That site is pretty disturbing. What kind guys build those machines? I
thought metalworking was a nice, innocent hobby :-).
Incindentally, those trolls don't think like engineers. Why spend
hundreds of bucks building something with your Kmart tools when you
could just rent a jackhammer for a day?
"To announce that there must be no criticism of the president,
or that we are to stand by the president right or wrong, is not
only unpatriotic and servile, but is morally treasonable to the
American public." -- Theodore Roosevelt
"Far better it is to dare mighty things, to win great triumphs, even
though checkered by failure, than to rank with those poor spirits who
neither enjoy much nor suffer much, because they live in the gray
twilight that knows neither victory nor defeat." --Theodore Roosevelt
On Fri, 29 Jul 2005 10:45:25 +0000, Ignoramus27279 wrote:
My RPC went through several enclosure schemes before I was happy with it.
This is a 2 HP RPC. I started out trying to build the whole thing inside
of an old Heathkit DX-100 Ham Radio cabinet, thinking that keeping everything
self-contained was a smart idea. It turned out to be too big a bulky for
my tiny workshop. I finally found a smallish metal utility box, like the
kind you might have on a desk to keep paperwork or keys and other odds and
ends in. It was kind of cramped but I mounted all of the components for
the RPC in it -- relays, balancing and start caps, start/run and on/off
switches, and fuses. I bolted this guts box to a plank and using EMT
connectors to run the wiring through I attached it to the idler motor
(which was bolted to the plank as well). On top of this guts box I mounted
two outlet boxes with a 4-wire twist lock outlet in each. This is 3-phase
output. Rather than cutting the shaft off of the idler motor, I decided
to make a shield for it out of aluminum wire mesh (stucco screen).
This arrangement has worked out great. Its much more compact and expandable
in the future. The only thing I'd like to change is to replace the
wooden plank with a frame made of welded angle stock and some rubber feet.
I'll get around to that one day.
I am building an enclosure right now:
I am doing that with various junk wood pieces lying around. I did not
want to use any more valuable materials. My objective is to make a
compact enclosure on wheels, with top that I can use to put stuff on,
like a mini cart of sorts. That way at least, it will not effectively
take any space away from me.
Forgot to say, I am hoping to use my phase converter as a base for my
Sears table saw. That way I will kill several birds with one stone, so
to speak, I'll have a phase converter and a base for table saw. This
way, the table saw will take up the minimum of space. I'll see how it
On Fri, 29 Jul 2005 18:33:58 GMT, Ignoramus27279
| thanks to everyone for your thoughts!
| Many pictures and the story in several chapters:
| http://igor.chudov.com/projects/Phase-Converter /
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
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.
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
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.
Hm, I am confused. I agree with you on the need for overload
protection. I will install something, for sure.
What I am curious about is, can I simply use motor rated fuses on both
incoming 240V legs? That should provide all necessary protections for
I do not mind installing an overload relay, as such, except that it is
a cost issue. Fuses are cheaper. (unless I can find something at that
junkyard). Realistically speaking, the idler is not going to bind. If
contacts to capacitors break, yes, I could have a stall issue, which
would be addressed by properly sized fuses.
A relay is more exciting and possibly a little more convenient (it is
resettable, I do not need to buy new fuses when a fuse burns out), but
in reality will provide about same protection.
Am I mistaken?
The overload units in a motor starter are have a very narrow operating
range like 57.5 - 61.3A that can be very closely matched to the motor
rating. Picking between a 50-60-70A fuse doesn't give much protection.
Also overloads are designed to allow the starting inrush and are
designed to match the thermal characteristics of the motor for overload.
Even time delay fuses have to be much larger than the motor rating or
they will blow on the starting inrush. Fuses can be sized at up to 175%
of the motor rating and circuit breakers up to 250%(NEC). The fuses in a
motor circuit are intended to provide short circuit protection, not
Fuses can be expensive and overload heater type contactors are even more
expensive. If you use this RPC and tool fed by it, are used only when you
are "in attendance", you wont need any fuses, your 60 amp breaker will trip
before you burn anything up. If the 3 phase tool motor stalls for any
reason, just hit the kill button on the RPC.
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