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Self-holding solenoids - with built-in permanent magnet
Self-holding solenoids are open frame solenoids with a built-in permanent magnet which exerts a magnetic attraction force between the armature and the pole and is thus held at the end positions without any electrical voltage being applied. They can be driven in either direction. Self-holding solenoids are well suited for applications where the "move time" is very short compared to the hold time in the closed position. Self-retaining solenoids are also ideal for applications where either the open or closed (retracted) position must be held for extended periods of time. They are very suitable for locking mechanisms and as shutter actuators in cameras.
1. Design and Features
Self-holding solenoids (also known as permanent magnets or magnetic locking solenoids) represent a product group of linear open frame solenoids that take advantage of a high-performance permanent magnet. The principle of operation is similar to all linear solenoids. When the coil is energized, the plunger core moves in the direction of the pole piece. The advantage of the self-holding solenoid is that once the plunger core is energized and has moved, it remains in the energized position without any further current being applied. This "hold" feature is achieved because the permanent magnet creates a magnetic locking field that keeps the plunger core in the closed or energized, i.e., energized position. There are two configuration types of self-holding solenoids: One is the design with the holding action in a single direction, the other with holding action in two directions. The single direction holding design uses a permanent magnet to hold the solenoid in the energized position. The design with the holding action in two directions uses a permanent magnet to hold the enclosed plunger core in two different positions at the two ends of the stroke. This is brought about by the use of two separate coils and pole pieces.
2. Stroke, Force and Holding Force
Self-holding solenoids can be used in both short and long stroke applications. As such, the pole piece designs are either tapered or flat, depending on the performance requirements. To improve efficiency, the stroke of the solenoid should be kept to a minimum in the application.
3. Operational Consideration
The data for the coils of electromagnets with permanent magnet show values at an ambient temperature of 20°C and a duty cycle of 25%. (For SH2LC0524 the duty cycle is 10%). The coil of the solenoid can operate at temperatures up to 105 "C. Most applications using self-holding solenoids are intermittent applications (since the permanent magnet is used for holding). Please pay attention to the maximum "ON" time to prevent possible thermal damage.
The permanent magnet electromagnet differs from traditional electromagnets in that electrical polarity is important to achieve proper operation. The lead wires from the coil are color coded and must be connected to the appropriate electrical terminals (positive and negative poles). When the current flows in one direction (as long as the electromagnet is energized), the magnetic field of the coil and the permanent magnetic field are additive. To release the electromagnet from the "hold" position, the coil field must cancel the permanent magnetic field, consequently the current must flow in the opposite direction to the direction of pull when current is applied.
C) Return spring
For the type with holding action in one direction, it is advantageous to use a return spring to prevent unintentional "pulling in" of the plunger core, which could be caused by the magnetic field of the permanent magnet.
D) Modifications to the plunger core and shaft.
It is not recommended that the customer make any modifications to the plunger core or shaft, as the shafts are manufactured and plated at the factory. Any special configuration can be supplied. Please consult the factory for further details.
A self-holding solenoid uses a permanent magnet that attracts metal particles. Therefore, special care must be taken to prevent such metal particles from "sticking" to the solenoid.
F) Installation of the electromagnet
The solenoids have threaded holes for mounting on the frame. Care must be taken to ensure that the mounting screws used to attach these solenoids are the correct length to avoid damaging the coil.
4. General Characteristics
Insulation class: Class A (105°C), lead wire class A (105°C)
Dielectric strength: WS 1000V 50/60Hz 1 min (at normal temperature and humidity)
Insulation resistance: More than 100 Megaohm at GS 500V insulation meter (at normal temperature and humidity)
Expected Life: 200000 cycles (The cycle life of an electromagnet depends significantly on the side load, frequency of use, and environmental conditions. Cycle life tests should be performed by the customer).
5. How to Select a Solenoid
Before selecting a particular permanent magnet solenoid, the following information must be determined:
A) Pull-in force and holding force.
The actual force required in the application for both "pull in" and "hold" should be increased by a safety factor with a multiplier of 1.5 to arrive at the force value that should be used in your specification.
B) Duty cycle
Most applications that use self-holding solenoids are intermittent applications (since the permanent magnet is used for holding). The duty cycle for each solenoid is listed in the catalog. Please pay attention to the maximum "ON" time to avoid possible thermal damage.
The stroke for a single direction permanent solenoid is determined by the requirements of the application. For a two direction self-holding solenoid, the stroke is determined by the selected
D) Operating voltage
The DC operating voltage is determined by the application and the available voltage. After these specifications are determined, the proper size solenoid for the application can be found by using the force-stroke characteristics tables and graphs. Coil data is also listed for various sizes of solenoid wire. If the exact operating voltage is not listed in the coil data table, then please consult the factory for further details. Note: For a self-holding solenoid of the single direction holding type, the appropriate degree of reverse current must be supplied to the coil to "release" the permanent magnet so that the plunger core can return to the de-energized, de-energized position. The data for this "release" current is shown with the coil data listed for each solenoid size. To determine the force output of the solenoid after the temperature has risen, please use the ampere-turn force graphs after which the ampere-turns have been calculated.
6. Ordering Information
To order a self-holding solenoid, the correct part number must be determined from the catalog information. The following example shows a complete part number:
SHILC0524-06 (holding action in one direction, conical surface and 6V coil).
SH2LC0730-12 (holding effect in two directions, conical surface and 12V coil)
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