What is contactor current in electromechanical relays? - Page 1
Forum section: electronics / beginners

author: Swapnil
author_rank: Contributor
timestamp: May 25, 2022, 11:50:39 am
content: I was going through DPDT configured electromechanical relay specifications and came across contactor current per pole.The contactor current for that DPDT relay was mentioned as 1A per pole. So, exactly what is that contactor current per pole, and how does it differ from the usual current rating of relays?

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 25, 2022, 12:02:13 pm
content: 1A seems to be in a strange range for a contactor. It's too much for the coil itself, and a very low value for the current handling capacity of a contactor.Can you post a datasheet, or at least a direct link to the info you found?

author: Psi
author_rank: Super Contributor
timestamp: May 25, 2022, 12:13:00 pm
content: It's probably a normally 1A relay but the manufacturer simply used the word contactor by accident or because their English wasn't that good.

author: Zero999
author_rank: Super Contributor
timestamp: May 25, 2022, 12:32:55 pm
content: Some contactors have auxiliary contacts, for switching low power levels.

author: Terry Bites
author_rank: Super Contributor
timestamp: May 25, 2022, 01:14:01 pm
content: The small current needed to pull in and latch the relay is far less than the main contactor contacts. It has to provide the coil with enough power to overcome the return spring. The latch / safety loop contacts are low current. A 100A contactor may only need an amp or less to actuate. 250-500mA is typical.The difference between contactors and relays is that a relay may haveno, nc or cocontacts and a contactor always hasnomain contacts. If its DPDT its not likely to be a contactor.

author: bdunham7
author_rank: Super Contributor
timestamp: May 25, 2022, 01:26:47 pm
content: Quote from: Swapnil on May 25, 2022, 11:50:39 amIt would help if you posted the specifications and the exact model of relay or contactor you're asking about. The term 'contactor current' is generic enough that there are multiple possibilities as to what it might mean.

author: BeBuLamar
author_rank: Super Contributor
timestamp: May 25, 2022, 01:48:26 pm
content: We do need to see the specifications but as the OP said it stated 1A per pole it seems to me that is the contactor rating and not the coil current.

author: rstofer
author_rank: Super Contributor
timestamp: May 25, 2022, 07:29:30 pm
content: First we need to know if we're discussing a relay or a contactor. If it's a relay, the rating may relate to a downstream contactor. It's not unusual to have a pilot relay control a much more capable contactor. It would almost certainly be the case for small PLCs.4 pole external relays might not be odd if there is some logic going on and one set of contacts drives an external contactor or motor starter.If it's a contactor, it's pretty darn small. When I think of contactors, I think of NEMA ratings like these:https://www.grainger.com/search/electrical/motor-controls/contactors/nema-magnetic-contactorsA size 7 contactor might be good for 810 amps. At $48,000 dollars it would be cheaper to motorize the upstream circuit breaker.

author: TimFox
author_rank: Super Contributor
timestamp: May 25, 2022, 07:43:01 pm
content: In that Grainger listing of high-current contactors, some of the devices have "auxiliary contacts" with much lower current ratings than the main contacts.Perhaps the original poster was confused between auxiliary and main ratings?

author: BeBuLamar
author_rank: Super Contributor
timestamp: May 25, 2022, 08:22:42 pm
content: You can call it contactor or relay but I am sure it's a relay because it's DPDT. I don't know of a contactor that is double throw. And since it's double pole and 1A per pole it seems obvious that the 1A is for the contact rating of the relay not the coil current. Also it doesn't see to indicate auxillary contact either.

author: TimFox
author_rank: Super Contributor
timestamp: May 25, 2022, 08:35:13 pm
content: In the Grainger listing, some of the high-power contactors are "reversing" contactors, which must be DPDT.

author: BeBuLamar
author_rank: Super Contributor
timestamp: May 25, 2022, 08:42:52 pm
content: Reversing contactor are 2 separate 3pole single throw contactors with mechanical interlocked with each other to prevent activating both at the same time. They are not double pole double throw.

author: TimFox
author_rank: Super Contributor
timestamp: May 25, 2022, 09:18:43 pm
content: Yes, that it is the internal construction, for safety reasons.The OP could help us all out by posting the actual listing for his "contactor".The ABB listings include 2 NO & 2 NC ganged contactors that break before make, but are not suitable for reversing:https://rexel-cdn.com/Products/ABB/AF09-22-00-13.pdf?i=3FCDE07F-E0CF-4BE6-A4C7-2705BF8BA97A

author: strawberry
author_rank: Super Contributor
timestamp: May 26, 2022, 08:48:35 am
content: remote operated contacting machine

author: BeBuLamar
author_rank: Super Contributor
timestamp: May 26, 2022, 01:56:12 pm
content: Quote from: TimFox on May 25, 2022, 09:18:43 pmThey are not DPDT either. They are 4PST with 2NO and 2NC.

author: TimFox
author_rank: Super Contributor
timestamp: May 26, 2022, 02:07:20 pm
content: Yes, that is the internal construction, but they can be wired as DPDT.I assume the original problem posted here is due to translation, since I'm sure some translation apps render a word that Americans would translate as "relay" into "contacter", since the difference in technical English is a matter of convention.

author: BeBuLamar
author_rank: Super Contributor
timestamp: May 26, 2022, 02:26:04 pm
content: Yes and thus I believe the OP has a relay so that it's specified as DPDT. And the 1A is for contact rating.It's a double pole double throw relay rated at 1A per pole.

author: Terry Bites
author_rank: Super Contributor
timestamp: May 26, 2022, 05:51:36 pm
content: I agree. I think that misdescribed product is the fact of the matter.So my advice is as always, post a chuffin picture OPs. No need to send the film off to crappy snaps any more.This is the internet, not teletext!

author: Swapnil
author_rank: Contributor
timestamp: May 28, 2022, 06:12:43 am
content: Quote from: bdunham7 on May 25, 2022, 01:26:47 pmIt was not a datasheet but just a requirement from a customer (and the customer won't reveal more information). The requirement was:-a)28V Electromechanical Relay (Make: STPI/Leach/Teledyne or equivalent relays)Configuration: DPDTContactor current: 1A per poleType: Latchb)28V Electromechanical Relay (Make: STPI/Leach/Teledyne or equivalent relays)Configuration: DPDTContactor current: 1A per poleType: Non-Latch

author: BeBuLamar
author_rank: Super Contributor
timestamp: May 28, 2022, 11:35:03 am
content: To answer your initial questin is easy. The current per pole is the sam as the relay current rating. Now you have problem because.Not too many latching relays are made28V DC or AC?

author: bdunham7
author_rank: Super Contributor
timestamp: May 28, 2022, 03:03:05 pm
content: Quote from: Swapnil on May 28, 2022, 06:12:43 amOK, you simply don't have enough information to specify a relay."Contactor current" here appears to have no specific meaning, just that the relay contacts are rated for 1 amp. But that doesn't tell you what voltage the contacts need to be able to break. Typically the contact ratings for a relay will include the current, the voltage at AC or DC (or both, usually with different numbers) and often the load type--resistive, inductive, etc.Also, as already stated, if the "28V" refers to the coil voltage, is it AC or DC?And 'latch' is also incomplete as there are multiple ways a relay can be configured to latch. I think the most common are single-coil set/reset where it changes state with each input pulse and dual-coil set/reset which should be self-explanatory. But there are others setups as well.Good luck with your customer.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 28, 2022, 06:21:56 pm
content: That's the problem with customers.You have to beat them into submission without hurting or offending them too much.It can be a long and arduous road to even get some decent specifications in written form, so keep track of the extra hours you need for this (including time spend on this forum thread) and bill them for it.