Solenoids convert electrical energy into force and motion. When the coil is energised with electric current an electro-magnetic force is created around the coil. Enclosed solenoids such as the LISK tubular solenoids are designed to direct that magnetic force through the steel housing and into the stop and armature (plunger). The stop becomes a north or south pole face depending on the polarity of the coil. The armature becomes the opposite pole face. These opposite poles are attracted to one another and this creates the force and motion in the armature.
The amount of force created is related to the amount of electrical current applied. Other factors such as the number of turns of wire in the coil, the size of the solenoid, and the magnetic character of the steel used will affect the amount of force developed.
The force is also dependent on the air gap or stroke of the solenoid. The force is lowest at the maximum air gap and highest when the pole faces are fully seated. In general the force is inversely proportional to the square of the distance (gap) between pole faces.
Before deciding on a solenoid for a particular application, there are a number of points that should be considered to assure that the smallest, most efficient, and proper type is selected. These points all influence the proper selection and some thought should be given to each.
Pull Force – When energised, the plunger retracts pulling the load. De-energised, the plunger is extended to specified stroke distance.
Push Force – When energised, the push rod extends pushing the load. A push solenoid is actually a pull solenoid with the addition of a push rod that rides against the internal face of the plunger extending through the fixed pole piece.
Hold Force – Plunger is pulled in either electrically or by external force and contacts the opposite pole piece. With power applied, the plunger remains fixed resisting the external pull or push load.
Combination – Two or all three of these functions can be combined into one solenoid.
The total movement expected of the solenoid when electrical power is applied. It is also often defined as the air gap between moveable plunger and the fixed pole piece in the de-energised condition.
For greatest efficiency and smallest size, design for the shortest stroke possible. When a given amount of “work” is required, try to use a short stroke / high force combination rather than longer stroke and lower force.
The load the solenoid is capable of pulling, pushing, or holding at the start of specified stroke when energised under a specific set of conditions: of voltage, temperature, and duty cycle. The force rapidly increases as the pull or push stroke length decreases. This again points to the importance of designing for the shortest possible stroke.
Keep in mind that the solenoid force will decrease as coil temperature increases. For any given stroke, the force will reduce to about 65% of the chart values when the coil reaches the 350° F (177° C) stabilised temperature.
Flat pole face (short stroke) – The plunger face (internal end) and face of the fixed pole piece are flat. A flat face construction should be selected for relatively short strokes or where maximum hold forces are required.
Conical Pole faces (long stroke) – Plunger face and face of fixed pole piece are matching 60° concave and convex cones. For relatively long strokes with lower holding forces, a conical pole face version should be selected.
Duty Cycle % = [(ON time) / (ON time + OFF time)] x 100.
For a given force / stroke requirement, the smaller the duty cycle percentage, the smaller the solenoid.
Continuous duty – Solenoid ON continuously without interruption for periods of about 30 minutes or longer.
With no OFF time to allow the coil to cool, continuous duty solenoids must have coils wound to limit the current drain and must be large enough to provide for adequate heat radiation to prevent coil burn-out. This means that a continuous duty solenoid will be considerably larger than an intermittent design to produce the same force / stroke.
One way to get around the continuous duty unit size problem is to go to a two-coil work / hold design.
Intermittent Duty – Solenoid ON for only a short time, usually not more than 2 or 3 minutes, then followed by an OFF time which is normally at least as long as the ON time. This ON time and OFF cycling can be repeated continuously over total life of the solenoid. As the ON time decreases and the OFF time increases, more current can be applied to the solenoid without causing coil overheating. By being able to use larger current drain (increased power), a smaller size solenoid can be selected to produce the same force / stroke output.
Pulse Duty – An intermittent duty unit with extremely short ON time. A duty cycle normally in the 10% to 25% range. This is maximum input power for that size solenoid.
Special Power – Applications which require different power levels than those shown for standard, intermittent, or pulse duty cycles. This covers applications at ambient temperature significantly above or below 76° F (25° C), or a desired current drain, below standard value, or a special duty cycle, or other design requirement affecting power.
All coils for our standard solenoids are wound to order on demand. Therefore it is no problem to accommodate a special winding for “in between” duty cycles or special voltages or temperature conditions in our standard solenoids.
All units are rated on the basis of maximum allowable input wattage the solenoid can draw without exceeding the 350° F (177° C) stabilised coil temperature when operated at its rated duty cycle in a 76° F (25° C) ambient atmosphere.
So that the solenoid operates at its designed power level; coils are wound to various resistance values depending on specified voltage and duty cycle.
Coils can be wound for any DC voltage. Those most commonly specified are 6, 12, 24, 28, and 120 VDC. For the very small size units operating at the higher voltage levels, special coils may be required.
The nominal coil resistance at 76° F (25° C) can be determined as follows:
Resistance = Voltage^2 (as specified) / Wattage (as specified)
In operation, the coil resistance will increase due to the heating of the coil wire.
For any given solenoid, the current draw at 76° F (25° C) can be determined as follows:
Current = Wattage / Voltage.
Ambient Temperature – The stable coil wire temperature with no electrical power to the solenoid.
Heat Rise – The stable increase in coil wire temperature during solenoid operation at rated voltage and duty cycle in designed ambient conditions. Standard units are designed for a 274° F (134° C) maximum rise in a constant 76° F (25° C) ambient environment.
Stabilised Temperature – The final stable temperature the coil wire reaches during operation – ambient temperature plus coil heat rise. Standard units are designed for 350° F (177° C) maximum stabilised temperature.
Heat Sink – It is important to consider the size of the heat sink when selecting the solenoid size and applicable wattage.
For any given stroke, solenoid force decreases with coil temperature increase and, conversely, increases when coil temperature is lowered. This is the result of changes in coil wire resistance with changes in wire temperature. The higher the wire temperature, the higher the resistance will be. For estimating purposes, for every 7° F (3.9° C) change in coil temperature from ambient output force changes about 1%.
A solenoid operated in a 76° F (25° C) environment at rated voltage and duty cycle long enough for the coil to reach the 350° F (177° C) rated stabilised temperature will have an output force reduction of approximately 65%.
Standard construction for LISK solenoids provides options for both loose and fixed push rods on push style units. Unique push rod needs can be easily accommodated upon request.
With standard construction the plunger is removable in all styles of solenoids. On push units the push rod is also removable, either as a separate piece or attached to the plunger, depending upon the option selected.
Either the plunger, or the push rod, or both can be configured to be retained in the solenoid if required.
Return springs are often used to return the plunger and / or push rod back to its original position. Return springs can be internal or external to the solenoid and custom fit for a customer’s unique needs.
Standard units are designed to meet normal environmental and operating conditions encountered in conventional industrial applications.
Humidity / water splash – Coil area is protected and exposed surfaces plated to withstand occasional water splash and normal in-plant ambient humidity conditions. For continuous high humidity exposure or water immersion, special encapsulated or molded coil constructions are available. For extreme environmental conditions, hermetically sealed designs are also available.
Sand / Dust / Dirt – Under normal in-plant ambient conditions, standard designs should perform satisfactorily over expected life. Unusual conditions of airborne contamination may require protective boots to seal off the plunger cavity. Additional protection of exposed surfaces may be required. Special sand and dust sealed designs are available.
Temperature – Since solenoid output force will continue to improve (though current draw will increase) with ambient temperature decrease, operation of standard designs in an ambient temperature as low as -65° F (-53° C) presents no problem. It is when solenoids will be operated in an ambient environment above 76° F (25° C) that some caution must be taken. If the temperature is significantly above 76° F (25° C), coil burning may occur even if solenoid is operated at its rated duty cycle and voltage. Special high temperature coil designs are available.
A common option for extended environmental protection is a potted or encapsulated coil. The usual procedure is to flood the gap between the coil and the housing with epoxy. Another less costly encapsulation that would be applicable to high volume applications would be overmolding the coil.
Potting or overmolding help in making the coil humidity / splash resistant in applications where this is common. If more severe conditions exist further sealing of the coil cavity can be done with o-rings and special connectors. The coil will also be very shock and vibration resistant.
Another benefit potting / overmolding provides is the ability to conduct the heat, generated when operating, more efficiently to the housing where it can be dissipated easily. This will allow more power to be applied to a given size solenoid.
Standard construction nominally rated for 1,000,000 cycles. In actual service, cycle life exceeding this figure is constantly being experienced. Periodic cleaning and lubrication will help in extending life.
Severe operating conditions – a heavy side load on the plunger, for example may shorten cycle life. Since many factors other than the solenoid construction itself have their effect, the rated life expectancy is valid only for the laboratory conditions under which life tests were run. Special long life designs are available.
Response time, usually measured in milliseconds, is from the point at which power is applied, until the plunger reaches the end of its design stroke. Two main factors contribute to the overall response time. One is time it takes the current to overcome coil inductance and develop the required magnetic flux field. The other is the time it takes for the plunger to actually travel the stroke distance. Flux build-up normally takes more than half of the total response time. Generally speaking, the response time varies between 5 milliseconds for small size units at short strokes up to 250 milliseconds for long stroke, larger sizes.
To achieve faster response times the solenoids must be overpowered. A solenoid size must be selected which will produce a force at the start of the required stroke several times as large as would be needed under normal speed operation. Stroke should be as short as possible to keep plunger travel time at minimum. Special high-speed designs are available.