Electrical Engineers help! Whats the point of 3-phase power?(rep)(srs)

[Spetsnazos]

I did some research and read up on it but I am a little confused as to why someone would want their tool to receive 3phase power input.

I know that 3phase basically means 3 wires, 120 degrees out of phase of each other, same amplitude.

But what is the point? Is it to maintain steady power output of the device? Does it really make a difference from 1 phase to 3 phase?

Thanks, repping

[paul.09]

google brah

I can't post links cos of low post count but google 'advantages of 3 phase power' and click the first couple results and look at the wikipedia page.


My house has 3-phase, our aircon/heating/some lighting takes advantage of it, pretty much though you won't see it used for consumer appliances very often, not quite sure why my house has it

It means there is more power available if needed, 3phase can support heavier loads. Also appliances designed for 3phase can be smaller, more efficient, cheaper (specifically motors) look at the wikipedia diagram

[fcdaniel]

From what i understand large compressors need to run at 240, so the power won't spike causing wear on the motors.

[fenguin]

Originally Posted by Spetsnazos

I did some research and read up on it but I am a little confused as to why someone would want their tool to receive 3phase power input.

I know that 3phase basically means 3 wires, 120 degrees out of phase of each other, same amplitude.

But what is the point? Is it to maintain steady power output of the device? Does it really make a difference from 1 phase to 3 phase?

Thanks, repping

Think of each phase as being a single alternating current, or a single wave.

Normally to supply a certain amount of power (say P) to an electric tool, you could use one-phase which would use one wave with maximum amplitude P. This would result in the power seen by any point along the circuit varying from 0 to P, which could result in (1) heating or (2) undesirable magnetic fields being generated.

For three-phase power, you're instead using three waves with maximum amplitude P/3, with the property that the currents at any point in time sum to zero. This means the power seen by any point along the line will remain constant throughout operation, which will eliminate problems (1) and (2) above and result in much safer operation of the device.

[Spetsnazos]

Originally Posted by paul.09

google brah

I can't post links cos of low post count but google 'advantages of 3 phase power' and click the first couple results and look at the wikipedia page.


My house has 3-phase, our aircon/heating/some lighting takes advantage of it, pretty much though you won't see it used for consumer appliances very often, not quite sure why my house has it

It means there is more power available if needed, 3phase can support heavier loads. Also appliances designed for 3phase can be smaller, more efficient, cheaper (specifically motors) look at the wikipedia diagram

will hit on recharge

So basically if I have single phase motor then V^2/R and my voltage is 240 then as the sine wave goes to zero the motor will slow down and then spike up again to the max and go through this kind of cycle over and over?

And with 3 phase it is basically getting maximum power constantly because as one current goes down to zero, another one is reaching the peak?

I get the feeling that this will mean that you use 3 times the power, however, the websites I read say that 3phase is slightly more efficient.

Thanks guys

[SoUptheEXTREME]

3 phase is typically used in commercial/industrial applications. Its rare to have it going to a residential location.

With your "tool" receiving 3 phase input, it can run on a smaller more efficient motor, yet produce equal or greater steady power for a longer period of time. Each phase essentially "repumps" the system as opposed to a constant single phase.



Could be wayyyy off yet thats what I was kinda of told over the summer as we didn't have 3 phase going to our shop and had to find a single phase machine only.

edit:: don't think it gives 3x the power

[gixxer0.6g]

I'm a mech engineer and from what I know it delivers more power.

When we're specifying a motor, anything over 1.5-2 hp, 3 phase power is usually required.

[Spetsnazos]

Originally Posted by fenguin

Think of each phase as being a single alternating current, or a single wave.

Normally to supply a certain amount of power (say P) to an electric tool, you could use one-phase which would use one wave with maximum amplitude P. This would result in the power seen by any point along the circuit varying from 0 to P, which could result in (1) heating or (2) undesirable magnetic fields being generated.

For three-phase power, you're instead using three waves with maximum amplitude P/3, with the property that the currents at any point in time sum to zero. This means the power seen by any point along the line will remain constant throughout operation, which will eliminate problems (1) and (2) above and result in much safer operation of the device.

ty so you dont really get the asme power, you get P/3 from each wave. I see. and since its V^2/R, R being the same, the voltage can be lower?

Say V = 120V and R = 1k(for simplicity)

P = 14.4W for single phase

but with 3 phase

P/3 = 192W, V = 70V??

So you need about 60% of the voltage with 3 phase to meet the same power demand?

[Spetsnazos]

Can someone confirm that this is correct? From yahoo answers. Going to ask my instructor also

There are several advantages to three-phase power:

* Constant power delivery. Single-phase delivers zero power each time the voltage crosses zero (120 times per second in the US), while with three-phase each time a phase crosses zero there is still power being delivered. This leads to three-phase motors in machinery running more smoothly.

* Easier motor wiring. Three-phase induction motors don't require brushes, start capacitors, or any of the complexities of single-phase motors, and are easy to reverse as needed.

* Smaller wires required for connecting three-phase equipment. Because the current is divided among the three phases instead of between just two, the wires can be smaller and therefore are often easier to handle.

1) understandable

2) I see

3) dont seem like a big deal, how thick are the wires...

[sssick]

Originally Posted by Spetsnazos

Can someone confirm that this is correct? From yahoo answers. Going to ask my instructor also



1) understandable

2) I see

3) dont seem like a big deal, how thick are the wires...

When you are running thousands of feet of wire, it's a big deal to run a small wire. Easier labor, and costs less.

[Spetsnazos]

Originally Posted by sssick

When you are running thousands of feet of wire, it's a big deal to run a small wire. Easier labor, and costs less.

good point but running three thinner wires as opposed to 1 thicker wire is more efficient?

[sssick]

Originally Posted by Spetsnazos

good point but running three thinner wires as opposed to 1 thicker wire is more efficient?

Yes. Three thinner wires are easier to work with than one (Or two if it's 240V) larger wire.

[supy]

Originally Posted by paul.09

google brah

I can't post links cos of low post count but google 'advantages of 3 phase power' and click the first couple results and look at the wikipedia page.


My house has 3-phase, our aircon/heating/some lighting takes advantage of it, pretty much though you won't see it used for consumer appliances very often, not quite sure why my house has it

It means there is more power available if needed, 3phase can support heavier loads. Also appliances designed for 3phase can be smaller, more efficient, cheaper (specifically motors) look at the wikipedia diagram

Never seen a house with 3 phase there would be no need for it unless your house is 30,000 sq ft.

[brl131psu]

motors

[supy]

Originally Posted by sssick

When you are running thousands of feet of wire, it's a big deal to run a small wire. Easier labor, and costs less.

This^^^^^^^^^. Im a construction electrician with 20yrs exp. Saves a lot in labor and the price of copper has skyrocketed in the last 3-4 yrs

[paul.09]

Originally Posted by supy

Never seen a house with 3 phase there would be no need for it unless your house is 30,000 sq ft.

I'm in australia

And yeah it is unnecessary, not sure why we have it but we do House was not built by us, maybe previous owners needed it for something