H. Kuhnke Ltd, Andover, UK
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Solenoid Company Glosary

Glossary

These definitions or explanations are based on the questions we are most frequently asked. In some cases we have added more detail to help you understand how a solenoid is specified, and the way variations in certain criteria affect performance.

Duty Cycle

The Duty cycle is expressed as a percentage. It represents the proportion of time that a solenoid is energised, compared with the total time of an "on" and "off" cycle. So if a solenoid is energised for 15 seconds and switched "off" for 45 seconds before being energized again - then total on/off cycle time is 60 seconds, expressed as 25% duty cycle.

If a solenoid is to be constantly energized then it will have a 100% duty cycle.

If the energized period is longer than five min's continuously, then despite an "off" period, the solenoid will need to be rated at 100%.

A solenoid is normally rated at a duty cycle based on a standard ambient temperature of 35°C and a particular, chosen voltage. A solenoid with a standard duty cycle of n% will be equivalent to a deviating duty cycle of (n+/-x)% if the ambient temperature or voltage is changed. A solenoid can be manufactured for any duty cycle.

Duty cycle is generally quite easy for clients to determine. Many solenoids available ex-stock from Kuhnke are standard 100% duty cycle.

Force

Force is the power output usually measured in Newtons, that represents the energy available to do work.

Once energised, a solenoid will develop a force to start the movement of the plunger. This is defined as the "pull-in force", as the plunger moves the force available will increase until the maximum force is achieved at the end of stroke. This is the "force" of the solenoid which is shown in our technical data. Other forces which are defined in a solenoid include the "holding force" of bistable solenoids with the armature in the fully home position, and at the start position. The available force may depend on the mounting position and if a return spring is used.
Note: 1 Newton = 0.1Kgf = 0.225 lbf

Rotary solenoid power output is defined as available "Torque" see elsewhere in this glossary.

The force required of a solenoid is often difficult to determine for new applications, and has to be estimated, then measured in empirical prototype tests.

Force output is effected by temperature, the higher temperature, the lower the force. The Solenoid Company quoted torque or force figures are given at 90% of the rated voltage and with a warm winding. With a cold winding and the rated voltage, the value is significantly higher. This variance can be 15%-50% higher depending on solenoid type, voltage etc.

Voltage

You should specify the operating voltage of your solenoid. Most solenoids are DC and can be wound for any voltage. The standard tolerance DIN IEC 38 is +6% -10%. Non-standard is available.

A solenoid wound for a specific voltage can be "over-volted" to achieve a greater force than in the specified design. However the over voltage will create additional heat, thus reducing the duty cycle. For example a 24VDC wound coil might be over volted by 300%, but only for a few milliseconds, giving a duty cycle of a%. This extreme example is included to demonstrate the principle and should not be taken as a design parameter.

Direct AC solenoids are of a different design to DC solenoids. Please see elsewhere in this glossary.

Power

In solenoid construction, "power" is usually defined as the rate of energy transfer, expressed in watts. In solenoids it defines the amount of energy/power availible to do work. The power transfer required to achieve a (mechanical) force output will be very much the same irrespective of operating voltage; but the current consumption will vary. It is the current which defines the size of wire used in the coil windings, and which will effect the electrical driving circuit of the solenoid. A solenoid may draw more power when moving from rest than when in its holding position.

Life expectancy

Life expectancy for solenoids is dependant more on the application than on the design of the solenoid itself. Temperature, loads, moving positions, frequencies of operations, will all effect the total life expectancy. Typical standard solenoids can be expected to have operating lives of 10⁷ -- 10⁸ cycles, with certain heavy duty designs capable of 10¹² cycles. But this is a guideline only, as the solenoid will only be part of a more complex mechanism.

Temperature

The reference temperature of a solenoid is the temperature of the device when cold. Kuhnke standard reference temperature is an ambient 35°C.

The "warm operating" condition is the condition at which a steady temperature is reached.

The effect of temperature on a solenoid is significant. For example a solenoid rated for 100% duty cycle can only be used at 80% if the ambient temperature increases to 50°C. If 100% duty cycle is required then the same design solenoid used at this higher temperature will develop around 20% less force than at the standard 35°C.

Pull in Time (Actuation Time)

The full time it takes from switch-on to the moment a linear solenoid completes its stroke, or a rotary solenoid has moved through its rotation angle. This includes the coil excitation time.
It is sometimes possible to reduce over all pull in time by over-exciting (over volting) the coil for a very short period (milliseconds).

Drop out Time

The total amount of time taken for the solenoid to return to its rest position after current is switched off. The drop out time will depend on the mass being moved and the influence of any springs. Drop out times are not shown in our data tables.

Torque

Torque is the power output, usually measured in Newton centimeters (Ncm), of a rotary solenoid.
Torque of Kuhnke solenoids is measured at 90% of the rated voltage and with warm windings (see "force").
A rotary solenoid has a set angle of travel, normally 25°, 35°, 45°, 65° or 95°. The highest torque is achieved at the end of the movement.
The lower the duty cycle, the greater the opportunity for cooling means the coil can be wound more powerfully, thus the higher the torque will be available for any given size solenoid.

Operation Frequency

This is the cycle rate usually per minute or per second made up of on-time and off-time. It has a bearing on the duty cycle and life expectancy.

Surface Preparation

Standard surface finish is galvanised zinc. For special finishes please ask.

IP rating

Is the degree of protection as set out in European standards IEC 529 and DIN 40 050s typically IP x.x. The first code defines the protection level against foreign bodies, and the second against water. For a full explanation of the codes used please click here.

Plunger (or shaft)

This is the part of the solenoid which is moved by the armature, on energisation of the coil. It is fitted to the armature.
Most solenoids can be customised to suit each customer's application, say with special threads, chamfers, slots, flats etc.

Armature

The part of solenoid which moves, within the magnetic field generated by the coil. Normally separated from the coil by the minimum air gap possible.
Armature systems vary, combining flat face and conical shapes to achieve different stroke/force combinations.

Coil

The winding of many turns of fine wire around a hollow core, which when energised (or "excited") by current will develop a magnetic field inside the core.
In solenoid design, the coil is defined by the specific operating voltage, for a given duty cycle. The more opportunity there is for the coil to remain cool then the greater the number of turns can be wound onto any given single spool. This way a stronger magnetic field is developed to achieve higher operating forces and torques. Similarly the current measured in mA will be greater for any given voltage, if the duty cycle % is lower.
A coil can be over "excited" or over-volted beyond its specified voltage, in which case a stronger magnetic field will be developed, and/or quicker pull in times achieved. But this can only be done under strict control for very short periods (ms). Otherwise the coil is likely to be permanently damaged.

Frame or Body

There are different body shapes, or frame types. Obviously some design aspects affect movement, mounting and the available space envelope. What is not always apparent is the need to create a magnetic force, and the fact that the more mass, the greater the force may in turn determine a "square" design. Check out the difference in design that appears in these pages as you see the smallest open frame linear solenoids used for many locking vending machines, to the heavy duty Kuhnke E series rotary solenoids and RM heavy duty linear solenoids.

AC or Laminated Solenoids

Most solenoid applications use DC. This allows compact design, long life, accuracy and versatility. But your application may be one where only AC is available. If this is the case it may be possible to use a silicon bridge rectifier. A 230VAC supply for example will operate a 205VDC wound coil in thuis way. The coil is still DC, but the application is AC. This will give all the benefits of a DC coil without the disadvantages of an AC.
Other quite different designs are used for direct AC operation. These designs pre-date the DC devices which form the majority of applications. They are not necessarily more expensive however and may suit some applications. Ten pin bowling, and remote electrical buss bar switching are two examples.