View Full Version : where does the neutral come from?

22-09-2012, 01:19 AM
this maybe a simple question but here goes,you supply 3 defrost heaters in an evap with a phase to each heater from a 3phase supply and gang/join the other ends together as one and hey presto they work so i guess they form there own neutral likewise you do the same on a 3phase compressor the star point becomes neutral you can touch it no power,can someone explain because its baffling me?!!!!!!!

install monkey
22-09-2012, 03:44 PM
yep as you say like a 3ph motor- ur creating a star point- also summit to do with the frequency of the phases- 1 high , 1 medium and 1 low- at any 1 time.
as long as it works -thats the main thing

Rob White
22-09-2012, 04:55 PM

As you say 3 phase has no neutral that is because of the way the
electricity is generated and supplied.

In a single phase you need a live (line) and neutral but the live
is switching + and - 50 times a second. That is the Alternating
Current (AC) and that is the way our power is generated.

In a 3 phase, instead of one winding producing one supply you have
three windings producing three supplies. All contained on a rotary
shaft. So like single phase they produce AC current 50 times a second.

Because there are three windings on a rotor they all produce electricity
at slightly different times and because the rotor is rotating the difference
is one third of a rotation.

When winding one is producing electricity, winding number two
is 120 degrees behind (one third of a full rotation) and so only
part producing electricity and the third winding is 120 behind that
and is not producing electricity.

It is doing this 50 times a second across the three phases, plus - minus,
plus - minus, plus - minus, plus - minus, plus - minus, plus - minus, plus - minus
do you get the drift.

Every time one phase is at a positive the other is negative, so on and so on.

So the 3 phase motor or heater array that is star or delta configured can only
work with an AC supply.

Think of a kids roundabout at a park and the roundabout has 3 handles
evenly spaced around it. If one person was to stand at the side of it and
push the handle, the roundabout would rotate. If that person pushed
every handle as it passed him he could work up to a speed. Now just
imagine he could do that 50 times a second, he would have to wait
for the handle to come to him and then push it away 50 times a second.

Now think of the same round about with the 3 handles on it and 3 people
stood at the side.

If person one pushed handle number one the roundabout would rotate
and handle number one would move to person number two. Person one
would have to wait for the next handle to come to him before he could
push it and therefore he will never touch the same handle as person number
two at the same time as him. And don't forget person number three waiting
his turn.

All three people pushing each handle 120 degrees apart and 50 times a
second. Three phase......




install monkey
22-09-2012, 05:11 PM
cant give you rep points for that post rob, nicely worded, but thinking about kids on round abouts-could be misinterpreted! :o

22-09-2012, 05:53 PM
You must be popular Rob, neither can i give you points. Good answer all the same.
Thanks for the good question cadwalader.

22-09-2012, 08:03 PM
Might also be worth noting that its mostly only balanced 3 phase loads that don't require a neutral, like the 3 indentical windings in a motor or in your case 3 heating elements with the same resistance

22-09-2012, 08:13 PM
thanks rob,well explained!!

22-09-2012, 08:33 PM
good one rob.
If the heating elements do not have the exact resistance, you can measure a few volts at the point they are connected, only on Y setup.

23-09-2012, 06:24 PM
Hi Rob
nice answer!!!
best regards