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The solenoid can be useful for positioning, stopping mid-stroke, or for low velocity actuation; especially in a closed loop control system. A uni-directional solenoid would actuate against an opposing force or a dual solenoid system would be self cycling. The proportional concept is more fully described in SAE publication 860759 (1986).
In physics, the magnetomotive force (abbreviated mmf or MMF, symbol ) is a quantity appearing in the equation for the magnetic flux in a magnetic circuit, Hopkinson's law. [1]
In order for the total number of field lines to be conserved, the field outside must go to zero as the solenoid gets longer. Of course, if the solenoid is constructed as a wire spiral (as often done in practice), then it emanates an outside field the same way as a single wire, due to the current flowing overall down the length of the solenoid.
A solenoid The longitudinal cross section of a solenoid with a constant electrical current running through it. The magnetic field lines are indicated, with their direction shown by arrows. The magnetic flux corresponds to the 'density of field lines'. The magnetic flux is thus densest in the middle of the solenoid, and weakest outside of it.
In a typical application the term "flux linkage" is used when the flux is created by the electric current flowing through the coil itself. Per Hopkinson's law, =, where MMF is the magnetomotive force and R is the total reluctance of the coil.
The term reluctance was coined in May 1888 by Oliver Heaviside. [1] The notion of "magnetic resistance" was first mentioned by James Joule in 1840. [2] The idea for a magnetic flux law, similar to Ohm's law for closed electric circuits, is attributed to Henry Augustus Rowland in an 1873 paper. [3]
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That is, the back-EMF is also due to inductance and Faraday's law, but occurs even when the motor current is not changing, and arises from the geometric considerations of an armature spinning in a magnetic field. This voltage is in series with and opposes the original applied voltage and is called "back-electromotive force" (by Lenz's law).
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