Bruce (or anyone qualified).....
Three phase power ----- isn't each leg 120 degrees from the others?
Single phase power - 220v - isn't each leg 180 degrees from the other?
If the above is correct (and I'm hoping I understand it correctly) it
would stand to reason, that if you needed to hook up a 220v single
phase machine and you had 3 phase available at that location you
could, (and I am not saying it's correct) connect to two of the three
phases and run the machine, even tho the sine waves wouldn't be 180
out, it probably wouldn't hurt anything. Am I correct in this
assumption??? Any and all comments welcome
Single phase means there is one leg only. Think of single phase as
line segment (one line) and three phase as a triangle (three lines).
Each leg is single phase.
Read wikipedia article on Three Phase Power, it is very good.
If your three phase power is 220/230/240 volts phase-to-phase, which is
common, you get normal 220/230/240 volts single phase between any two
phases. So your machine will not only work, it will be connected to its
This topic generally starts a really long and involved thread, so stand by.
There is quite a bit more that can be learned about the voltages, currents,
and phase angles and there is some disagreement about the terminology
You question is really "can I run a 3 phase motor on single phase?"
Answer is a qualified maybe: since you will be running only 2 of the 3
phases, you can only get 2/3rds of the rated power. Plus you will need
to run some capacitors to pick up the 3rd leg to get it to start in the
desired direction. This is the approach used by a static phase
converter. Real answer is to run a rotary phase converter or a VFD.
Ken Sterl> Bruce (or anyone qualified).....
Re-read his question please, it's the exact opposite of what you
The short answer is that yes, a 220V single phase machine will generally
run just fine on power from a three phase service.
In some rare cases the fact that the typical three phase supply in a
commercial environment is 208/120V wye service and therefore only
provides 208V phase to phase can be an issue and the machine may have a
transformer with adjustable taps to account for that, or you can use
buck/boost transformers to compensate. In industrial environments you
may have 220V three phase delta service where you actually get 220V
phase to phase and there isn't a voltage issue. The 120 degree phase
angle has no practical effect generally.
Right. Figuring the sine wave overlap, it's more like six-phase
power - every 60 degrees of an electric motor rotation one of the
phases is at peak voltage, either negative or positive. That's why
it's so darned efficient for running motors, since there's always
smooth forward motion. It's the difference betweeen a one-lung hit
and miss Johnny Popper and a Packard Merlin.
And vibration sensitive things like surface grinders should only run
on real 3-phase, for the same reason. Try running it on a phase
converter and the surface finish on the products can go straight to
Residential style 120/240 single phase is one center-tapped
transformer winding, with the center tap grounded for the Neutral.
The '180 degrees' is a convenient way to explain it because the
polarity /appears/ to be opposite when viewed/measured from the center
tap. In reality there's no differential at all.
Absolutely - you are going across one secondary core winding of the
utility distribution transformer, and there's no worry about phase
Two times to worry: One is if you have 120/208V Wye power, because
it is only 208V phase to phase. Most small motors are dual-voltage
rated 208V/240V and will gladly run on 208V but at a higher current
draw. But there ARE pieces of gear that do not take kindly to running
on 208V - this is when you connect a simple 16V/32V Buck-Boost
transformer between the utility and the load to kick 208V up to 240V.
The other: If you have 120/240 Open Delta or "High Leg" power - the
High Leg (usually coded Orange and connected as B phase, but not
always) is 208V to ground, not 120V.
When you run across Open Delta panels in the field, it is
considerate to put a big note on the breaker panel to warn the less
enlightened among us that 'The B Phase is 208V to ground, and
connecting 120V loads to the B Phase is a Very Bad Idea.'
You can connect 240V loads between any two phases on Open Delta, but
you can only connect 120V light loads between A-N and C-N. Try
placing them on B-N and they won't live very long - but they glow
really bright before they blow...
Or 277V/480V. 277 between Phase to Neutral, and 480V Phase-Phase.
Found in larger high-rise buildings, large shopping malls, and light
industrial, because the higher voltage means much smaller wiring for
the same work produced.
When you get close to the end-use location for 120V/208V/240V power
you place a step-down transformer. That keeps the voltage drop to a
Heavy industry (automobile plants, steel mills, forges, foundries,
mines, etc.) buys their power "in bulk" at the higher utility
distribution voltages in use in their area - 2,400V (almost extinct)
4,800V (LADWP Residential) 9,600V 14,500V 34,500V (LADWP Industrial)
The utility installs fuses or breakers and a KWH Meter recording the
power usage on the line going into the complex, and the end user takes
The industrial user gets the power cheaper, but has to supply their
own underground or aerial cabling, step-down transformers and
switchgear, and knock the medium voltage down to the needed voltage at
the point of use. And they have to absorb the cost of all the
transformer and transmission losses that occur after they 'buy' the
power at the KWH Meter.
Indeed. Even if they dont have a light source..they can glow really
Then the magic smoke comes out.
Gunner, who has been putting orange tape on all manner of things the
last few months......
Orange County is nearly all Open Delta and 240 vts
A lot of the confusion in power phases is that you need to look at where you
are measuring the voltages and where the ground is.
If you are just connecting the two hot leads of the 240 line into your
house, you have one 240 volt phase.
If you also connect the ground, you have two 120 volt phases that are 180
Three phase power has the three phases 120 degrees apart but the voltages
and ground can be referenced different ways. You can also refer to them in
different ways, depending on how you are using the power.
The common ones are Y with the voltage measured between each phase and the
ground centered between the phases and Delta where the voltage is measured
between the ends of the phases.
208/120 (Y) is common for large buildings because it gives three 120 volt
phases using the ground to each phase. If you connect directly between two
phases (withoug connecting to the ground) you have a single phase 208 volt
connection. A third option is to connect all three phases (but not the
ground) and have a 208 three phase delta power.
240 Delta with a ground at the mid point of one phase is common in small
industrial buildings. It gives the 240 three phase power that many machines
want , allows you to connect single phase 240 equipment (if it doesn't need
a ground) to any of the phases, connect single phase 240 equipment (if it
needs a ground) to the phase with the ground, and two 120 phases on the
If the available three phase power is the common 240 V delta with the ground
in a leg, your 240V single phase machine is absolutely going to work when
connected to the leg with the ground.
If the machine does not need the ground as reference, it can work on any 240
If you have 208/120 Y three phase power and the machine is not rated to run
on 240/208, then it becomes messier. If you don't need a ground, you can
run a 208-240 boost transformer (autotransformer that just makes up the
difference between the 208 and 240) to run it.
If you need a mid phase ground, you could run a 208-240 center tapped
transformer. You connect the input to a 208 phase and the output's center
tap to the ground.
Isn't this "fun".
It may help to think of a "phase" as a pair of conductors rather than
a wire. A pair of wires comprises a phase. With threephase, there
are three wires, call them A, B, and C. (Ignoring ground for the
moment). There are therefore three phases: AB, AC and BC. The
sinusoidal voltages on these phases are 120 degrees apart. The
voltages on each of these wires relative to ground will also be 120
degrees apart but line-to-ground voltage in a wye connection is .867
* line-to-line voltage.
Residential single phase power in the US is 120 volts relative to
ground, or 240 volts line-to-line because one line has polarity
opposite (180 deg phase shift) to the other w.r.t. ground. Household
120 volt power has one or the other of these lines on the black wire,
and neutral (grounded somewhere) on the white wire. 240 volt loads
like stoves are connected line-to-line, often do not use the neutral.
Others have treated the various voltages found on each phase of
commonly-found three-phase power distribution setups.
To visualize phase angles draw 3 parallel lines the same length, put
a dot at the left end of the top line, and a dot at the right end of the
middle line, and a dot in the middle of the last line. Now imagine those
dots moving at the same time, the same speed from one end to the other.
Their timing is 120 deg apart and if you look only at the top and middle
lines, they are 180 deg apart.
120/208 is star or y just imagine each leg of the y is a coil of a
transformer and the middle of the y is grounded. Each end of the y is a
phase, the voltage is phase to ground 120, phase to phase 208. On a
120/208 bank all 3 transformers will be the same size.
120/240 is delta imagine a triangle, each side is a coil of a
transformer, each point is a phase, in the middle of one side draw a
ground. The transformer you ground is the lighting transformer. It has
all the single phase load. The voltage is hot leg to ground 120, hot leg
to ground 120, power,high,or stinger leg to ground 208, because it is a
coil and a half away from ground. Between any 3 hot is 240v. In a
120/240 bank the middle or lighting transformer will be larger to handle
the 3phase plus the single phase load. To visualize an open delta erase
one side of the triangle. The only way you can tell if you have an open
delta versus a delta is to look at the bank, 2 transformers is open
delta, 3 is delta.
Open delta is not very efficient. It is good for temporary repairs. I
could rebuss a bank to open delta so a farmer could milk his cows and
not have to wait 4hr for a crew to change out 1 transformer.
The secondary coil of the transformers is not center tapped. It is two
coils, you have them serialed for 240 and paralleled for 208.
You can see some pages from a transformer manual at
120 * SqrRoot 3 (1.73)= 208
Hmmm I did the math, then I stuck the probes of my Fluke on the 3
phase into our shop, and almost every other large distro I've ever had
to work in and found 208/120. If your little corner of the world is
different, it ain't my fault.
But then again, so much of your little world doesn't agree with reality,
why should this be different.