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VFD037CH23A-21
Руководство по эксплуатации Страница: 633

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Delta Electronics
VFD037CH23A-21
Руководство по эксплуатации Страница: 633

Delta Electronics VFD037CH23A-21 — эффективность экранирования электромагнитных полей и методы защиты [633/638]

Превью страниц Страница 633 / 638

Electromagnetic fields

Wall of

shielded

enclosure

Greater leakage

form bigger

apertures

(gap)

(depth)

"Waveguide below cut-off"

doesn't leak very much

(does not have to be a tube)

G=gap

(aperture dimension)

d=depth

(distance that fields

have to travel)

0.05 0.1 0.2 10.5 2 5

GHz

Shielding effectiveness

(SE)in dB

d=18"

g=6"

d=12"

g=6"

d=6"

g=6"

d=6"

g=2"

d=4"

g=2"

d=2"

g=2"

F<0.5Fcutoff SE is approximately 27d/g

4.2 How to reduce EMI by Shielding?

Iron and other metals are high conductivity materials that provide effective shielding at extremely low

frequencies. But conductivity will decrease as:

1. High frequency signals are applied to the conductor.

2. Equipment is located in a strong magnetic field

3. The shielding frame is forced into a specific form by machines.

It is difficult to select a suitable high-conductivity material for shielding without the help from a shielding

material supplier or a related EMI institution.

Metallic Shielding Effectiveness

Shielding Effectiveness (SE) is used to assess the applicability of the shielding shell. The formula is:

SEdB=A+R+B (Measures in dB) where A= Absorption loss (dB)

R= Reflection loss (dB)

B= Correction factor (dB) (for multiple reflections in thin

shields)

The absorption loss refers to the amount of energy loss as the electromagnetic wave travels through the

shield. The formula is:

AdB=1.314(fσμ)1/2t where f= frequency (MHz)

μ= permeability relative to copper

σ= conductivity relative to copper

t= thickness of the shield in centimeters

The reflection loss depends on the source of the electromagnetic wave and the distance from that source.

For a rod or straight wire antenna, the wave impedance increases as it moves closer to the source and

decreases as it moves away from the source until it reaches the plane wave impedance (377) and shows

no change. If the wave source is a small wire loop, the magnetic field is dominant and the wave

impedance decreases as it moves closer to the source and increases as it moves away from the source;

but it levels out at 377 when the distance exceeds one-sixth of the wavelength.

Delta Electronics VFD037CH23A-21 — эффективность экранирования электромагнитных полей и методы защиты [633/638]

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