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Using an AC Rated Contactor for DC Circuits

07/20/2016 11:55 PM

I have a few questions:

1) Could we use an AC rated Contactor for DC switching? I have heard that typically the switching voltage a AC contactor can handle at DC for the same current is 10 times less. How do we arrive at this scaling factor of 10 times? Is it a practical figure or a properly calculated number?

Does it mean that a contactor rated to switch 690V AC, 20A can handle 69V DC, 20A?

2) What is the fundamental limitation on switching DC vs AC. Is it just the longer sustained arc? If so, in case I use proper snubbers/ arc suppression circuits could I make use of AC rated contactors for DC?

3) Most relays and contactors are rated for resistive and moderately inductive loads. In case I am switching capacitive loads the switching becomes very bad and leads to welding of contacts. What is the best recommended approach to overcome this?

4) What is the fundamental difference between force guided relay and a contactor. Let's say both are at the same current rating.

Cheers!

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#1

Re: Using an AC rated contactor for DC circuits

07/21/2016 12:13 AM

You need to become a google-dude!

A Google search of "contact ratings ac vs dc" yielded this (it's just the first page):

About 756,000 results (0.62 seconds)

Search Results

AC switch vs DC switch - AnandTech Forums

forums.anandtech.com › Hardware and Technology › Highly Technical AnandTech Nov 20, 2005 - 11 posts - ‎10 authors I found an artice explaining that AC vs DC amperage ratings are very ... As you open thecontacts, you'll draw an arc until there's a current zero ...

[PDF]Switch Ratings, What's it all Mean? - AeroElectric Connection

www.aeroelectric.com/articles/Switch_Ratings.pdf with respect to AC versus DC ratings. ... Various combinations of AC or DC voltage along ... loaded, over-center mechanisms that provide higher contact.

power - Purchasing a Switch - AC vs DC Current Specifications ...

electronics.stackexchange.com/.../purchasing-a-switch-ac-vs-dc-curre... Stack Exchange Nov 13, 2011 - Purchasing a Switch - AC vs DC Current Specifications? ... Does this currentrating apply to both the rated AC and DC voltages? .... help blog chat data legal privacy policy work here advertising info mobile contact us feedback ...

switches - Switch current rating AC vs DC - Electrical Engineering ...

electronics.stackexchange.com/.../switch-current-rating-ac-vs-dc Stack Exchange Feb 24, 2015 - AC ratings are typically much more lenient and DC much more strict ... AC arc than an DC arc, and therefore the contacts in an AC switch do not ...

[PDF]Electrical Ratings - NKK Switches

www.nkkswitches.com/pdf/electricalratings.pdf original current ratings at 125V AC. NKK has not ... Closing the switch contact on a motor ... DESIGN FOR INDUCTIVE DC LOAD MODELS S800D & SW3800D.

Converting AC amp ratings to DC amp ratings - CandlePowerForums

www.candlepowerforums.com › Forum › Flashlights › General Flashlight Discussion Jan 14, 2009 - 16 posts - ‎6 authors If I have a simple on-off switch rated at 5 amps 125v AC, then how many amps will it berated at 12v DC? ... there is an arc-over when the contact is broken, it is quickly stopped from the .... You're starting to get into Relativity vs.

Amp Ratings, HP, Volts | carlingtech.com

www.carlingtech.com › Resource Center › Product Learn More › Switch Learn More This rating reflects the amount of current the switch contacts can handle at the ... For example a 125V rating would be read as 125 volts AC and 125 volts DC.

12 volt DC switch vs. 110 volt AC switch - Frihost

www.frihost.com › Frihost Forum Index › Science › General Science AC and DC switch contacts are supposed to be manufactured with different ... by carefully inspecting ahigh-quality switch carrying both AC and DC ratings.

Dc on Ac switch - Mike Holt's Forum

forums.mikeholt.com › Active Forums › Electrical Calculations / Engineering Apr 28, 2009 - 8 posts - ‎7 authors Q: What DC current and voltage will an AC rated switch handle? A: The DC ... The electrical difference is that DC is much harder on contacts.

DC Switch ratings | Electronics Forum (Circuits, Projects and ...

www.electro-tech-online.com › ... › Electronics Forums › General Electronics Chat Oct 25, 2007 - 14 posts - ‎10 authors Secondly is the breaking capacity of the switch, as the contacts open ... If I know the AC rating, is there any way to decide the DC capacity if the ...

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#5
In reply to #1

Re: Using an AC rated contactor for DC circuits

07/21/2016 12:42 AM

Thanks for the reply MikerHo! This was too much of a judgemental remark which is not appreciated!

I have tried all this (google search) and am done with vague and imprecise answers from various threads. I am not looking for people's opinions but factual and fundamental reasons.

Anyway, just a piece of advise. If you do not know something do not reply. I have been able to reach this forum since I know how to "google".

Take it easy!

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#17
In reply to #5

Re: Using an AC rated contactor for DC circuits

07/21/2016 6:21 PM

It was not meant to be judgmental.

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#2

Re: Using an AC rated contactor for DC circuits

07/21/2016 12:20 AM

Yes, it quite commonly done. Many smaller MIG welder power supplies use ordinary multi pole AC power contactors to switch their low voltage high current DC side output power on and off.

It works fairly well but it does take it's toll on the contacts after a while (Usually they fail just after the waranty is up) expecially in machines that were ran hard being many used 40 - 50 amp rate contactors in machines that could put out up to and over 150 - 200+ amps of current.

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#3

Re: Using an AC rated contactor for DC circuits

07/21/2016 12:31 AM
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#4

Re: Using an AC rated contactor for DC circuits

07/21/2016 12:35 AM

If you put snubbers and arc suppression devices on an AC contactor, then wouldn't it become a DC contactor? So why not get a suitably rated DC contactor in the first place?

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#6
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Re: Using an AC rated contactor for DC circuits

07/21/2016 12:51 AM

Thanks for your reply Tornado!

I am actually having a hard time finding DC rated contactors for my requirements at a reasonable price. Typically the DC rated contactors I am getting can accomodate several 100's to 100V DC and are quite expensive. My requirements are not very high otherwise. I need to handle 20~25A @ 48V DC. Now if I could get an AC-1 - AC-3 contactor which can handle this, it would be great! These contactors are available in plenty and are much cheaper (5 times). So, if I could use a 20A, 690V AC contact to switch 20 A, 48 VDC, I am sorted at a fifth of the cost. Just to keep the contacts happy I am thinking of using the snubbers. Let me know what you think.

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#7
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Re: Using an AC rated contactor for DC circuits

07/21/2016 1:17 AM

Maybe just try one and see how it performs. If you need to switch only one pole, you might loop it through two or three phases of a multipole AC contactor.

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#8
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Re: Using an AC rated contactor for DC circuits

07/21/2016 1:59 AM

Forklift and arc welder manufacturers might be a source of contactors that operate in your desired voltage/current range. Some of their amperage needs are higher than yours, but others might be a decent fit.

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#9
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Re: Using an AC rated contactor for DC circuits

07/21/2016 2:40 AM

Thanks for the advice Tornado!

Actually, we are also doing similar AGV activities. We got hang of cheap automotive DC relays which are able to handle even 80A DC without a problem and they work well for our 24V systems but the catch is once you get into 48V DC they aren't that many options. The 24V 80A DC relay handle only a few amps at 48V! They are like 7 -10 $ for an 80A DC relay. An AC-1 contactor for 20A is about 40$ and a 20A DC/ 48V and above relay is 200$. Let me check if the welding/ fork-lift guys have 48V rated relay.

Cheers man!

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#10
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Re: Using an AC rated contactor for DC circuits

07/21/2016 2:48 AM

Lots of golf carts and electric pallet jacks are 24V, many forklifts are 36V, and many forklifts are 48V. In any inquiries, be sure to specify your voltage.

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#14
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Re: Using an AC rated contactor for DC circuits

07/21/2016 11:31 AM

I have used regular relays and contacts in higher voltage DC application many times without problems.

The trick is to use a several uF nonpolarized capacitor with a resistor and diode in series with it across the contacts plus a flyback diode in parallel with the load.

The concept is that the diode in series with the capacitor lets the load current rush into the capacitor while the contacts are opening essentially creating a zero cross moment, like what an AC cycle has, and the resistor in parallel with the diode limits the discharge current down to a level below the contacts ratings for when they close again.

The value of the components I typically went with was diodes that have a peak repetitive current ratings at least double what the load current is and a resistor that would limit the capacitor discharge to about 10% of the contractors current rating.

Overall if set up right there is almost no arc flash when the contacts open even if running several hundred volts DC @ several tens of amps.

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#11
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Re: Using an AC rated contactor for DC circuits

07/21/2016 3:09 AM
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#16
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Re: Using an AC rated contactor for DC circuits

07/21/2016 12:51 PM

Does it have to be mechanical? There are MOSFET's that can handle that kind of voltage and current all day long with no sweat.

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#18
In reply to #16

Re: Using an AC rated contactor for DC circuits

07/21/2016 10:37 PM

Actually, This is an E-STOP controlled switch. The problem with MOSFET is that they are highly susceptible to shorting. We have had issues with them on over current / short circuiting. You would require an active current monitoring circuit to cut the MOSFET upon short. Even a fuse blow takes a while to blow off. We can't use fast acting fuses here. Also the safety regulations do not encourage the use of semi-conductor based switching. In-fact, even these relays will be sequenced to prevent welding off all contacts together.

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#20
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Re: Using an AC rated contactor for DC circuits

07/22/2016 1:01 AM

Yup. I've designed power distribution units for military applications with MOSFETs protected by a Linear LTC4366 to protect loads from voltage transients as well as taking advantage of the circuits ability to limit current as well. I'm not saying this works well in your application especially if you have certain safety requirements to meet. Innovative is not usually a desirable trait when meeting safety regs.

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#22
In reply to #6

Re: Using an AC rated contactor for DC circuits

07/22/2016 5:01 PM

Have you tried using vacuum tube thyratrons? Or solid-state relays? The latter does not really sound like much of an electrical safety device to me.

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#12

Re: Using an AC Rated Contactor for DC Circuits

07/21/2016 8:26 AM

The plant that I worked at used to manufacture DC relays and such. We spent significant effort getting the contact materials sorted for breaking DC.

Switching AC has a zero crossing where current normally goes through a zero point and the arc extinguishes, while DC does not have that feature.

The "vague" responses that you will see from others is due to the uncertainty about what contact materials may have been used.

Another criteria for DC devices is a greater breaking force, since as you indicate there is a tendancy to "weld" during the "make" phase due to inrush currents (Capacitors and incandescent fillaments).

The "Vague" responses that you will see from others is due to uncertainty about the load that is faced by the contacts and whether they have enough breaking force to overcome the weld.

The next criteria is contact material migration. The arcing that happens in DC always moves material in the same direction to deposit on the other contact. With no zero-crossing,this always happens at load current and voltage. (Every time is worst case scenario.) Contact geometry can reduce this concern as also can contact material composition. For low current,we used fine silver, for higher current we had additives to harden the contact and also provide arc suppressing capabilities.

The"Vague" responses that you will see from others is due to uncertainty about contact geometry and metalurgy.

Unless you actually know the design details of the contact set that you are proposing to use and have the design skills to extrapolate from AC to DC there is no means to figure the DC capability.

Now, I ask you this. What is the failure cost of a relay/contactor in your situation? Will it mean that customer never buys another device from you? Will it mean they return a "failed" unit to you and requiest refund AND compensation?

Will it mean that a factory area becomes unsafe? Will someone be stranded and unable to safely complete a task?

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#19
In reply to #12

Re: Using an AC Rated Contactor for DC Circuits

07/21/2016 11:00 PM

Thanks a ton man for taking out time to share your valuable experiences!

You are absolutely right in saying that using an AC rated contactor for DC would not be the most optimal approach. The data sheets leading almost nowhere, I actually opened these contactors to figure out their contact geometry. Ran tests with capacitive loads and observed the contact deterioration.

The contactors here are for main power switching (high power) and E-STOP (For E-STOP we have force-guided safety relays) however the main power control contactor is where this whole trouble is. As this explains that it is actually the safety circuit so failure is not an option. If the contactor completely burns out, it is ok since the brakes will kick in however if the contacts get welded, it will be a serious issue. Now, the entire safety does not totally depend on on contactor but it still is a very crucial element in the safety chain.

Thinking more about this problem, I am kinda more inclined for going in for a contactor I found (though a bit expensive) which has the DC-3 rating. I don't want to get stuck with the certifications.

Cheers!

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#13

Re: Using an AC Rated Contactor for DC Circuits

07/21/2016 10:44 AM

A couple of observations.

1) If this contactor is to be installed in a product that has 3rd party certification, the use of a AC rated device on a DC source would more than likely cause the unit to fail its certification tests or should cause it to fail.

2) If this is part of a system that will be inspected the inspector should fail this installation, because the use of this equipment is not certified to the conditions of use.

3) Sounds to me like you need to look at using a solid state switching element for these DC loads. Bet they would be cheaper in the long run.

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#21
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Re: Using an AC Rated Contactor for DC Circuits

07/22/2016 2:29 AM

Hi!

I agree with your point for the certification. That is the reason I am trying to find the real reason behind the mysterious conversion factor of 10. How do we arrive at the DC voltage 10 times less than the AC voltage for the same current switching? My objective here is not to bring up a prototype on somehow get something to work but to build a certified product.

The solid state switching has one major issue. It does not do well with very high current shorts. Like a 260AH, 48VDC battery shorting through it. Before the fuse blows the MOSFET's turn into a dead short. For the same situation, the contactor contacts will melt and break which is highly desirable (I thermal).

Cheers!

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#15

Re: Using an AC Rated Contactor for DC Circuits

07/21/2016 11:33 AM

Google White Rodgers they manufacture DC contactors for electromove equipment.

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