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Posted by Robert Swinney on June 13, 2008, 5:42 pm
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I think my news server is acting up, but here goe again . . . .
Karl,
About as close as you can get to "soft start" with a conventional RPC is to
properly design and
balance one. A RPC, with sufficient start and run capacitance to bring the load
motor up to speed
within 1/2 second which is fed from a correctly sized breaker provides a form of
soft start.
Implicit in understanding this is the fact that a RPC is an electrical network
consisting of idler
motor, load motor(s), and capacitance. In such a network, single-phase current
is fed through an
appropriate circuit breaker to run three-phase motors. The RPC circulating
current path is not
through parallel motors as is commonly thought. Phase relationships vary widely
throughout the
network, but if properly designed, current and voltage will be in phase across
the input to the
network. Circuit breakers for motor service are designed to stand large surge
currents so long as
those currents are of short duration. Analysis of the currents flowing in a RPC
is quite complex
and touched on briefly toward the end of my article. Generally speaking,
problems occur with RPC's
because of design shortcuts. It is a common mistake for one to short-cut the
amount of start and
run capacitance used in a RPC.
Bob Swinney
> Make that November / December 2001 Ed. of Homeshop Machinist. The article
> also appears in Metal Web
> News along with a lot of others....
And a very fine article it was too. I remember reading it when it came out.
But, IIRC, not a dang thing in it about soft start devices.
Karl
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> also appears in Metal Web
> News along with a lot of others....