­tive power of 5.74 kW in this case. The power factor, i.e. the quotient of effec­tive power to apparent power, is a poor 0.64, while the total harmonic distor­tion (THD(I)) of the current is around 120%. This means that the geometric sum of all harmonic currents is larger than the funda­mental compo­nent itself. It is easy to conceive how such currents might place a signif­i­cant strain on all compo­nents required in the power supply chain, i.e. trans­formers, the UPS system, gener­a­tors, and even cables. The distor­tion reac­tive power must be main­tained at the infeed point. In the interest of opti­mizing the power supply and backup system, the current distor­tion should be as low as possible. In other words, the power factor should be close to a value of 1.

A digres­sion to the world of stan­dards

The values stip­u­lated by stan­dard EN 61000-3-2, which applies to conven­tional indus­trial systems, are insuf­fi­cient in this case. This stan­dard clas­si­fies devices with a phase current of up to 16 A and defines harmonic-current limit values for these classes. Fans fall within equip­ment group A, which contains house­hold appli­ances along­side symmet­rical, three-phase devices, for example. The higher the input currents become, the more diffi­cult it is to adhere to the limit values of Class A because these are absolute limit values (i.e. not rela­tive to the respec­tive output) defined for the indi­vidual clas­si­fi­ca­tions of the harmonic spec­trum. Only active filter solu­tions can be of help from a certain input current level. For three-phase devices, however, these are extremely intri­cate and not econom­i­cally attrac­tive for “normal” indus­trial appli­ca­tions. This is perhaps the reason why three-phase devices with outputs of over 1 kW are not covered by the stan­dard. As a result, there are simply no limit values at all for three-phase devices with outputs of between 1 kW and approx. 10 kW.

Figure 3: Reducing circuit feed­back by inte­grating a line choke upstream.

On the other hand, if multiple devices for which no limit values apply as indi­vidual devices are inter­con­nected, and if the phase current is in a range of between 16 A and 75 A, an addi­tional stan­dard applies: EN 61000-3-12 defines the limit values for indi­vidual clas­si­fi­ca­tions of the harmonic spec­trum and the total harmonic distor­tion of the current. The inter­esting aspect of this is that the permis­sible values are depen­dent on the grid quality. The “softer” the grids are – that is to say, the higher the complex internal resis­tance of the grid is – the lower the limit values for the harmonic content of the current will be. This is only logical: After all, a non-linear current distorts the voltage wave­form more signif­i­cantly as the internal resis­tance of the grid increases. In order to satisfy demands in this case, measures also need to be taken for classic vari­able-frequency drives with a three-phase supply. Because a low level of internal resis­tance can usually be assumed in most indus­trial grids, a limit value of 48% applies for the harmonic content in accor­dance with EN 61000-3-12.

Line chokes and DC-link chokes only help to a certain extent

This value, which still falls well short of satis­fying the needs of today’s data centers, can be achieved with rela­tive ease, such as by installing a line choke upstream (Fig. 3). Nonethe­less, such chokes require space and addi­tional cabling. For the example mentioned above featuring 2 fans, each rated at approx. 3 kW, values of around 0.9 and 45% can be achieved for the power factor and total harmonic distor­tion (THD(I)) respec­tively with a 2% line choke.

Figure 4: Reducing circuit feed­back by inte­grating a DC-link choke.

Even better values can be achieved if a DC-link choke (Fig. 4) is incor­po­rated into the power elec­tronics (which is a stan­dard feature of ebm-papst EC fans with a three-phase supply and highly capac­i­tive DC link): The require­ments of stan­dard EN61000-3-12 are satis­fied with a power factor of approx. 0.94 and a THD of 34%, at least as far as the limit values for “hard” (i.e. low-imped­ance) indus­trial networks are concerned. However, in cases where the grid voltage distor­tions are prob­lem­atic and/or grid imped­ance levels are high, the harmonic content in the current must be reduced to a minimum. Conse­quently, target values of over 0.95 for the power factor and a THD(I) of below 5% are now frequently stip­u­lated for many data-center appli­ca­tions.

The target value for THD(I) is below 5%

Ballast solu­tions in the form of harmonic filters (Fig. 5) are one theo­ret­ical option for approaching such values. These filters are quite impres­sive with regard to their char­ac­ter­is­tics and the quality of mate­rials used – a fact which is also reflected in the price tag. Nonethe­less, the achiev­able values do come very close to hitting the target: In the example here, a power factor value of 0.98 and a harmonic content of 7.5% in the phase current are achieved, while the current wave­form is more or less sinu­soidal.

Figure 5: Reducing circuit feed­back by connecting an expen­sive harmonic filter upstream.

Despite their bene­fits, such filters often have the draw­back of only being opti­mally designed for one single oper­ating point that gener­ally corre­sponds to the rated output. In partial-load oper­a­tion, the THD(I) values are often twice as high. The power factor also dimin­ishes at lower output levels. As a result, the reac­tive currents become very high in partial-load oper­a­tion. What is more, there is a consid­er­able voltage drop through the filter; the nominal speeds and output values of the fans might not actu­ally be reached under certain condi­tions.

The disad­van­tages of passive filters can be largely avoided by employing an active filter solu­tion. Ballast devices are avail­able in this case, although these are also more expen­sive and require addi­tional space and wiring effort.

Active PFC in fans as a plug & play solu­tion

With this in mind, the motor and fan special­ists at ebm-papst have made the fan manu­fac­turer the first to incor­po­rate a three-phase, active PFC stage into their EC fans in the form of an active recti­fier with the aim of saving the user addi­tional effort (Fig. 6). This over­haul started with motor size 150 (3 kW), which can be found installed in the new RadiPac fan sizes 450, 500, and 560, in addi­tion to RadiCal fan sizes 500, 560, and 630. The values which can be achieved actu­ally exceed require­ments: With a power factor of over 0.99 under the rated load, the THD(I) value is typi­cally around 2%. The THD(I) value even stays below 5% all the way down to 10% of the rated output.

Figure 6: Reducing circuit feed­back through active power-factor correc­tion. A three-phase active PFC is incor­po­rated into the EC motor’s elec­tronics.

In the axial direc­tion, the motors are only slightly longer due to the inte­gra­tion of the three-phase active PFC, while the loss of effi­ciency at the rated output is low at around 2%. Not only that, but the oper­ator still enjoys the advan­tage of having the pulsed input current of the EC motors found in conven­tional solu­tions converted to a sinu­soidal input current without needing to wire in addi­tional compo­nents and without filters and motors needing to be matched to one another. The highly effi­cient, plug & play-capable EC fans reduce energy consump­tion in data centers while also granting the wish of only having to design the power supply of such systems for the rated effec­tive power of the fans. 

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