Yep, Carl is right about the protection considerations.
The overload protection devices aren't just to protect against the motor
developing a short to ground, they react to protect the motor from being
overloaded by a jam or machine malfunction/breakage, stalled or any other
condition that causes the current to rise to a point where the motor would
be damaged/destroyed by the resulting heat in the windings.
The OLP's heaters are selected from a chart of currents for the specific
motor being used.
Some OLPs have adjustable trip settings so that heaters don't need to be
selected.
Each type of OLP will cause all 3 phases to be opened/interrupted.
The OLPs are also available as a separate device, but are commonly
integrated into the overall starter/contactor box.
The best setup for an RPC would be a magnetic starter/OLP rated for the size
of the RPC motor, and a separate (specifically sized) starter/OLP for each
machine motor that's powered from the RPC.
Choosing not to use a second, separate OLP for the (usually) smaller machine
motor will not offer any protection for the machine motor.
WB
.................
"carl mciver" wrote in message
ink.net...
I don't know jack about RPC's, so I just ask this as a sounding board
for those among us who can verify my concern. Should your contactor have
an
overload on it? The overload's job is to protect the equipment if an
overcurrent exists on any or all legs. In essence, it shuts all three
phases off even if one has shorted to ground. Your supplier will have
some
or you can trade the contactor in for one that has it. Overloads come
with
a holding contact that turns off power to the contactor if an element
overheats.
I know that current in one leg will be lower in an RPC, but as long as
it doesn't exceed the overload element's rating it will work just fine.
----== Posted via Newsfeeds.Com - Unlimited-Uncensored-Secure Usenet News==----
http://www.newsfeeds.com The #1 Newsgroup Service in the World! 120,000+ Newsgroups
----= East and West-Coast Server Farms - Total Privacy via Encryption =----