when reversing a 3-phase motor 2

[durf208]

When reversing rotation of a 3-phase motor. Does it matter wich 2 wires are reversed? I was told any 2. But after what Iv'e learned here and in books , it states A B C or
C B A. So if you reversed switching A&B OR C&B could this throw off the timing of the motor? I am new at this and when I was trained it wasn't by "professionals". So sorry in advanced if this was a dumb question. thanx

 

[durf208]

We have had trouble with motors running hot. That's why I ask.

 

[OperaHouse]

It's like the genetic code, except the same leters keep repeating. Put them in any order so they keep repeating and you will always be able to pick out an ABC or a CBA. Switch any two wires the result is the same. Motor overheating is generally the result of voltage imbalance. Just 5% imbalance will overheat a motor 25%. This is generally caused by a lot of single phase loads placed unevenly on a three phase line.

 

[corrosivo]

durf208

Your question is not "dumb" is a perfectly valid question, you were told right, any 2 phases can be exchanged to reverse rotation in a 3 ph. motor, no harm will be done to it, but be careful because the load it drives might not tolerate reversal of rotation like a centrifugal compressor as you know.
What is hot to you? A class H insulated motor can be very hot to the touch and it is running ok! Can you explain your last statement?

 

[SteveKW]

Have a look at this link...

http://www.franklin-electric.com/Manual/AIM_33.htm

 

[brupp]

Many motors should not be operated in reverse because their cooling fans only cool them properly when turned in one direction. When this is the case there should be a direction arrow stamped on the motor. Most smaller motors (for example almost all 460 volt motors) are bidirectional and their cooling fans are designed to cool them properly regardless of the direction of rotation.

 

[jraef]

If you don't understand what Franklin Electric was saying in SteveKW's link, let me point it out. In SOME 3 phase distribution sysyems in the US, the utility uses a transformer configuration called "open delta" where they only need to supply 2 of the 3 phases, and reference the 3rd phase to ground. This saves the utility the cost of the 3rd primary wire which can be significant in far-flung areas, but it is not necessarily good for the motor since the voltage is usually slightly unbalanced. Couple that with the naturally small amount of unbalance inherent in AC motor windings and you can get in trouble. If you "roll" the phases as Franklin suggests, you can often match the motor's unbalance with the line's unbalance, thereby achieving better performance.

How that applies to your situation is that in the process of reversing your motor, you may aggravate any system unbalance conditions that were corrected in the original phase sequencing, so your motors could conceivably run hotter in reverse. This however would only apply to being hooked up to an open delta utility source. For any other service connection, OperaHouse is absolutely correct. And as far as I know, other countries do not use open delta configurations, however maybe some other members can shed light on that.

corrosivo has a good point as well. How hot is hot? It's all relative. Many people assume that if the frame is too hot to leave your hand on it, it is in trouble. That's a poor test method, because most insulation designs will allow temperatures far in excess of what humans can tolerate.

Unless you are considering very large specially made medium voltage motors, I would not worry about what brupp mentioned. 99.999999% of all 3 phase motors are designed to cool themselves in either direction. The arrow on the case is often there just to show the normal direction of rotation when A-B-C phase rotation is applied or is applied after installation to indicate proper rotation for future service personnel.

"Venditori de oleum-vipera non vigere excordis populi"

 

[durf208]

thanks guys for your help,I just needed to know the phase sequence didnt matter. I didnt think it did, but ive bought alot of snake oil from my "trainers".Not their fault though, they didnt know what they were selling. As for the motors running hot,we're running an unbalanced voltage 472,479&481 I just wanted make sure sequence didnt matter before I evened them out. Thought it would be easier to track the three legs while they were out of balance. thakx jraef(your a mind reader)I wasnt sure which system they were talking about.

p.s.
this will be the third time I've tried to post this hopefully this time it will go through.

 

[mc5w]

That is not a whole lot of voltage imbalance - about 2% which is actually very good.

It might just be that the motors are heavily loaded right below the nameplate rating or even pushing service factor amps. Do an amp check or otherwise check the loading.

I have encountered a few machines that required a larger motor such as an alledgedly 3 horsepower pump that was really 3.5 horsepower. Somebody at the manufacturer ( Dunham-Bush ) thought that they could drop a decimal point and get away with it. More details about the problem in another thread.

Mike Cole, mc5w@earthlink.net