AC Relay Coil 2

[czping]

Hi all, just a theory question to ask.

We know that relay coils make use of electromagnetic principle (DC) for activation. But what about AC relay coils? Is there any rectification circuit inside the relays? I am referring to those tiny auxiliary relays.

Thanks in advance for enlightening!

 

[nevakar]

there is no need to rectification the source. in this relay, there is a shade pole (a copper ring) as same in shaded-pole induction machine which is located in relay core. This ring eliminate the vibration when ac sinusoid goes to zero.

 

[jbartos]

Suggestion: There is no rectification in AC relay. Electrical current can develop force via AC as well as DC coil. However, there are differences between DC and AC relays in the electrical parameters, physical design and magnetic circuit materials.

 

[jbartos]

Suggestion: Visit

http://www.schrackrelays.com/pdf/C0_v4bg_2.pdf

http://electiger.51.net/100pop_articles/11001.htm

etc. for more info

 

[Skogsgurra]

czpin

I feel that all answers to your question are correct, but they do not really make life easier for you. Let's try a practical approach (although you had a "theory question" to ask.

Imagine a piece of iron. With a coil on it. Below it, there is another piece of iron. Connect the coil to a battery (a DC source). If voltage and number of turns and such things are OK, then the piece of iron with the coil will attract the other iron.

What if you had connected the other way round? With opposite polarity? Yes, the same thing would happen. The reason is that the force between an electro-magnet and a piece of soft iron is proportional to current squared (at least as long as the iron doesn't get saturated).

And what do you get when you square a sinewave? Right! A DC component with a superimposed sinewave with the double frequency. It is the DC component that does the work. The double frequency sometimes does the "hum" that can be heard from transformers and AC relays/contactors.

So, your assumption that some rectfication was needed is correct - in a way. Only that the "rectification" happens mathematically.

BTW, this is why transformers "hum" with 100 Hz in Europe and 120 Hz in the USA. Not 50 Hz and 60 Hz as many people think.

 

[czping]

Thank you, nevakar, jbartos & skogsgurra,

Would like to ask skogsgurra, are you saying that there r 2 coils in the realy, coiled in opposite directions?
If that is what you meant by "squaring" the sine wave?

If not, are you saying that electromagetic theory also applies to AC circuit? Hmm...ponders...

Thanks!

 

[Skogsgurra]

czping,

"2 coils in the realy, coiled in opposite directions?"

How do you get that impression?

The equation for the force between an electro-magnet and a piece of iron is

F = k*I**2

Where k depends on a lot of things, but remains constant as long as the iron does not get saturated and the distance between the two iron pieces is constant.

That's where the squaring takes place and how it is done.

 

[jbartos]

Suggestion: Normally, rectification is associated with rectifying devices. Squaring function results in DC with superimposed fluctuations. However, the magnetic circuit of AC relay is AC type, and the magnetic hardware is designed for AC. The mathematical expression could easily accommodated I x I = I x Vcoil/Zcoil, which would remove the I**2 from the expression for the force. The [sin()]**2 would not appear in the expression since the I=Irms=Imax/sqrt2=1.11 x Iaverage = 1.11 x Idc not I x sin(wt).

 

[czping]

Hmmm...pardon me for my poor knowledge. I may need a bit of time to digest the formula. What about the arrangement physically? Is the arrangement as simple as a coil coiled round a iron piece, just like DC electromagnetic theory? Any drawing/picture to refer me to?

Sorry for so much trouble just to satisfy my curiosity.

 

[Skogsgurra]

czping,

Yes. It is that simple. A piece of iron and a coil around it.

I think that the fact that you ask is very encouraging. When micro-processors were introduced in the early 1970 I often heard people saying that they did not like things that they couldn't understand.

I then asked them if they could understand the working of an AC relay. To their own surprise, they didn't. But they used them. In hundreds.

Your question is a sign of sound curiosity and it is by really trying to understand that an individual develops herself. Or himself.

 

[czping]

Skogsgurra, thanks for the encouragement!
If it is that simple, it the physical arrangement the same as that of a DC relay? Or there is a shade pole (a copper ring)in AC relay but not DC?
I am asking because we are always being reminded that application of electromagnetism is always on DC.

Then, when do we use a AC coil and when to use a DC coil? Is it dependent on the control source available or is there any major consideration?

Thanks!

 

[Skogsgurra]

czping

There are several differences between an AC and a DC relay.

First: An AC relay core has to be laminated. That is to avoid eddy currents that would heat a massive iron core (that can be used in a DC coil).

Second: It is the impedance of the relay coil and its armature that determines the current drawn by the coil. When the armature halves are separated, the impedance is low because of the low inductance (big air gap) so the starting current gets quite high and helps making the pull-in force high enough to accelerate the moving core quickly into working position. When it has pulled in, the air gap is close to zero and the impedance high so that the operating current of the coil goes down to a low value - and it is this value that is specified on the data sheet.

A DC relay has a constant coil resistance and that means that there is no inrush current - rather the contrary - the current increases like I x (1 - exp(-t/T)), where T is the time constant of the coil (L/R). That is why DC relays often are slow starters (slow current rise + weak force (distance dependent) at the start of the travel).


Third: An AC coil does produce a varying force with a constant mean value and a superimposed sine function with double the mains frequency. The superimposed sine has the same amplitude as the DC mean value. That means that the force moves between zero and double the DC mean. That is why many relays hum at twice the mains frequency and also why a short-cicuiting ring is placed in the pole surface (covering half the area). The ring delays the flux so that it gets phase-shifted. The result is that the original flux and the delayed flux never go to zero at the same time, so the resulting force never goes to zero and the hum is greatly reduced.

There's also a lot to be said about snubbers, tranzorbs, varistors and free-wheeling diodes, but I think that this will be enough for now.

 

[czping]

Thanks Skogsgurra
I will try to do some reading up on that!