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Fans with a wear-free run-down brake

More safety and air flow, less turbu­lence and oper­ating noise

Fans are part of the stan­dard equip­ment in almost all indus­trial facil­i­ties today. They cool, suction, regu­late humidity and do much more. The benefit is plain to see, but compact dimen­sions and high blower output with low power consump­tion also bring some limi­ta­tions to safety. A stan­dard fan today runs at a rela­tively high speed. The fact that the bear­ings move easily means that the fan has a long run-down time when shut off. Since rotating parts always entail danger of injury, safety is guar­an­teed via guard grilles. These, however, cause aero­dy­namic disad­van­tages. A new approach that avoids these disad­van­tages with a greater level of safety is an active brake in the fan. If the fan gets switched off, the wear-free elec­tronic brake reduces the run-down time to less than two seconds. The kinetic energy is quickly dissi­pated, thereby dras­ti­cally reducing the risk of injury.

Figure 1: The new fan with inte­grated Action-Brake is just as compact as the conven­tional models

Rotating masses store energy. This prop­erty, which is desired in flywheels, becomes a source of danger in other rotors. This also applies to fans whose rotors transfer the energy of the motor to the air through blades. Large volumes of air can be moved at high speeds, even with compact dimen­sions. Simul­ta­ne­ously, however, the energy stored in the impeller also increases, and with it the poten­tial hazard. For protec­tion, the fan specialist ebm-papst from St. Georgen now provides – in addi­tion to guard grilles – a new, aero­dy­nam­i­cally better alter­na­tive: Action­Brake, the active brake inte­grated in the fan motor. In less than two seconds it reduces the speed to harm­less values without requiring addi­tional instal­la­tion space or gener­ating addi­tional drag, as with a guard grille (Figure 1). By way of compar­ison, a stan­dard fan without a brake requires 20 seconds to reduce the speed to harm­less values.

State of the art of tech­nology

Figure 2 (left): Clearly visible, guard grilles not only stand out visu­ally, they also impede the air flow

If a service tech­ni­cian switches a device off and opens it, the rotating rotors of the fans pose a substan­tial poten­tial hazard. This applies partic­u­larly to modern compact fans, which operate at high speeds in order to over­come high coun­ter­pres­sure or to generate a large air flow. At the same time, low-fric­tion bear­ings operate in the fans to increase the energy effi­ciency and service life. Conse­quently, the energy stored in the rotor makes for long run-down times. If the rota­tion is suddenly stopped by an object or even a finger, the kinetic energy abruptly discharges. This leads to damage or, in the worst case, injury. To prevent this, regu­la­tions stip­u­late the use of protec­tion against acci­dental contact with quickly acces­sible fans, in other words, guard grilles (Figure 2). Precisely with high-pefor­mance fans, however, this negates a portion of the sophis­ti­cated aero­dy­namic prop­er­ties. The grille gener­ates turbu­lence and thereby noise and coun­ter­pres­sure, resulting in lower blower output. Thus the grille acts as an energy dissi­pator, like a gently and contin­u­ously applied brake. The higher speed here gener­ates even more addi­tional noise (Table 3a, Figure 3b).

Safety without external compo­nents

Table 3a: Compar­ison of Sound Power Level; Fans with and without a guard grille

The new approach proceeds from inte­grated rapid braking via the fan drive. This means that no more disrup­tive obstruc­tions are imple­mented in contin­uous oper­a­tion; the fan can operate at optimum effi­ciency. This also reduces instal­la­tion effort and eases the instal­la­tion. The prin­ciple behind the wear-free brake is simple as well as tried-and-tested: The motor becomes a gener­ator and the gener­ated current is then converted into heat. The tech­nology offers various options for the design. Higher outputs require external brake resis­tors; small outputs use the motor wind­ings as a brake resistor by means of a short-circuit switch. There­fore the fan experts were set the task of opti­mally designing the Action­Brake to meet the needs of the fans, while doing without external compo­nents wher­ever possible. Through a series of tests, the optimum variant was found to be the short-circuit switch for the motor wind­ings. Initially in this case, however, very large currents can arise at high speeds by means of the induc­tion. This means that the wind­ings have to endure a high but short-term heat load, and the control tran­sis­tors have to be designed for these pulse currents. Addi­tion­ally, high currents in coils always means there will be a strong (counter) magnetic field. Thus there is a danger of the magnets in the rotor demag­netising. These prob­lems are solved by a suit­able selec­tion of compo­nents and config­u­ra­tion of the magnets. There­fore the brake lifting magnet oper­ates as reli­ably and as long as usual.

Figure 3b: Compar­ison of char­ac­ter­istic curve; Fans with and without a guard grille

If the oper­ating voltage is switched off, the elec­tronics enter the braking mode. In doing so, they draw their power supply from the loaded filter capac­itor. If the voltage decreases, the capac­itor is recharged by short charging pulses from the “brake gener­ator”. This is done to main­tain the func­tion, and thus the acti­va­tion of the short-circuit tran­sis­tors of the elec­tronics, until the rotor comes to a stand­still. To do so, the brake short circuit is temporarily discon­nected. With a typical fan, this enables braking times of under three seconds. The inner heat capacity of the compo­nents and the maximum braking energy then produce a temper­a­ture level that reli­ably lies below the maximum oper­ating temper­a­ture of the compo­nents used. This ensures long-time oper­a­tion that is free of malfunc­tions and main­te­nance. The new active braking upon switching off the power supply at the fan opens new options to the user. In short, the brake lifting magnet brings a whole series of bene­fits all at once: elim­i­na­tion of guard grilles and their instal­la­tion, which trans­lates into less noise, better output and lower costs by means of lower power consump­tion. Also in terms of logis­tics, the lower number of parts is a notice­able plus. Thus the new brake lifting magnets save time and money while main­taining the level of safety.

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