What are the pitfalls of Designing a piece of Equipment which purposely unbalances a 3 phase circuit

[Eng8492]

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.

 

[dpc]

It is customary to use a small single-phase control power transformer to derive 120 V ac control power in package systems. This can be 480-120 V. The imbalanced current on the 480 V (or whatever it is) supply will not be an issue.

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.

 

[Eng8492]

What do you define as large amounts of power? In this case, one of legs will hamust supply 120% of the current that would have flowed in that leg if the equipment had been designed with a balanced 3 phase loads and a seperate 120V circuit.

What is the magic number for unbalanced systems that creates problems, and what are those problems?

 

[dpc]

The main problem is that unbalanced current creates unequal voltage drops which create voltage imbalance. Induction motors really do not like voltage imbalance. You haven't mentioned the main three-phase voltage, but generally if you are putting in a transformer to derive the 120 V, you will use a phase-to-phase primary connection. I doubt that 20% would be an issue.

 

[controlsdude]

I have never had an issue deriving control power 120Vac from the incoming 3 phase that is used for all the motors. Designed motor panels for over 20 years.

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.

 

[jraef]

I have zero doubt that 20% would be a non-issue. This is standard practice for motor control systems. Think about it this way; if you set it up for a separate feed of 120V control power, where is that 120V ultimately going to come from, a different utility generator? No, it all will be coming from the same 3 phase source anyway. Whatever infintesimal effect it might have on voltage balance will take place no matter what.

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"

 

[waross]

Further to DPC's comments. It depends on the relative size of the loads. With a 120 Volt current greater than the three phase currents you may have protection and wire size issues. Not really a technical problem, but possibly larger than needed and more expensive conductors on the phases with only three phase loads to satisfy protection requirements.
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

 

[Eng8492]

Thanks for all the advice.

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.

 

[dpc]

nuisance tripping of 3 pole breakers since 1 pole may have higher current

That is not nuisance tripping. The breaker is tripping to protect the conductors. You have to size for the total current.

 

[Eng8492]

Yes we will have to augment the current rating on the name plate for all 3 phases even though only 2 phases may be affected.

 

[mikekilroy]

I assume from your comments thus far you are not putting a small 3ph transformer INSIDE your machine and then plan to add the 120 transformer off ITS secondary; you can see if you did this then your 20% may potentially approach an issue level; it would depend on that transformer's design. Of course in that case you would simply wire it to the PRI side of YOUR internal 3ph transformer and solve it.

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.

http://www.KilroyWasHere<dot>com