Product manual

Description
2-4
Part 1105120-01
E 2013 Nordson Corporation
Magnetron Operation and Life
Magnetrons consist of a cathode built into the center of a circular vacuum
chamber, surrounded by permanent magnets. The chamber itself is the
anode. Like a vacuum tube, a filament in the chamber is heated and bleeds
off electrons which are attracted to the anode, the positive outer part of the
chamber. This starts the magnetron. A high voltage power supply then
apples a high negative potential to the anode, which accelerates the free
electrons generated by the filament.
A magnetic field parallel to the filament is imposed by the permanent
magnets. The magnetic field causes the free electrons to spiral outward
from the cathode in a circular path. Spaced around the rim of the chamber
are cylindrical cavities open along their length and connected to the
common cavity space. As electrons sweep past these openings, they induce
a resonant, high-frequency electrical field in the cavity. A short antenna
directs the high frequency (2.4 GHz) microwave energy into the waveguide
which contains the energy and directs it to the UV bulb.
Although magnetrons have no moving parts, they do eventually wear out
and fail. The filament will slowly lose its ability to emit electrons to the point
where the magnetron cannot start. The filament can fail open, or short to the
anode. Magnetrons generate a lot of heat, and the heat can eventually
cause the permanent magnets to lose energy and cause the magnetron to
fail. Heat can also cause the ceramic antenna cap to crack and lose
vacuum causing magnetron failure. An imbalance in the frequency output
from the magnetrons can shorten their life as they then try to couple energy
into one another. This typically causes the ceramic antenna cap to break,
permanently damaging the magnetron.