Three Phase Mains: Current in the Neutral?

This is a favorite interview question of mine:

Given a three phase suppy at 63Amps per phase, standard Star 5 wire configuration with all three phases fully loaded. How much current flows in the neutral?

A. 63 Amps

B. 189 Amps

C. Zero Amps

.... well there are three phases L1 L2 L3 and they all share the same neutral wire.... hmmmm

And the answer is ....

C. Zero amps

Assuming the phases are balanced in terms of current load and the power factor per phase is the same. The relative phase offsets of 120 degrees per phase means that if you sum the voltages at any point in time over the three phases you end up with a total voltage of zero volts, so zero current flows.

This is what it looks like:





Another question follows: If the loading is hopelessly unbalanced... what is the worst case current in the neutral?

Here is a clue... how much bigger is the neutral wire compared to the three live wires?????

Care to comment?? ;-)

Mark SFL 1, West Ham United 0

Some technical audio notes form my sound design for Global Day of Prayer at West Ham United football stadium London UK. 27th May 2007

Sound, Lighting, LED and Video production by SFL.

Ave it!

I Win!

I win, no more than a 5 dbA drop from front row in the ground blocks to the back top seat at the back of the west stand. It was actually better than I calculated. The reverberant field lifted the levels up in the gods but the intelligibility was still spot on.

Why Fly?

There were no people to be on the pitch so what is the point of flying when you can achieve front row level control by taking the system back 20M?

Design Objectives

As always my design objective (for contemporary stuff) was to come up with a 100dBA (1min LEQ) capability with 10dB of amplifier headroom and achieve no more than a 6dB level drop from front to back.

Opps Lots of boxes

I used over 100 boxes of d&b Q series (Q1 and Qsub) with some additional B2 in infra mode. All the sub systems were in CSA (cardioid sub array). SFL's Mark Young mixed the band and Graham Preston mixed the orchestra, they both nearly wet themselves over the system/stage separation provided. They were able to mix the orchestra up at ANY level they wanted.

Cardioid Sub Array

The CSA took the sub energy away from the stage and tightened up the bass in general.


No Time

We had to rig the systems through the night, in the wet (nice). Qcalc did a great job as normal but the distances were large and small angle errors were going to have a big effect on array performance. I lasered the boxes into the stands which really helped to get it all right from the off. The production schedule gave me only 30 mins to align the whole system with real noise so I had to have the mechanics of the arrays right from the off. No time to physically tweak.

Things That were Magic

Some things that really worked on this gig:

• Radio (Mike Wilson) did a great job for me as system No2. Everything we rigged worked first time. Brilliant.


• RopeC... you cannot do a gig like this with distributed amps without remote control. RopeC is a piece of genius. Gotta love CanBus


• AES/EBU .. I went digital to all the amps from the PM5Ds at FOH. It's great but what a lot of lesons I have learnt. Beware of what happens over 100M. In preparation I found all sorts of problems that I might write up later.


• My own private wirless network allowed VNC control of my FOH PC. This enabled me to play sources, control the PM5D's and tweak the system with RopeC from the stands on my (damp) laptop.
  

Trust no-one

"Trust no-one" part 1.. never build anything on site that you have not previously tested. I was worried about these ground stacks. The last two boxes are a little tricky to get on. A bit of a risk assesment was going on here.


"Trust no-one" part 2.. hmmm thats a lot of digital cable. better check it... opps it does not work very well!. Then I learnt a lot about impedance matching and conversion. Also the difference between reclocking DAs and simple (no latency) edge conditioning buffers in the D12 amps... The fix on the cheap is if you want over 100M you convert to from 110Ohm to 75Ohm and run it on video coax before converting back again. Glad I learnt these limitations in the wearhouse, got my trusty HP osilloscope out again!








"Trust no-one" part 3. Our consoles for both FOH and monitors were preloomed and preprogrammed at SFL. The whole 96 channel split system was configured, patch checked and line checked from stage box to three ends (FOH, Mons and TV).















Some other pics:

People comming in

Craig's LED Screens

When it rains, stuff gets wet.

SFL People:

Craig Lawrence, Overall SFL Technical Production Manager and LED
Mark Payne, Sound Systems Design
Mike Wilson, FOH Speaker Systems
Mark Young and Graham Prestion, FOH Sound
Colin Hounsome, Monitors
Tom Jeffery, Lucy Kay, Dan Appleby, Stage setup and management.
Adam Evans, Choir Monitors
Monkey Tim Pitman, double shift crew madness
James Cremer Evans, Rigging
Ollie Jeffery, Lights
Mike Simpson, Camera Director
Clair Stroud, Graphics Op
Nick Pemberton, Sound wanderer and water management!
Audrey Payne, Crew Nanny