Mo­tors & drives

DEMM Engineering & Manufacturing - - CONTENTS -

To­tal cost of own­er­ship anal­y­sis demon­strates high po­ten­tial for sav­ings, even for sim­ple ap­pli­ca­tions, by re­plac­ing pneu­matic drives with elec­tric lin­ear drives

Elec­tric lin­ear drives are re­plac­ing con­ven­tional pneu­matic cylin­ders in more ap­pli­ca­tions.

The rea­sons in­clude poor ef­fi­ciency, high costs for com­mis­sion­ing, re­con­fig­u­ra­tion, ser­vice, and main­te­nance, and the lim­ited con­trol ca­pa­bil­i­ties of pneu­matic sys­tems.

A to­tal cost com­par­i­son demon­strates that elec­tric lin­ear drives, at cur­rent prices for com­po­nents and elec­tric­ity, pay for them­selves within a few months – even for sim­ple point-to-point mo­tions with two end po­si­tions. This also helps to re­duce the carbon foot­print sig­nif­i­cantly. In ad­di­tion, they pro­vide greater f lex­i­bil­ity in the de­sign of pro­duc­tion pro­cesses and pro­duc­tion mon­i­tor­ing sys­tems.

Pneu­matic drives are char­ac­ter­ized by lower ac­qui­si­tion costs, ro­bust­ness against ex­ter­nal influences (e.g., tem­per­a­ture vari­a­tions and dust) and high re­sis­tance to over­load. They are also sim­ple to op­er­ate and do not re­quire hold­ing cur­rent when in­stalled in a ver­ti­cal ori­en­ta­tion.

Com­pressed air is also used for trans­port and clean­ing tasks in many shop f loor and in­dus­trial en­vi­ron­ments, so that com­pres­sor sys­tems need to be pro­vided in any case. So it’s no won­der that pneu­mat­ics are used in a wide range of ap­pli­ca­tions and can be found in many fac­to­ries.

Com­pressed air, how­ever, is one of the most ex­pen­sive en­ergy me­dia, be­cause com­pres­sors can con­vert only a small por­tion of the in­put en­ergy into use­ful power. The great ma­jor­ity is dis­si­pated as heat loss.

The lat­est tech­nol­ogy can achieve an ef­fi­ciency of about 30 per­cent. Fur­ther in­creases are nearly im­pos­si­ble, as the phys­i­cal lim­its have prac­ti­cally been reached. In ad­di­tion to the al­ready high costs for the mo­tor, com­pres­sor, startup and run-on losses, and losses from com­pressed air han­dling, in prac­tice ad­di­tional oc­cur losses due to leak­ing dis­tri­bu­tion sys­tems.

In re­al­ity, there­fore, af­ter ad­di­tional con­ver­sion loses in the ac­tu­a­tor (with­out op­ti­miza­tion) only about five per­cent of the in­put en­ergy is avail­able as use­ful power. Op­ti­mal de­sign of the pipework sys­tem and ac­tu­a­tors, prompt track­ing of leaks, and heat re­cu­per­a­tion sys­tems can in­crease the ef­fi­ciency. The Ger­man En­vi­ron­men­tal Min­istry rates the po­ten­tial en­ergy sav­ings at 20 to 40 per­cent, while other ex­perts cal­cu­late sig­nif­i­cantly greater po­ten­tial sav­ings.

Pneu­mat­ics

Even if all of th­ese po­ten­tial sav­ings can be re­al­ized, how­ever, com­pressed air sys­tems still use this in­put en­ergy very in­ef­fi­ciently, with a max­i­mum achiev­able over­all ef­fi­ciency of 10 per­cent. This can also be seen in the to­tal cost cal­cu­la­tion (TCO, To­tal Cost of Own­er­ship) of a com­pres­sor. While about 10 per­cent of to­tal costs must be spent on pro­cure­ment and another 10 per­cent or so for main­te­nance of the sys­tem, the en­ergy costs are typ­i­cally 70 to 80 per­cent of the to­tal costs over the ser­vice life of the com­pres­sor.

It should be no won­der, then, that more and more com­pa­nies are at­tempt­ing, in times of ris­ing en­ergy prices and in­creased en­vi­ron­men­tal aware­ness (par­tic­u­larly CO2 emis­sions), to elim­i­nate com­pressed air from their fac­to­ries, or at least to re­duce it to an ab­so­lute min­i­mum.

To­day there, al­most with­out ex­cep­tion, al­ter­na­tives that do not use com­pressed air avail­able for com­pressed air drives. For lin­ear mo­tions in many ap­pli­ca­tions, the very ef­fi­cient, all-pur­pose elec­tric lin­ear mo­tor in tubu­lar form is a good sub­sti­tute. Th­ese are avail­able from LinMot in var­i­ous de­signs and power classes.

Cost com­par­i­son

Elec­tric drives are in­deed more ex­pen­sive to buy than sim­ple pneu­matic cylin­ders, but an anal­y­sis of the to­tal costs over their ser­vice life shows that in­dus­trial lin­ear mo­tors from LinMot in par­tic­u­lar can pay for them­selves within a few months or even weeks, even in sim­ple point-to-point mo­tions be­tween two po­si­tions.

The fol­low­ing ex­am­ple, with a hor­i­zon­tal point-to-point stroke of 400mm and 15kg of mass in mo­tion, op­er­at­ing at 30 cy­cles per minute and 50 per­cent duty cy­cle (= 2,000ms cy­cle time), makes this clear.

The re­quired po­si­tion­ing time of 500 ms for this task above is achieved with an ac­cel­er­a­tion of 10 m/s² and a travel speed of 1 m/s. The ac­cel­er­a­tion time, dur­ing which the lin­ear mo­tor does use­ful work, is then 100 ms.

This means that the ef­fec­tive power draw takes place dur­ing just one-fifth of the po­si­tion­ing time. When stopped and when trav­el­ling at a con­stant speed, the mo­tor does not draw any power be­yond that needed to over­come fric­tion.

The ki­netic en­ergy in­curred dur­ing brak­ing is con­verted to elec­tri­cal en­ergy in the mo­tor (via the gen­er­a­tor ef­fect) and stored in the in­ter­me­di­ate ca­pac­i­tors of the servo con­troller, where it can be used for the next cy­cle.

This ap­pli­ca­tion can be im­ple­mented us­ing a LinMot lin­ear mo­tor, size P01-48x240F in com­bi­na­tion with a LinMot servo con­troller, model E1100-XC/B1100-XC, with a con­tin­u­ous power draw of less than 100 W.

As­sum­ing 8,000 op­er­at­ing hours per year (three-shift op­er­a­tions) and an elec­tric­ity price of 0.12 EUR/kWh (price for large in­dus­trial con­sumers, in­clud­ing taxes, per EURO­STAT) the

to­tal an­nual en­ergy cost is 96 eu­ros. A pneu­matic so­lu­tion would be much more ex­pen­sive.

Pneu­matic cylin­der so­lu­tion

If a load mass of 15kg is trans­ported pneu­mat­i­cally at a (max­i­mum) speed of 1 m/s, as re­quired by the ap­pli­ca­tion ex­am­ple, an anal­y­sis of the ap­pro­pri­ate char­ac­ter­is­tic curves for de­sign­ing pneu­matic cylin­ders from a fa­mous man­u­fac­turer in­di­cates that a pneu­matic cylin­der with a 50 mm pis­ton di­am­e­ter must be used. In con­trast to the lin­ear mo­tor, the en­ergy (com­pressed air) must be fed in through­out the en­tire mo­tion. The ki­netic en­ergy from brak­ing must also be ab­sorbed by shock ab­sorbers, and can­not be stored in­ter­me­di­ately for the next mo­tion.

Ac­cord­ing to its data sheet, the se­lected cylin­der con­sumes 0.02529dm³ of air at six bar for each mil­lime­tre of travel in a dou­ble stroke. For a stroke of 400mm, this re­sults in con­sump­tion of 10.37dm³ per cy­cle.

At 30 cy­cles per minute, the pneu­matic cylin­der thus re­quires a to­tal of 150,000 Nm³ of com­pressed air per year for con­tin­u­ous op­er­a­tion (8,000 h/year). Con­sid­er­ing pres­sure drop, re­duc­tion, and leak­age losses on the or­der of 25 per­cent, the com­pres­sor must com­press and feed a to­tal of about 190,000 Nm³ of air into the pipe­line.

A nor­mal com­pres­sor (750 kW mo­tor, 7,500 Nm³/h air ca­pac­ity) can use 0.130 kWh of elec­tri­cal en­ergy to com­press 1 Nm³/h to 6 bar, in­clud­ing start-up and run-on losses and com­pressed air han­dling. The to­tal an­nual en­ergy cost is thus about 3,000 Euro (0.12 Euro/kWh*0.130kWh/m³*190,000 m³), or more than 30 times that of the elec­tric equiv­a­lent. At a higher cy­cle count, this ra­tio would be even worse for the pneu­matic cylin­der.

Con­tact John Brooks Ltd, 0800 484950.

See the next edi­tion of DEMM mag­a­zine for the con­clu­sion of

this fea­ture, which in­cludes de­tails on cost cal­cu­la­tions.

At 15 kg load and 1m/s travel speed, a 50 mm pneu­matic cylin­der is re­quired.

The mea­sured con­tin­u­ous power draw of the lin­ear mo­tor in the ap­pli­ca­tion ex­am­ple is 92 W.

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