Why do out-of-phase power connections matter for sensitive equipment?

[schmitgreg]

Hello, I am an Electrical Engineer for the Marine Corps, and I ran into an interesting situation that I honestly do not fully understand.

I was setting up a small power grid for another shop recently to operate 4 systems, using "house power," which means we are uusing power from the building, standard 3 phase power L1, L2, L3, and grounded neutral. We have it set up to have a 300-amp cable going from the load studs on the building power box going to a 100kw "turtle" (Basically, a circuit breaker box), which then uses 4 60amp cables running from the turtle to the gear.

I ran into a problem because on the gear itself there is a phased indicator lamp which according to the manual 'will illuminate when phases A, B, and C are in phase, and will not illuminate when they are not in phase.' These lamps were not illuminating and was interrupting power from actually going to the gear even if the main cb was closed (I am assuming it is a protective device to prevent the gear from being damaged by voltage spikes, because it is sensitive intel/comm gear.).

After I looked at the wiring (another engineer had set it up so I had to check it over, I wanted to see if i understood the problem) So at the turtle, I simply switched phases B and C (to my knowledge, you can switch any 2 phases and it will put it back in-phase) and voila, the indicators illuminated and the gear worked fine. So I knew that somewhere the wiring was messed up. So I switched back B and C on the turtle, and checked all the wiring. Sure enough, on the 300amp cable running from House Power to the 100kw turtle, the guy who had initially attached the cable had attached it wrong. They had it installed A-B-C Black-White-Red on the House power, but A-B-C Red-White-Black on the turtle; phases A and C were flipped. I switched it up and it worked fine.

So, finally to my question. I don't understand why it mattered if the gear was hooked up backwards to the House Power because each load stud was still putting out 120V 60Hz. Why does this put the power "out-of-phase." If anyone can help me understand this, that would be great!

thanks!

 

[phyzguy]

I think the answer is that three phase induction motors use the three phases to create a rotating magnetic field. This requires that the peak voltage of each phase follow a specific sequence (i.e. A-B-C-A-B-C...) If the order is messed up (for example A-C-B-A-C-B...) then you won't get a properly rotating magnetic field and the motor won't work properly. Your protection circuit was probably checking for this so that any three phase induction motors would not be damaged. If you are not using three phase induction motors, but just powering single phase motors, computers, ovens, etc, then it probably doesn't matter. I think this is the answer - anybody else want to weigh in?

 

[schmitgreg]

Yeah, that makes sense. And I believe when the manual says "in-phase" it means too say "properly phased where the sequence goes A B C. I remember in our electric motors claass if you switch any 2 phases it would make the motor operate backwards, and then you just switch any 2 phases back to make it operate properly again.

Thanks for the help!

 

[stewartcs]

There must be some type of check specific to the "gear" your connecting the power to because 3-phase power is always out of phase by 120 electrical degrees! Hence the 3-phase part.

The motor analogy is a very good example of why it is checking. If only two phases are available to a 3-phase motor, the motor can become damaged (if running already). The phase "checker" device most likely serves as a safety device to prevent damage to the load.

CS

 

[ThomasEdison]

In the USA:
House power is generally a single phase 120/240 service. ( This means two phases are present.. oddly enough.) So I would not call 3-phase power, even if the individual circuits were 120v, "House Power." I suppose you could say it was like house power.. because of the 120v part but if you are speaking about all phases then no (one has 2 the other has 3.)

Transmission lines and commercial buldings and industrial buildings are 3 phase; "house power" is single phase.

In a commercial or industrial building you generally see three phase services which once transformed are most commonly sent to 120/208 or 277/480 panels. There is also sometimes a service with delta high leg 240v (but not common.) It all depends on the transformer types used in distribution onsite.

In residential:
You might see three cables entering your weatherhead or from an underground service into your residential home but chances are good that the three cables you see are Phase A, Phase B and a neutral. If you open up your electrical panel and look inside you will only see two buses one for "A" and one for "B" be extremely carefull-- and don't touch anything.

The two phases together are often referred to as "single phase power." Don't ask why 2=1phase it just is.


There are special cases where some (wealthy) homes have three phase and often large condos or large apartments have three phase but it is not standard to have three phase power in a typical family dwelling house.

A 120volt circuit on a commercial sight by itself behaves the same as a "house voltage" 120v circuit from residential but you will have 3 different 120 circuits in a commercial building out of phase with each other and only 2 different in a residential home so calling them both "house voltage" may lead to confusion when talking about phases.

 

[schmitgreg]

@CS
yeah, I shouldve known that from the start because they taught us that at school and showed us how reversing 2 phases changes the polarity to make an electric induction motor run backwards, and can mess up other gear so that's probably why that fault indicator/device is there to protect the equipment.

Thanks!

@thomasedison
sorry about not being clear on our terminology. When we say "house power" it's actually coming from a commercial building, but we differentiate it from gen power when we use our generators. Yeah, it's three phase power L1, L2, L3, G and N. We have no gear that operates under single phase power.

Thanks!

 

[ThomasEdison]

@CS
yeah, I shouldve known that from the start because they taught us that at school and showed us how reversing 2 phases changes the polarity to make an electric induction motor run backwards, and can mess up other gear so that's probably why that fault indicator/device is there to protect the equipment.

Thanks!

@thomasedison
sorry about not being clear on our terminology. When we say "house power" it's actually coming from a commercial building, but we differentiate it from gen power when we use our generators. Yeah, it's three phase power L1, L2, L3, G and N. We have no gear that operates under single phase power.

Thanks!

Kind of odd that your motors require a nuetral. Generally, the three phases in a motor are a balanced load. The purpose of a nuetral when running multiple phases to one load is often to carry back the imbalance between the two. Sometimes for extra safety measures there will be a nuetral present too (extra safe I guess but not neccesary since if you metered that nuetral it might simply be carrying no current.)

In cases where one phase separates out a tiny bit to power up an instrumentation panel I can understand the purpose of a 4-wire system (What you call ground I call equipment ground or even mechanical ground and don't mention it as a number in a four wire system.) One phase of a four wire can sort of split off from the rest to be treated like an individual 120v or 277 to run control panels or lights for the motor. In that case that phase will not be completely balanced with the rest and that imbalance gets taken back with the nuetral.


Yes there is an amazing amount of trade speak with electricity. I have been an electrician for 12 years now and I still hear new "trade" words for how tools and materials are described even today. Outside Linemen talk about electricity different as well and so do engineers. Even electricians from other regions within the USA use different words for tools and material and some different terminology than each other. I try to mostly use terminology from the code book that way inspectors understand, since that is what they are supposed to use too.

 

[stewartcs]

In the USA:
House power is generally a single phase 120/240 service. ( This means two phases are present.. oddly enough.) So I would not call 3-phase power, even if the individual circuits were 120v, "House Power." I suppose you could say it was like house power.. because of the 120v part but if you are speaking about all phases then no (one has 2 the other has 3.)

Transmission lines and commercial buldings and industrial buildings are 3 phase; "house power" is single phase.

In a commercial or industrial building you generally see three phase services which once transformed are most commonly sent to 120/208 or 277/480 panels. There is also sometimes a service with delta high leg 240v (but not common.) It all depends on the transformer types used in distribution onsite.

In residential:
You might see three cables entering your weatherhead or from an underground service into your residential home but chances are good that the three cables you see are Phase A, Phase B and a neutral. If you open up your electrical panel and look inside you will only see two buses one for "A" and one for "B" be extremely carefull-- and don't touch anything.

The two phases together are often referred to as "single phase power." Don't ask why 2=1phase it just is.


There are special cases where some (wealthy) homes have three phase and often large condos or large apartments have three phase but it is not standard to have three phase power in a typical family dwelling house.

A 120volt circuit on a commercial sight by itself behaves the same as a "house voltage" 120v circuit from residential but you will have 3 different 120 circuits in a commercial building out of phase with each other and only 2 different in a residential home so calling them both "house voltage" may lead to confusion when talking about phases.

In US residential homes it's actually called split-phase. The pole or pad transformer has a center tap neutral that allows 120V from line to neutral on one leg and 120V from line to neutral on the other leg. Due to the center tap, they are 180 degrees out of phase. Thus, if line to line is connected one gets 240V.

It's still one phase power coming in but you can combine it in the way I just described to get a split-phase that gives a higher potential when referenced between the two legs instead of leg to neutral.

CS

 

[stewartcs]

@CS
yeah, I shouldve known that from the start because they taught us that at school and showed us how reversing 2 phases changes the polarity to make an electric induction motor run backwards, and can mess up other gear so that's probably why that fault indicator/device is there to protect the equipment.

Thanks!

@thomasedison
sorry about not being clear on our terminology. When we say "house power" it's actually coming from a commercial building, but we differentiate it from gen power when we use our generators. Yeah, it's three phase power L1, L2, L3, G and N. We have no gear that operates under single phase power.

Thanks!

I was actually referring to a 3-phase motor that loses one phase and starts single-phasing (i.e. running on only two phases). If it is already running it will continue to run but at a derated capacity. The motor will see a sharp increase in temp and most likely will become damaged if allowed to continue to run. Hence my comment about the safety cut-out.

CS

 

[schmitgreg]

@ThomasEdison
No, we aren't setting up motors, I was just using it as an example of what improper wiring can do to certain equipment. When we provide power to their equipment, we don't know what phases they are using, they probably don't even know that. But to set it up, all three phases are available to them including a grounded neutral line (I say grounded neutral because they are two separate wires but are the same electrically because of a bar that connects the two at the breaker box. I'm not sure why this is, but i believe it is to send excess neutral to ground instead of the operator (me). this ground is also connected to the external grounding system set up with all of the equipment (usually, six grounding rods set up in a circle around the equipment, all attached to a central grounding rod). But like I said, their gear might only be using L1-N, L2-N, and L3-N for different equipment, or it might be running a three phase motor, or anything really, I just make all three phases and N available to the equipment. Rarely do I have to wire it specifically for certain gear, although there was one time when we had to make three separate power grids and wire each grid to only one phase of the power available and the neutral line.

One of the main things I have to pay attention to is there needs to be load balancing between each phase, ESPECIALLY when using gen power because if all of their gear was running from L1-N, that would cause the generator's rotating armature to wobble inside the stator. If there is too much wobble, I've seen damage caused when the stator actually rubbed against the stator windings; huge arcs, turn-to-turn shorts inside the stator windings, and damage I couldn't even explain.

 

[denni89627]

I use phase protection relay's all the time in my line of work. (Elevator mechanic). Everybody is correct that your manual would be better written if it said "in proper phase relation" rather than "in phase". If my elevator motors lose a phase they begin to sound like a sci-fi movie UFO sound effect, and shortly after self-destruct. This is why we use reverse phase relay's at our controllers. Many smaller 3 phase motors in a building (HVAC, fans etc,) are not protected, which is where your turtle steps in.