3 phase heater with imbalanced loads

[BroadReach]

Hi All

I would appreciate your help with the following.

I am familiar with standard calculations such as calculating the current draw on a three phase heater battery when the loads are balanced across the three phases. I have recently come across some equipment where it appears the heater battery has been deliberately designed with a load imbalance. I believe this is to enable the manufacturer to use their standard elements to produce alternate heat outputs for special purposes.

The heater is as follows:

2 off 21 Ohm heaters and 1 off 27 Ohm heater wired to a 400V 3 phase supply in star formation with floating centre point.

Now I know that 3 off 21 Ohm heaters wired this way would result in about 10.9A/ph and 7.5kW heat
and 3 off 27 Ohm heaters wired this way would result in about 8.7A/ph and 6kW heat

How would I calculate the current draw and heat output of the hybrid heater battery?

Any help appreciated

 

[Knobhead]

You'll have to do a vector diagram of it.

 

[BroadReach]

Tony, thank you.

Just have to remember how to now.

 

[Knobhead]

Well I'm not doing it. Sounds to much like homework

 

[GLENNSPARK]

Well I'm not doing it. Sounds to much like homework

and since when were you scared of puttin `pen to paper` Tony....lol...

 

[Knobhead]

Only use a pen to do the crossword.

 

[BroadReach]

I have been thinking about this since Friday and am asking for pointers to work it out rather than just an answer to this example.

OK so if there was a neutral to the star point then it would be easy as the phase to neutral voltage on a 400V 3 phase system is 230V and I could then use Ohms law to work it out. In my example above the answer would be 2.5 + 2.5 + 2 = 7kW with 10.9A down two phases and 8.7A down the third with a neutral current of 2.2A.

What I am thinking is how to work out the phase to centre point voltage as with no neutral this will float away from Zero I believe. Once I know this then I think Ohms law is my friend again?

Edited to add:

Perhaps another way is to work out the current draw on each phase and then calculate the power from there?

Problem is I know how to use a vector diagram to work out neutral current in an unbalanced TP&N system but cannot visualise how to use one to work out either of the above in an unbalanced TP system.

 

[Engineer54]

Only use a pen to do the crossword.

More like a pencil with a rubber at the end of it!! lol!!

 

[Knobhead]

Tipex falls off when you fold the paper :devil2:

 

[BroadReach]

I'm not an electrician
I'm not a student looking for an answer to my homework
I'm not a DIYer looking to do something on the cheap

I am a Mechanical Engineer who would like a better understanding of the electrical bits of the kit I sell/specify.

Any help appreciated.