EMC Testing

fifty60

Senior Member
Location
USA
What electrical components are considered to have the most effect on EMC? Is a transformer more than a motor? How about a motor with a VFD, what is more electrically noisy than that?

What components effect the radiated emission and what effects the conducted emissions?
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
What electrical components are considered to have the most effect on EMC? Is a transformer more than a motor? How about a motor with a VFD, what is more electrically noisy than that?

What components effect the radiated emission and what effects the conducted emissions?
old style fluorescent lights are pretty bad.
 

GoldDigger

Moderator
Staff member
What electrical components are considered to have the most effect on EMC? Is a transformer more than a motor? How about a motor with a VFD, what is more electrically noisy than that?

Whatup components effect the radiated emission and what effects the conducted emissions?
Would you like to start the 6 week online course today?

Seriously, you can start by splitting the effects up into line frequency components and higher frequency components.
Then recognize that one man's signal is another man's noise.

Any pwiring that is not shielded will generate 60Hz electric fields. Any wiring with space between the supply and return conductors will generate 60Hz magnetic fields . Unless you are dealing with audio or instrumentation, these will not be a problem.
The rest of the course will examine the higher frequency components. :)
These can only be produced by switching or non-linear elements.
 
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fifty60

Senior Member
Location
USA
Yes please, sign me up for the 6 week course. Any time you feel like explaining more I will certainly read it and absorb.
 

GoldDigger

Moderator
Staff member
Yes please, sign me up for the 6 week course. Any time you feel like explaining more I will certainly read it and absorb.
Chapter 2: Introduction to higher frequency components of voltage and current:
Any time you make or break a current path, there will be transient rapid changes in voltage and current.
1. Rapid changes in voltage produce rapidly changing electric fields. These changes can induce voltages and currents in nearby objects via capacitance. These electric fields can be shielded fairly easily. (Ultimately via a Faraday cage if you want really good shielding.) But if you do not provide any shielding they can still pass through transformers via the inter-winding capacitance.
2. Rapid changes in current produce rapidly changing magnetic fields. These changing magnetic fields also produce voltages remotely. Magnetic fields are much harder to shield.
3. If the changes in voltage and current get to really high frequencies the changes in electric and magnetic fields together travel long distances in the form of electromagnetic waves (radio waves, x-rays, etc.)

A transformer or a brushless motor will not generate harmonics at very high frequencies. Those low order harmonics that exist will result from non-linear effects such as saturation or hysteresis in magnetic materials. And they will not travel very far as harmful emissions (except as in the audio and instrument discussion earlier.) Only arcing contacts (including brushes), gas discharge tubes (fluorescent, metal vapor, etc), rapid electronic switching and similar sources can produce very large high frequency emissions.
 

fifty60

Senior Member
Location
USA
Is there any way to know in advance what adding, for example, a contactor that will switch on and off a 15A heater load? How would that contactor compare to an SSR which is switching much faster? I know the initial arc will consist of a multitude of frequencies and the subsequent harmonics...should the manufacturer have this data published somewhere?
 
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GoldDigger

Moderator
Staff member
Is there any way to know in advance what adding, for example, a contactor that will switch on and off a 15A heater load? How would that contactor compare to an SSR which is switching much faster?
Not without looking at the specs and doing an analysis, AFAIK.
This is complicated by the fact that an SSR will often include some form of transient protection to prevent damage to the SSR itself from inductive loads. That will have the side effect of reducing EMC.

And in terms of the load component being switched, either a transformer or a motor would act as a partially inductive load, making switching noise worse than for a comparable resistive load, not to mention the effects of initial surge.

Worst case for switching on would be if there is mechanical contact bounce in the contactor. That would not be an issue with an SSR.
 
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