What are the pitfalls of Designing a piece of Equipment which purposely unbalances a 3 phase circuit
What are the pitfalls of Designing a piece of Equipment which purposely unbalances a 3 phase circuit
(OP)
I am designing a piece of machinery for factory production of electronic parts. I have always been taught that it is "bad" to have unbalanced 3 phase loads, but my supervisor has suggested that I steal power frr the 120V circuits off one of the phases of the 3 phase input.
My supervisor is not an engineer but just heard this from someone.
I had planned to have a 3 phase input for the 3 phase loads and a separate 120V input for the 120 V loads. With my alternative no neutral is required (the 3 phase load has absolutely no provision for a neutral), and the circuit can be wye (grouned or ungrounded), delta (grounded or ungrounded or even corner grounded),
With the other alternative, I am not sure whether we have the same flexibility.
My supervisor is not an engineer but just heard this from someone.
I had planned to have a 3 phase input for the 3 phase loads and a separate 120V input for the 120 V loads. With my alternative no neutral is required (the 3 phase load has absolutely no provision for a neutral), and the circuit can be wye (grouned or ungrounded), delta (grounded or ungrounded or even corner grounded),
With the other alternative, I am not sure whether we have the same flexibility.





RE: What are the pitfalls of Designing a piece of Equipment which purposely unbalances a 3 phase circuit
If you are talking about large amounts of 120 V power, then that's another story. It is generally preferable to have a single power source into your tool or machine.
RE: What are the pitfalls of Designing a piece of Equipment which purposely unbalances a 3 phase circuit
What is the magic number for unbalanced systems that creates problems, and what are those problems?
RE: What are the pitfalls of Designing a piece of Equipment which purposely unbalances a 3 phase circuit
RE: What are the pitfalls of Designing a piece of Equipment which purposely unbalances a 3 phase circuit
BTW, the neutral on the 120vac is derived from the secondary not the primary. Like was pointed out above its 480VAC phase to phase on primary.
RE: What are the pitfalls of Designing a piece of Equipment which purposely unbalances a 3 phase circuit
Using a "Control Power Transformer" provides the exact same benefits you describe in allowing flexibility of the configuration of the incoming supply, but without the user having to provide two separate circuits. Separate circuits by the way also introduces a couple of safety issues with lock-out and isolation that must be addressed in the control panel, using a CPT tapped off of the main incoming power takes care of that too. When you kill the main power disconnect to open the cabinet, you inherently kill the control power too.
"Will work for (the memory of) salami"
RE: What are the pitfalls of Designing a piece of Equipment which purposely unbalances a 3 phase circuit
As DPC suggests use a transformer across two phases. That reduces the current and unbalanced voltage drops and allows you to avoid adding a neutral conductor.
The unbalance is more of a system issue affecting the whole service. A given KVA load across two phases causes less unbalance than the same KVA load connected from one phase to neutral.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: What are the pitfalls of Designing a piece of Equipment which purposely unbalances a 3 phase circuit
I have done some reasearch that introducing the equipment on a pre-existing 3 phase circuit may cause the following pitfalls:
= pre-existing motor loads on that 3 phase circuit to overheat,
- cause nuisance tripping of 3 phase protection circuits
- nuisance tripping of 3 pole breakers since 1 pole may have higher current
But if everyone thinks that a current imbalance of less than 20% wont create these issues its good to know.
RE: What are the pitfalls of Designing a piece of Equipment which purposely unbalances a 3 phase circuit
That is not nuisance tripping. The breaker is tripping to protect the conductors. You have to size for the total current.
RE: What are the pitfalls of Designing a piece of Equipment which purposely unbalances a 3 phase circuit
RE: What are the pitfalls of Designing a piece of Equipment which purposely unbalances a 3 phase circuit
But visualize what Jraef said once more: your putting 20% of YOUR load imbalance (let's put numbers to it - say 10kva your 3ph load so 2kva your imbalance load) - on a transformer in your building -probably rated 500kva! So your 20% is really only .4% imbalance to the transformer. Others in the building are doing the same so it all comes out in the wash.
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