# what can happen if you reverse two phases.



## McCready00

except for motors.. is there anything bad that could be happening for lighting equipements.. or sound equipments.. ?


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## photoatdv

What do you mean by reverse two phases? That is a very different thing if you're talking about 2 phases of a single (usually 2- 120v hots)/ 3-phase (3- hots) or if you're talking about phase in audio/speakers. If you reverse the phase on 2 speakers they'll cancel themselves out .

I'd assume you're talking about mains power phases though. To start with if you are asking, you shouldn't be wiring anything you might have in mind that is causing you to ask this question. If you're just curious however, it's unlikely to matter if you switched say the red and blue phases on a 3-phase wye connected (120-120-120 and 208 between phases) but if you were to switch a 120 and the "wild" or "#@$%^@#" leg (usually 208) on a delta you'd have a BIG problem.


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## SHARYNF

The problem would be as noted if you were using the three phase to supply a three phase motor, since the feeds to the windings would be out of sequence

Sharyn


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## McCready00

sorry, I got to say my question was really incomplete!

Well.. what would happen if I would switch 2 phases in a three phases 120/208v circuit. Cables switched from the generetor or the main power distribution panel.


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## photoatdv

Okay... I really need to stop clicking on someone's profile in the middle of writing a response...

Alright-- a wye setup with 120V phase-N and 208V phase-phase swaping two of the phases would not affect single phase loads (like your lighting console or cd player) at all. The waveforms (better) look the same other than the timing (the phases being 120' offset from each other). A 208V load, such as a moving light, simply gains a higher voltage by using 2 phases, so again since any phase-phase combination would be 208V, it wouldn't matter. However in a true 3 phase load, like the motors, could be a problem. I don't know quite what would happen, but it would make a difference.

This being said, I certainly would not recommend swapping phases around! And again, this is by no means any sort of how-to... I'm just explaining the science/ physics of this as I understand it.


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## Footer

With a motor, the power has to go in just like the generator put it out (a generator in its basic form in a backwards motor).

Heres a good illustration from the wikipedia....


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## epimetheus

The rotation of a 3 phase induction motor can be reversed by swapping 2 of the 3 phases. Technically, I suppose, with a wye-connected source (as opposed to a broken delta, or high-leg delta), swapping phases on a typical A/V/L distro wont have any effect. I still wouldn't do it though.


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## ajb

A three phase motor will simply run in reverse. If the normal sequence is ABC (ABCABCABC...), and you swap, say, A and B, you wind up with BAC (BA*CBA*CBAC...). This is true for swapping any two of three phases, and is exactly how you reverse a three-phase induction motor. Or more generally, for N phases, you have to rearrange at least N-1 to completely reverse phase order. You just have to be careful that the motor has a bidirectional cooling fan (not all do).


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## mstaylor

Without the science, it won't make any difference to anything on a sound or light rig except the motors. Switching phases will reverse the rotation of the 3 phase motor. This is why there is a phase reverse switch on motor controlers, it saves swapping wires around. All this assumes that you are not using a wild leg 3 phase which creates all kinds of interesting things to happen. 
As advised in the first post, if you need to ask don't do tie-ins. Get more training before you are the guy doing that portion of the job. However, it is important for anybody using equipment to know what bad can happen by altering things.


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## PeteEngel

i haven't seen the most i mportant issue with swapping phases brought up yet...

a tear in the fabric ofthetime/space continuum..


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## Morpheus

PeteEngel said:


> i haven't seen the most i mportant issue with swapping phases brought up yet...
> 
> a tear in the fabric ofthetime/space continuum..



no, that's only if you cross the beams.


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## epimetheus

Morpheus said:


> no, that's only if you cross the beams.



Cross the beams...


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## Morpheus

epimetheus said:


> Cross the beams...


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## Chris15

Apart from the motor reversal notedf, absolutely noting will change, with a small caveat - anything with a three phase motor - saw a large table saw or an air con compressor or something like that may be similarly affected.

IF you are trying to balance load across phase and have individual components tying into separate phases then those sums will be affected.

I would not stress over it - There's every chance that there is a phase reversal somewhere in your installation - I can think of a lab at uni where half the outlets are reverse wired, deliberately, so that people remember that you can't assume phase rotation at an outlet. If you need to know, go buy a phase rotation detector - Fluke make a nice one...


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## DimmerDotCom

McCready00 said:


> except for motors.. is there anything bad that could be happening for lighting equipements.. or sound equipments.. ?



Some dimmers actually calculate the phasing for the dimmers fed from B and see using A as a ref. They assume B is 120 degrees out and C is 240 out. A famouse example is the Strand System 6 dimmer rack. It only samples the A phase and calculates the remainder instead of bringing in all 3 phases for sampling.

In the System 6 case, swapping 2 phases would result in 4 dimmers working great, 4 dimmers acting like non-dims, and 4 dimmers having a quite interesting curve. 

Addendum: most dimmers that perform this type of calculated phase reference usually have a switch somewhere to reverse B and C phase referencing. I believe late firmware System 6 dimmers, as an example again, had such a thing added oto its dip switches.


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## FMEng

The answer varies by type of three phase....

WYE is the most common, providing 208 V phase to phase, and 120 V from any phase to neutral. Only three phase motors will care about phase order.

DELTA is 240 V phase to phase, and two of the phases are 120 V to neutral. The third phase is 208 V to neutral!! Mix up the phases with a delta, and the "wild leg" will damage or destroy 120 V loads! Motor rotation is also a factor here.

Delta isn't very common in stage environments, but you need to be aware of it. Be very cautious when you encounter it. This is why you should always measure all of the voltages before hooking up in an unfamiliar situation.

Even the power utility companies sometimes mess up their phases. On our college campus, the power company put in some new switch gear during a planned outage, and forgot to check rotation. Several big motors in the building HVAC systems were running backwards, until the campus electrician called it to their attention hours later.


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## DimmerDotCom

FMEng said:


> The answer varies by type of three phase....
> 
> WYE is the most common, providing 208 V phase to phase, and 120 V from any phase to neutral. Only three phase motors will care about phase order.
> 
> DELTA is 240 V phase to phase, and two of the phases are 120 V to neutral. The third phase is 208 V to neutral!! Mix up the phases with a delta, and the "wild leg" will damage or destroy 120 V loads! Motor rotation is also a factor here.
> 
> Delta isn't very common in stage environments, but you need to be aware of it. Be very cautious when you encounter it. This is why you should always measure all of the voltages before hooking up in an unfamiliar situation.



Common thing to watch out for when working on cruise ships.


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## epimetheus

DimmerDotCom said:


> Common thing to watch out for when working on cruise ships.



Or older church/school buildings. Really any older building. My previous church was built in the 1940's and has a "wild leg" 240V delta configutation. It was at least a full three phase source. Some substations I've worked on have had an open delta configuration with only 2 transformers.


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## zuixro

When I first started my old job, I was told that swapping the phases on amps could cause hum. I didn't really know what 3 phase power was back then though, so I may have misunderstood them.


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## tjrobb

We had TWO-phase power (and still do to a very limited degree) when we were flooded last year. Doing the post-flood walk-through with the electricians I mentioned this to them and they looked at me like I had 3 eyes (hasn't even been in the NEC in years). Crazy buildings from 1928...


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## ajb

tjrobb said:


> We had TWO-phase power (and still do to a very limited degree) when we were flooded last year. Doing the post-flood walk-through with the electricians I mentioned this to them and they looked at me like I had 3 eyes (hasn't even been in the NEC in years). Crazy buildings from 1928...



Two phase? How did that work? Presuming you mean two proper phases and not split single phase (I'd guess from your signature that you know the difference!).


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## Chris15

tjrobb said:


> We had TWO-phase power (and still do to a very limited degree) when we were flooded last year. Doing the post-flood walk-through with the electricians I mentioned this to them and they looked at me like I had 3 eyes (hasn't even been in the NEC in years). Crazy buildings from 1928...



REAL two phase or just two thirds of a three phase supply?


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## tjrobb

Two phase, 180d separate. Was the 5-wire style (2h, 2n, g). I'd have to look at the motors for more specifics. Probably was installed due to the needs of incandescent lighting in the '20's.


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