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Re: Relay question




> From: Steve Roys <sroys-at-umabnet.ab.umd.edu>
> To: tesla-at-grendel.objinc-dot-com
> Subject: Relay question 
> 
> What is the difference between a relay, a contactor, and a motor starting 
> relay?  I ask this because I was ready to buy a motor starting relay that 
> I thought would have worked just fine for my Tesla coil control 
> cabinet (proper voltage and current ratings), but the person who was 
> selling it said that it would not be possible to use it as a simple high 
> current relay, and that I really needed a regular relay.
>
 
Relay is kind of a generic term that could be applied to both starters and
contactors, but is normally used for smaller units in low power control 
circuits.  Contactors are high current relays for switching circuits 
controlling anything but motors and are used with external overcurrent
protection.  Motor contactors include overcurrent protection via heating
strips made of a melting metal alloy.  A 90 amp motor starter will
not provide 90 amps continuous because the heater strips are usually
carefully chosen to protect its associated motor with full load current of
the motor, ambient temp., and type of motor load being deciding factors.  
Starters don't come with the heating strips and the strips have to be 
purchased seperately and cost a heck of a lot more than a normal fuse.
The salesman was probably right, you want a contactor. not a motor 
starter.

-Mike McCarty