Comparison of Field Generation
Field excitation by air coils
To achieve the required magnetomotive force Θ = n * i, the magnetic field must be generated by a current impulse from a capacitor discharge. Because of the short-time current flow, the heat rise per field excitation is limited.
Nevertheless the increasing of the OHM´s resistance by heating the field winding may be problematic in particular with fast sequential erasures.
The capacitors must be (partially) recharged after every discharge.
It is not common to measure the size of the field strength H or the flux density B = μ0 * H.
For this reason it is difficult to assess information on the size ( B = 1.775 .. 2.20 T ) or the
inhomogeneity ( Bmax ≈ 1.8T at Bcore = 1.0T) of the fields.
The size of stray fields from the degausser is discussed in several cases.
Other issues which are caused by the power supply are breaks of coercion for charging the capacitors
or for cooling overheated copper windings as well as short circuits in capacitor banks with gas escape.
Field excitation in yokes with air gap
In the air gap of a ferromagnetic yoke can be generated a strong magnetic field for powerful degaussers, which are suitable for a failure-free continuous operation.
Due to the low magnetic resistance Rmagn. yoke = (l/μ*A)yoke of a ferromagnetic yoke
- a large magnetic flux Φyoke = BFe*AFe in the yoke and
- because of Φyoke = Φgap also in the air gap
can be achieved with a low flux Θ = n * i.
The achievable strength of flux density B in the air gap is smaller than in the surface of the poles because of spreading of the air gap field.
This undesirable reduction of the flux density can be influenced and limited by
- the shaping of the poles
- the geometric dimensions of the pole faces
- an excitation modulation of the poles to the saturation magnetisation
To achieve flux density values Bgap ≥ 1.2T at the given gap length l = 30mm, it is necessary to choose a large cross section of the yoke and larges poles. For this reason the material usage is large too (yoke, copper winding wire).
There are not stray fields outside of the degausser.