Relay outputs for solenoid valves

[Leadfoot]

Now, just to add fuel to the fire, are we talking conventional current or electron current? LOL

I have found on solenoids that need a rapid off, therefore CANNOT have a flyback diode on them, I use 2 or 3 poles of the interposing relay wired in series. This provides 2 to 3 times the air gap helping quench the arc associated with the collasping field in the solenoid. The current is interrupted faster and you have NO current in the windings and the solenoid drops out the fastest.

ALWAYS use interposing relays between a PLC and a solenoid or motor starter coil. The relay takes 30 to 45 seconds to replace and you or some maintenance person will be replacing it at some point in time.

Coils are a dead short to current when voltage is first applied. the magnetic field that is built up around the windings develops the CEMF that sets the holding current. AC has less issues than DC but both fry relay contacts DC just faster than AC. Some DC contactor assemblies have an internal resistor that is switch in series with the coil after the contactor closes to limit holding current and extend the life of the coil.

I have measured as high as 500 volts DC kick on some 120 volt coils when the power is removed. I have found using 1kv prv diodes last the longest. 1N4007, 1N5551 if you can find them, seem to last the longest. If you have high cyclic rates on the coils, a full wave bridge rated at 1KV 20a holds up even better.

My $00.02 on the subject.

 

[DFJ]

you are correct, they are triac suited cards. oa16.

 

[nashcp21]

Hello, got a question that relates to the title of this thread. I have 1746-OW16 relay card on my SLC-500 and I want to use it to control 8 solenoid valves.

According to AB's website:

Continuous
Current per Output, Max. inductive load = 1.5A @ 120V ac

backplane current load = 170 mA @ 5V

http://www.ab.com/en/epub/catalogs/12762/2181376/2416247/1239758/2554063/tab4.html

The valves i want to use are 15W 120VAC.

P=V*I so 125mA of current?

So the question is do i need an interposing relay for the spike?

Thanks

Nashcp21

 

[ianingram]

Hello, got a question that relates to the title of this thread. I have 1746-OW16 relay card on my SLC-500 and I want to use it to control 8 solenoid valves.

According to AB's website:

Continuous
Current per Output, Max. inductive load = 1.5A @ 120V ac

backplane current load = 170 mA @ 5V

http://www.ab.com/en/epub/catalogs/12762/2181376/2416247/1239758/2554063/tab4.html

The valves i want to use are 15W 120VAC.

P=V*I so 125mA of current?

So the question is do i need an interposing relay for the spike?

Thanks

Nashcp21

Things to consider....

1)How often are you are switching the solenoids? Several times per minute, or once a day?

2)For AC coils you could have a "varistor" to lessen the spike. It may be installed at your control panel, or some solenoid plugs will have them built in.

IMHO You will most likely not require the interposing relay

 

[AutomaticLeigh]

Coils are a dead short to current when voltage is first applied. the magnetic field that is built up around the windings develops the CEMF that sets the holding current.

Hmmm..
What does this mean? A simple rule for working out what happens in switching circuits with RLC elements is that the voltage across a capacitor cannot change instantaneously, and by duality, the current through an inductor cannot change instantaneously. Applying a voltage to an inductor results in a current that rises exponentially to a value limited by the resistance. Real devices are more complicated, because the magnetic circuit changes with the moving parts, hence the inductance may change.
I would use a 1N4007 as well, but only because they are dirt cheap. When you switch off, the diode is forward biased. The reverse bias is only the supply voltage.

If you want faster operation, use an RC snubber or varistor instead.

 

[brucechase]

Just thought I would add my 2 cents worth. I have over 500 each 120 VAC solenoid valves, over 200 each 120 VAC MCC starters, and over 250 each 24 VDC solenoid valves all on the AB SLC 500 family. On the 120 VAC, I use a 130 volt MOV across the output to neutral. I don't have anything on the 24 VDC system. I do not use interposing relays ( although if I had to I would use a terminal block relay like this http://www.ab.com/en/epub/catalogs/12768/229240/229266/229643/229703/tab3.html ).

I have probably replaced only 2 or 3 output cards in the past 6 years. I cycle the solenoid valves thousands (up to 7,000) times per day and I never (bad thing to say) have any problems. My real world experience is not to use relay cards or interposing relays, just drive the device. I use triac cards and sinking cards.

 

[OkiePC]

I concur with Bruce. The PLC was invented to eliminate relay logic, and relays too. I only use them when absolutely necessary. Most of my solenoid valves have built in spike or arc protection. I use MOVs on the ones that don't. I HAVE had problems with SLC 1746-OW16 relay cards causing rack faults with inductive loads even with spike protection installed (driving 120vac solenoid valves). Avoid outputs 7 and 8 on this card, as they are closest to the backplane. I have seen two occurences of noise getting onto the SLC backplane because of one or both of these outputs. Triac outputs don't have thic concern, but will only handle 0.5a.

I am very interested in the "theory vs. reality" discussion about inrush currents in DC coils. I had always assumed there was inrush in any inductive load. I knew it was worse for ac loads. The textbook pure theory examples are good proof.

Where's the bitmap of a scope trace from the real world to defend Terry's statement?

The bottom line is...
There is Rush Current in ANY kind of coil under ANY kind of voltage - IT'S THE LAW!

 

[nashcp21]

Things to consider....

1)How often are you are switching the solenoids? Several times per minute, or once a day?

2)For AC coils you could have a "varistor" to lessen the spike. It may be installed at your control panel, or some solenoid plugs will have them built in.

IMHO You will most likely not require the interposing relay

Thanks for the input,

These valves are being switched a few times day, if that, so it sounds like direct connection might be possible.

 

[rsdoran]

The difference between AC and DC circuits is that in AC, the voltage is constantly changing in both level and polarity so a steady state is never reached. The inductance is always having an effect in the circuit.

I wrote a bunch of stuff about shader coils, magnetism, etc etc but changed my mind, its been done before.

 

[Archie]

Current in an inductor

Only because this thread had taken me way back to circuits class from college, I just had to throw in some input on inrush current.

Straight to the correct answer.... AC coils have high inrush current and DC coils do not. The reasoning has already been explained, but I'll recap.

AC coils have high inrush only because they start out without a core(valve spool) in the coil. The core increases the inductance and therefore reduces the current. If you wanted to test this, just remove an AC solenoid from a valve, then measure the current while energizing it. The current will stay very high. That is why you can burnup coils and outputs on a double solenoid valve if both sides are energized simultaneously, the core never gets to one side. The inrush time is dictated by how fast the core moves into the coil.

DC coils do not have a high inrush because of the basic laws of inductors. Inductors resist a change in current. When a DC solenoid is switched on, the voltage goes from 0 to full voltage, but the inductor resists the instant increase in current as the magnetic field increases. Once at full current an inductor only appears to be a length of wire when a steady DC voltage/current is applied to it, therefore the core moving into the solenoid does not affect the current at steady state.

If you look at an AC voltage and current through a coil on an oscilloscope you will see the current lags the voltage. That is because the coil resists the current increase as the voltage increases and vise versa.

This resisting of change of current is why you use diodes or snubbers accross the contacts of coils. The coils wants to resist the change from full load current to zero current. AC voltage is interesting in this perspective because if it is shut off at the zero crossing of current, there will be no arcing of the coil.

It was mentioned before, but you do have to be cautious when using diodes because the solenoids will stay energized for a short period of time after the output is turned off. This is because the current decays slowly through the diode. For example, if you have a DC brake on a motor and use a diode on the brake coils, the motor may coast a little further before the brake fully engages.

Well, that is enough of my food for thought.

 

[Orn Jonsson]

Any comments on using NMOS FET outputs. Automation direct 4 point module D2-04TD1? Should I use diodes with this one? I'll be simultaneously driving 4 2A DC Solenoids, (common for this card is 8A.
Thanks,

 

[rsdoran]

I use 2 or 3 poles of the interposing relay wired in series. This provides 2 to 3 times the air gap helping quench the arc associated with the collasping field in the solenoid.

How does it do that, last I heard a series circuit still carries the same current. The potential difference created from a collapsing field would be most noticeable (likely to arc) on the set of contacts nearest to the field, that potential does not change. The air gap does not change.

I do not see how this would help.