In article ,
jim rozen wrote:
In article , DoN. Nichols says...
And another question is what kind of load is planned? Assuming
that he really *did* mean 220V single phase to something like a
reversible lathe motor, then he will need all three sets of contacts (on
a typical drum switch) to be able to start the motor in either
direction.
One can do this with three contacts as long as the motor is a
120/240 volt motor, because of the peculiarity that when those
are wired for the higher voltage, the start winding is tied
to the *center* *tap* of the run windings (which are in series
when wired for 240) and can thus be reversed with a single
contact, as shown in this diagram:
http://www.metalworking.com/DropBox/_2001_retired_files/wire.jpg
Agreed. I have posted ASCII schematics in the past to show
this.
I think that reversing a single-voltage 240 volt motor would
in principle require four contacts.
Nope -- as long as you have one end of the 240 as neutral (as in
the UK, where single-voltage motors are more common) or are willing to
leave one side of the motor hot (unswitched).
Let's see -- with ASCII drawings for the switching (as usual,
view with a fixed pitch font, like Courier, to avoid distortion):
o------+----------------------------------------------+
L(240V) A | |
o----+-----o------o | |
| |
o------+ +-------Start Cap |
| | |
+--------------------+ V +--Centrifugal Switch |
| Y V 3
| o--------+-|(--o---o-+ 3
| B | | Run 3
+-----o------o | 3 Winding 3
| 3 3
o----+ | 3 Start 3
| | 3 Winding 3
o----+ | 3 3
N (0V) C | | 3 3
o----+-----o------o | | 3 |
| | | 3 |
| o--------+ 3 |
| | | |
| +----------------+ |
| Z |
+-------------------------------------------------------------------+
The center position will probably not have any contacts associated with
it, but I drew them to show that there was an off position for the
switch.
Switch section A switches power to everything (except the
neutral side of the main winding. Switch sections B and C essentially
reverse the connections to points Y and Z, and are sometimes pre-cross
wired to normally reverse two of the three wires on a three-phase motor.
Obviously, if using this on US 240V line, you want to especially
make sure that everything is unplugged before working on the wiring, as
there will be more points still hot than otherwise would be found.
Note -- this probably violates code in the US, but it shows that
it *can* be done with three switch sections. As a matter of fact, I
suspect that this was how my Clausing was wired when I received it. I
very soon rewired it for 220V because it was in the habit of popping the
breaker when I started it (say one time out of fifteen -- enough to be a
nuisance. :-) I didn't bother tracing out how they had done it, I just
went straight for 220V operation.
Enjoy,
DoN.
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