Operating a 24VAC coil on a valve or contactor (relay) with a DC coil voltage.

[sms]

I have a project that requires an electrically operated gas valve
<http://www.emersonclimate.com/Documents/White-Rodgers/instruction_sheets/0037-6159.pdf>
(36G22-254) and a 30A contactor (Square D 36G22-254). Both have coils
that are specified for 24VAC. The gas valve draws about 280mA, the
contactor coil current is not specified, but it's less, about 200mA.

What makes these coils "AC coils?" What is the downside of operating
them on D.C.?

I read the article at
<http://ecmweb.com/cee-news-archive/using-ac-coils-dc-power-1> and it
seems like there's no harm in operating the coils on DC, in fact they
claim that they'll work better and last longer.

[Phil Allison]

sms wrote:

> I have a project that requires an electrically operated gas valve
> <http://www.emersonclimate.com/Documents/White-Rodgers/instruction_sheets/0037-6159.pdf>
> (36G22-254) and a 30A contactor (Square D 36G22-254). Both have coils
> that are specified for 24VAC. The gas valve draws about 280mA, the
> contactor coil current is not specified, but it's less, about 200mA.
>
> What makes these coils "AC coils?"

** The design of the iron core is special - to prevent mechanical chatter with 50/60Hz AC current.

What is the downside of operating them on D.C.?

** They may get too hot or not release due to the iorn core magnetising.

> I read the article at
> <http://ecmweb.com/cee-news-archive/using-ac-coils-dc-power-1> and it
> seems like there's no harm in operating the coils on DC, in fact they
> claim that they'll work better and last longer.

** Reads like a pile of crap to me.

YOU need to know if YOUR solenoid will operate with DC.

Only way is to try it out, with a variable voltage PSU.


... Phil

[Tim Williams]

"sms" <scharf...@geemail.com> wrote in message
news:mcoc1i$2i5$1...@dont-email.me...

> What makes these coils "AC coils?" What is the downside of operating
> them on D.C.?

Are you familiar with shaded pole motors? There's a copper clip around
part of the core, which attenuates and phase shifts the AC. If the
contactor armature were free to rotate (and made of more copper), it would
spin.

It's been so long since I wired a panel, I don't remember what the typical
specs of those sorts of things are. As Phil said, check the datasheet.

Tim

--
Seven Transistor Labs
Electrical Engineering Consultation
Website: http://seventransistorlabs.com

[meow...@care2.com]

Its been ages since I did this. ISTR some wanting less volts dc for the right current, some not, but may be remembering wrong.


NT

[sms]

Yeah, that article basically says to figure the current needed on AC and
then determine the DC voltage where the same current is being drawn. I
did that and the coil operates fine, but I don't know what any other
implications may be.

It's not a big deal for me to operate them on 24VAC so I will probably
do that. I will just use a 24VAC/40VA power supply and generate my 5V
and 3.3V with a switcher and an LDO respectively.

[John Larkin]

On Thu, 26 Feb 2015 16:00:30 -0800, sms <scharf...@geemail.com>
wrote:

A 24 volt AC relay or solenoid will probably fry if run on 24 DC. AC
relays draw a lot of current when they are first energized, when the
magnetic loop is open. After they close and the mag loop is closed,
the inductance increases and the current drops. AC relays are desiged
with this change of current in mind. Coil current won't drop with DC
drive.

You could use it if you arrange to have full voltage initially, but
reduce it after it pulls in. There are several ways to do that.


--

John Larkin Highland Technology, Inc
picosecond timing laser drivers and controllers

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

[Paul E Bennett]

sms wrote:

> I have a project that requires an electrically operated gas valve
> <http://www.emersonclimate.com/Documents/White-Rodgers/instruction_sheets/0037-6159.pdf>
> (36G22-254) and a 30A contactor (Square D 36G22-254). Both have coils
> that are specified for 24VAC. The gas valve draws about 280mA, the
> contactor coil current is not specified, but it's less, about 200mA.

These are, I take it, pre-existing in the project and you are doing a new
controller. In which case, if you can supply the AC, do so.

Square D is now part of the Schneider Group and I was unable to find he
details of that product on their web-site. So, it looks like it is
discontinued now. Many of their contactor offering have a selection of coils
that can be fitted (230VAC/120VAC/24VAC/24VDC etc).

> What makes these coils "AC coils?" What is the downside of operating
> them on D.C.?

Others have adequately answered that question and I have nothing to add to
their comments. If you decide on DC energisation I would conduct testing on
reduced voltage (and hence current) after energising to maintain the ability
for it to release when de-energised. This complicates the drive circuitry
somewhat though. If you really want just a simple DC drive to the solenoid
and contactor then seek out alternative DC operated components.

--
********************************************************************
Paul E. Bennett IEng MIET.....<email://Paul_E....@topmail.co.uk>
Forth based HIDECS Consultancy.............<http://www.hidecs.co.uk>
Mob: +44 (0)7811-639972
Tel: +44 (0)1392-426688
Going Forth Safely ..... EBA. www.electric-boat-association.org.uk..
********************************************************************

[sms]

On 2/27/2015 1:48 AM, Paul E Bennett wrote:
> sms wrote:
>
>> I have a project that requires an electrically operated gas valve
>> <http://www.emersonclimate.com/Documents/White-Rodgers/instruction_sheets/0037-6159.pdf>
>> (36G22-254) and a 30A contactor (Square D 36G22-254). Both have coils
>> that are specified for 24VAC. The gas valve draws about 280mA, the
>> contactor coil current is not specified, but it's less, about 200mA.
>
> These are, I take it, pre-existing in the project and you are doing a new
> controller. In which case, if you can supply the AC, do so.

There is nothing pre-existing. The valve and contactor (2 versions of
the product) are being selected by me. I'd rather use super-standard
24VAC contactors and valves and operate them on 24VAC than spend even $5
more for ones with DC coils.

For devices with AC coils the power supply will be something like this:
<http://www.mpja.com/24VAC-40VA-Plug-Transformer/productinfo/31005%20PA>
which will simplify regulatory approval of the system, though this does
raise the component cost slightly. A switching regulator with up to a
36VDC input is very cheap, an and LDO will give me 3.3V from that.

[John S]

That sounds pretty much correct. You want to match the RMS current it
needs to the DC current you will supply. Make them the same and the
temperature rise of the coil will be within specifications. If it works,
you are good to go. However, you might need to estimate the contact
force in both situations. I don't know how to tell you to do that
because I've never gone that far with my experiments. Good luck.

[sms]

This is something that just isn't worth it. I wanted a 5VDC/2.1A power
supply, since these are only a few dollars (since they are used for
tablet charging), but there are two many unknowns to risk it. Also,
these coils take a lot of current so a boost regulator to get sufficient
voltage to operate the coil would add some cost and complexity.

The 24VAC supply is only around $6 in small quantities
<http://www.vitekcctv.com/ProductDetails.asp?ProductID=97> and I'm sure
that if I went to the manufacturer in China it would be $2-3, if the
quantities I need ever reach that sort of volume.

[Jon Elson]

Won't work! The scheme is that when the armature is out of the
coil, the inductance is low, high current flows, pulls the armature
into the coil, then the inductance is raised a LOT, and the current
draw drops greatly. The current is enough to hold the armature
in place. If you run it on 24 V DC, or run it on AC without the
armature being able to pull in (coil removed from valve, armature
jammed) the coil will burn up. It may take a couple hours, but
it will happen for sure. Don't ask how I know this.

The onyl way to do this on DC is to have a circuit that reduces the
current once the armature has pulled in. There's no way to do
this with a simple circuit that guarantees the plunger pulls in
first, but some experimentation ought to get you there with reasonable
repliability. What you need is a BIG electrolytic capacitor and
a series resistor. Put this in series with the coil. When the
cap is discharged, and the circuit connected to 24 V DC, the
capacitor applies 24 V DC to the coil for a moment. As the capacitor
charges, the coil voltage (and thus, current) is reduced. Adjust
the capacitor value until the the armature is solidly pulled into
the coil. adjust the resistor value until the holding current is
about twice the minimum needed to hold the valve open. That should
work with pretty good reliability.

Jon

[Jon Elson]

sms wrote:

> I have a project that requires an electrically operated gas valve
> <http://www.emersonclimate.com/Documents/White-
Rodgers/instruction_sheets/0037-6159.pdf>
> (36G22-254) and a 30A contactor (Square D 36G22-254). Both have coils
> that are specified for 24VAC. The gas valve draws about 280mA, the
> contactor coil current is not specified, but it's less, about 200mA.

Oh, on a contactor with a NC contact, you can rig it so the
coil gets full voltage until the armature pulls in, then the
contact opens to cut in a dropping resistor in series. In the
worst case, the thing will start cycling, but if the values are
right, it ought to work. the R-C trick is for solenoid valves
where you have no contact to detect the pulled-in state.

Jon

[sms]

Just saw that I screwed up the contactor part number, it's a Square D
8910DP32V14 <http://www.zoro.com/i/G0937912/>. This is sufficient for
the electric version of the product I'm designing.

[Kevin McMurtrie]

In article <mcoc1i$2i5$1...@dont-email.me>,

Linear motors may or may not have a conversion to DC. Conversion only
works if the motor locks into position rather than needing adaptive
strength based on inductance provided by the core.

You can take a look at old pinball machine schematics for a zillion ways
to use a linear motor. The flippers use a parallel RC pulse circuit
because their cores strongly magnetically bind, metal to metal, with the
coil's case. Large solenoids that pull with no specific stopping point
use AC so that the current drawn is proportional to the core's position.

--
I will not see posts from astraweb, theremailer, dizum, or google
because they host Usenet flooders.

[P E Schoen]

"Jon Elson" wrote in message
news:_-KdnR1qsdvRL23J...@giganews.com...

> sms wrote:

>> What makes these coils "AC coils?" What is the downside of operating
>> them on D.C.?

> The onyl way to do this on DC is to have a circuit that reduces the
> current once the armature has pulled in. There's no way to do
> this with a simple circuit that guarantees the plunger pulls in
> first, but some experimentation ought to get you there with reasonable
> repliability. What you need is a BIG electrolytic capacitor and
> a series resistor. Put this in series with the coil. When the
> cap is discharged, and the circuit connected to 24 V DC, the
> capacitor applies 24 V DC to the coil for a moment. As the capacitor
> charges, the coil voltage (and thus, current) is reduced. Adjust
> the capacitor value until the the armature is solidly pulled into
> the coil. adjust the resistor value until the holding current is
> about twice the minimum needed to hold the valve open. That should
> work with pretty good reliability.

I have done this in a test set I designed, which draws high current surges
from the mains, and thus an AC supply derived from it can sag enough for the
AC contactors to drop out, which can have bad consequences for the contacts
when they start to open at maximum current. Actually, you need a capacitor
and resistor in parallel, feeding the coil. The time constant of the
capacitor and the DC resistance of the coil is chosen to exceed the
operating time of the relay, typically 30 mSec, and the resistor such that
it supplies the rated holding current.

Recently I redesigned the boards to use a MOSFET driving the coil, and
controlled by a PIC12F1822. It applies full voltage for about 100 mSec and
then uses PWM to reduce the current. This works pretty well, but I had some
problems with the MOSFETs shorting out after a few operations. I used a
self-protected MOSFET designed for inductive loads and that worked better,
plus I added a capacitor across the coil and a TVS diode across the MOSFET.

It's also possible that the shorted turn on the coil to keep it from buzzing
on AC, may cause a very high current when the PWM (2-10 kHz) is applied. It
was difficult to get good scope readings because of noise spikes, which may
indicate high current surges, but the circuit seems to work reliably.

A similar PWM circuit appears to be used in some recent ABB contactors, so
they will work on DC without drawing excess current and not requiring an
"economizer resistor" as some need. This is attached across a pair of
special delayed opening NC contacts which do not open until the armature has
pulled most of the way into the coil. You can't use ordinary NC contacts
because they open too quickly.

Paul