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Selection Factors

To obtain the optimum performance, reliability and life of a solenoid, selection considerations should include the following factors:


1. Force or Torque

Pull,  push or rotary load, developed by plunger when the coil is activated by an external voltage.

2. Stroke

The distance a plunger must travel before it is stopped.

In general, the force initially developed by the solenoid is dependent upon the stroke; i.e., the longer the stroke, the smaller the force.

The force versus stroke relationship must be known for any particular solenoid to be used. This relationship is usually shown as a characteristic curve.

This relationship can be changed for a given solenoid, by changing some geometries of the interior components.

For example, by changing the plunger tip geometry, the force versus stroke relationship can be totally altered.

Most solenoids listed in our catalog have "non-captive" plungers. This means that the plunger is free to travel within the solenoid until it is totally removed from the body of the solenoid. (See "Solenoid Construction" for more details.)

The length of the stroke is set by the user by designing an outer stop within the user's mechanism. The inner stop is built within the solenoid where the plunger bottoms out (at zero stroke).

There are only a few solenoids in our catalog where the outer stop is built into the solenoid as well. These solenoids are identified as having a "Captive Plunger".

3. Temperature

When a solenoid is energized by an outside voltage supply, some heat is generated which increases the temperature of the coil. This temperature rise has some undesired effects, since resistance of the coil winding varies with temperature: i.e., by increasing temperature, resistance rises, which in turn, reduces the electrical current. This will result in a reduction in force output.

Resistance of copper changes by approximately 0.39 percent per degree centigrade, in the neighborhood of 20C. The following formulas show the relationships between resistance and temperature rise:

An extreme increase in temperature can result in damages to the winding and other components. Usually the limiting factor for operating temperature is the rated temperature of insulating material used in the solenoid.

The standard rated temperatures are as follows:

Class "A" 105C maximum

Class "B" 130C maximum

Class "F" 155C maximum

Class "H" 180C maximum

The solenoids shown in this catalog are rated as Class "A", although in most cases Class "B" insulation is used for protection at higher temperatures. Other classes of insulation are available for most solenoids for higher operating temperatures.

Temperature rise in a solenoid can be reduced by dissipating the heat generated in the coil. This can be done by mounting the solenoid on a metal surface (heat sink) large enough to dissipate the excess energy, or by forced air cooling, or where the space permits, by using a larger size solenoid.

Two other methods for inputting a smaller amount of energy and as a result generating less heat are to use the solenoid intermittently, or to use a solenoid with multiple windings. Both methods will be discussed in detail in the next sections.

4. Duty cycle

Higher performances may be obtained from a solenoid by introducing higher input power, in which case more heat will be generated.

One way to reduce the amount of excess heat generated at a higher input power is to use the solenoid intermittently, that is, the solenoid is used in "ON-OFF" intervals.

Duty cycle is the ratio of "ON" time over the "TOTAL" time for any one cycle of operation. Duty cycle is usually expressed in percentage.

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