PDA

View Full Version : Trouble understanding a/c current in relation to a single phase supply.



mburns
05-12-2010, 11:21 AM
Hello long time reader first time poster from an apprentice.
I'm have trouble understanding a/c current flow when a single phase 240v load is connected to a 3 phase distribution grid. (australia)

To my knowledge current flows from - to +. So for example if phase A was at -240v and phase c was at +240v current would flow from that phase of the transformer through the load through the neutral to the star point of the transformer and back on phase C and then now the phases change polarity over time and for example current now flows from phase c at -240v through the star point of the transformer and back through the neutral to the board and the other way through the load and to the transformer phase a leg which is at 240v or have i got this completely wrong?

It's just got me bugged that how current flows back through the neutral which is at 0v to the active which is at 240v? surely there's a simple explanation because i think i'm trying to over analyze things cheers guys!

nevgee
05-12-2010, 12:33 PM
Three phases are three seperate single phase supplies 120deg out of step with each other. In the UK transmit at 50Hz (cycles/sec) Therefore the supply on each line goes through a 360 cycle from max to min. say +250V to -250V ever 1/50th sec for each phase. Unlike Direct Current, there is no directional flow only a "pulsing" or alternating current.
Without over complication then, the other 2 phases are following in step behind. Imagine watching waves on a beach.

A very complicated subject .. try searching the net for answers to your question ...

try this to start with
http://www.study-center.com/femp/content/demo/basics/bag3p.htm

paul_h
05-12-2010, 02:17 PM
To my knowledge current flows from - to +.
Thats DC not AC.

So for example if phase A was at -240v and phase c was at +240v current would flow from that phase of the transformer through the load through the neutral to the star point of the transformer and back on phase C and then now the phases change polarity over time and for example current now flows from phase c at -240v through the star point of the transformer and back through the neutral to the board and the other way through the load and to the transformer phase a leg which is at 240v or have i got this completely wrong? I thought you were talking about
single phase AC, so why are you talking about +240V and -240V and star point?
BTW 240V is RMS voltage much higher peak, which is why true RMS multimeters are chosen. As said above, it's a sine wave of volts to neutral, there's no + or -. Hence it will fry you but DC voltage probably wont. I'm no electrician though, and don't play one.
If you are australian and don't know where to ask and want to speak to a few electricians to explain it, try asking at www.whirlpool.net.au (theres a least 3 sparkies there (its a internet/hobby forum)) or www.overclockers.com.au (it's a computer enthusiast/internet/car/hobby forum) post in the electronics forum and there's about 5 sparkies there.
I'm just posting from an AU persepective, where the electrical trades are way separated from refrig, maybe here you'll find some dual trade people, or fridgies with knowledge of 240v AC anyway. But if not, all I'm saying is places I'll know of that you'll get an electrician reply pretty quick

Sly Jenkins
06-12-2010, 10:47 AM
Don't let the fact that it's a 3 phase dist grid bother you, you will only be tapping from one phase to neutral, getting a phase voltage of 240v. The other two phases shouldn't come into it. If you connect across two phases then you would get the higher line voltage.

NoNickName
06-12-2010, 07:17 PM
240Vac is the voltage from live conductor to neutral (or to ground is neutral is tapped to ground). Live to Live, the voltage is 415Vac. In fact 240 x square root(3) = 415.

Contactor
06-12-2010, 08:29 PM
Are you confusing a 3 phase motor which may not need a neutral with a single phase load which will (usually) be wired back to the neutral on the transformer?

Contactor
06-12-2010, 08:34 PM
Or are you thinking of a singe phase load balanced between two of the three phases supplied by its own transformer?

Quality
06-12-2010, 08:56 PM
In fact 240 x square root(3) = 415.

?

Sorry mis-read you quote

NoNickName
07-12-2010, 09:20 AM
The square root of 3 is 1.732
240 Vac x 1.732 = 415 Vac.
That is the relationship between phase voltage and line voltage

mburns
07-12-2010, 12:07 PM
Sorry guys. I'm just meaning in terms of when a single phase load (say your home) is picked off a 3 phase grid. Does the current come say from phase A come through the loads in your house go out the neutral (which is connected to the star point in your transformer on the street) and then flow out on phase C? And then when the current reverses it does the exact opposite (Back through phase C to the star point then back through the neutral going to your home and back through the load other way then back on phase A? I'm just trying to get an understanding of the direction in which a/c current flows. thanks :) Your help is appreciated

NoNickName
07-12-2010, 02:04 PM
Why do you care about the flow? Anyway, when you connect one phase, the other two phases don't come into play. It's the neutral that takes the field back to the center of the three phases. At any given time, the neutral is supposed to have the same potential than ground, because the vectorial sum of the three phases should be zero (when the neutral is tapped to ground).

Contactor
07-12-2010, 04:42 PM
understand that alternating current does not flow backwards and forwards as such, think of it more as a series of waves with crests and troughs moving in the same direction.

currents on single phase loads all pass back down the neutral from to the transformer while the primary usually returns star point, it think this assumes the phases are well balanced though

regards

Contactor
08-12-2010, 12:30 AM
paul h's point about ac electrocution may help:

a 60ma 50 hz ac shock through the heart is more likely to kill you than a 200ma dc shock

could this be the waveform ? ??

mad fridgie
08-12-2010, 12:59 AM
I think this is a very good question, or is he asking!
What is Electricity?
Looking at his point of view, if you look at 3 phase moter, star or delta, there is no neutral, So it would appear that flow from one phase must go back down, the other phases. So it would seem to make sense at the transformer that the neutral is just a common point for this type of flow to occur.
So what is electricity?

nevgee
08-12-2010, 01:12 AM
read the replies .....

"Unlike Direct Current, there is no directional flow only a "pulsing" or alternating current. "

chilliwilly
08-12-2010, 02:39 AM
The distribution system in Oz is the same as the UK which is typically 230/240/400/415v 50hz, and most of the rest of the, world except North and South America, which are typically 120/208/240/ 270/480v 60hz. AC current does travel in two directions down any conductor. If you have a load utilizing a two (one missing phase) or three phase supply you don't need a neutral. The generator supplying the load has three windings, (A B & C or 1,2 & 3) consisting of conductors wound on a stator, or rotor/armature. That in turn rotates through 360 degrees, between two magnetic poles north and south facing each other at 180 degrees. There are actually 3 pairs of poles on a three phase generator, one for each winding or phase, and in turn displaced at 120 degrees. Theoretically its the same as a three phase squirrel cage motor, but performing the reverse.

Basically for not a too technical explanation, when a generator is connected to a load. The rotor or armature is turned starting at 1 degrees towards 120 degrees, the voltage in one of the windings/phases starts to ramp up from 0 volts to whatever the design output voltage is, say 415v for easy reconning. As it cuts through the lines of the magnetic poles flux, it causes exitation thus inducing a voltage and current to flow in a forward direction.

When it reaches 120 degrees the voltage is at its maximum, say 415v. The second winding is also turning in the same direction, from 120 degrees towards 270 degrees and cutting through the other two pairs of poles, and is in turn exited by the magnetic flux and resulting in a current to flow in it also. Because the magnetic poles are of opposing faces, north and south (+ and -). The other winding is moving away from facing a north pole because of its position on the rotor, towards a south pole, the maximum voltage isn't reached at this point. But instead a voltage less than the maximum is started to be induced into it. But then starts to ramp down as the winding moves away from the north facing pole. The voltage in that winding falls to 0 volts, then starts to ramp up towards the maximum voltage as it moves towards the south facing pole, but the current flow is in the opposite direction due to the lines of flux having a different polarity to the north facing pole.

The third winding is also moving in the same direction but from 270 degrees towards 360 degrees. And the same action is taking place as in the other two windings. As its position is already moving away from a south facing pole and moving towards a north pole. The voltage induced causes the current to flow in the reverse direction of the first and second winding. As it moves away from the south facing poles lines of flux, the voltage induced in it drops to 0 volts then starts to ramp towards the maximum, but in the forward direction because its moving toward the north facing pole.

As you can envisage, when the first winding rotates through the whole 360 degrees revolution, it produces a forward, reverse, and then a forward current flow. As do the other two windings, giving a three phase output voltage at any one time, that's out of phase with each other and in turn causes a potential differrence between the phases. Due to the continous differential polarity of the current flow.

When a supply is taken to a house from a 415v three phase supply, (execpt North and South America) its usually single phase and neutral at 240v to earth/ground. The active/live/hot supply is simply taken from one of the phases, and a neutral is taken from the star/wye point of the distribution transformer, which is in turn connected to earth/ground, to help main a float free neutral.

When a load is connected to the single phase supply, the current flows down the live/active/hot conductor in a forward direction, then back towards the star/wye point on the distribution transformer. Then flows back through the other two phases via the transformer windings that are out of phase with the respective phase supplying the single phase supply, back to the generator which is rotating and constanly changing the polarity of the phases.

Then when the respective phase that supplies the single phase supply drops to 0 volts then ramps toward a maximum voltage, the current is now in reverse flow. The current then flows back through the neutral through the load and back through the live/hot/active conductor. Back to the distribution transformer via the windings the back to the generator.

Again as you can envisage, an unbalanced load such as a single phase supply only needs a neutral. When a motors windings aren't matched, they cause a phase load imbalance due to the differing load effect with each mismatched winding. And it causes each winding to draw different current values, causing further problems. As there is a three phase supply connected to the motors windings, it still doesn't need a neutral even though the load is imbalanced. The current just flows back through the respective phases that are in opposite polarity or out of phase with each other via the impedance of the windings.

Any three phase load can be connected without a neutral, although is isn't uncommon to find star connected loads to have a neutral connected to it to deal with current from mismatched/unbalanced loads that should be equal in value. A neutral connected to such loads, can maintain a 0 volts potential at star points. And can prevent further damage to connected components in the event of a winding or element burnout.

Hope this helps.

mburns
08-12-2010, 08:56 AM
Hey guys thankyou very much! Chilly Willy your explanation seems to coinicide with what i had thought myself from all the researching i've done, just couldn't word it well in a question i think. It helps alot just to get a decent understanding of how it all works even though it's not really something that helps me on the job.