3 phase kiln questions?

[HappyHippyDad]

A customer has asked me to install a supply for a Wenger Webcot kiln.

The faceplate is very worn and I cannot see the kW. They state the kiln it type 3670w and is rated at 12kW.

The paperwork does have 3670w on it but when I look at the spec sheet (see picture 2, SpecSheet) ) there is no sign of a kiln which is 12kW. This model has 12 elements therefore it must be one of the last 6 models on the spec sheet, but again no sign of 3670w. It looks like there is one called 3060w though but this is not 12kW. Does anyone have any ideas how I could find the kW, or does anyone recognise the kiln details and know the kW?

The wiring diagram (see picture 4) shows that it can be wired as single phase or 3 phase. This will be wired as single phase. My questions...

1. Do I just join the 3 phases together as per the diagram?
2. Do I need to increase cable size compared to the existing flex on the Kiln as I am joining 3 x 10mm cables into one conductor? Although the neutral carries the same current and there is only 1 neutral conductor! Why is this?
3. On the spec sheet (pic 2) it has 2 columns for electricity supply required. One for single phase and one for 3 phase. The last 4 columns for single phase have a dot. looking below it says treat as nominal rating 14kW for single phase, yet the actual column labelled nominal electrical .loading states different, higher values, why is this?

Pictures below show the almost unreadable faceplate, spec sheet, cable and part of the wiring diagram.

Picture 1, faceplate.


Picture 2, Spec sheet.


Picture 3, Cable


Picture 4

 

[HappyHippyDad]

Yes.

One thing that deserves explaining here is the actual connection configuration of the element groups. We can ignore that each group is actually made of four physically separate sections and just consider them as single elements.

Each element group is connected from one line terminal to neutral (via the main switch and contactor.) In other words the kiln contains three identical, single-phase 230V heaters, just like three fanheaters sat on the floor with separate flexes and plugs. It wouldn't matter whether you plugged those fanheaters into three sockets on the same phase, or three sockets on different phases, because they are independent. Same with the kiln - the three line terminals can be connected to the same phase or different phases, so long as each terminal is 230V from neutral.

This is a different situation to (e.g.) a 3-phase motor, which is not like three independent single-phase motors because the magnetic fields of the three windings interact with each other. The windings must be connected to alternating voltages that vary in time with the correct phase relationship to each other. You cannot simply connect all three to one phase.

Likewise, a different situation obtains in a heater consisting of three identical elements connected in star (i.e. each between a line terminal and a star point) but where the star point is not connected to the supply neutral. This is a handy configuration for industrial supplies where a neutral might not be available When connected to a 3-phase 400V supply, the three elements form a balanced load in which each element receives 230V between the line terminal and the star point. Through symmetry, the star point will lie at a voltage close to that of the supply neutral, hence the element currents and voltages would be just like those of the klln (although note that the 230V control ccircuit of the kiln absolutely requires the neutral to be present). But clearly, if the star point of our heater is not connected to neutral, wiring all three line terminals to the same phase won't work because now there is no complete circuit. Like the motor, this needs a 3-phase supply.

Returning to the kiln and considering the currents in the circuit conductors; on 3-phase the currents from the three elements sum to zero at the neutral terminal (except for the small control circuit load) unless one element fails O/C in which case the neutral current will equal the line current. On single phase the line and neutral currents are inherently equal. The line and neutral currents can be taken as equal both on single-phase and 3-phase supplies, with the current on single phase supply three times that on 3-phase.

What I hope to have shown in the above explanation is that the kiln elements, by virtue of being connected L-N, are less like a 3-phase load and more like three single-phase ones, for which ordinary single-phase calculations can be applied.

You can supply your own sketches to accompany the above

Oh, BTW


R=V/P ???
For this, you are to beat yourself with a big lump of SWA until it really hurts.

God, that's awful isn't it ?.
I was just getting too excited with all the different maths and new information floating around and wrote complete nonsense!