Center Tap Transformer

Center tap is center tap

The voltages from one end to the other are 0,120,240 and the two voltages are in phase

It is your option what wire of the secoondary you want to connect to

a) Primary neutral

b) Earth

c) Nobody in particular (ie floating neutral)

The secondary voltages are relatively created and are in phase for all practical purposes.

For it to be a 2 phase line, there should be a relative phasor angle (usually 90^o) between them. here you don't have them, only the voltages. The two phases, wrt neutral are at 180^o phase shift, ie in simple terme you have +V and -V across the taps at any moment.

You should also note that power transformers have a Kva rating .

Generally input Kva = output Kva + Kva losses, the secondary windings of small transformers would probably be wound with the same Dia wire [ not guaranteed], with larger transformers the 240V leg would be wound with smaller Dia wire than the 110V leg, this saves cost of production. The amp output of the 240V leg would be half that of the 110V leg. Taking the center tap as common will work but the output amps would have to be limited on the 240V leg to half the Kva rating.

Garth,

??? A center tap is just that; a tap brought out at the midpoint of a single winding. Therefore, the winding is the same gauge of wire. If your connected load is exclusively L1-Center or L2-Center, then this load cannot be allowed to exceed the rated transformer secondary current. This means that such a load is usually only using one half of the transformer's capacity. A well-designed application will balance the load as much as possible so the current flow in L1 is close to that in L2. This can be by only using L1-L2 loads (ignoring the Center), or having nearly equal loads L1-Center and L2-Center, or any combination of these two. With a balanced load, the output power of the transformer is its full rated capacity.

--JMM

On thing missed here though, you will probably get two phases here. If the two voltages referenced to the neutral are 180° apart then you have two phases. Verify this with an oscilloscope. But don't forget, if you don't ground the neutral for at least this test, the grounded oscilloscope may give you a false reading.

Sir,

I respectfully and completely disagree with characterizing this as a two-phase output. If you have a dual-trace scope, connect it so one trace is L1-Center and the other is L1-L2. You will find that the waveforms are in-phase with each other and differ only by a factor of two in amplitude. That is a single-phase transformer output.

--JMM

Right

Phases 180 degree apart do not constitute a 2 phase circuit (in fact you do not have any phase-symmetric even phase circuits)

It depends on how the secondary is wired. Think of it this way, consider the secondary windings as two separate output windings with a total of four leads. (This is not the configuration of the problem so no complaints please.) Now using a battery driven oscilloscope with isolated inputs so that no scope lead is connected to the other or earth, connect each probe tip to the dot end of each secondary winding and the scope's return lead to the non-dot secondary lead (remember this is not a grounded configuration.) You will see that the two waveforms are in phase and with identical amplitudes (assuming perfect winding matching of course.) Now if one and only one of the scope probes switches connection with probe tip and return so that the return is connected to a dot and the probe tip is connected to non-dot end, this inverts the waveform for one trace on the scope. Viola, a waveform 180° out of phase, a second phase. This configuration is also called a split phase configuration.

Another, more common two phase waveform though has instead a 90° phase angle between the two waveform signals. If this waveform is desired then just a simple transformer will not suffice for single phase being transformed to this type of two phase power.

Two waveforms 180 degree out of phase is not a two phase (split phase yes)

IEEE definition

single-phase circuit: An alternating-current circuit consisting of two or three intentionally interrelated conductors that enter (or leave) a delimited region at two or three terminals of entry. If the circuit consists of two conductors, it is intended to be so energized that, in the steady state, the voltage between the two terminals of entry is an alternating voltage. If the circuit consists of three conductors, it is intended to be so energized that, in steady state, the alternating voltages between any two terminals of entry have the same period and are in phase or in phase opposition.

two-phase circuit: A polyphase circuit of three, four, or five distinct conductors intended to be so energized that in the steady state the alternating voltages between two selected pairs of terminals of entry, other than the neutral terminal when one exists, have the same periods, are equal in amplitude, and have a phase difference of 90 degrees. When the circuit consists of five conductors, but not otherwise, one of them is a neutral conductor.

NOTE—A two-phase circuit as defined here does not conform to the general pattern of polyphase circuits. Actually, a two-phase, four-wire or five-wire circuit could more properly be called a four-phase circuit, but the term two-phase is in common usage. A two-phase three-wire circuit is essentially a special case, as it does not conform to the general pattern of other polyphase circuits.

three-phase circuit: A three-phase circuit is a combination of circuits energized by alternating electromotive forces that differ in phase by one third of a cycle, that is, 120 degrees.

NOTE—In practice, the phases may vary several degrees from the specified angle.

six-phase circuit: A combination of circuits energized by alternating electromotive forces that differ in phase by one-sixth of a cycle, that is, 60 degrees.

NOTE—In practice, the phases may vary several degrees from the specified angle.

Cool, I knew that eventually if I harped long enough defending a common misconception, that instead of hearing just an unsubstantiated chorus of "no," somebody would actually site a source. Bravo sb and my apologies to you for escalating this as far as I did. The point I'm trying to make here is that to improve the quality of answers given here people have to start backing their statements whenever possible. People come to this forum to get guidance on technical matters. If you can't back it up, then no matter how amusing its just an opinion.

Again, Bravo and my apologies.

sir,

You say, This configuration is also called a split phase configuration. No, no, and no. I might allow "opposed phase". The phrase "split phase" is most commonly associated with windings in motors, which have a phase angle difference so that the motor has a direction of rotation imposed upon its shaft any time single-phase power is applied.

People who insist on calling a 3-wire single-phase circuit a "two phase circuit" are misusing the terminology. I agree with your observation about 'scope connections. However, getting the traces to be inverted with respect to each other does not mean you have split or shifted the phases.

Thanks--JMM

Approach this with a bit of caution. Back when the world was young, men were made of steel, ships were made of wood etc. it was common for one of the secondary leads of a transformer to be connected internally to the transformer case. Then when you bolted the transformer to a steel chasis, that lead became ground. so before you go and call the center lead neutral, or ground, or Fred or anything else, spend a few minutes with a volt ohm meter and make sure that none of the leads is connected to the transformer case.

From the OP's post he refers to the centre tap as common, thus this is a two phase transformer, regardless of the IEEE definition. The simple fact is that L1 & L2 are 180^o out of phase so they are two phases. There can be no other way to describe the situation.

This transformer appears to be for standard US domestic supply where low power 120V loads are taken from L1 or L2 to N, usually protected by a singe pole breaker and heavier loads (say >2kW) are 240V taken across L1 – L2 protectected with double pole breakers.

I suppose houses in the US are connected this way as we see this on US spec'd yachts.

Regards

Chas

Wiki:

A split phase electricity distribution system is a 3-wire single-phase distribution system, commonly used in North America for single-family residential and light commercial (up to about 100 kVA) applications. It is the AC equivalent of the original Edison 3-wire direct current system. Its primary advantage is that it saves conductor material over a single ended single phase system while only requiring single phase on the supply side of the distribution transformer. Since there are two live conductors in the system, it is sometimes incorrectly referred to as "two phase". To avoid confusion with split-phase motor start applications, it is appropriate to call this power distribution system a 3-wire, single-phase, mid-point neutral system

Also refer the link

http://www.allaboutcircuits.com/vol_2/chpt_10/1.html

where the 180^o out of phase is discussed. Go through the pages and there are lot of interesting electrical knowledge to be brushed up.

Hi SB,

I stand corrected, the definition as a split phase system is also valid. Your link to "All about circuits" I found a little strange as it does not have any provenance. No author name, commitee or society (or at least I couldn't find it).

regards

Chas