
A-72937EN/11
To meet the requirement of the EMC Directive
TMN06007E_A-72937EN_11.DOC
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1) Units that have oscillators in it should be put in an enclosed metal
cabinet.
2) Ground all the conductive stuffs that are electrically floating.
3) Stuffs that could be antenna (e.g. signal cable with high frequency)
should be surrounded by grounded metal. (Shielding)
The countermeasure for EMC issue is very delicate. Narrow gaps, condition of
cable shielding and other conditions influence on the result largely. Bear in
mind this situation and take the countermeasure as strictly as possible.
3.1 Cabinet
Cabinets are, as well as cables, most important element to eliminate radiated
noise. If our products are put in a totally enclosed metal box with no hole, it
will pass the test. But it is not realistic because we need a door, holes for
cable inlet and outlet, holes for unit mounting and others. Electric wave gets
out through these holes and it prevents products from passing the test.
Therefore, the point is how to close the holes electrically.
3.1.1 Fundamental consideration about cabinets
A basic condition is cabinets should be made of metal. Then, connection
between walls, floor and ceiling is by welding or, in case of screw assembling
type cabinet, is sure to eliminate paint in the contact area so as to have
conductivity.
3.1.2 Treatment of gaps
In case of screw assembling type cabinets, a gap at connecting portion is the
issue. Even if there is no mechanical gap, electric wave can come out if the
walls are painted because there is electric gap. As described in section 3.1.1,
eliminate paint in the contact area to get electric contact. Further, it could
happen screwing bends a panel. Pay attention to this problem as well.
Normally there is no problem in welding type cabinets.
3.1.3 Biggest allowable hole
Size of minimum hole where electric wave can go through differs dependently
on wavelength. It is said the guideline is a tenth of the wavelength. The
regulated frequency in the EMC Directive is from 30MHz to 1GHz. Also it is
known, from the experience, level of electric wave come out of the cabinet is
very small in frequency of 300MHz or higher, when FANUC products are put
in a metal cabinet. Wavelength of 300MHz electric wave is 1m and a tenth of
that is 10cm. Therefore, 10cm is a maximum diameter of allowable holes and
take countermeasure to close them if holes are bigger than 10cm.
Nowadays CNC has high clock frequency to get high performance. The
higher clock frequency becomes, the upper the harmonics shifts to.
Frequency range of the harmonics has spread over 1GHz. For today’s
high-speed CNC, we can no longer estimate the definite allowable
maximum aperture diagonal. We can tell only that the maximum
aperture diagonal is never excess 100 mm but the value of limit is not
enough any more. Please understand that allowable aperture is rather
small. Rev. 11