DPDT Rib Relay Controlled By 3-Way Switch

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chevyx92

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VA BCH, VA
Does anyone see an issue with using this set up to control a Mechanically Held contactor? One 3-way switch with 120V on the common terminal, one traveler terminal connecting to the "Common" of a DPDT rib relay and the other traveler terminal connecting to the second "Common" of the DPDT rib relay. Then from the rib relay, one "NC" contact will connect to the "Latch" terminal of the contactor and the second "NC" contact will connect to the "Unlatch" terminal of the contactor. I just want to make sure I am not missing anything with this configuration. I know I could use a double throw center off momentary switch and do the same thing.
 
Are you using the Rib relay because the contactor coil voltage is different? Most latching contactors have clearing contacts, and if not, the manufacturer usually has a control module that converts it to a two wire input.
 
Generally the Mechanically held contactor will come with the 2 wire control module.

All you would need would be a momentary spring return center off switch.

Jap>
 
It's a 12 pole square d mechanically held contactor. Square D has said the the unlatch coil is not rated for constant voltage and to use an opposing relay with the 3-way switch. My OP was what I was thinking would work. Using relays because can't have constant voltage applied when switching.
 
One simple solution would be to combine an NO push-button with a 3-way switch.

Wire it in series with the common. Flip the switch, push the button.

Or you could just use two push-buttons.
 
I could. Or I could just use a momentary center off switch too. I was trying to figure out how to incorporate the relay like the Square D rep said. I am now thinking the DPDT relay won't work and would actually need two SPDT relays.
 
I'm thinking you're thinking of using the relays to convert the steady state into a pulse?

Could they respond so quickly that the pulses don't activate the contactor?
 
LarryFine said:
I'm thinking you're thinking of using the relays to convert the steady state into a pulse?

Could they respond so quickly that the pulses don't activate the contactor?
Click to expand...
Yes. That is what I am thinking, a pulse to activate the latch and unlatch. Never thought about if it responds faster than it can activate the contactor. Hmmm.
 
chevyx92 said:
It's a 12 pole square d mechanically held contactor. Square D has said the the unlatch coil is not rated for constant voltage and to use an opposing relay with the 3-way switch. My OP was what I was thinking would work. Using relays because can't have constant voltage applied when switching.
Click to expand...
It’s not a Square D LX series then, those have clearing contacts. it’s probably made by ASCO labeled as Square D, those have a two wire module that can be added. If you don’t want to add the module, a momentary double throw center off switch is easily available, no relay needed.
 
This is the model # SQD 8903LXG1200V02. Again, the Square D tech guys said the unlatch is not rated for constant voltage and to use an opposing relay with the 3-way switch. I'm just going by what he said. The momentary center off switch is probably the best option.
hillbilly1 said:
It’s not a Square D LX series then, those have clearing contacts. it’s probably made by ASCO labeled as Square D, those have a two wire module that can be added. If you don’t want to add the module, a momentary double throw center off switch is easily available, no relay needed.
Click to expand...
 
chevyx92 said:
This is the model # SQD 8903LXG1200V02. Again, the Square D tech guys said the unlatch is not rated for constant voltage and to use an opposing relay with the 3-way switch. I'm just going by what he said. The momentary center off switch is probably the best option.
Click to expand...
The Square D guy is correct that the coils are not rated for continuous voltage, but that contactor has built in clearing contacts. I’ve hooked up 100’s of them. As soon as it changes state, it opens the power to that coil.
 
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I would think the Sq D Tech rep meant the latch and unlatch "coils" are not rated for constant voltage.
That's because they are only meant to be energized for the short period of time it takes to latch or unlatch the contactor.

As Hillbilly said, the coil clearing contacts remove the voltage to the coils when the contactor changes states from latched to unlatched or vice versa.
You can't have true coil clearing contacts if the contacts are not physically activated by the contactor itself.

Even with a momentary button there is nothing to keep from burning the coils out other than coil clearing contacts should someone decide to sit there an hold the switch in one position or the other.

As far as producing a latch and unlatch signal from a single switch, there are numerous ways to go about achieve that.

There's no need to try and manufacture a "pulse" signal from the switch to the contactor if coil clearing contacts are in place.
The coil clearing contacts are creating that pulse signal to the coils for you by changing state when the contactor pulls in or out.

Jap>
 
I don’t know if they are still around, but Andover energy management systems used two separate relays to latch and unlatch the contactors. Their downfall was both outputs could be energized at the same time. Replaced a lot of contactors that beat their self’s to death because the latch and unlatch inputs were energized at the same time for several minutes causing the contactors to chatter. Bad design.
 
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jap said:
There's no need to try and manufacture a "pulse" signal from the switch to the contactor if coil clearing contacts are in place.
The coil clearing contacts are creating that pulse signal to the coils for you by changing state when the contactor pulls in or out.

Jap>
Click to expand...
OK. So if that is the case, then a simple 3-way switch will suffice to activate the latch and unlatch function of the contactor then. I will have to try that out.
 
chevyx92 said:
OK. So if that is the case, then a simple 3-way switch will suffice to activate the latch and unlatch function of the contactor then. I will have to try that out.
Click to expand...

The coil clearing contacts are the key.

Think about a mechanically held contactor that is controlled by a single pole timeclock.
The timeclock doesn't need a momentary pulse signal, it can be simply an on or off signal sent to the 2 wire control module on the contactor.
The 2 wire control module is nothing more than a glorified ice cube relay that has a Common, NO & NC.
This allows a single input to create 2 outputs. No different than a 3 way switch.
The common of the relay has constant power on it and the NO and NC go to the respective latch and unlatch terminals on the contactor.
The timeclock does nothing more than pull in and out the coil on the control module and allow the contacts to change state sending the voltage to the latch or unlatch coil just like you turning on and off a 3 way switch.
The coil clearing contacts on the contactor itself remove the voltage being sent to the coils from the 2 wire control relay.
As long as you have the coil clearing contacts to remove the voltage from the coils between whatever is sending the signal and the coil itself you're good.

Jap>
 
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