3 phase question

[John D v2.0]

Only a few amps through the neutral then, should be fine. Max would be 13ish amps with separately switched loads.

 

[John D v2.0]

Except I'm still not sure if the "3 circuits" mentioned in post #6 are on the same phase or not....

edit wrong post number

He said it is spread over the 3 phases in the op

 

[industryspark]

The neutral will be ok from the sounds of your description. Id be a bit more concerned with the fusing/protection for the elements and the motor. Surely its not relying solely on the TP 16A mcb?

 

[athej]

Brown Phase L1 Element 230 Volt 3KW
Black Phase L2 Element 230 Volt 3KW
Gray Phase L3 Motor 230 Volt 2.2KW

 

[endecotp]

OK, so this just boils down to "how does the neutral work in a 3-phase system"? Answer, the current in the neutral is never greater than the current in the highest-current phase. Google will find some nice vector diagrams showing why, if you're curious.

 

[athej]

Ok So The Current will never Be more then the largest load ie one of the 3 kw Element

Thanks

 

[ban-all-sheds]

They have run L1 to one Element 230 volts L2 to second Element L3 to single phase motor All relying on the one neutrel in the 4 core SWA

Who are "they", and why does this concern you?

 

[athej]

I have to Repair The Machine and I think The Neutral is to small for total load

 

[ban-all-sheds]

OK, so this just boils down to "how does the neutral work in a 3-phase system"? Answer, the current in the neutral is never greater than the current in the highest-current phase.

523.6.3
A4 5.5

 

[JohnW2]

OK, so this just boils down to "how does the neutral work in a 3-phase system"? Answer, the current in the neutral is never greater than the current in the highest-current phase. Google will find some nice vector diagrams showing why, if you're curious.

That's certainly true if the load is totally resistive. However, without doing some scribbling I wonder if that necessarily remains true when PF starts wandering - particularly if (as in the OP's case) each phase has a separate L-N load, with those three loads differing markedly in PF (two resistive and one inductive in the OP's case)??

Kind Regards, John

 

[athej]

No seprate Neutral all using the one core of 4 core SWA

 

[Robin0577]

Ok So The Current will never Be more then the largest load ie one of the 3 kw Element

Thanks

In most cases that is true.

However it is possible, if electronic controls are involved to regulate the motor speed or element power, for strange harmonics and phase shifts to be introduced and cause a higher neutral current. Stage lighting dimmers are often fittet with an oversized neutral conductor for this reason.

 

[JohnW2]

In most cases that is true. ... However it is possible, if electronic controls are involved to regulate the motor speed or element power, for strange harmonics and phase shifts to be introduced and cause a higher neutral current. Stage lighting dimmers are often fittet with an oversized neutral conductor for this reason.

Indeed - but, more simply, as I've just written, could not the fact that L1-N and L2-N loads are both resistive, but L3-N highly inductive cause the neutral current to be higher than one would expect if all loads were resistive? (as I said, I need to do some scribbling, but maybe you can answer the question before I get there ).

Kind Regards, John

 

[endecotp]

True John, let's try to quantify it...

Say the PF for the motor is 0.8 (http://www.engineeringtoolbox.com/power-factor-electrical-motor-d_654.html), phase lag = 36 degrees.
Consider the worst case: a resistive load A on one phase and an inductive load of the same magnitude A on the adjacent phase that lags towards it. Draw the vectors; the angle between them is 120-36 = 84 degrees. Size of the resulting vector = 2A cos 84/2 = 1.48 A. So you might have 50% more current in the neutral than you would expect.

In practice, 3-phase neutrals that I've seen don't seem to allow for this.

Of course in this case it's not an issue as the two resistive loads are well balanced. I tried to work it out but I realise we don't know if the 2.2kW motor is actually 2.2kW or 2.2kVA.

 

[JohnW2]

True John, let's try to quantify it... So you might have 50% more current in the neutral than you would expect.

Thanks - as I expected, and you got there before I got around to reaching for my 'scribbling pencil'

Kind Regards, John