Reversing a 3 phase motor.

[Stevareno]

In one of Mike Holt's exam prep books, it says to reverse a 3 phase motor you can switch any two leads. It goes on to say that the industry practice is to switch L1 and L3.

This goes against what I was taught in my early days, which was to reverse L1 and L2 or L2 and L3, but not L1 and L3.

Why would I have been taught not to reverse L1 and L3 if it doesn't make a difference which two leads are swapped?

 

[charlie b]

Why would I have been taught not to reverse L1 and L3 if it doesn't make a difference?

That I do not know. All I can offer is that if you reverse any two, the motor will spin in the opposite direction. Perhaps it has something to do with physical positioning. If, for example, the three connections are in a straight line, then L1 and L2 are next to each other, and L2 and L3 are next to each other, but to swap L1 and L3 you have to move both sets of wires past L2. That might make the job a bit harder to do, and might make the final connections look a bit ugly. That is my best guess.

 

[Hv&Lv]

I have always picked the two easiest. Never really cared which one was L1, L2 or L3.

 

[augie47]

If thats the only bad teachin' you get, you will be way ahead in this game.
You've been here for 5 months. We're slipping if we have not given you more bad advice than the reversing teracher.

 

[masterinbama]

I look at it this way. Say you have a 480 volt motor with your leads marked brown-orange-yellow. when looking at them in the motor starter they are in sequence left to right. if you swap L1 and L3 the will be brown-orange-yellow in sequence right to left. It just makes it easier to tell at a glance that that particular motor needs to be running in the opposite direction from similar motors in your facilities.

 

[Besoeker]

That I do not know. All I can offer is that if you reverse any two, the motor will spin in the opposite direction. Perhaps it has something to do with physical positioning. If, for example, the three connections are in a straight line, then L1 and L2 are next to each other, and L2 and L3 are next to each other, but to swap L1 and L3 you have to move both sets of wires past L2. That might make the job a bit harder to do, and might make the final connections look a bit ugly. That is my best guess.

Another thought.
The motor terminal box, in my experience, has a cable/conductor entry point located symmetrically. That means that L2 will be located closest to that entry and L1 and L3 equally displaced from it..
That means that the conductors from entry to L1 and L3 would be equal to each other and longer than from entry to L2.
Swapping two conductors of equal length just might be easier and neater than two of unequal length.

Just a thought and not asserted with any degree of certainty but it may be what's behind the L1-L3 swap.

 

[rt66electric]

reversing tecniques.

reversing tecniques.

One of the best tricks to play one the new guy is to:: turn the fuses "upside" down to make the motor run backwards. Its easy -- oh no the motor is running backwards, pretend to be befuddled...... send the new guy for "parts" (a skyhook or something)...... quickly switch the leads.... when he gets back , act surpised to see that the fuses are upside down, Then, While he is watching - reverse the fuses and watch the motor spin.

 

[Stevareno]

If thats the only bad teachin' you get, you will be way ahead in this game.
You've been here for 5 months. We're slipping if we have not given you more bad advice than the reversing teracher.

Well, that bad(?) lesson was taught to me 20+ years ago, and I'm sure over the years I've passed the myth along. :ashamed1::angel:
Instead of being taught wrongly, I was hoping that maybe it had to do with a special situation (where reversing L1 and L3 doesn't have any effect) that someone here might have heard of.
That's the beauty of this forum. A vast wealth of experience. If no one here can think of something, I will be able to mark it off as just being mis-informed.
BTW, I was union schooled if that makes any difference. I've had my fair share of fetching left handed benders and wire stretchers.

 

[WIMaster]

In my part of the world it is standard practice to reverse the motor at the load side of the disconnect/starter.
It is also not unusual in my part of the world to encounter a grounded B system.
With the grounded b system you will have a white wire on B phase, so moving that wire to A or C would be a code violation.
Hence our standard practice of swapping A & C, 1&3.

 

[jumper]

Well, that bad

Possibly, they let me post here....

 

[jstagner]

Grounded B?

Grounded B?

It is also not unusual in my part of the world to encounter a grounded B system.

WIMaster: By a "grounded B", do you mean a delta secondary is grounded at the B phase, for the ground fault return path to the source?

Where I work, we have always swapped any two, with no preference.

 

[weressl]

Why would I have been taught not to reverse L1 and L3 if it doesn't make a difference which two leads are swapped?

Physical proximity. In terms of physics, any two wire will work.

 

[weressl]

I look at it this way. Say you have a 480 volt motor with your leads marked brown-orange-yellow. when looking at them in the motor starter they are in sequence left to right. if you swap L1 and L3 the will be brown-orange-yellow in sequence right to left. It just makes it easier to tell at a glance that that particular motor needs to be running in the opposite direction from similar motors in your facilities.

Doesn't the coloring supposed to mark the phase rotation, not the wire sequencing? Shouldn't one be changing the marking when swapping wires?

 

[weressl]

Well, that bad(?) lesson was taught to me 20+ years ago, and I'm sure over the years I've passed the myth along. :ashamed1::angel:
Instead of being taught wrongly, I was hoping that maybe it had to do with a special situation (where reversing L1 and L3 doesn't have any effect) that someone here might have heard of.
That's the beauty of this forum. A vast wealth of experience. If no one here can think of something, I will be able to mark it off as just being mis-informed.
BTW, I was union schooled if that makes any difference. I've had my fair share of fetching left handed benders and wire stretchers.

This what happens over the years when we do things the way we've always done them and the "why" is not passed along. (So nobody can tell you for shure WHY the 1/3 restriction was imposed, albeit for completely legitimate reasons.) Unfortunately even our own 'bible' the NEC is written that way, hence the handbook was developed, but even that can't answer all the misteries of electrical installations....:lol:

 

[Besoeker]

Physical proximity. In terms of physics, any two wire will work.

On the basis of physical proximity, wouldn't L1 be closer to L2 than L3

 

[weressl]

On the basis of physical proximity, wouldn't L1 be closer to L2 than L3

That WAS - or at least attempted to be - my point in response to the question: "Why would I have been taught not to reverse L1 and L3 if it doesn't make a difference which two leads are swapped?"

 

[kwired]

Instead of being taught wrongly, I was hoping that maybe it had to do with a special situation (where reversing L1 and L3 doesn't have any effect) that someone here might have heard of.

Reverse any leads as many times as you want on the supply side of a VFD and motor will always run same direction.:happyyes:

In my part of the world it is standard practice to reverse the motor at the load side of the disconnect/starter.
It is also not unusual in my part of the world to encounter a grounded B system.
With the grounded b system you will have a white wire on B phase, so moving that wire to A or C would be a code violation.
Hence our standard practice of swapping A & C, 1&3.

I know they commonly call it grounded B phase - reality of it is any phase could be grounded and it works the same. I don't know of any requirement to place the grounded conductor in any particular pole of a breaker or motor starter, but maybe there is one.

Doesn't the coloring supposed to mark the phase rotation, not the wire sequencing? Shouldn't one be changing the marking when swapping wires?

Exactly:thumbsup:

 

[WIMaster]

My point was to keep whichever grounded conductor that is required by code to be gray or white on the same terminals to remain code compliant as to color coding, one could also change any 2 phases at the motor to remain compliant but it is usually easier to do at the starter/disconnect.

There is mention in the code as to which phases go where,(left to right, front to back), I think it is in the panelboard section.

 

[kwired]

My point was to keep whichever grounded conductor that is required by code to be gray or white on the same terminals to remain code compliant as to color coding, one could also change any 2 phases at the motor to remain compliant but it is usually easier to do at the starter/disconnect.

There is mention in the code as to which phases go where,(left to right, front to back), I think it is in the panelboard section.

408.3(E).

It does not mention grounded phase systems. It does mention that B phase should be the phase that is higher voltage to ground on those particular systems. Existing equimpent can be an exception and equipment containing metering equipment can be an exception.