2 single pole contactors instead of one 2 pole contactor

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LMAO

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Will there be a problem if I use 2 single pole contactors instead of one double pole one? My application is 1000VDC and there are not whole lot of options out there. I found these cheap single pole contactors and I am thinking of using two of them in parallel to isolate my DC bus from inverters. contactors will be energized/de-energized simultaneously.
Any problem? comment?
Thanks
 
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Jraef

Jraef

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There is no such thing as "absolutely" simultaneous and with electricity, seemingly insignificant differences in time make a big difference. So if you have 2 coils, they will never release at the exact same time, at least as far as preventing the contacts from burnng. Won't work, at least not for long.

There are very few contactor rated for 1000VDC, but they do exist. Most of them use 4 pole contactors that have each pole wired in series with each other, so in effect it becomes a 1 pole contactor.
 
L

LMAO

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Jraef said:
There is no such thing as "absolutely" simultaneous and with electricity, seemingly insignificant differences in time make a big difference. So if you have 2 coils, they will never release at the exact same time, at least as far as preventing the contacts from burnng. Won't work, at least not for long.

There are very few contactor rated for 1000VDC, but they do exist. Most of them use 4 pole contactors that have each pole wired in series with each other, so in effect it becomes a 1 pole contactor.
Click to expand...
Even a 2 pole contactor can not close both poles "absolutely" simultanously. But let's forget that.
How and why a time delay between closing 2 poles of a DC contactor will result in contacts burning??
 
Jraef

Jraef

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LMAO said:
Even a 2 pole contactor can not close both poles "absolutely" simultanously. But let's forget that.
How and why a time delay between closing 2 poles of a DC contactor will result in contacts burning??
Click to expand...
if (when) one opens before the other, it will attempt to interrupt the entire arc. By the time the second set opens, the damage is done. You are right that even on a common armature they are not going to be exact, but that has been proven (in the case of a tested unit) to be sufficiently close and sufficiently repeatable. You cannot guarantee either when there are two coils involved.

You can in theory get away with that on AC because the power is crossing zero 120 times per second anyway. But even then, you cannot get a HP rating on parallel contactors for similar reasons. Twin contactors at a higher rating can only be used on non-inductive loads.
 
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L

LMAO

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Jraef said:
if (when) one opens before the other, it will attempt to interrupt the entire arc. By the time the second set opens, the damage is done. ......
....
..
Click to expand...

So, if I am not mistaken, you are saying that the worst case scenario is when one contact opens before another and has to absorb the "breaking arc" all by itself. BUT, each single pole contactor is ALREADY rated to open the circuit by itself. The only reason I need a two pole is to completely isolate my inverters from DC bus. in other words, using a single pole contactor is equivalent of the worst case scenario you just described.
I think there is a misunderstanding; I am not using the second contactor to increase make/break rating!
 
Jraef

Jraef

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I guess I am misinterpretting your use of two contactors then. If you use a contactor rated for the full load and voltage, I don't see the point of two. Are you switching both the + and -? If so, are you sure you are allowed to do that? In the US you are not allowed to switch grounded conductors, and all DC systems over 12VDC must have one side grounded. I don't know what it's like in other countries though. If this is INTERNAL to the inverter (which you never aluded to until now), and this is not switching a solar PV array output, then it may be fine.
 
G

gar

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111913-0932 EDT

See http://relays.te.com/appnotes/app_pdfs/13c3236.pdf for a good general discussion.

If you do not need quick turn off, then consider a diode to shunt the arc.

If you must accept the arc, then consider a separate relay that has its state toggled when no current is flowing, and is used to alternate the direction of current flow thru the primary switching relay. This will keep reversing the direction of metal flow.

.
 
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jim dungar

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Jraef said:
In the US you are not allowed to switch grounded conductors, ...
Click to expand...
Not true.
The code requires the grounded and ungrounded conductors to be switched at the same time.

DC lifting magnet contactors are (2) single pole devices switched at the same time. One method used is to put the two contactor coils in series (each coils is rated for 50% of the control voltage).
 
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petersonra

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I seem to recall a prohibition in the code from using contacts in parallel unless they are listed for being used that way.

I am not sure that the situation you are describing is really a parallel thing though. It sounds more like they are opening two different conductors.

I think it is a bit cheesy to do it with 2 cheaper relays, but off the top of my head as long as they are rated properly, I don't see an issue with doing so. I do something similar for power isolation on some 4 wire equipment we make. A properly rated 4 pole contactor is pretty pricey in larger sizes. It is always cheaper to use 2 3 pole contactors. I put one 3 pole on the 3 hots and 1 on the neutral. The coils are in parallel so they get actuated at the same time.
 
L

LMAO

Senior Member
Location
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Jraef said:
I guess I am misinterpretting your use of two contactors then. If you use a contactor rated for the full load and voltage, I don't see the point of two. Are you switching both the + and -? If so, are you sure you are allowed to do that? In the US you are not allowed to switch grounded conductors, and all DC systems over 12VDC must have one side grounded. I don't know what it's like in other countries though. If this is INTERNAL to the inverter (which you never aluded to until now), and this is not switching a solar PV array output, then it may be fine.
Click to expand...

DC leads are not grounded. DC bus is about 810VDC which feeds a bunch of inverters. I am trying to isolate the DC bus from inverters.
I understand that even one contactor will open the circuit, but the reason I want to open both leads is to completely isolate the inverter to be safe for maintenance. I just want to have complete isolation, electrically and mechanically.
 
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petersonra

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LMAO said:
DC leads are not grounded. DC bus is about 810VDC which feeds a bunch of inverters. I am trying to isolate the DC bus from inverters.
I understand that even one contactor will open the circuit, but the reason I want to open both leads is to completely isolate the inverter to be safe for maintenance. I just want to have complete isolation, electrically and mechanically.
Click to expand...

You cannot use a contactor for this purpose. You need a proper disconnecting means.
 
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petersonra

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engineer
LMAO said:
I see...
So, does NEC say anything about this?
Click to expand...
You may not find it in the code as this kind of equipment may not require a disconnecting means.

However, if the maintainers are subject to OSHA rules, you have to have a proper disconnecting means that is open before you can work on something.
 
L

LMAO

Senior Member
Location
Texas
petersonra said:
The OP mentioned an 800V DC Bus. These contactors do not appear to be rated for that high of a voltage..
Click to expand...
in fact I have about 1200VDC in regen mode (when motors are supplying power).
Funny thing, our switches are electrically interlock but I heard that some electricians are too lazy to run the interconnections and, to bypass the interlock coil, their stick their finger in a 1200VDC disconnect to push the pin!
:jawdrop:
 
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