Re: Definite Purpose Contactors

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: Jim Lux <jimlux-at-earthlink-dot-net> 

At 08:05 AM 9/4/2003 -0600, you wrote:
>Original poster: "Dave Kyle" <dave-at-kyleusa-dot-com>
>I am building a general purpose Tesla control panel where I want to use a
>master latching relay for applying main power with an emergency cut off
>switch which unlatches the relay. Current will not exceed 25 amps at 125
>volts AC.
>
>I am very familiar with wiring smaller relays for this function but I have
>come across some high power relay nomenclature I am unfamiliar.
>
>I believe what I want is a Definite Purpose Contactor. Can someone
>illuminate if this is correct and possibly explain why it is called a
>Definite Purpose Contactor?


Definite Purpose refers to regulatory approvals.  You can buy generic 
relays which are approved for general use (i.e. they might be UL listed), 
or you can buy relays that are only approved in certain specific 
applications (that is, a specific model or mfrs larger assembly, of which 
the relay is a part). The latter is cheaper, generally.  For instance, the 
contactor in an airconditioner would typically be a "definite purpose" 
item.. it could be designed just to meet the requirements for that unit, 
and not the more general requirements.

This pops up all the time with such mundane things as terminal 
strips.  There are UL listed terminal strips, which can be used for 
anything, and then there are UL recognized component ("RU") terminal 
strips, which can only be used as part of a larger UL listed device, but 
not standalone.

For your purposes, you probably don't care about regulatory approvals.

Do check the specs carefully... A standard motor operating contactor might 
have fairly high peak current ratings (to correspond to the locked rotor 
current or starting surge) compared to the average current rating.  A 
definite purpose contactor, intended, for example, for running an electric 
heater, might not have the same surge ratings.

However, most of the time, the differences are things like connector 
configurations, shields over screw terminals, and that sort of thing.


>Dave
>
>=========================================
>Dave Kyle
>Austin, TX USA
>Email: dave-at-kyleusa-dot-com