# What's the "Definite Purpose" of a contactor? **URL:** \ **Category:** Factual Questions\ **Created:** [March 13, 2009, 5:29pm UTC](https://boards.straightdope.com/t/whats-the-definite-purpose-of-a-contactor/489348 "2009-03-13T17:29:34Z")\ **Posts on this page:** 14\ **Page:** 1
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [March 13, 2009, 5:29pm UTC](https://boards.straightdope.com/t/whats-the-definite-purpose-of-a-contactor/489348/1 "2009-03-13T17:29:34Z")
I’m working on a design that requires the use of contactors. There are several different categories - Motor Start, Lighting, and “Definite Purpose.” What the Hell is a “Definite Purpose,” and where did that term come from? ---
**Author:** ![Really\_Not\_All\_That\_Bright](https://avatars.discourse-cdn.com/v4/letter/r/e8c25b/32.png) [@Really\_Not\_All\_That\_Bright](https://boards.straightdope.com/u/Really_Not_All_That_Bright)\ **Post date:** [March 13, 2009, 6:06pm UTC](https://boards.straightdope.com/t/whats-the-definite-purpose-of-a-contactor/489348/2 "2009-03-13T18:06:11Z")
[http://www.squared.com/us/products/motor\_control.nsf/unid/44CE1438482BD8E985256A6A006330B8/$file/dpcontactor.htm](http://www.squared.com/us/products/motor_control.nsf/unid/44CE1438482BD8E985256A6A006330B8/$file/dpcontactor.htm) ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [March 13, 2009, 6:08pm UTC](https://boards.straightdope.com/t/whats-the-definite-purpose-of-a-contactor/489348/3 "2009-03-13T18:08:14Z")
> [@Really\_Not\_All\_That\_Bright](#): > > [http://www.squared.com/us/products/motor\_control.nsf/unid/44CE1438482BD8E985256A6A006330B8/$file/dpcontactor.htm](http://www.squared.com/us/products/motor_control.nsf/unid/44CE1438482BD8E985256A6A006330B8/$file/dpcontactor.htm) Thanks, but I know where to find them. I want to know where that odd terminology came from. ---
**Author:** ![Really\_Not\_All\_That\_Bright](https://avatars.discourse-cdn.com/v4/letter/r/e8c25b/32.png) [@Really\_Not\_All\_That\_Bright](https://boards.straightdope.com/u/Really_Not_All_That_Bright)\ **Post date:** [March 13, 2009, 6:13pm UTC](https://boards.straightdope.com/t/whats-the-definite-purpose-of-a-contactor/489348/4 "2009-03-13T18:13:35Z")
I think that comes from the fact that they’re _designed_ to be used in HVAC applications. ---
**Author:** ![Nametag](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/nametag/32/406_2.png) [@Nametag](https://boards.straightdope.com/u/Nametag)\ **Post date:** [March 13, 2009, 6:18pm UTC](https://boards.straightdope.com/t/whats-the-definite-purpose-of-a-contactor/489348/5 "2009-03-13T18:18:59Z")
I think you were directed to the product description. More info from Eaton: > [@](#): > > Definite Purpose Contactors and Starters are electrically operated switching devices specifically designed for the heating, ventilation, air conditioning and refrigeration (HVAC) industry. They are typically controlled by automatic, i.e. thermostat, control or manual, i.e. pushbutton, interfaces. > > They are most commonly found in applications such as refrigeration, air conditioning and resistance heating (HVAC). They are suitable for a wide range of other low-cycle duty applications requiring low cost switching of single- and three-phase loads. And from Wikipedia: > [@](#): > > general purpose machine tool contactors generally emphasiz[e] simplicity of application while definite purpose … emphasizes design for the intended life cycle of the application ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [March 13, 2009, 6:23pm UTC](https://boards.straightdope.com/t/whats-the-definite-purpose-of-a-contactor/489348/6 "2009-03-13T18:23:13Z")
> [@Nametag](#): > > I think you were directed to the product description. More info from Eaton: And from Wikipedia: That’s interesting. I wonder what the difference, if any, is these days. I suspect that there may have been lifecycle differences 70 years ago - maybe technology has improved since then. ---
**Author:** ![Really\_Not\_All\_That\_Bright](https://avatars.discourse-cdn.com/v4/letter/r/e8c25b/32.png) [@Really\_Not\_All\_That\_Bright](https://boards.straightdope.com/u/Really_Not_All_That_Bright)\ **Post date:** [March 13, 2009, 6:32pm UTC](https://boards.straightdope.com/t/whats-the-definite-purpose-of-a-contactor/489348/7 "2009-03-13T18:32:24Z")
What are you designing, exactly? ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [March 13, 2009, 6:44pm UTC](https://boards.straightdope.com/t/whats-the-definite-purpose-of-a-contactor/489348/8 "2009-03-13T18:44:28Z")
> [@Really\_Not\_All\_That\_Bright](#): > > What are you designing, exactly? A custom 40-output lighting controller for a very controversial art project: [http://www.echelman.com/site/phoenix\_project.html](http://www.echelman.com/site/phoenix_project.html) ---
**Author:** ![Really\_Not\_All\_That\_Bright](https://avatars.discourse-cdn.com/v4/letter/r/e8c25b/32.png) [@Really\_Not\_All\_That\_Bright](https://boards.straightdope.com/u/Really_Not_All_That_Bright)\ **Post date:** [March 13, 2009, 6:50pm UTC](https://boards.straightdope.com/t/whats-the-definite-purpose-of-a-contactor/489348/9 "2009-03-13T18:50:07Z")
Whatever the hell that thing is, it looks cool. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [March 13, 2009, 7:01pm UTC](https://boards.straightdope.com/t/whats-the-definite-purpose-of-a-contactor/489348/10 "2009-03-13T19:01:57Z")
> [@Really\_Not\_All\_That\_Bright](#): > > Whatever the hell that thing is, it looks cool. Hopefully, that’s the way everyone will react. The artist works in woven materials, among other things. She has designed other large “floating” sculptures around the world. This one is a 2.4 Million dollar project that has received a fair amount of publicity - both good and bad. Our company does lighting controllers for Parks and Recreation- this is a custom design based on our standard controller (we normally only handle eight outputs - this requires 40). I’m doing the hardware and software for it, and of course, we are running into last-minute snags (we just found out it’s 208V (requiring DPST switching, as opposed to our standard SPST)), which is where the contactors come in. ---
**Author:** ![HongKongFooey](https://avatars.discourse-cdn.com/v4/letter/h/c4cdca/32.png) [@HongKongFooey](https://boards.straightdope.com/u/HongKongFooey)\ **Post date:** [March 13, 2009, 10:37pm UTC](https://boards.straightdope.com/t/whats-the-definite-purpose-of-a-contactor/489348/11 "2009-03-13T22:37:44Z")
> [@beowulff](#): > > I wonder what the difference, if any, is these days. I suspect that there may have been lifecycle differences 70 years ago - maybe technology has improved since then. HVAC equipment often includes positive displacement compressors which may start under heavier load than a regular motor depending on the starting position of the compressor. Supposedly the definite-purpose contactors have contacts that can handle the higher starting current without oxidizing too much because of the low duty cycle of most HVAC equipment. I think they’re coated with cadmium or something, they also have the coil shielded to protect from moisture. Whether they’re really any different from regular contactors in actual construction I’m not sure. ---
**Author:** ![Kevbo](https://avatars.discourse-cdn.com/v4/letter/k/e47774/32.png) [@Kevbo](https://boards.straightdope.com/u/Kevbo)\ **Post date:** [March 14, 2009, 3:01am UTC](https://boards.straightdope.com/t/whats-the-definite-purpose-of-a-contactor/489348/12 "2009-03-14T03:01:59Z")
> [@beowulff](#): > > That’s interesting. > I wonder what the difference, if any, is these days. I suspect that there may have been lifecycle differences 70 years ago - maybe technology has improved since then. HVAC compressors often have pretty high starting current, and can often be expected to cycle 2-3 times per hour. Relays in general tend not to survive long under such service. The subject contactors use oversize contacts tied to largish copper plates to dissipate the heat. They are designed so that the armature has a running start to build speed before the contacts actually seperate, and the current path when the contacts seperate is U shaped to allow Lorentz forces to extinguish the two arcs quickly. All this costs money, so relays not intended for such service won’t have these features, modern technology notwithstanding. Of course thyristor based Solid State Relays (SSRs) take the normal loads quite well, but tend not to survive voltage surges, lightning induced spikes, and shorts as well as mechanical contactors…and they mechanical devices are a bit cheaper and run cooler. ---
**Author:** ![Crafter\_Man](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/crafter_man/32/458_2.png) [@Crafter\_Man](https://boards.straightdope.com/u/Crafter_Man)\ **Post date:** [March 14, 2009, 3:53am UTC](https://boards.straightdope.com/t/whats-the-definite-purpose-of-a-contactor/489348/13 "2009-03-14T03:53:26Z")
A couple tips on using contactors: - Use small relays to energize the coils of contactors. - If the contacts are switching AC, stick a snubber circuit (resistor in series with a capacitor) in parallel with the contacts. Google this for more info. ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [April 19, 2009, 3:17pm UTC](https://boards.straightdope.com/t/whats-the-definite-purpose-of-a-contactor/489348/14 "2009-04-19T15:17:09Z")
The project is finished. I ended up using the cheapest contactor I could find - a 30A unit designed for the HVAC market. Everyone makes them, and they only cost around $11 in quantity. Since we are only switching 2A, they should last forever. I posted photos in this thread: [Her Secret is Patience - Miscellaneous and Personal Stuff I Must Share - Straight Dope Message Board](http://boards.straightdope.com/sdmb/showthread.php?t=514560)