Equivalent 3 phase vs single phase power draw
 

In article ,
lid says...
Suppose that I have a 3 phase device (say, a resistive load) that
draws X amps. What would be the current draw of a single phase device
that consumes the same amount of power.

My hunch is that a single phase device would draw 1.5 times the
current of the 3 phase device. In other words, a 3 phase device that
draws the same amps would produce 1.5 times as much energy as a single
phase device drawing the same amount of amps.

Is that correct?


Close. 1.732 = square root(3)

Ned Simmons

If I understand your question correctly:

You are asking for an answer in amps and giving conditions in power
and/or amps. For a resistive load.

So for a single phase resistive load and a 3 phase resistive load with
the same power consumption, the total amount of amps would be the same.
The three phase load would simply be 3 loads, each a third of the
single phase load.

For example with a single phase load of 1, the three phase load would
be three loads of 3. The same power would be dissapated(sp?) but the
current through each of the 3 phase loads would be 1/3 of the single
phase load.

With a resistive load any current shifts would not be a factor. Things
get interesting when the load is inductive and/or capacitive.

Bob AZ

On Sun, 31 Jul 2005 23:09:20 GMT, Ignoramus6304
wrote:


I do not think that 3 times the single phase load is the correct
answer. It would involve double counting of current. I am not claiming
to be a big expert in this, but 3 times cannot be the right answer.

i

If line-to-line voltage is the same in both cases, singlephase line
current will be 1.732 x threephase line current for same total power
-- like Ned sed!

But we are dealing with a resistive load. No inductance or capacitance.
A resistive load will not concern itself with phasing. So the 3 phase
thing is moot.

If we had a 3 phase device that has an ouput function, motor,
transformer or the like then we would be dealing with other things like
is the 3 phase Delta or WYE? And is the loading on the 3 phase,
phase-to-phase or phase to neutral/ground.
Bob AZ

Bob,
The premise under discussion here is multiple phase transmission of power.
You might be confusing that with the concept of power factor. Correct me if
I'm wrong.

Bob Swinney
"Bob AZ" wrote in message
oups.com...
But we are dealing with a resistive load. No inductance or capacitance.
A resistive load will not concern itself with phasing. So the 3 phase
thing is moot.

If we had a 3 phase device that has an ouput function, motor,
transformer or the like then we would be dealing with other things like
is the 3 phase Delta or WYE? And is the loading on the 3 phase,
phase-to-phase or phase to neutral/ground.
Bob AZ

In article .com,
Bob AZ wrote:
:But we are dealing with a resistive load. No inductance or capacitance.
:A resistive load will not concern itself with phasing. So the 3 phase
:thing is moot.
:
: If we had a 3 phase device that has an ouput function, motor,
:transformer or the like then we would be dealing with other things like
:is the 3 phase Delta or WYE? And is the loading on the 3 phase,
:phase-to-phase or phase to neutral/ground.
:Bob AZ

Whether the load is connected delta or WYE does not matter. What
_does_ matter is how the voltage is measured, and the article you
responded to (and neglected to quote!!) explicitly stated, "If the
line-to-line voltage is the same ... ." A WYE connected load will
see a line-to-neutral voltage that is (1 / sqrt(3)) times the
line-to-line voltage, so the total power for 3 such identical
resistive loads is V*I*sqrt(3), where V is the line-to-line voltage
and I is the current in any one line.

--
Bob Nichols AT comcast.net I am "rnichols42"

"Bob AZ" wrote in message
oups.com...
But we are dealing with a resistive load. No inductance or
capacitance.
A resistive load will not concern itself with phasing. So the 3
phase
thing is moot.

If we had a 3 phase device that has an ouput function, motor,
transformer or the like then we would be dealing with other things
like
is the 3 phase Delta or WYE? And is the loading on the 3 phase,
phase-to-phase or phase to neutral/ground.
Bob AZ


OK, for ease of calculations, letrs say we have a 2400 watt 240 volt
single phase heater. The line current is 2400/240, or 10 amps.

Now, a 2400 watt 3 phase heater is 800 watts per phase. The three 800
watt elements are wired in delta. The load current for each element is
800/240, or 3.33 amps.

By drawing the vector diagram of the 3 load currrents and and some
trig, the line currrent for each leg is 3.33 * square root of 3, or
5.77 amps

And 5.77 * square root of 3=10


Resistance can be used if you prefer. Say we have 3 heater elements
that measure 15 ohms. If one was connected across a 240 volt single
phase line, the power dissipated would be 240 * 240/15, or 3840 watts,
with a current of 16 amps. Three of those in parallel would be 11,520
watts total, with a current draw of 48 amps.

Now, wire those three elements in delta. Each element is still across
240 volts, so each one still dissipates 3840 watts (for a 11520 watt
total) and draws 16 amps. But since it's three phase, the line current
is 16 * square root of 3, or 27.7 amps.

And 48 /square root of 3 also =27.7...

It's still early-I hope all those numbers are right, lol...