balancing 3 phase

[ILCML]

If I have 208/230 3phase coming into a control ciruit to run a 3 HP motor, and am pulling a leg to use as a 120v supply (with the neutral) for control circuitry, a) will this unbalance the load to an unacceptable level b) if so how can I "rebalance" it?

 

[dpc]

"208/230 V" doesn't exist - I'll assume you're talking about 208/120V. There's no problem having 120 V loads provided you have a neutral conductor back to the transformer. There may be some current imbalance, but it won't be a problem.

If you're talking about something other than 208/120V please clarify.

 

[ILCML]

sorry, 230v 3-phase....

 

[aolalde]

The control circuit will demand such a small load current that it will not cause perceptible unbalance on the three phases.

 

[ILCML]

is there a formula or a percentage to use to determine whether additional use of the incoming leg will cause unbalance? If so, what could I do to even it out? My guess would be to use a transformer on 2 of the legs and step it down (although that is what I was trying to avoid to begin with)

 

[waross]

A common practice is to use a small transformer (50 VA for smaller starters) across two phases to provide control power. On 230 V or 208 V systems 230 v or 208 V starter coils are often used to avoid the transformers. The coil load or the transformer is so small as to be insignificant and may be safely ignored.
respectfully

 

[dpc]

If you have 230 V three-phase, you can have 120 V (115 V) only on one leg of the transformer. So any 120 V load means the system will not be perfectly balanced.

But as long as 120V loads are small fraction of the three-phase loads, the imbalance does not cause any signficant problems.

 

[ILCML]

the 120 leg will have a total load nearly that of the motor using teh 3 phase. At what point should I be using a transformer, 50%? 25%? 5%?

 

[waross]

The transformer will not balance the load. You use a transformer for either or both of two reasons;
You want to limit or isolate the energy in the control circuit.
You need to change your plant voltage to your chosen control voltage.
The unbalance to the system from a starter coil is negligible.
On the subject of balanced loads:
If you have a 120/208 volt star system you balance your load on the panel by moving loads from one phase to another phase.
If you are using a 240/120:240 volt 4 wire or center tapped delta system it depends.
Four wire delta systems often run unbalanced. They are often a mix of single phase loads and three phase loads. The transformers may be sized accordingly. If you don't know what you are doing, DON"T DO IT.
respectfully