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240VAC single phase from three-phase?

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Posted by Ulysses on March 29, 2010, 10:41 pm

Hi.  I've been googling and read a lot about three phase power but didn't
find the answer to my specific question...

I have a vacuum-heat press that operates on 240 VAC single phase.  The plug
is a 20 amp twistlock so I'm assuming it draws less than 20 amps at that
voltage.  The vacuum motor runs from 120 volts and is included in the
circuitry.  I can't seem to find the wiring diagram so I'm guessing that
either one leg of the 240 supplies the pump and the other supplies the the
heating elements OR one leg supplies the pump and one heating element and
the other leg only one heating element (this seems unlikely to me).

So, I have three-phase power in my store and the voltage across any two hot
lines is about 209.  BUT, the voltage between any hot line and ground or
neutral is 120 volts.  SO, since I'm not going to be running any 240 volt
motors etc, and the pump is running from 120 volts anyway, can I simply
connect the vacuum-heat press to two legs of the three-phase plus neutral
and ground?  As far as I can figure I will be supplying the pump and heating
elements individually with 120 anyway.  Unless for some reason the heating
elements require 240 but that seems like it would be an unbalanced load with
the pump drawing only 120 from one of the lines.

The wiring diagram is somewhere and I'll consult it before doing anyway.  If
this won't work what are my options?

Posted by hubops on March 29, 2010, 10:52 pm

 Consult your professional -- or use your own judgement -
my uneducated opinion is that you will be fine - on the 208.
( 208  is what it's called at work  -  can measure diff )
 I wouldn't know a vacuum-heat-press  if I tripped over it.
but if the 240 is only the heat elements - you'll just be a little
cooler ...    like James Dean .
  John T.

Posted by Gordon on March 30, 2010, 12:27 am

Most likely the heating elements are wired across the 240 volt legs
and the vacuume pump goes from one leg to neutral. Sure it's
but that is not a problem.

The reason that two legs of a 3-phase supply read 208 volts is because
they are 120 degrees out of phase. A single phase 240 volt (residential)

secondary.  The two legs are 180 degrees apart.

So on a 3 phase supply, one leg will rise up to 120volts, but the other
leg will not be at -120 volts. It will be at about -88volts (if i did
the head math correctly).

Any way, hooking that machine up the 3 phase will cause the heating
elements to run cooler.  that may or may not be a problem.

Posted by danny burstein on March 30, 2010, 4:53 am


misc note: That "120 degrees out of phase" is only coincidentally
the same number as 120 Volts.

Misc note #2: in some areas of the country, such as NYC, you
don't get that "single phase 240 volt", but rather, you're likely
to get the 208 volt deal.

The way the wiring works is that you have four wires from
the street. You've got "hot legs" of "A", "B", and "C", each
of which are 120 degrees apart. You also have a neutral.

If you tap from any of the legs to ground (neutral) you'll
get 120VAC.

If you tap hot-to-hot, you'll get the equivalent of 208V.

In a typical setting the first apartment has legs "A" and "B"
(plus neutral and a safety ground). The next one has "B"
and "C". The third has "A" and "C". Rinse lather, repeat.

All three legs go to the roof to feed the elevator motor
and the central AC chiller.

Note that it's NOT a mere 14 percent difference (208 vs. 240),
but is in the case of resistance heaters, about a 25 percent
reduction. That's because with the voltage dropping,
you get a corresponding reduction in amperage...

So you've got roughly 75 percent of the heating capability.

If you've got, say, a typical kitchen oven, it simply means
it'll take longer to heat up. But oce it does, everything
will be fine.

But if you're trying to boil water on the top heater, the lower  
voltage will take about 33 percent longer...

Knowledge may be power, but communications is the key
[to foil spammers, my address has been double rot-13 encoded]

Posted by amdx on March 31, 2010, 12:20 am

 Your math is ahh, hmm, depends how you think about it :-)
You almost need to think in peak and peak to peak terms when you
are discussing 3 phase and measuring phase to phase.
When one phase peaks it is 169.68 volts above ground, (1.414 x 120V)
 The other leg must be -124.43V, because 124.43 + 169.68 = 294.11 Vpp.
And 294.11V  x .707 = 208 V
 And yes you're correct 88V x 1.414 = 124.43Vp
What I'm a bit puzzled by is I can't find a proper graph.
The negative leg is never at the correct point to show what I have said.

What should be 208 on this graph calculates out to 192Vrms (assumes 0.6 on
the graph)

Ands this one shows 240Vrms (assumes 70.7 on this graph) note labels change
from page to page.

I think both graphs are wrong, ie, poor representation of a sinewave.
The line is in the wrong position at 120 degrees.

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