280V motor on 230V circuit



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).
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wrote:
| 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.
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snipped-for-privacy@ipal.net writes:

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.
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wrote:

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
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|
| wrote: |> |> | 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?
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----------------------------
wrote:

------------- 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.
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wrote:

Yep. Seen those types of units and was about to mention them. One model had a core that had a space in it much like a D'Arsonval meter movement. The space was filled with a 'bobbin' that when cross-ways left two large air-gaps and when aligned would neatly bring the gap between the two sides of the core. A weight and lever would turn the 'bobbin' into/outof the core to control the current.
Problem with those is, if you get a loose connection or arc, the unit will just keep pumping power to the system no matter what.
daestrom
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The only place I've seen those used was for regulating current in 6.6A (usually) series loop streetlighting. Lots of this still left in the Los Angeles area and a few other pockets but most is gone by now. It was very common from the 20s up through the 60s though, incandescent at first, but 6.6A matching transformer "ballasts" are available for HID lamps as well. Most airfield illumination is still 6.6A series, I suspect the modern control gear is solid state.
Westinghouse had a design where the secondary was on a linear mechanism with a counterweight and would float above the primary. Current was adjusted by moving the counterweight.
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| ----------------------------
| wrote: |>> |>> | 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. |> |> daestrom | ------------- | 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.
The thought is to change the core in some way. Maybe that can be done in a gradual way, as opposed to winding taps that have to be either BtM or MtB.
| 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.
I'm thinking more along the lines of a motor drive to move the coil, and that be controlled by the same authority that would have controlled the steppable taps.
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On 14 May 2008 01:35:23 GMT, snipped-for-privacy@ipal.net wrote:

Some years ago I worked at an Air Base in Northern Thailand. the airfield lighting was a constant current series circuit and used transformer as you describe - a movable core winding that was driven in and out of the outer windings by a motor controlled by a current sensing system.
I believe that street light systems are similar.
Bruce-in-Bangkok (correct Address is bpaige125atgmaildotcom)
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I've never seen one that used a motor, but then I haven't looked at a lot of them. I do have a Westinghouse manual for one of the old streetlighting constant current transformers if anyone is interested in the pdf.
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writes:

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
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writes:

You mean a secondary and a tetriary? The transformer for the hotel load of a 300 MW unit is powered directly from the turbo alternator (21 kV) and has a secondary of 6.6 kV and a tetriary of again 6.6 kV. This is done because it has wye-wye-wye connection (IIRC). The hotel load of such a unit is 10%, also 30 MW, including 7 brown coal mills. Typical size of a 6.6 kV motor is 1 MW.

Quite the same principle is done with diesel locomotives and is called diesel-electric transmission, and also in pure electric locomotives (E-Lok in german, for Elektrische Lokomotive). The diesel engine, 2-stroke and usually 600 to 900 rpm at full throttle, is coupled to a generator. The generator has small windings, connected in series for the last notch, higher voltage and relatively smaller current, and in parallel for start, higher amperage and smaller voltage. The traction motors are directly coupled on the wheel shaft, and are air cooled. An E-Lok has a trasformer, with the primary directly supplied by the cetenary, 15 kV 16 2/3 Hz in Germany, and 25 kV 50 Hz in Greece, The secondary uses the same principle. The typical size of a traction motor is 1 MW, 4 (one each shaft) and maximum voltage 700 volts, and are series wound motors with special construction to operate at 16 2/3 Hz (or 50 Hz with today's technology). Typical power of a diesel locomotive is 2850 HP, while an electric is 6000 HP. with 1500 HP at each shaft, also ~1MW. There is a heavy duty 12,000 HP diesel engine in USA(with 6 shafts, also 2000 HP at each shaft). The high speed ICE train (InterCityExpress) in germany is 13,000 HP, has a normal travelling speed of 200 km/h, 2 locomotives, 3-phase induction motors, electronic drive.

The one we have here operates with a motor.

I had no idea how it really works, but I got the general idea.>
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Tzortzakakis Dimitrios
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writes:

In US, diesel-electric used to always be DC machines, but modern ones are now AC generators with thyristers to regulate the power flow to the traction motors. Traction motors are still DC however to allow for their use in dynamic braking.
I suppose in Europe the better way to go would be regenerative braking, putting the braking power back into the overhead line, but that would need a static inverter. Probably the transformer secondary has a four-quadrant converter to allow reversal of power flow ??
Nice thing about the newer solid-state control systems (AC-Generator/ DC-Traction) is the ability to control wheel-slip. In the old days it took a skilled engineer (the train-driving kind) to get maximum power without slipping a lot (and wasting a lot of sand). Now modern units have speed sensors on each individual wheel set and control the power flow to individual traction motors. As soon as a wheel set starts to slip it can redirect power flow to other traction motors to prevent the slipping set from 'polishing the rail'. This prolongs life of the wheels and rail and actually improves the maximum tractive effort a locomotive can deliver. And when hauling 100+ cars of coal in a unit train up grade, tractive effort is what keeps you moving.
You forgot to mention that traction motors often have separately powered blower motors for air-cooling. This is because the motor may spend hours operating at low speeds and shaft-mounted cooling fans are not enough. The motor blower is usually mounted up inside the engine house and connects to the traction motor via a large flexible duct.
Some diesel-electric unitl have six axles and six traction motors. The trade-off is between how much power you can get to the traction motors and how much weight you can keep on the wheels to keep them from slipping. Sand is okay for starting and some special situations, but you can't carry enough to use it for an entire run. But of course too much weight and you need more axles to protect the rail from damage (depending on the size of the rail being used).
daestrom P.S. As you can see, I've seen a few railroad locomotives as well. Mostly just the older EMD's though, not GE's newer 'green' units.
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writes:

This is for sure in ICE, where they get 15 kV 16 2/3 Hz AC from the cetenary, and they convert it to 3 -phase AC for traction motors (3 phase induction), and they also use regenerative breaking.There's also the french TGV (Tren de Grand Vitesse) and the just new by Alstom (www.alstom.com) AGV (Autometrisse de Grand Vitesse). Classic E-Loks have regular breaking, and AC motors with series excitation, designed to work at 16 2/3 Hz. (Just like the ones you'll find in a drill, but much larger, at 1 MW or more). They are called universal motors, in the small scale, because they can work both in AC and DC. I'm wondering, how large their brushes are... In the 300 MW turbo generator, the brushes that suplly the excitation current, are as large as bricks. Newer type of turbo generators are brushless. The speed record for a classic E-Lok is held by Siemens' Taurus, IIRC 180 km/h with 12,000 HP.

I have no idea about train driving, but in Germany I got a local train from a small city to Mannheim, and the Lokfuehrer (train driver) was driving it like a race car... He accelerated fully to 130 km/h, and when he was close to the next stop, he braked fully, too. It had one E-Lok, and two cars. Also, the ICE starts like a race car. It's longer than 500 m, 12 cars, and I think it accelerates to 100 km/h in 10 seconds.

Yeah, right, and the transformer is cooled by active oil cooling (that means that the oil cools the trasformer, and there's a separate oil cooler. Like the intercooler used in the tanks where I served at army, but that's a differrent story).

But isn't a locomotive by itself heavy enough? Like 120 tons and above, with fuel and all? (Check at www.wartsila.com some large diesels). In our new power station, they have installed two 50 MW, 70,000 HP two-stroke diesels. To see how 2-stroke diesels work, look in www.howstuffworks.com.. The ships that travel from Iraklion to Piraeus (the harbour of Athens) the new ones, have 4 Wartsila 12 V 46 4-stroke diesels. 12 is number of cylinders, in V, and with a diameter of piston, 46 cm. When they travel normally at night, they fire up 2 engines. But, when they make a day trip, they fire up all 4 engines at full throttle, and the whole ship vibrates. A ship is the only place you can get free electricity. In my last trip, I saw young students plugging their laptops to the ship's receptacles. A free lunch, after all:-)

I have no idea what they are doing in continental Greece, they *should* have electrified all routes.
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Tzortzakakis Dimitrios
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<snip>
There is little doubt that electric trains are faster than other types as far as acceleration and overall speed. :-)
<snip>

I'm quite aware of how a 2-stroke works, as the large EMD's (654 series, up to V-20 cylinder) that have been around for years are exactly that. Also how the turbo-charger works, the four different lube-oil pumps (scavenging, piston-cooling, main, and soak-back). Not to mention the fuel injectors, overspeed trip, high-crankcase pressure shutdown, and air-start systems to name a few of the various components. And Westinghouse air brakes with several variations, and the MU (multi-unit) interface used to connect several locomotives together and allow them all to be 'driven' from one cab.
But the trouble with overall weight is the combination of weight, power and rail capacity. When you get to larger units, the rail used on a lot of roads can't handle more than about 50,000 lbm per wheel set. That means you're limited to about 100 tons for a unit with just 2 axles per truck (4 total). Go up to a 120 ton and you need 3 axles per truck. But a 100 ton, 4-axle unit has 12,500 lbm per axle, while a 120 ton, 6-axle unit has only 10,000 lbm per axle. If the wheel friction coefficients are the same, the 4-axle unit can develop 25% more tractive effort when starting before slipping wheels.
Of course if the 120 ton, 6-axle unit has more overall horsepower, then even though it develops less tractive effort at low speeds, it can achieve a higher speed when loaded to it's rated tractive effort. Below a certain speed, the maximum you can pull is dictated by wheel slip. Then you're limited by tractive motor cooling up to a second point. Beyond that, the overall horsepower becomes the limit. Once you're 'horsepower limited', you can go faster, but only if you can reduce the amount of tractive effort needed (i.e. you want to go faster, you have to pull fewer cars or not climb as steep a grade). This 'hp limited speed' is in the range of just 15 to 20 mph for a lot of 4-axle units, somewhat faster for 6-axle units.
With typical freight trains in the US, they look at the steepest grade on the road and figure out enough locomotive units and maximum cars to just be horsepower limited on that grade. So while the train may go faster on less steep sections or level grade, it'll be at notch 8 (full throttle) and struggling to make about 15 mph up the steepest part of the route. And stalled if one of the locomotive units dies.
So more hp means you may be able to pull it faster, but you can't always pull as much.
Kind of 'weird' until you work out a few problems, but that's how it works.
daestrom
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daestrom wrote:

lbm?
I'm not sure on your units.
In another life I used to calibrate railroad electronic weigh bridges.
4 axle locomotives were about 265,000 pounds (US).
6 axle locomotives were about 360,000 pounds.
One 3 axle drive truck weighed 65,000 pounds. (In a second other life, hauled it on a flatbed truck.)
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pounds (mass), lbm, as opposed to pounds (force), lbf, or lb.
It is necessary to distinguish between mass and force but they are both measured in pounds in the english system.
Metric is 'much simpler' with grams(mass) and newtons(force).
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bz 73 de N5BZ k

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bz+ snipped-for-privacy@ch100-5.chem.lsu.edu says...

The "English" system uses the "stone" as the measurement of mass. The pound ('lb') is the unit of *FORCE*.

Evidently *you* think the "English" system is too complicated. ;-)
--
Keith

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The 'Stone' is a unit of mass, not "The unit of mass"
All the engineering I ever learned in the British (Imperial) system used pounds.

No - the modern Metric system uses the kilogramme as its fundamental unit.
--
From KT24 - in "Leafy Surrey"

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