VSD noise is­sues

BY STEPHEN BROAD BENT-RE­GIONAL MAN­AGER DAN FOSS NEW ZEALAND

DEMM Engineering & Manufacturing - - FRONT PAGE - DETAILS: WWW.DANFOSS.CO.NZ

When it comes to the world of Vari­able Speed AC Drives, there are two vari­a­tions on the topic of noise: au­di­ble noise and elec­tro­mag­netic noise em­a­nat­ing from the AC drives on var­i­ous fre­quency bands. We’ll fo­cus on the lat­ter.

Many of us be­yond a cer­tain age prob­a­bly re­mem­ber see­ing ran­dom dots and lines (com­monly called ‘snow’) on our CRT TV screens when a vac­uum cleaner was op­er­at­ing in the same room or a two-stroke mo­tor­bike rides by. To­day, although less com­mon, some in­ter­fer­ence can be heard in speak­ers when our mo­bile phone changes its re­ceiv­ing sta­tion or a re­ceives a call.

These are mi­nor ex­am­ples of some symp­toms of elec­tro­mag­netic in­ter­fer­ence (EMI). But imag­ine what could hap­pen if EMI were to cause mal­func­tions i.e. med­i­cal equip­ment in hos­pi­tals. Along­side the up­take of com­mu­ni­ca­tion sys­tems in in­dus­try – and par­tic­u­larly the In­ter­net of Things – with de­vices con­nected via one com­mon pro­to­col the need for seam­less in­ter­fer­ence-free com­mu­ni­ca­tion is para­mount.

In­cor­rect data or de­lays in trans­mis­sion and con­trol causes pro­duc­tiv­ity loss in in­dus­try or se­ri­ous po­ten­tial life threat­en­ing sit­u­a­tions in a hospi­tal. Ev­ery piece of equip­ment should be com­pat­i­ble with the elec­tro­mag­netic en­vi­ron­ment it is des­tined for. (EMC Com­pli­ance.) This in­volves not in­tro­duc­ing in­tol­er­a­ble dis­tur­bances into its en­vi­ron­ment and also hav­ing re­sis­tance to in­com­ing dis­tur­bances gen­er­ated from other pieces of equip­ment. The qual­ity of the in­stal­la­tion is vi­tally im­por­tant to a de­vice’s EMC com­pat­i­bil­ity. Close at­ten­tion should be payed to the man­u­fac­turer’s in­stal­la­tion guide and the length of mo­tor ca­ble that a VSD can tol­er­ate to con­firm to EMC stan­dards.

STAN­DARDS ARE IN PLACE TO COVER THIS

Dis­tur­bances in the range of 150 kHz to 30 MHz (ra­dio fre­quency in­ter­fer­ence, RFI) is cov­ered in the IEC 61800-3 stan­dard be­cause these dis­tur­bances in­ter­fere with pub­lic ra­dio broad­cast­ing. IEC 61800-3 dif­fer­en­ti­ates be­tween first (home/ of­fice) and sec­ond (in­dus­trial) en­vi­ron­ments. And de­vices are di­vided into four cat­e­gories (C1 to C4), de­pend­ing on the en­vi­ron­ment they are in­tended for, the EMC knowhow of the in­staller and the volt­age/ cur­rent level of the de­vice. Dif­fer­ent emis­sion lev­els are de­fined for both con­ducted and ra­di­ated emis­sions for each cat­e­gory with C1-rated de­vices hav­ing the low­est emis­sion lev­els. Dif­fer­ent im­mu­nity lev­els are de­fined for first and sec­ond en­vi­ron­ments (lower for first en­vi­ron­ment) and in­clude im­mu­nity against elec­tro­static dis­charges, in­com­ing elec­tro­mag­netic field ra­di­a­tion (ra­dio, mo­bile com­mu­ni­ca­tions), burst tran­sients (switch­ing of con­tac­tors/ re­lays), surge tran­sients (light­ning) and ra­dio fre­quency com­mon mode.

Steep- edged pulses are gen­er­ated in the out­put be­tween the AC drive and the mo­tor due to Pulse Width Mo­du­la­tion (PWM) of a drives out­put tran­sis­tors. These pulses con­tain high-fre­quency com­po­nents, which cause un­de­sir­able ra­di­a­tion from the mo­tor ca­ble, so it’s nec­es­sary to in­clude an ap­pro­pri­ate RFI fil­ter in the AC drive to ful­fil the emis­sion lim­its of IEC 61800-3 and also in­crease the im­mu­nity of the drive.

This prob­lem re­ally comes into fo­cus when in­stalling sev­eral AC drives in an in­stal­la­tion, even in IEC 61800-3- com­pli­ant ones. This is be­cause their emis­sions will add up and could po­ten­tially sur­pass the lim­its of the generic EMC stan­dards, IEC 61000- 6-3 and IEC 61000- 6- 4, ap­pli­ca­ble for the in­stal­la­tion as a whole. Due to the im­pact that many sources have on the sys­tem, these EMI prob­lems can be, in some cases, quite dif­fi­cult to de­tect and even more dif­fi­cult to solve. Pow­er­ful sources may cause EMI over long dis­tances; elec­tro­mag­netic noise can be trans­mit­ted ei­ther through con­duc­tors or elec­tro­mag­netic waves, and cou­pling can be gal­vanic, ca­pac­i­tive, in­duc­tive or elec­tro­mag­netic. As a re­sult, there are many things to con­sider when look­ing to min­imise the im­pacts of EMI on your sys­tems. It’s not just VSD’s that pro­ce­dure EMI ef­fects. With the growth in So­lar con­vert­ers, Elec­tri­cally Com­mu­tated mo­tors (EC Mo­tors) and other switch mode power sup­ply de­vices, our sup­ply grid is ever more stressed with po­ten­tially dis­rup­tive EMI. This also raises the ques­tion of the cor­rect op­er­a­tion of new tech­nolo­gies such as Eth­er­net trans­mis­sion over power lines.

BEST PRAC­TICES IN­STAL­LA­TIONS

As a re­sult, the ideal ap­proach is al­ways to fol­low a code of best EMC prac­tices when in­stalling AC drives, in or­der to avoid mal­func­tion­ing of de­vices in the in­stal­la­tion:

• Use shielded ca­bles for mo­tor and con­trol wiring. The task of the shield is to ‘cap­ture’ the high-fre­quency com­po­nents and con­duct them back to the in­ter­fer­ence source, in this case the fre­quency con­verter. Among them, much bet­ter at­ten­u­a­tion is achieved with braided cop­per shields than with wound cop­per ones. For con­trol wiring, us­ing shielded ca­bles with twisted con­duc­tors im­prove even more the at­ten­u­a­tion.

• Make a cor­rect ground­ing of the shield in both ends of the ca­ble. Use EMC ca­ble glands for this pur­pose. They fully sur­round the shield and con­nect it to ground over a large area. All other con­nec­tion meth­ods de­grade the ef­fec­tive­ness of the shield. As the high fre­quency noise trav­els on the out­side of a con­duc­tor, skin ef­fect, a braid with a high sur­face area con­nec­tion is an ideal so­lu­tion to cap­ture this noise and con­tain it within the VSD’s fil­ter net­work. In­stall­ers of­ten in­cor­rectly twist the shield braid into a pig­tail and use a clamp­ing ter­mi­nal to con­nect it to ground. This form of con­nec­tion cre­ates high trans­fer im­ped­ance for high-fre­quency sig­nal com­po­nents. As a re­sult, the shield­ing ef­fect may be re­duced by as much as 90 per­cent.

• In­ter­fer­ence ef­fects de­crease sig­nif­i­cantly with in­creas­ing ca­bling dis­tance: re­spect a min­i­mum clear­ance of 20cm be­tween con­trol, mo­tor and mains ca­bles. Route them separately as much as pos­si­ble.

• If ca­ble cross be­tween the power and sig­nal wires can­not be avoided, it should be made with a 90° an­gle.

• TN-S grid dis­tri­bu­tion, with sep­a­rate neu­tral (N) and pro­tec­tive earth (PE) con­duc­tors, is pre­ferred to a TN-C grid in terms of EMC.

• Make a good ground­ing of the in­stal­la­tion. En­sure that metal­lic sur­faces are earthed with low-im­ped­ance con­nec­tions. In terms of EMC, the de­ci­sive fac­tor is not the cross-sec­tion of the con­duc­tor, but in­stead its sur­face area, since high-fre­quency cur­rents flow on the sur­face due to the skin ef­fect.

• Emis­sions over 1 MHz can be re­duced by in­stalling com­mon mode fil­ters, pri­mar­ily in­tended to re­duce elec­tri­cal dis­charges in the mo­tor bear­ings.

SE­LECT­ING THE AP­PRO­PRI­ATE AC DRIVE AND EMC/RFI FIL­TER CLASS

When se­lect­ing your AC drive, you must con­sider, in terms of EMC, in which lo­ca­tion it is go­ing to be in­stalled and the mo­tor ca­ble length re­quired. The lo­ca­tion will carry the com­pli­ance of a cer­tain level of emis­sions, ac­cord­ing to a spe­cific IEC 61800-3 cat­e­gory. So, C1- rated AC drives could be re­quested for EMCsen­si­tive lo­ca­tions, such as hos­pi­tals. C2 would be the stan­dard for all other pub­lic grid in­stal­la­tions ( in res­i­den­tial and com­mer­cial ar­eas), and C3 for in­dus­trial grid in­stal­la­tions, even if C2 could be re­quested in­stead in cer­tain in­dus­trial ar­eas.

An AC- drive model can be con­fig­ured with dif­fer­ent RFI fil­ter op­tions to match the lo­ca­tion and ap­pli­ca­tion re­quire­ments. For a spe­cific se­lec­tion of AC- drive model and RFI fil­ter, there is a max­i­mum mo­tor ca­ble length for shielded ca­bles that must be con­sid­ered in or­der not to ex­ceed the IEC 61800-3 emis­sion lim­its for a spe­cific cat­e­gory. For ex­am­ple, an H1 RFI fil­ter in the Danfoss VLT AQUA FC202 VSD al­lows up to a 150m ca­ble for C2 con­duct­ede­mis­sions-limit com­pli­ance, but a 50m ca­ble for C1 com­pli­ance.

Some AC drives are in­stalled in IT grids, for ex­am­ple in the ma­rine in­dus­try. In this case, the drive must be or­dered with­out an RFI fil­ter or it must be dis­con­nected from earth. This is to pre­vent the in­ter­me­di­ate cir­cuit of the drive from be­ing dam­aged in case of an earth fault on the mains side. This in­creases the emis­sion level of the drive, and so re­duces the max­i­mum mo­tor ca­ble length with EMC com­pli­ance.

As you can see, even some­thing as ‘un­seen’ as EMI or RFI can cause a large amount of in­ter­fer­ence is­sues with the equip­ment around it if the proper in­stal­la­tion steps and fil­ter class se­lec­tions aren’t made. In most cases, you can re­move the in­ter­nal EMC/RFI fil­ters with a screw or re­moval of a jumper. But, in many cases, it’s not so sim­ple to add the EMC/RFI pro­tec­tion back in.

All in all, con­sid­er­ing the in­stal­la­tion en­vi­ron­ment and se­lect­ing the proper fil­ter lev­els from the be­gin­ning will al­ways be the safest bet.

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