Stray voltage/current and small coils

[jdbrandt]

Actual field case of stray voltage/current for comments...

A PLC relay contact output is to energize the coil
of a pneumatic solenoid. This solenoid is a dual coil,
4 way valve, the type used to extend and retract a
cylinder.

With the PLC output OFF, approx. 28-50 volts AC can be
measured across the coil. [Attaching a small test load reduces,
but does not eliminate the voltage.]

The symptom is that when coil 'A' is attempted to be
energized, coil 'B' is seeming to hold the spool of the
solenoid sufficiently that coil 'A' never gets control
of it. (The same symptom is NOT true when 'B' is attempted
to be energized...'A' does not hold the spool)

The solution was to replace all the field wiring with
twisted shielded wire, and ground the shield at the
solenoid base.

Facts of the case:
1.) The solenoid's rating is 120VAC, 8.3 VA & 67VDC, 8 watts.
2.) The solenoid reads ~890 ohms with a digital multimeter.
3.) The total length of wiring run, from PLC output terminal
to solenoid coil is 3 meters.
4.) Operating voltage is 121 volts, AC.
5.) Solenoid is new; installation is new.

Q1: Has anyone ever seen solenoid coils dual-rated for both
AC and DC? What must they look like inside????
Q2: What the heck...how do you avoid this kind of problem
in future designs?

 

[cnrservices]

If your PLC outputs are TRIAC based, then you are getting a leakage current - the solenoid coil isn't pulling enough current to fully shut off the TRIAC in it's off state.

This also happens when using a proxy on an input to a "cheap" PLC or smart-relay.

If you can, change the output to a relay card.

Otherwise, power a general purpose relay with the ouput and power the solenoid with the relay - along with a fuse.

Normally, I always control solenoid coils through relays and not directly by the PLC output, this adds a little cost to the panel, but saves a lot of PLC outputs due to coils and wires that get shorted.

 

[CaseyK]

As cnrservices mentioned, you can always go to a relay card. As a form of standardization, I started using relay output on all my projects. I don't remember if all we had in stock was relay cards, or maybe we didn't know what the machine was going to be wired with when it came in, but it just stuck. Since relays are universal, and generally have higher current ratings, it worked out well. While I don't like to mix AC and DC on the same card, I do often have several relay cards for AC and 1 or 2 for DC.

For some unknown reason, I started using 16 point input cards and 8 point output cards, and that stuck too. A lot of personal preference in this industry.

Regards....casey

 

[Eric Nelson]

The symptom is that when coil 'A' is attempted to be
energized, coil 'B' is seeming to hold the spool of the
solenoid sufficiently that coil 'A' never gets control
of it. (The same symptom is NOT true when 'B' is attempted
to be energized...'A' does not hold the spool)

I remember reading in a pneumatic catalog (Festo or SMC?) about the availability of 3-position vales with a 'priority' coil. Basically, if both coils were actuated, one would have priority over the other. A 2-position, dual coil valve like yours might have a similar priority effect, depending on the design. Especially with your weak voltage scenario. What brand/type are you using?

Q1: Has anyone ever seen solenoid coils dual-rated for both
AC and DC? What must they look like inside????

Festo offers coils rated 12VDC/24VAC and 24VDC/42VAC. I've never seen this for any other brand. Don't ask me how they work...

Might be time to abandon your 'old school' 120VAC control circuitry, and join the 'hip' 24VDC crowd, Jeff... :nodi:

We don't have these 'weird' problems (and don't get shocked near as much either)...

beerchug

-Eric

 

[Gerry]

But he said

A PLC relay contact output is to energize the coil

Maybe there's a R/C snubber across the contacts??

Has anyone ever seen solenoid coils dual-rated for both
AC and DC? What must they look like inside????

Perhaps there's a rectifier inside??

 

[jdbrandt]

thanks, so far

Thanks to all the replys. Here are a few additional items in
response to your (collective) replys.

a. I've never heard of a priority coil, but, it makes sense to do,
when you think of it. The data sheet does not mention this, but,
then again, the data sheet does not mention the dual rating
either, so, additional research is required here.

b. As the panel builder/programmer, I have absolutely no control,
say, or influence in the voltage level of the coil as supplied
by the machine builder. Yes, I have seen plenty of 24VDC out there,
especially for our out-of-the-US jobs. I don't see how using
24VDC coils would have reduced the stray voltage on the output.

c. No R/C snubbers are installed that I can tell. And, even if they
were, how would that account for stray voltage in the field with
the output wires from the PLC disconnected at the PLC?

d. I didn't get shocked. Since the human body is a 57k resistor (although I'm about a 59k since I got married), there is not enough
potential behind the stray voltage I'm seeing to do any damage. And,
if there HAD been, I'd still be at the site looking for the trouble.

 

[Rick Densing]

I have seen a similar situation with a loose neutral. The two coils were tied together, but had a high resistance connection to neutral.

 

[godfrey]

We used to use triac digital outputs to drive ice cube type relays like Square D or IDEC. These coils have comparitively low DC resistance. When we changed to slim profile Phoenix Contact relays we found that due to their coils higher DC resistance, the relay indication light would be dimly lit even when the triac was off. The triac output would float at 120 VAC even when turned off by the PLC. After changing to Relay type digital outputs, the problem of the Phoenix relay being dimly lit went away.