advantages of 3 phase as opposed to single phase?

[sephiroth]

what are the advantages of three phase system supply over single phase supply?

 

[sephiroth]

could really use some help here my manager wants to know

 

[Skogsgurra]

Also, if you need to run normal motors, you need three phases. At least if you want more than a few HPs out of your motors. Single phase motors are either inefficient, have poor life - or both.

If you need a few lamps only and perhaps a fridge and a TV set - then single phase is OK.

Gunnar Englund

http://www.gke.org

 

[LionelHutz]

I'd say the biggest advantage is that a 3-phase induction motor is cheap, has good efficiency and is low maintenance. Seems to be the best way to convert electrical power into mechanical power. There are millions of them in use in the world.

A single-phase induction motor isn't as efficient and has capacitors and speed switches that are prone to fail.

 

[sephiroth]

thanks that should keep him happy for a while

 

[Electic]

Some answers:

1: because of motor pole geometry, three phase motors are self starting and do not require such adders such as split phase starting, capacitor starting or shaded pole starting. This becomes critical at higher horsepower levels and generally at about 7.5 hp one starts thinking 3 phase.

2: on a balanced 3 phase distribution system, the phases will cancel at the load and you do not need to consider return current to the source. No return path voltage drop (potential rise) and no return path I2R losses.

 

[bacon4life]

Three phase power transmission is not inherently more efficient than single phase. Consider instead a household 240V load fed from two 120V conductors with opposite phasing. There is no "return path", but rather two wires that are both supply paths. It is just that in most applications, the second supply path supplies no power as it is the neutral at (approximately) zero volts. A High Voltage DC (HVDC) transmission line is another example. Assuming that there is a maximum current and voltage each wire can handle, the type of transmission (3phase AC, Single phase AC or DC) make no difference on the efficiency as long as each conductor is at it's full potential. The catch is that most single phase systems waste the insulation on the neutral or it is more convenient to have part of the device at neutral potential. Unbalanced 3 phase would be 4 supply paths, with the neutral path supplying zero power but still causing losses.

Expanding on the comments of thread238-139362, it is not only the voltage that never goes to zero, but rather that the instantaneous power supplied is constant with three phase. In single phase the instantaneous power goes to zero, meaning that something has to store power if the load needs instantaneous power.
In singe phase rectifiers this means the larger capacitor/inductors have to store enough energy to supply the load while the source is at zero power. In three phase rectifiers the capacitors/inductors only have to store enough energy to ride through commutations.
In motors, the energy is stored in the rotation of the machine. Unfortunately, if the machine stops at a point where the load requires more power to start moving than the single phase motor can supply instantaneously, it will not turn. The devices Electic mentions are energy storage devices. They allow some of the energy to be stored electrically for those times when the load average power exceeds supply instantaneous power.

Triplen harmonics cancel in a three phase system.

On some applications more than 3 phases are beneficial. 12 pulse rectifiers even use 6 phases so that more harmonics also cancel. I also heard about a experimental six phase transmission line used so that the electromagnetic fields would cancel out and be below some threshold on a narrow right of way.

 

[davidbeach]

Triplen harmonics cancel in a three phase system.

?????? Excuse me?????

Triplens are the harmonics that always ADD in a three phase system.

 

[Skogsgurra]

Right - and wrong.

Triplen currents add in the neutral if non-linear loads are connected from phases to neutral. That is where you are right.

Triplen voltages cancel in a three-phase system when voltage is measured phase-phase. That's where you are wrong.



Gunnar Englund

http://www.gke.org

 

[davidbeach]

skogsgurra, right. Too much time spent working with building loads where triplens are the bane of power systems. Your are absolutely correct that measuring phase-to-phase you can't see them since all three phases are doing the same thing at the same time. Any phase to ground or phase to neutral voltage measurement would show the triplens.

 

[bacon4life]

I was thinking of harmonics trapped in a delta winding, forgetting that they must of course exist in order to be eliminated. I was also thinking about the 6th, 9th, 12th .. harmonics and if they canceled out the same way?

 

[Skogsgurra]

Yes bacon,

That is valid for all harmonics with 3xN frequency. The reason is that Nx3x120 degrees is Nx360 degrees. So the harmonics have the same phase angles in all three phases.

Gunnar Englund

http://www.gke.org

 

[tinfoil]

I think this was already implied, but..

3 phase power delivers energy more efficiently than 1 phase:

A 4-wire 3 phase system, if reasonably balanced, delivers approximately 3 times the power for only 2 times the conductors (single phase must have 2 wires), while only experiencing half of the voltage drop (as neutral only carries the vector sum of the 3 phases, which should be near zero, and Ohm's law shows that negligible current = negligible voltage drop on the return path).

So, for the same guages of cables, twice the conductors allow triple the power for half of the voltage drop over a given distance,

or

A (4 wire) three phase system can carry 150% more energy than a (2 wire) single phase system, using wires with only half as much current-carrying material (i.e. smaller guages) over the same distance.

A three-wire 3 phase system can be even more efficient, but comes with its own drawbacks.

This is over and above the 'power usage' advantages described above.