Switching 8 parallel a.c. Contactors with one pressure switch

[K4SC]

I am redesigning a circuit which currently uses two DPDT pressure switches; both at the same setpoint. Each switch pole actuates two contactors, for a total of eight contactors,each with a 120VAC coil rated at 96 VA.

I need to modify the circuitry so that a single pressure switch pole simulateously energizes all eight coils. I am trying to avoid adding an interposing relay to the circuit if at all possible. The pressure-switch contact is rated at 15 amps for 120/240 VAC, so I don't think there is a capacity issue.

What gives me pause though is operating all eight coils in parallel. Where there are currently two contactor coils in parallel for each pole of each pressure switch pole, quadrupling the number of coils makes me wonder if the inductive kick might be too much, or if some other unknown (to me) reason exists to make this a poor engineering decision. These devices operate very infrequently, perhaps once or twice a year during plant start-up or shutdown.

Any thoughts or comments?

 

[waross]

Not much inductive kick at 120 volts AC, when energizing. Much worse when de-energizing DC circuits.
You will have inrush issues but a pressure switch with contacts rated for 15 amps @ 120/240 volts AC is frequently used for direct control of a pump motor or compressor motor.
If your contacts have an HP rating they will probably be suitable for the inrush.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

You will have problems.

Use the interposing relay...

Or, add auxiliary contacts to some of your contactors and have them close other contactors.


Keith Cress
kcress -

http://www.flaminsystems.com

 

[bogeyman]

Use a contact arc suppressor or RC snubber (100r + 0.1u) across the switch contacts and they wil last much longer.

 

[LionelHutz]

I'm with Bill. If that switch does not have a HP rating then forget about it. If that 96VA is the holding or steady state rating then these contactors will likely require around 400VA to pull-in. That puts the pull-in VA of all 8 around 3200VA which is well above the 15A rating of the switch.

Overall, it is a difficult load to switch. If you use an interposing relay then you just have to swap a relay out when it fails instead of changing the pressure switch. They are usually plug-in devices that are simple to change.

 

[dpc]

I'm pretty old school, but I would not be comfortable trying to control 8 contactors from a pressure switch contact. It's asking a lot of what is probably a small snap-action microswitch. (if it isn't a snap-action switch, I wouldn't even consider it.)

So I agree with itsmoked - put in the interposing relay AND also the surge suppressor.

If your concern is reliability, put in two interposing relays.

What is your objection to an interposing relay.

 

[ScottyUK]

If you are determined to avoid an interposing relay - and I can't see why it is a big problem to be honest - then use auxiliary contacts on one (or two) of the contactors to drive the next contactor(s) in line. Most contactors will take at least four auxiliary contacts, each rated for about 10A or so.


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If we learn from our mistakes I'm getting a great education!

 

[K4SC]

I have considered your responses and you have confirmed my gut reaction that it was a poor design. The reason for trying to avoid the interposing relay is lack of power supply capacity/margin in the cabinet where the relay would be located. In this case I am going to specify the interposing relay and the plant will just have to run an external power source to supply the coil, or sharpen their pencils on the existing supply; which is probably what will happen. The eight relays ultimately control steam drain valves on a 1300MWe turbine generator, so the cost/trouble of finding the needed power is miniscule compared to putting a slug of water through the steam turbine.

Thanks to all for taking time to reply. Chuck

 

[dpc]

Since this is related to water induction issues, I would seriously consider use of a dc interposing relay running from your station battery and also use of redundant interposing relays with a contact from each running to each contactor.

(I'd also use dc motor operated valves, but that's just me.)