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Effect of not using Zero-crossing on line

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alphacat

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I was looking for articles/Information about the effect of unplugging/turning off an apliance without using Zero-crossing, but since i'm new to it, i'm not familiar with related terms. (i'm not sure if EMI or EMC have something to do with it).

Could you please refer me to text and perhaps tell me the related terms?

In somewhat that matter, what is Electromagetic Conduction? (I know what Electromagnetic Induction is, but am not familar with the first term).

Thanks.
 
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alphacat

New Member
Is this example that i wrote correct please?

zc-jpg.34513
 

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ericgibbs

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Code:
[quote="alphacat, post: 802628"]Is this example that i wrote correct please?

Hi alpha,

I think you are getting yours 'terms' crossed.
Look at this link for definitions.

Electromagnetic compatibility - Wikipedia, the free encyclopedia

If a triac/scr is designed to switch at ZC of the mains wave, the power to load increases from zero as the mains phase sinewave voltage increases.

Without ZC detection, as you say, switching at mains peak could occur which would try to drive a high current thru the load.
This can cause electromagnetic radiation from the wiring and the load, especially if its an inductive load.
 
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MikeMl

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The problem that switching at zero-crossing attempts to fix is L di/dt where the L is inside the appliance (motor, transformer, ballast), not in the AC line wiring.
 

ericgibbs

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The problem that switching at zero-crossing attempts to fix is L di/dt where the L is inside the appliance (motor, transformer, ballast), not in the AC line wiring.

hi Mike,
I was considering the 'ac line wiring' as acting as a radiating aerial for any Ldi/dt generated EMI signals from the 'inductive' load , rather than the small inductance of the wiring.;)
 
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MikeMl

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hi Mike,
I was considering the 'ac line wiring' as acting as a radiating aerial for any Ldi/dt generated EMI signals from the 'inductive' load , rather than the small inductance of the wiring.;)

Ok, but the zero-crossing switching is usually added to protect the switching device itself; reducing EMI is usually secondary.
 

ericgibbs

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Ok, but the zero-crossing switching is usually added to protect the switching device itself; reducing EMI is usually secondary.

Totally agree the primary and secondary purposes of ZCD.
 

alphacat

New Member
Hello,
Thank you very much Mike and Eric.

I'm trying to figure it out step by step.
I'm currently reading as much material about it as i can (starting from the EMC link you posted Eric), and trying to break it into small cases.

So, about the example i've given,
is it correct that the rapid change of current (due to de/activating an appliance at the sinewave's peak) will cause the jwL of the loop closed by the LIVE and NEUTRAL wires to grow,
and therefore the distribution board will experience a voltage drop? (Or am i exaggerating and no voltage drop will occur?)
 
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ericgibbs

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Hello,
Thank you very much Mike and Eric.

I'm trying to figure it out step by step.
I'm currently reading as much material about it as i can (starting from the EMC link you posted Eric), and trying to break it into small cases.

So, about the example i've given,
is it correct that the rapid change of current (due to de/activating an appliance at the sinewave's peak) will cause the jwL of the loop closed by the LIVE and NEUTRAL wires to grow,
and therefore the distribution board will experience a voltage drop? (Or am i exaggerating and no voltage drop will occur?)

hi,
Look up Lenz's Law.
http://hyperphysics.phy-astr.gsu.edu/Hbase/electric/farlaw.html#c2

Faraday's Law
 
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crutschow

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The problem that switching at zero-crossing attempts to fix is L di/dt where the L is inside the appliance (motor, transformer, ballast), not in the AC line wiring.
But the rapid voltage rise-time of an SCR/Triac turning on at other than the zero crossing will generate significant EMI, independent of the load type. I'm sure you've heard the buzz from a lamp dimmer in an AM radio, for example. So reducing EMI for any load, even resistive, can be a a reason to use zero-crossing turn-on.
 

MrAl

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Hi there,


For a slightly inductive load the rise of current would be a bit slower, but geeze
for a somewhat capacitive load the current would shoot way up until the capacitive
part became charged. Lucky most loads are not capacitive but i guess power
supplies can look like that during turn on.
Also, turning on during the zero crossing means less switching losses assuming
some output current, because the switching loss is higher when the output
power has to be switched...at zero crossing the output power is zero and starts to
actually turn on at fairly low voltage like maybe 2v or 3v which means less power
dissipated during the switching time.
The IE curve during turn on for a device turning on with higher voltage can be
roughly approximated as a ramp, where the voltage across the device ramps down
and the current through the device ramps up. If you look at the power dissipated
during this time (I*E) it can be very high for that short time interval because of
the high E and high I somewhere within that switching interval and lasting for
some time. With E=0 (or very low like E=3v instead of E=170v) the switching
loss has to be much much lower meaning the device stays cooler.
Luckily here the switching frequency itself is usually low like 120Hz.

Also as many others have already mentioned, the radiated power is higher too
because of the steep and high turn on ramps (both E and I) which can actually
interfere with other devices. The steep rise or fall of the waveform contains high
harmonic frequencies (Fourier) and these frequencies propagate wherever they
feel like it :) and sometimes end up being picked up by nearby appliances.

Even given the various reasons for zero cross turn on, many dimmer controls do
not use that technique because it would not allow them to be able to effectively
lower the total power getting to the appliance being controlled. The dimmer
has to be able to turn on mid cycle so that it can regulate or at least lower the
power getting to the appliance. Zero cross turn on is a good idea for things like
solid state relays where the appliance only has to turn on and turn off, and so
many of those relays have this feature already built in.
 
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crutschow

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Even given the various reasons for zero cross turn on, many dimmer controls do
not use that technique because it would not allow them to be able to effectively
lower the total power getting to the appliance being controlled. The dimmer
has to be able to turn on mid cycle so that it can regulate or at least lower the
power getting to the appliance. Zero cross turn on is a good idea for things like
solid state relays where the appliance only has to turn on and turn off, and so
many of those relays have this feature already built in.
The zero crossing approach is not used for dimmers since, to vary power, you need to vary the duty-cycle and that would mean removing whole cycles from the waveform so that all waveform cycle turn-ons are at the zero crossing. That would cause intolerable flicker in an incandescent lamp for low duty-cycles (several cycles off and one cycle on).
 

MrAl

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The zero crossing approach is not used for dimmers since, to vary power, you need to vary the duty-cycle and that would mean removing whole cycles from the waveform so that all waveform cycle turn-ons are at the zero crossing. That would cause intolerable flicker in an incandescent lamp for low duty-cycles (several cycles off and one cycle on).

Hello,

Didnt i say that in the fourth paragraph?
 

crutschow

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Hello,

Didnt i say that in the fourth paragraph?
You said it was not effective because it couldn't control the power, which is not true. A zero-crossing switch can indeed control the power to a load but, in the case of a lamp, will cause noticeable flicker. The do use zero crossing switches to control loads, such as heaters, where flicker is not important.
 

MrAl

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Hi Carl,


Oh ok, good point. I read my post over again and yes it does sound like there
was no other way to regulate the power when yes you can eliminate one or
more cycles with a load that can tolerate that (and humans that can tolerate
that too :)

I've never seen a dimmer do this though. They always regulate sub cycle.
 

ericgibbs

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Hi Carl,


Oh ok, good point. I read my post over again and yes it does sound like there
was no other way to regulate the power when yes you can eliminate one or
more cycles with a load that can tolerate that (and humans that can tolerate
that too :)

I've never seen a dimmer do this though. They always regulate sub cycle.

hi,
As you say 'light' dimmers use random phase switching, full cycle switching creates a flickering light, which is not desirable.:)
 

MrAl

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Hi Eric,

Yes, i've seen some LEDs running off of the 60Hz line that have a very
slight flicker and they are not even regulated. That can be very annoying.

I've designed several types of dimmers in my time using triacs and i've
always used sub cycle regulation because they were sometimes used for
lighting and sometimes for brush type motors. The brush type motors work
very well with this kind of regulation too.
 

crutschow

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Hi Eric,

Yes, i've seen some LEDs running off of the 60Hz line that have a very
slight flicker and they are not even regulated. That can be very annoying.
LEDs have more noticeable flicker than incandescents since they have fast response with no thermal lag to smooth the pulses as incandescents do.
 

MrAl

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LEDs have more noticeable flicker than incandescents since they have fast response with no thermal lag to smooth the pulses as incandescents do.

Yes i agree. LED's have fairly low capacitance while models for
incandescents have the thermal persistence which partly keeps the
filament glowing in between line peaks.
 
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