3 Phase Suppling a Lift and CU

[Newboy1996]

I overheard a job be discussed recently where the incoming supply was a three-phase cut-out and a domestic supply was also needed for the premises.
What resulted was the domestic supply taken off one of the phases at the cut-out and not the installation of a second (single phase) cable for the flat.
Would this put the lift motor out of balance with current being drawn from one of the phases??

 

[Spark123]

Probably not a problem providing they did their sums right.
It won't make the motor go out of ballance - as long as the motor is getting the required 400v across each of its windings it will be fine.

 

[ricicle]

Providing the supply is big enough, the lift will draw the current it needs from all 3 phases and the single phase will draw its demand from 1 phase and one should not affect the other.

EDIT: Doh - too slow for 'im

 

[Lectrician]

Sounds like a fairly normal situation to me

 

[Stoday]

It's the neutral connection that keeps the phases in balance.

 

[ricicle]

It's the neutral connection that keeps the phases in balance.

You might want to re-word that statement............

 

[Stoday]

It's the neutral connection that keeps the phases in balance.

You might want to re-word that statement............

OK, it's a bit cryptic.

The effect of the neutral connection back to the transformer means that the voltage of each of the three phases is much the same even though the phases are unequally loaded.

Happy now?

 

[ricicle]

It's the neutral connection that keeps the phases in balance.

You might want to re-word that statement............

OK, it's a bit cryptic.

The effect of the neutral connection back to the transformer means that the voltage of each of the three phases is much the same even though the phases are unequally loaded.

Happy now?

Yes, I thought you were trying to say that the Neutral keeps the phase currents in balance (equal). Much better

 

[Spark123]

How does that work with delta systems?

 

[Stoday]

How does that work with delta systems?

All normal distribution systems are delta primary/star secondary. A delta secondary is only suitable for a balanced load or for a single phase load connected through a Scott transformer. The benefit of a delta winding is that it attenuates third harmonics introduced by loads such as computer power supplies. Anyway you can't have a mixture of delta star and delta delta transformers on a distribution network because you could never phase out between them.

 

[Spark123]

But even you don't have a neutral on a delta system, you can still draw current from two of the phases and leave the 3rd phase unloaded. Arguably the same with a CTE site transformer - no neutral.
I thought the dno used a mixture of delta delta and delta star transformers, there is a phase shift with delta star transformers.

 

[Stoday]

There is indeed a phase difference between the HV (11 or 6.6kV) system and the LV distribution system. But you're never going to connect them so any phase difference doesn't matter.

There are two way of connecting the HV side of a distribution transformer, such that the electrical rotation can be reversed. You will never be able to connect two LV distribution systems together if they are fed from transformers that rotate in opposite directions.