Solenoid Theory 101

Linear Solenoid Fundamentals
Solenoids are electromechanical devices, which convert electrical energy into mechanical motion. Generally, this motion is used to move a load a specified distance. Magnetec solenoids produce linear motion.

All linear solenoids are basically pull-type, that is they pull the plunger into the coil when energized. However, most linear solenoids can be equipped with a push rod mounted to the inner end of the plunger, which extends through a hole in the back stop. Thus, when energized, the plunger is pulled into the coil, and the rod extends producing a pushing motion.

Return motion, upon de-energizing the coil, is provided by the load itself (i.e., the weight of the load) and / or by a return spring which can be provided as an integral part of the solenoid assembly.

Making the Right Choice
A number of mechanical, electrical and thermal factors should influence the decision-making process when you are selecting the most effective and efficient solenoid for a particular application. Striking the right balance among these factors is critical to making the right choice. This Web-page is designed to aid you with this process, but our resources and understand extend far beyond these basis. Our applications and engineering experts will work as and extension of your staff to make sure you get the most effective solution, on time. We focus all our resources and skills on providing you the best combination of price and performance available.

We help you evaluate your needs in terms of:

• Force / Stroke requirements
  
• Electrical requirements
  
• Duty cycle
  
• Maximum envelope dimensions
  
• Environmental factors
  
• Temperature extremes
  
• Operating environment
  
• Termination requirements

Ensure the Right Force / Stroke
Solenoid force is the pull or push force developed by the plunger when the coil is activated. A number of interrelated factors influence the solenoid force / stroke relationship. Voltage, temperature, and duty cycle all affect the load your solenoid is capable of pulling or pushing. Also, the force capability increases as the stroke length decreases.

Variations in nominal voltage greatly affect the force / stroke characteristic of a solenoid. It is good practice to select a solenoid force / stroke based upon the lowest anticipated supply voltage as this equates to the wattage that is available for the separation of the solenoid.

When selecting the proper solenoid, remember to consider all forces against which the coil must work to move and seat the plunger. In addition to the external load to be moved, often the effects of a return spring must be considered. If one is used, its force must be subtracted from the force available to do work to arrive at net force available. If the solenoid is to be used in the vertical mode, and the weight of the plunger has not been considered in determining force-stroke curves, it too must be subtracted from the force available.

Mechanical Life
Our engineers achieve longer solenoid life by ensuring that the force the solenoid generates coincides closely with the demands of the load. Solenoids that generate excess force are subject to stressful hammering that can cause mechanical failure. Proper alignment of the plunger also helps ensure optimum performance and longer life expectance.

Understanding Force / Stroke Performance Curves
Industry has long used force / stroke characteristic curves to evaluate solenoid performance and to help make solenoid selection. These curves can be useful tools; however, you must use care when comparing different manufacturers' product lines because their data may be based on different design criteria. Many factors influence these curves, including:

• Plunger geometry
  
• Plunger diameter
  
• Coil temperature
  
• Coil ampere turns
  
• Applied power
  
• Duty cycle
  
• Solenoid orientation

The Performance graph underscores the importance of understanding how force/ stroke curves are determined. These curves represent the minimal performance for a model designation. As you evaluate different manufacturers' solenoids configurations, make sure you fully understand the frame of reference before making a final selection. We can provide you sample solenoids for testing under actual application conditions.

Balancing the Power / Temperature Formula
Operating coil temperature greatly influences the force a solenoid delivers at a given applied voltage. Coil resistance increase as coil temperature goes up. This causes a reduction in applied voltage and resulting mechanical force. Coil temperature can be reduced by:

• Mounting the solenoid to a heat sink to conduct heat away from the solenoid
  
• Providing forced air cooling
  
• Reducing the applied voltage after the solenoid has "pulled in"

Balancing Load and Power
Solenoids can be designed to operate from either AC or DC power. DC power is preferable for most applications because of design flexibility, solenoid reliability, less electrical noise and more consistent operating speed. Variations in nominal voltage greatly affect solenoid force / stroke characteristics, so it is important to select force / stroke performance based on lowest anticipated input voltage. The current and the number of wire turns determine the solenoid's magnetic flux. The current limitations are determined by your specifications requirements. The number of wire turns are limited by the physical constraints of the mechanical package and by coil temperature. Magnetec solenoids ensure the most efficient design by decreasing size and weight as allowable temperature increases.

Optimizing "On-Off" Cycle Time
Duty cycle is the ratio of time "on" to total cycle time for solenoid operation, and it should be kept to a minimum. It is expressed as: