280V motor on 230V circuit

You're confusing two uses of the term 'regulation'. Tap changers and voltage regulators actively sense the terminal voltage and adjust 'something' to maintain the voltage within some design limit. That's a 'regulator' and provides 'regulation' of the sensed voltage.

But 'regulation' also is a term used to describe the inherent voltage drop in some devices. For example, if you review DC generators, you'll find that simple shunt-wound generators have fairly good 'regulation' and their output voltage only drops a few percent from no-load to full-load when supplied with a fixed field. A cumulatively-compound DC generator (which has a series field and a shunt field), can have a nearly flat voltage curve from no-load to full-load with just a fixed shunt excitation, or even have a voltage rise depending on the degree of compounding. (of course, an active voltage regulator can counteract whatever inherent regulation a machine may have)

In the case of simple fixed-tap transformers, the term 'regulation' can be used to describe how much the output terminal voltage changes from no-load to full-load if the primary voltage is held constant. This use is less than perfect as it is much better to use the transformer's impedance along with the load's power factor to get a more precise answer.

In the US, voltage regulation is accomplished with load-tap-changers, capacitor banks, and other 'voltage support services'. But just like in Europe, it is done at the substation or higher level and not done at the typical distribution transformer. There are exceptions for rural areas though where the line length of the primary leads to some issues.

daestrom P.S. In the US, a 'tap-changer' may be built for either for unloaded or loaded operation. The 'unloaded' type can not be stepped to another tap while there is load on the unit (although it can still be energized). It's switch contacts cannot interrupt load though, so if you try to move it while loaded, you can burn up the tap-changer. The classic 'load-tap-changer' is actually several switches that are controlled in a precise sequence to shift the load from one tap of the transformer to another while not interrupting the load current.

P.P.S. Load tap changers typically have a significant time-delay built into the controls so they do not 'hunt' or respond to short drops in voltage such as starting a large load. 15 seconds to several minutes is typical. So even with load-tap-changers, starting a single load that is a high percentage of the system capacity will *still* result in a voltage dip.

Sorry, should be '...contribute to whatever inherent regulation a machine may have'

daestrom

? "daestrom" ?????? ??? ?????? news:482725ae$0$30509$ snipped-for-privacy@roadrunner.com...

I know that, but it was a temptation to post this:-)

A shame that Tesla won the infamous "battle" and we don't have DC:-() But then, we would be having a power plant at each neighborhood, instead of the

300 MW ones.

We have here capacitor banks, too, connected at the LV side of the substation, 15 kV line-to-line voltage. But just like in

Yeah, the ones we have here are automatic, live and even have a shaft for manual control.

I know, I know, my answer was a bit provocative:-) And of course there are DC regulators.... You're talking about DC generators;the one a 300 MW uses for excitation is 220 V, 1000 A DC and probably shunt field. I have seen here in some machine shops the old type welding generator, which is a 3 phase induction motor coupled to (usually) a compound field DC generator, which provides the welding current. The modern ones are, maybe, not larger than a shoe box and powered by a higher wattage 230 V 16 A receptacle. (Usual receptacles are 230 V 10 A;16 A for washing machines, dryers and the like).

Are the load tap generators configured make-before-break? Break-before-make would mean a (very short) power outage every activation but make-before-break would mean a momentarily short-circuited winding and the break would involve interrupting a large short circuit current.

Certainly modern ones likely use thyristors and zero crossing detectors.

When I was a kid living in a rather rural area, there would be a pair of these on poles every few miles, connected open delta. (all transformer primaries were connected phase-phase then).

| A shame that Tesla won the infamous "battle" and we don't have DC:-() But | then, we would be having a power plant at each neighborhood, instead of the | 300 MW ones.

And the latter make easy terrorism targets, too.

| I know, I know, my answer was a bit provocative:-) And of course there are | DC regulators.... You're talking about DC generators;the one a 300 MW uses | for excitation is 220 V, 1000 A DC and probably shunt field. I have seen | here in some machine shops the old type welding generator, which is a 3 | phase induction motor coupled to (usually) a compound field DC generator, | which provides the welding current. The modern ones are, maybe, not larger | than a shoe box and powered by a higher wattage 230 V 16 A receptacle. | (Usual receptacles are 230 V 10 A;16 A for washing machines, dryers and the | like).

You don't use 400 V for anything heavy duty like an oven?

And so does that 20 gallons of gasoline parked in front of your house. And that 500 gallons of diesel fuel in your basment. And that 20,000 or so gallons in the nearby gas station.

Yawn.

| Are the load tap generators configured make-before-break? | Break-before-make would mean a (very short) power outage every activation | but make-before-break would mean a momentarily short-circuited winding and | the break would involve interrupting a large short circuit current.

I wonder how much regulation could be managed through the use of variable leakage inductance in the transformer windings.

| Certainly modern ones likely use thyristors and zero crossing detectors.

With zero crossing detection, then the switching is not happening on all phases at the same time.

In North America, 240V 50A is pretty standard for ovens, some are 40A, clothes dryers are 30A, most other stuff plugs into a 15A 120V receptacle.

Good question.

Since the ones I've seen are 3 (or 2) independent autotransformers, this is true without zero crossing detectors, and the power supplied may not always be of equal voltages 120 degrees apart.

I figured someone would 'bite' :-)

Typical large power load-tap-changers have a primary winding and two secondaries. One secondary produces about 100% of 'rated' secondary voltage. The second secondary produces about 15% to 20% of the rated voltage, but has numerous taps from end to end, about 2.5% 'steps'. (for a total of about eight taps). The cental tap of the boost/buck winding is tied to one end of the main secondary. The boost/buck can be used to step from 90% to 110% of the 'design' output. I suppose some can step over a wider range, but I haven't run across them.

*TWO* rotary switches have each tap tied to one of the positions of each rotory switch, and each 'wiper' is tied to single heavier contacts that are opened in the operating sequence. The output side of these two interrupting contacts are tied to each end of a large center-tapped inductor.

So, normally both rotary switches are aligned to the same transformer tap, both interrupting contacts are shut, and load current flows from the boost/buck winding tap, splits and flows through both rotary switches, both interrupting contacts, enters both ends of the inductor and out the inductor center tap. Because the current flows into both ends of the inductor and the mutual inductance of the two parts cancel, there is little voltage drop in the inductor.

Begin step sequence:

1) Open one interrupting contactor. Now load current doubles through half the inductor and is zero in the other half, so the voltage drop across the inductor actually makes output voltage drop, even if trying to step 'up'. 2) Move associated rotary switch to next step of transformer bank. 3) Close interrupting contactor. Now, the two rotary switches are across different taps. The inductor prevents a excessive current, otherwise you have a direct short of the two winding taps. Some tap changers can stop at this point and are called 'half-step' units. Obviously, the inductor has to be rated for sustained operation across a step of the boost/buck winding plus load current in order to survive sustained 'half step' operation. 4) But for tap changers that can't operate 'half-step', the sequence continues. And opens the other interrupting contactor. Now the other half of the inductor has full load current. 5) Move second rotary switch to next step (now both switches are on the new step) 6) Close the second interrupting contactor. You're back in the initial configuration, but with both rotary switches on a new transformer tap.

Older units do this whole thing with a fancy cam/gear arrangement circa

1940's. Just takes a single reversable motor to drive the unit and some limit switches to be sure it can only stop at full 'steps' (or 'half steps' for those capable of running 'half-step')

Because the system intermittently inserts an additional voltage drop through the inductor, the control circuits typically have time-delays that prevent it trying to reverse direction or something while stepping.

As far as zero-crossing and thyristors, I suppose it's certainly possible, but I haven't run across them for large substations. I have seen such a setup in power-conditioners for computer complexes and such, but that's only a few kVA (one unit I know of was rated for 25 kVA).

The mechanical-switch tap changer is well-matured and has the nice advantage that when they 'fail', they 'fail' at the last 'step' and power continues to flow (albeit perhaps the wrong voltage).

Those are smaller than the units I'm thinking of. I'm talking about multiple MVA rated units.

daestrom

I suppose you could, but increasing leakage inductance means you're increasing losses aren't you? Just a percent or two on a unit rated for 250 MVA can be too much to tolerate.

daestrom

Thing about DC generators used for welding, they are often *differentially* compounded, whereas those used for conventional power production are

*cummulatively* compounded.

For power production, the series winding is arranged so that additional load will add MMF to the shunt field and help to compensate for the various internal factors causing a voltage drop.

But for welding, you don't want a constant voltage so much as a constant current. By using a differentially connected series winding, any increase in current in the series winding opposes the shunt winding, rapidly dropping the terminal voltage. So with no arc, the shunt winding gives you a nice, fairly high voltage to strike an arc, and as soon as you do, the voltage drops to whatever level is needed to maintain a specific current. Tap settings allow the welder to adjust what amount of current he gets so he can adjust for different welding. Often the 'course' adjustments are done with different taps to the series winding, and a final 'fine' adjustment is done with a lower-wattage rheostat controlling the exact amount of shunt-field current.

DC machines are often under-appreciated :-) (not to mention that some of this technology is older than either one of us and probably older than both of us put together)

daestrom

Some parts of Europe do. You find ovens can be strapped to run from one or two phases, depending what's available on the premises. Some parts of Europe use 3-phase 400V domestic water heaters.

Continental Europe used to have 220 volts (before that it was 127 volts in some places), the UK used to have 240 volts. Nowadays, the common voltage is 230 volts -10% +6%.

In alt.engineering.electrical James Sweet wrote: | | snipped-for-privacy@ipal.net wrote: |> In alt.engineering.electrical Tzortzakakis Dimitrios wrote: |> |> | A shame that Tesla won the infamous "battle" and we don't have DC:-() But |> | then, we would be having a power plant at each neighborhood, instead of the |> | 300 MW ones. |> |> And the latter make easy terrorism targets, too. |> |> |> | I know, I know, my answer was a bit provocative:-) And of course there are |> | DC regulators.... You're talking about DC generators;the one a 300 MW uses |> | for excitation is 220 V, 1000 A DC and probably shunt field. I have seen |> | here in some machine shops the old type welding generator, which is a 3 |> | phase induction motor coupled to (usually) a compound field DC generator, |> | which provides the welding current. The modern ones are, maybe, not larger |> | than a shoe box and powered by a higher wattage 230 V 16 A receptacle. |> | (Usual receptacles are 230 V 10 A;16 A for washing machines, dryers and the |> | like). |> |> You don't use 400 V for anything heavy duty like an oven? |> | | | In North America, 240V 50A is pretty standard for ovens, some are 40A, | clothes dryers are 30A, most other stuff plugs into a 15A 120V receptacle.

But we don't have an easy option for any higher voltage. In many parts of Europe, three phase 400/230V is delivered to homes. Then using 400V, either

2 lines or all 3 lines, is an option.

|> | Are the load tap generators configured make-before-break? |> | Break-before-make would mean a (very short) power outage every |> activation |> | but make-before-break would mean a momentarily short-circuited winding |> and |> | the break would involve interrupting a large short circuit current. |>

|> I wonder how much regulation could be managed through the use of variable |> leakage inductance in the transformer windings. |>

| | I suppose you could, but increasing leakage inductance means you're | increasing losses aren't you? Just a percent or two on a unit rated for 250 | MVA can be too much to tolerate.

Isn't it just inductance in series? Shouldn't that just be a phase shift as seen from the primary side?

Yes -you are shorting a part of the winding but the switching is a bit more complex than that so that short circuit currents are limited to reasonable values. It is a multistep operation with reactor switching. On-load tap changers are expensive and are generally limited to applications where this is absolutely needed (I have seen one where the tap changer was nearly as large as the transformer).

--------------

Possibly but probably not- I am out of date on this but I would expect that the old way of good switches plus reactors might still be the better way. It saves a lot of control wiring plus a lot of money to operate thyristors at

300KV and 500A or more and I doubt whether they would be cost effective or technically advantageous otherwise. --------------------------

"on load tap changers"? Not likely. These were applied to transformers only where it was worth the effort. Definitely transformers in rural areas- typical pole pigs- would have to be de-energized as the tap changer is a manually operated switch inside the tank. Some larger transformers did have off-load but live changers operated from ground level. What you saw could have been somethng else altogether. Delta primaries as you indicate were around when you were a kid, would, in most areas mean that you are now a pensioner. I remember cases of conversion from delta to star for distribution primaries in small towns being done about 60 years ago and use of delta for transmission died much before that.

------------- I don't see changing leakage inductance having much effect on losses ( a great effect on voltage regulation -likely all to the bad) but the problem is one of changing leakage inductance. Does this mean changing a gap in the core? Does it mean moving one winding with respect to another? In any case it does mean some fiddling with the core or winding. This has been done for series lighting circuits where the load current was kept constant by using a transformer which balanced the forces between coils against a fixed weight. If the current changed the secondary coil moved so that there was more or less leakage. The units that I have seen were rather cumbersome.