Snubbers for 208v single phase

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Steve Kelly

Member
Location
Manteca, Ca.
Hi all,

I'm new to this forum and this is my first post. My question has to do with hooking up RC snubbers on a contactor with 208 single phase inductive load. Do I go from phase to phase or from each phase to ground? :confused:

Thanks
Steve
 
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ELA

Senior Member
How is the coil switched?
The snubber provides a path for the stored energy in the coil to dissipate.
Sometimes you will see the snubber across the switch opening the load. Or it may be more directly in parallel with the the load.

The concept being that when a switch opens a coil a large voltage is generated since the open circuit impedance is so high. The snubber limits the open circuit impedance in order to limit the large voltage from being produced.
The RC network allows the stored energy (current) to flow through it to dissipate the energy.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Hi all,

I'm new to this forum and this is my first post. My question has to do with hooking up RC snubbers on a contactor with 208 single phase inductive load. Do I go from phase to phase or from each phase to ground? :confused:

Thanks
Steve

the RC network gets wired across whatever load you are trying to suppress. If you are trying to suppress the coil, the Rc network gets wired across the coil.
 

Steve Kelly

Member
Location
Manteca, Ca.
Hi ELA,

It's a contactor with a 24vdc coil which in turn gets a signal from a PLC. The DC coil has a freewheeling diode. I'm considering snubbers for the load to extend the life of the contacts.

Thanks
Steve
 

ELA

Senior Member
Hi ELA,

It's a contactor with a 24vdc coil which in turn gets a signal from a PLC. The DC coil has a freewheeling diode. I'm considering snubbers for the load to extend the life of the contacts.

Thanks
Steve

The same principle applies whether it be the coil or load. The level of protection depends upon the robustness of the switching device.
Snubbers or varistors (or diodes) are often made internal parts of solid state switches to protect the switching devices. They cannot handle voltage spikes very well.

As already implied ... I would select a contactor designed for the load and call it good.
 

mxslick

Senior Member
Location
SE Idaho
Hi ELA,

It's a contactor with a 24vdc coil which in turn gets a signal from a PLC. The DC coil has a freewheeling diode. I'm considering snubbers for the load to extend the life of the contacts.

Thanks
Steve

Bold added by me....

Guys, what he needs is snubbers across the contacts.


There are snubbers (RC networks) that you wire in parallel with the contacts that will suppress arcing.

In my experience, Varistors (or MOV's) do not work for that application.

Max protection would be provided by the use of the snubbers across the contacts and Varistors properly installed at the load.
 

ohmhead

Senior Member
Location
ORLANDO FLA
Well what i see you are asking is load contacts for a single phase 208 volt motor or what ever load is connected to or thur contacts?

Yes its been done it helps the life of contacts if you want to do it go for it .

You need a capacitor and a resistor in series then install it in parallel across contact when contact opens it eliminates arc between when closing or opening but you need the correct size resistor and cap for voltage used and high wattage resistor for load current used .

One snubber circuit for both contacts on that 208 volt .


As stated above MOV and such are just for voltage on spikes but they kinda been around before todays surge protection devices and were use for over voltages across power supplies you could install a single MOV on BOTH 208 volt legs on contactor side and then take each MOV to ground it will take a spike or arc to ground with no resistor needed.

But it must be rated at a much higher voltage then 208 volts like 1000volts or better if you design this .
 
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Steve Kelly

Member
Location
Manteca, Ca.
One more question

One more question

Hi guys,

Thanks to all of you for your help. What I would really like to know is, since this is 208v, do I hook the RC snubber between the 2 phases, or go I go from each phase to ground?

Thanks
Steve
 

ptonsparky

Senior Member
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
Electrical Contractor
IMO you would put the snubbers at the load coil. A & B or A1 & A2. You want to limit the inductive rush of the coil through the contacts of your plc. Both on start & stop. Line to ground has nothing to do with your coil circuit and contact longevity.
 

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
Hi guys,

Thanks to all of you for your help. What I would really like to know is, since this is 208v, do I hook the RC snubber between the 2 phases, or go I go from each phase to ground?

Thanks
Steve

I think you are making this too hard. For the most part, there is nothing gained by adding an RC network to an AC load. It may supress some noise, but it won't solve any actual problem in most cases, unless you are using a marginal contact for the switching. If that is the case, the best answer is to get a better contactor.

If you are trying to protect the contacts, put the RC network acorss the contacts, or across the load. Either will do the trick. Putting the Rc network from line to ground does not serve a good purpose.
 
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