Valves-Article
Solenoid Valves Accomodate Remote And Automated Operation
Unlike mechanical operation Solenoid Valves are usually lighter and more compact.
Solenoid Valves are most suited to accomodate remote operation with greater
reliability and accuracy. Fluid Power System designers and users of directional
control valves (DCVs) h ave a wide variety of operators to choose
froms : Levers, Pedals, Cam followers, Springs, Pilots, Ball-screw mechanisms
and Solenoids to name a few. As the most DCVs fluid-directing mechanism
moves on the principle of linear motion within the valve envelope, solenoid
valves are most suitable mode especially requiring remote operation. Most
of the DCVs being installed in equipment etc. are digital now a days.
The digital valves are so designed that smooth shifting is possible and
when they are in operation, the internal flow-directing element move quickly
to fully open or fully close a flow path. Solenoid valves are most suited
for this type of operation.
Solenoid Operation : The generation of enough force to shift the fluid-directing mechanism although can be met with manual, techanical or pilot operators, many applications demand the benefits accruing to solenoid-operated valves or with a conbination of these valves. Unlike mechanical operation solenoid valves are usually lighter and more compact. solenoid valves are most suited to accomodate remote and automoted operation with greater reliablity and accuracy. No relubrication or maintenance is required. They also do not suffer from wear of mechanical or pilot actuation. All related problems can be avoided with the use of solenoid valves as they can be checked quickly to determine if they have burned out. Unlike mechanical and pilot actuators, solenoids do not exhibit ‘grey areas’ where they only partially shift a flow-directing element. The force which solenoid generates mainly depends on the functional size as also the electrical current and voltage, owing to basically an electromagnetic quality of solenoid valves. This is why solenoids become impratical for shifting valves that require high actuation force directly, because the size of a high-force solenoid easily could exceed that of the valve. For applications requiring high actuation force, a small solenoid can be used to control a small flow of (pilot) pressure fluid that actuates the main valve. For example, energizing the solenoid might open a small valve that directs pressure fluid to a small, cylinder (Pilot operator) which in turn shifts the DCV’s flow-directiing element with a high force than could be generated by the solenoid alone.
AC or DC Power: Because of the ready availability of electric power in industrial plants, solenoid valves are especially popular for industrial machines. Most mobile machines also have electric power, often DC. The selection of an AC or DC solenoid usually depends on the type of power most conveniently available to the application. Years ago DC solenoids offered longer service life that AC versions, but improvements in AC designs have eleminated that advantage. Whether AC or DC, electronic control circuit generally operated at power levels too low to fit conventional bang-bang solenoids. (Small, pilot-type valves that are compatible with low power signals, however, are available). Ampllfiers or solid state switches can interfact electronic logic controls with fluid power system. While more expensive than conventional relays, solid-state switches offer silent operation, smaller size, no mechanical parts or wear, and almost instantaneous operation. A solenoid desinged to operate on AC can also operate on direct current. There are, however, some limitations. An AC solenoid operating on AC draws a high inrush current when open. As the solenoid closes, this current decreases to a low holding current when the solenoid is fully closed. This current charachteristic of an AC solenoid is extremely important. The high inrush current provides a high initial force that is usually desirable to overcome the load on the soleniod. On the other hand, when the solenoid is held closed, current is at the low holding current, which generates very little heat. The solenoid, therefore, remains cool. In short, an AC solenoid has a built-in current, which provides for high force pull-in and cool holding. When an AC solenoid operates on DC, current is constant regardless of whether the solenoid is open or closed. : inrush and holding current are identical. Because of this constant-current feature of DC, a compromise between pull-in force and holding temperature must be made. Specifically, if enough current is provided to give the same pull-in force as AC provides, the solenoid may over-heat if held energized. If the current is reduced to a level that will prevent over-heating when held closed, the pull-in force will be greatly reduced. Therefore, an AC solenoid’s success at operating on DC power depends upon how pull-in force and overheating can be balanced. In many cases, AC solenoids can be operated on DC by adding a switch and resistor. The switch is arranged to open when the solenoid closes. When the switch opens, the resistor is connected in series with the solenoid coil. This resistance reduces the coil current solenoid can be held energized without burning out. A high current that will provide a high pull-in force is then feasible. Within limits, DC soleniods can be operated on AC. For reasons of economy and flexibility, DC solenoids are usually made of solid iron parts. When operated on AC, high eddy-current losses can develop in the iron. These losses generate heat, so powering DC solenoids with AC should be limited to applications requiring low current to minimize this heat build-up. AC power can be practical with a DC solenoid if it cycles on and off in so short a time that the losses do not have enough time to generate appreciable heat. If a DC solenoid is to be used with both AC and DC power, it should be equipped with shading coils. These coils, common on AC designs, keep the solenoid from buzzing when the AC sine wave (frequency) passes through zero DC-design solenoids with shading coils are sometimes termed AC-DC units.
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