AC Coil Operation

[faulkma]

Just need some advice:

Have a customer who wants to operate a Coil in an extreme input voltage range (13 VAC to 190 VAC ). The design must have no DC or caps in it. Any ideas, a multi-stage coil design via a relay/resistor arrangment?

 

[Skogsgurra]

Is this a contactor/relay coil - or just any coil?

Gunnar Englund

http://www.gke.org

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[electricpete]

Does the coil remain continuously energized, or only momentarily energized?

Does it have to pull-in at 13vac or just not drop out at 13vac


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[faulkma]

The coil is part of a solenoid assembly. The device must operate down at 13 VAC and up to 190 Max. The coil is momentarly pulsed when needed for about 4 cycles mon.

 

[Skogsgurra]

Momentarily pulsed? You may actually be able to use a 13 V AC coil and use it all the way up to 190 V. A coil that is built with enough thermal mass will not heat excessively if pulsed. Just make sure the pulsing doesn't happen too often...

You may also use a thermistor (PTC) to reduce current after the initial high current. Google "PTC applications" for more info on that.

Gunnar Englund

http://www.gke.org

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[faulkma]

Thanks for the PTC idea. How fast do they react? Maybe at the norm voltage range (120V-190V) I can hit the 13 VAC coil direct, but at the under the low voltage, I can go through a relay circuit and a PTC to limit the IsqR heating of the coil, if the PTC is fast enough.

 

[Skogsgurra]

It all depends on PTC thermal mass, resistance and voltage. If you do the googling, you will find quite a lot of information. Typical times are 10 - 500 milliseconds. There are many PTC types to choose from and it is not easy to calculate the outcome. Start by trying a 240 V component with "large" dimensions (diameter around 1/2" and then some smaller ones). Coil impedance also has a lot of influence. It is a try-and-cut procedure.

Gunnar Englund

http://www.gke.org

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[electricpete]

"The coil is momentarly pulsed when needed for about 4 cycles mon."

What does "mon." means?

What does the solenoid do? Move something? 4 cycles seems like a very short energization time for solenoid.


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[itsmoked]

Not enough time for one to operate I'd say.

I would not give a PTC as big a chance as an ice cube in hell.. As that's what you'd be talking.

Remember at 190VAC you are talking about.. ah gesh..
If it is a 20W coil.
20W = 13² x R
R = 0.118 Ohms

At 190VAC you are talking P = 190² x 0.118

P = 4.2kW
Or I = V/R
I = 1,600 AMPS

We are talking flash bulb stuff.

Unless you give way more information you better start reviewing you reticence about capacitors and DC.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

How about a pair of zeners. You can regulate the current at 13 volts. Use a bank of 200+ volt incandescent lamps as a ballast. Take advantage of the ~ 10:1 hot/cold resistance ratio to limit current at higher voltages and still pass enough current at low voltages.

Another option may be the front end out of a switching power supply.
respectfully

 

[Skogsgurra]

Yes - a lot of time for some solenoids. I have actually used solenoids for half a cycle. OK, European half-cycles - 10 ms instead of 8.33 ms - but still quite short. And here we are talking six-seven times longer time. So that is not a problem.

Gunnar Englund

http://www.gke.org

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[Skogsgurra]

And, again, PTC are made to handle that kind of situations. Keith - go do some tests. You will be amazed how long an ice cube lives in Hell...

Gunnar Englund

http://www.gke.org

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[electricpete]

Out of curiosity, how big is the solenoid that operates for half a cycle, how far does it move and what is it's function?

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[Skogsgurra]

It is used to punch holes in plastic covers for IKEA book-shelves. The plastic is running continuously through a printing press and the holes are needed to allow some ventilation (prevents mould growing on wooden details). There is text on the plastic surface and the holes have to be placed in text-free areas. Also, it has to be fast. If it stays too long, the plastic will build up in front of it and create a general mess. No good.

I used a normal thyristor since the actuating pulse was never more than half a cycle. Actually somewhat shorter. The coil was something like 60 mm long and 30 mm wide. Do not remember exact peak current. Must have been one or two amps at 220 V (yes, 220 V - it was before we changed to 230 V).

Gunnar Englund

http://www.gke.org

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[electricpete]

Thanks gunnar. That's a little different than what I run accross at our power plant.

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[faulkma]

Thanks for your help guys. I should have paid attn. in electrical basics class. What I have is a 11.3 ohm coil that trips a breaker when the breaker trip contact is made, the trip contact stays on about 4 cycles. Help me out on my math by if the range is 13-190 VAC input to the coil, then at 13 vac= about 15 watts and at the top end (190AC) then that is 3194 watts. So, it does not appear that my 11.3 ohm coil is going to work at the high voltages. What if I power a 125VAC relay at the higher voltage and go through a resistor circuit.How would I calculate what resistor, I would need. And is there anymore coil information other than ohmic value , I need?
thanks

 

[Skogsgurra]

Is your coils DC resistance 11.3 ohms? Or is it AC impedance? It makes a helluvalotofdifference watt-wise.

Gunnar Englund

http://www.gke.org

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[faulkma]

I measured the coil with a fluke so I believe it would be the DC resistance?

We use to use a bridge rectifier with this coil and it would work fine as low as 16VAC . Now, with the spec changed to 13VAC-190VAC as a range, I not sure it will work at the low end without the rectifier. How do I calculate what kind of coil I need? or perhaps maybe a 2 coil arrangement?

 

[CJCPE]

DC resistance is what you measure with an ohmeter. AC impedance might be found on a spec sheet or marked on the breaker and is resistance plus the effect of the coil inductance at 50/60 Hz. The coil may be rated for a wide voltage range. Try to get specifications from the manufacturer. Does the breaker open the shunt trip circuit when it trips? Does the voltage applied to the coil come from a single device with a wide range of output voltages or is there more than one source?

If the coil has a DC rating or if you can change to a DC coil, this problem might be easier to deal with. What prevents using DC?

 

[Skogsgurra]

You usually cannot make a DC coil work on AC. I think that there are too many loose ends in this project. Time to have a straight talk with the customer.

What is this crap about not being allowed to use certain components while others seem to be OK?

If you were allowed to use a switcher, no sweat!


Gunnar Englund

http://www.gke.org

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