Additional Three Phase Power & Loading Questions

[Jezza]

Another member recently asked for a simplified explanation of 3 phase power. Here are my questions for further discussion:

-When loading a three phase system, I know it is especially important to keep the loads balanced across all three phases when using a generator. However, does one need to worry about balancing the load on each phase in a system that is drawing power "from the street"? It would seem that the electrical companies have a huge enough system that any type of unbalance, even on a 400amp service panel for our dimmers, wouldn't have much of an effect at all.

-Also, can some one please explain exactly what it means to be "out of phase?" I know it has to do with the sine or cosine curves of electricity originating at different times and thus not following the same waveform, but why is this important, how is it regulated, etc. Are there any good books out there that explain this type of electrical theory that you don't need a PH.D to understand? Thanks alot!

-Jeremy

 

[dbn]

When loading a three phase system, I know it is especially important to keep the loads balanced across all three phases when using a generator. However, does one need to worry about balancing the load on each phase in a system that is drawing power "from the street"?

The utilities don't like it when you draw unbalanced power from their system. Yes, it does tend to average out. The system design is for balanced consumption, though. Additionally, _some_ systems used to have what's called a "reduced neutral". Given that evenly distributed loads on a three phase system cause no current to flow in the neutral, it used to be acceptable to reduce the size of the neutral (to save copper) since it _almost_ never carries the full rated current of the circuit.

It is a bad idea to draw a substantially unbalanced load from a three phase circuit with a reduced neutral. The NEC (in the US) doesn't allow reduced neutrals anymore, in most applications, although the utilities sometimes still use them on the street. If fact, many three phase power circuits for solid state dimming systems use "double neutrals", to account for harmonic distortion. Harmonic distortion is a phenomenom related to the switching nature of SCR dimmer systems, coupled with the properties of power step-down transformers, that can cause _more_ than 100% of the rated capacity of the circuit to flow in the neutral under certain load conditions.

Are there any good books out there that explain this type of electrical theory that you don't need a PH.D to understand?

Sure. Pick up any college level text on AC power system design.

 

[Jezza]

Thanks a bunch Dave!

 

[PhantomD]

-When loading a three phase system, I know it is especially important to keep the loads balanced across all three phases

I am under the impression that keeping phases fairly equally loaded reduces hum in the audio system under certain conditions.

 

[Chris15]

I tend to agree with what has been said. Balancing a load is good practice regardless. I would tend to think that it would mean that you got even wear and tear on things like breakers, wiring, etc. which is always a good thing. The grid will be able to take a certain amount of imbalance, it has to be able to. When you connect a single phase load, unless you connect another load of exactly the same size to the other two phases, you have an imbalance.Consider this on a slightly larger scale. Many houses only have a single phase connection to the grid. So different houses get connected to different phases. Now there is no way that the electricity authority could make it so that each house had the same load, especially since loads come and go, so the network has to cope with some imbalance. There is however a difference between say 40 amps imbalance and 400 amps imbalance. The more out of balance the load is, the less likely the grid is to be able to handle it.

Recall also that the grid is broken up into many components. Now ignore most of them. Immediately before the electricity feeds into your main switchboard, it passes through a transformer, be it out on the street and shared with other customers, a padmount, or a larger substation. It is at this level that much of the load balancing is done. The very nature of a transformer means that its primary will always draw the same amount of power regardless of load, I think. (Based on the fact that it is just a coil and the resistance of said coil is relatively constant. This is also why sizing the distribution to the intended load is a good thing - otherwise you are pulling from the grid to make available power that is not getting used) (Those bits based on what would seem logical, so no guarantees that it is right.)

Bottom line, balance you load where you can, but the grid has a certain amount of tolerance and it generally more forgiving that a generator. (But a generator with a serious load on it would react much the same way as the grid I would think.)

 

[dbn]

The very nature of a transformer means that its primary will always draw the same amount of power regardless of load, I think.

Sorry, that's not correct. With transformer windings, as with all inductors, the inductive reactance must be considered in addition to any DC resistance. The back-EMF reflected from a lightly loaded transformer secondary to the primary winding will increase the inductive reactance of the primary, and thus the power consumed by a transformer is not independent of load.

 

[Chris15]

No need to be sorry. When I included the words "I think" I meant it to imply that I was guessing that what I said based on logic etc. and that I could be wrong and that please correct me if I was wrong. All it proves it that I'm not perfect and you learn something new everyday. So thankyou for expanding my knowledge.

And that does make sense. And now that I think about it, I remember all the physics of back EMF in electric motors and since transformers are based on the same electromagnetic principles, then it is only logical that transformers have back EMF that depends on the load.

 

[leistico]

-Also, can some one please explain exactly what it means to be "out of phase?" I know it has to do with the sine or cosine curves of electricity originating at different times and thus not following the same waveform, but why is this important, how is it regulated, etc. Are there any good books out there that explain this type of electrical theory that you don't need a PH.D to understand? Thanks alot!
-Jeremy

This one I "think" I can handle.

Bear in mind, I'm speaking of U.S.A. voltages and frequencies, but the three-phase theory is fairly universal beyond specifics.

Think of music--more specifically, a waltz. When you're counting or dancing to it, it's "1-2-3 1-2-3 1-2-3 1-2-3..." You have three different sine waves of electricity peaking at 170 volts either side of neutral (though fancy root-mean-square mathematics makes it equate to 117 volts AC--close enough to call 110 or 120, whichever you prefer). One waveform crosses the neutral (zero volts) on the "1" count, another on the "2", and the last one on the "3". In terms of electricity, though, you do sixty "1-2-3" counts every second--a bit hard to dance to.

If you think of a circle, in terms of going once around the circle every "1-2-3 1" count, 1 happens at 0 (or 360) degrees, 2 happens at 120 degrees, 3 happens at 240 degrees. That's what's meant by "out of phase"--the waveforms are 120 degrees out of phase with each other. In a household supply of power, there are typically 2 legs (2 hot wires) coming in, and a neutral, and the hot legs are 180 degrees out of phase, i.e. the waveforms are exact opposites of each other. Connect either leg and a neutral, you have 117 volts AC (a maximum difference of 117 volts of potential between the two wires), but connect between both legs and you have 234 volts AC (234 volts maximum difference, since one waveform is "high" when the other is "low").

Three-phase power has two possible ways of being hooked up--either "wye" or "delta". Wye, or simply "Y", is like the letter Y: the transformer feeds you three legs of 117 volts, all 120 degrees out of phase, and a neutral (from the "center" of the Y). Delta (the greek letter that looks like a triangle) is wired up triangularly--I won't profess to know much about Delta hookups, as wye is more common in my personal experience. I do know enough about Y to to say, however, that from one leg to neutral is 117 volts AC, and between two legs is 208 volts AC (hence some gear or motors or machinery being labelled "208/230 volts").

Also, of major importance especially on some gear and extra-especially on three phase motors, you have to have the 1, 2 and 3 in the right order, lest you hook a motor up and wonder suddenly why it's running backwards (the three phases set up spinning magnetic fields in one direction or the other, dependent upon order of hook-up) or why your equipment rack just went "poof" (as some circuits are phase-dependent to sync up and function properly).

I hope this makes sense to you, and gives you some answer to what you're asking. Anyone else, please do correct me if I'm wrong, as I only know what I know from reading and asking questions and cooperative work on various apparatus.

sean

 

[icewolf08]

-When loading a three phase system, I know it is especially important to keep the loads balanced across all three phases when using a generator. However, does one need to worry about balancing the load on each phase in a system that is drawing power "from the street"? It would seem that the electrical companies have a huge enough system that any type of unbalance, even on a 400amp service panel for our dimmers, wouldn't have much of an effect at all.

I don't think that I have to point out that generally it is important to make sure your loads are balanced across all three phases no matter what your situation as multiple people before me have said this. One thing I will add is that if you are working in a venue with installed dimmer racks the standard practice for installing racks is to distribute the dimmers across all three phases such that an entire drop box or raceway is not all on the same phase. For example, with an installed ETC Sensor Rack Dimmers 1+2 are on phase A, 3+4 on B, 5+6 on C, 7+8 on A and so on. this setup generally averages the load on each phase. When you get into using portable racks you need to figure out how the rack is wired to see if you need to distribute the load yourself or if it is setup like an installed rack.

 

[Chris15]

Slightly dormant thread I know, but someone linked to it in a more recent thread. For what it may be worth, as far as I know, portable dimmer rack wiring is dependent on manufacturer. For instance, Strand and Jands amongst others at least in 12 pack varieties here will wire circuits 1-4 on phase A, 5-8 on B & 9-12 on C. On the other hand LSC and possibly others go 1A 2B 3C 4A 5B 6C...

So it would pay to check you manual so you can do your load balancing.