Phase Rotation

Tom Jenkins said:

I one did a plant near San Francisco. When I got on site the contractor complained that nothing would run because none of his fourteen solid-state starters worked! I read the error code on the starters. (Him- "Is that what those blinking lights are?"). You guessed it - phase rotation. I had to swap connections on one blower to prove it to him. Swapped the main transformer and all was good.

Then I had to convince him he had to swap back my test motor. Sigh.

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PG&E’s system phase rotation is ACB for whatever reason so most of the state of California is backwards from what people are generally accustomed to. I’m not sure how many people are aware of that. I only know from working in power plants where phasing had to match perfectly. It could be a real pain at times.

GaryS said:

“In the original case I stated, the contractor who phased the building should get a crack on the noggin for not phasing it correctly or for even thinking that it didn't matter. It does. That's engineering and the lack of it.”
I can see that you have never worked on a new install or you wouldn’t say that.

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As it goes, I've been an automation guy on greenfield startups for 30 years and a plant controls guy or system designer for the other 15 so it the kind of thing I check and get it corrected. When you have hundreds of small horsepower motors it makes a huge difference because I've never run into OEM equipment with phasing incorrect as all that equipment was verified on buy offs. That's a lot of labour and a lot of potential for smoke in the ****pit if it's not right.

The strangest one I ever ran into was in the Philippines. We were blowing rectifiers all over the place. Computers, DC drives, AC drives.. you name it and it would break if it had a power supply. Won't turn this into a tale but I suspected and discovered that the phasing was not exactly 120° apart. In fact it was something like 118, 122, 120. Nobody but my PM believed me until my PM went to the utility with the info I gave him and they admitted and correct the timing on their THREE single phase generators. Cwazy.

Szumi said:

My uncle gave me this a long time ago. Someday I'll see if it works.

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Pre-zip codes!


http://www.freepatentsonline.com/2027864.pdf

scameron81 said:

PG&E’s system phase rotation is ACB for whatever reason so most of the state of California is backwards from what people are generally accustomed to. I’m not sure how many people are aware of that. I only know from working in power plants where phasing had to match perfectly. It could be a real pain at times.

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Interesting - being a cheesehead I didn't know that. It was a California contractor though, so they should have!

It didn’t matter whether my position was an electrician, designer, electrical automation engineer, or consultant, L1 in the main service was the same L1 in each MCC, distribution panel, lighting panel, etc. The same with L2 and L3 being carried through all the way out to a motor where L1 was connected to T1, L2 to T2, L3 to T3.

If rotation needed to be changed on an individual motor it was on the load side of the last field disconnect (or load side of motor starter if no field safety disconnect).

If rotation was wrong on a VFD the CW/CCW parameter would be changed.

We didn’t often need a phase rotation meter unless we were replacing an existing MCC or other 3 phase panel. Even then we would always check one motor’s rotation before the panel change out and then verify that afterwards the motor was rotating the same way.

Phase rotation is 100% subjective, it is NOT absolute that it must be A followed by B followed by C. Case in point, PG&E, the largest utility in California and the 3rd largest in the US, is C-B-A rotation. Makes no difference though, so long as you know, and even then only for the few devices that care.

jraef said:

Phase rotation is 100% subjective, it is NOT absolute that it must be A followed by B followed by C. Case in point, PG&E, the largest utility in California and the 3rd largest in the US, is C-B-A rotation. Makes no difference though, so long as you know, and even then only for the few devices that care.

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Yeah! What he said!
As usual, jref has a way of succinctly summing up the situation.

scameron81 said:

PG&E’s system phase rotation is ACB for whatever reason so most of the state of California is backwards from what people are generally accustomed to. I’m not sure how many people are aware of that. I only know from working in power plants where phasing had to match perfectly. It could be a real pain at times.

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I missed this post, you beat me to it in saying that PG&E is ACB rotation. Must have been Saftronics Soft Starters, am I right? The way they did their Phase Lock Loop sensing to determine the firing triggers for the SCRs was based on looking only at the at A-B phase cross, then calculating everything from there. So if the incoming phase was not A-B-C, it would misfire, so it had a rotation check built-in that you couldn't defeat.


By the way, is this the same scameron from another site? If so, haven't heard from you in a while. Everything going good up there? I was thinking about you (assuming it's you) when the fires were raging again last year.

GaryS said:

I always like to think you have a 50-50 chance of getting the correct rotation the first time[...]

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I remember hearing I had a 50 % chance of getting the rotation right the first time, but 80 % chance of getting it wrong.

CommissioningMan said:

I remember hearing I had a 50 % chance of getting the rotation right the first time, but 80 % chance of getting it wrong.

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Try this - on one side of your toolbox lid put a check and the other an X

Every time you wire a motor if the rotation is correct put a tick mark under the check, if reversed put the mark under the X.

Watch over the years how much more populated the X side will become.

That 50/50 chance in actuality is 80/20 - or maybe 66/33 according to the YouTube videos on the Monty Hall Theory that you are only 33% right on the first guess and it stays that way.

All this reminds me of a story about a maintenance tech I worked with. Normally our jobs were pretty easy but every now and then we would run into an *** kicker. It was around half way through the 12 hour night shift and he came into the break room dirty and hungry to grab his first break. He said he had already changed 4 motors and still had one down.

He said "And every one of them ran backwards and had to be reversed. You would think that with a one in three chance, I would have gotten one of them right the first time."

After a pause, I spoke up to correct him "Actually, Harlan, your luck is even worse, since you have a 50/50 chance".

Without missing a beat, he said "No. It's one in three. The motor can run forward, reverse, or not at all like that last one I put in."

CommissioningMan said:

I remember hearing I had a 50 % chance of getting the rotation right the first time, but 80 % chance of getting it wrong.

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It's Newton's fourth law of motion: a three phase motor connected to three phases will run backwards.

We install buildings, incoming suppy, cabinets etc. The power lines are always checked to be correct rotation with instruments, all installed socket outlets etc. It's extremely important. Otherwise your average joe starts to switch phases in the extension coords, in portable tool etc and you are having a hell trying to make eveything right again. Imagine a open sanding machine gets connected and goes the wrong direction, the material will bounce off your hands and in you face. In electrical installation it must always be correct phase rotation , or do you say phase sequence?

Aabeck said:

I have seen so many mains reversed I began to doubt there was a standard.

The one standard I have seen followed by most shops is buss bars start at the top.

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There is a standard. New techs that install machines that have 2 motors on frequency drive and 2 on contactor, do not know what to do to install this machine. No need to buy them expensive tools that they will forget to bring with them every time that they need to.

Lately, I asked my new tech if he knew what a frequency drive is and how to change the direction of the motor if needed. Blank face, again.

A frequency drive (VFD, or how do you name it) is basically a converter of alternating current to direct current. So all phase direction information is lost.

A contactor never loses it direction.

If a machine is installed and only the motors on contactors are rotating in the wrong direction, then check the mains, change 2 phases, and try again. No meter needed.

Noli

ASF said:

It's Newton's fourth law of motion: a three phase motor connected to three phases will run backwards.

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That is soooo not true. A three phase motor connected to three phases will not run because one phase is loose, or because one phase is switched with the neutral conductor, and only after correcting this, the motor will run backwards