280V motor on 230V circuit



Thanks for your (and esp. daestrom's) explanation on how they work.

I'm not completely sure what these are other than being told that they were voltage regulators (tapped autotransformers) long ago. These are large cans with 3 bushings on top, taller and slimmer than most pole pigs, and they usually have a control box on the pole around eye level. I see the same style cans in substations between the stepdown transformer and the distribution system except they sit on the ground and come in sets of three.

While I'm hardly a kid, I'm no pensioner yet. In fact my father's place still has delta-connected distribution primaries in the area, at 7200 volts (I have an old fuse/switch holder from there labelled 7200V ??A).
Where I mentioned they had pairs of these "voltage regulators" (or whatever they were) every several miles was a long run along a state highway. At some point they upgraded it to a wye configuration, probably at a higher voltage. However, several side branches haven't been upgraded yet. On the side branch feeding my father's place there is a bank of 3 transformers connected wye-delta immediately followed by a pair of these "voltage regulator" cans connected open delta. From that point on the distribution system is visibly old.
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| 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).
I was thinking of what I might do to get some fine voltage control within a very limited range around 120 volts. The obvious option was a 0-140 volt variable transformer. But I wanted to make sure I had a setup that could be better limited, for example, to not allow an accidental too low voltage. I also didn't want to run all the power through the variable. So what I was going to do was get a smaller variable transformer, and two buck-boost transformers. One transformer would be wired 120->16 in buck mode to drop the voltage down to 104. The other transformer would be wired 120->24 and supplied via the 0-140 variable transformer, giving me a 0-28 variable boost. The end result is 104-132 over the full range of variable transformer control (assuming the boost transformer has no issues with being overfed at 140V).
So I might envision a transformer where the taps can be part of a boost transformer added to the main transformer. The first buck transformer in my above example would not be needed because the main transformer would be designed with a 1st secondary at the lowest voltage of the adjustable range. A 2nd secondary on the same main transformer would have the adjustable taps and it would feed a separate boost transformer which has a secondary wired in series with the 1st secondary of the main. So the taps would only be dealing directly with a fraction of the power (assuming there is no back feed issue involved) based on the needed adjustment range.
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| If I read you correctly, you want to use a second secondary (lower power | rating) which is tapped and put in series with the main secondary. Now once | you do this, you have in effect a single secondary with taps just as in a | conventional tapped secondary. Sure the "tapped section" is lower power- | because it is a lower voltage but it still has to handle the same current. | Nothing is gained. | The problem in tap changing is not "power" but the current being switched.
No, that is not what I tried to explain. I'll try again:
The main transformer would have 2 secondaries. These 2 secondaries are NOT wired in series with each other. The smaller of these secondaries will have taps. The tapped smaller secondary feeds another smaller transformer. The larger secondary of the main transformer, and the only secondary of the smaller auxiliary transformer, would be wired in series. So the taps are only dealing with the current of the lower power "tapping section". The smaller secondary of the main transformer, and the primary of the auxiliary transformer, can be wired for whatever voltage/current works out best.
| In either case the voltage driving short circuit current on tap changing is | that between taps | Delta V =A(delta n) Delta Z =B(delta n)^2. where delta n is the change in | turns between taps. The short circuit current on such a change will be | proportional to 1/(delta n). | | If you want fine control, then you could go to sliding carbon brush as in a | variac. The first idea of a separate transformer feeding a variac will not | solve the "too low" voltage problem of the variac because you are still | dealing with an autotransformer.
In that first scheme, adjusting the variac to the lowest voltage would be reducing the voltage contributed by the boost transformer. There is still the original supply voltage going around the variac, "plus" (actually minus) the buck voltage (to select the range I want). Since the variac is an autotransformer itself, it merely feeds the primary of the boost transformer. Note that in this case the "boost" transformer is wired as an isolation transformer. I should have mentioned that. If needed, I guess I could draw some ASCII diagrams or try to get something made graphically (all the tools I have to do that suck, except for Visio which needs Windows to run and I don't have a spare machine to do that at the moment).
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wrote:

--------------------------------- Actually I see added complexity without any gain. You may be doing the tap changing at a lower current and higher voltage but there will be no "lower Power" switching but there will be more losses during operation even when not changing taps. I suspect the complexity and the losses together would cost more than a conventional tap changer. There are some circuit factors involved which may be undesirable but I haven't done a proper analysis.
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| 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).
What about multiple parallel transformers, or at least multiple parallel windings on the same core (on whichever side the tapping is to be done), where the taps are stepped incrementally on each winding? Instead of a shorted winding segment, you'd have windings of differing voltage in parallel as each of the windings change their taps one at a time.
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wrote:

So when one is set for say 118V and the other is set for 120V, you have a 118V source connected in parallel with a 120V source and the only impedance is the transformer windings??
OUCH!!! I think the magic smoke will be spewing in no time
daestrom
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That's still essentially a shorted turn (or set of turns).

Phil, did you see daestrom's excellent explanation how they use an inductor to prevent a dead short but in a way such that the inductor is virtually not there during normal operation (counterflowing currents)?
If these tap changers are rather expensive, I'm wondering what those pole pig "voltage regulators" I mentioned are. I thought they were just tapped autotransformers.
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wrote:
| Phil, did you see daestrom's excellent explanation how they use an | inductor to prevent a dead short but in a way such that the inductor is | virtually not there during normal operation (counterflowing currents)?
I believe I missed that.
| If these tap changers are rather expensive, I'm wondering what those | pole pig "voltage regulators" I mentioned are. I thought they were just | tapped autotransformers.
Sounds like they may be more of a voltage selector.
One set of transformers I saw once had a voltage selector which also revealed the voltage to me. Even those these huge things were well guarded behind a chainlink fence with barbed wire on top, I could clearly read the instructions on the voltage taps. It listed 5 or 6 different voltages in the 4160 volt range (I believe that was a middle one). The secondaries were a thick bundle of insulated wires not on insulator standoffs, so obviously LV, possibly 480V or 208V. These were 3 single tank transformers in roughly the design style of a pole pig (round tank) with a control panel on them with the tap control and some gauge I guessed may be temperature (but I could not see it clear enough at the distance I was at to be sure). The instructions did indicate that the transformer must be de-energized (not just unloaded) when making the change. So I'm guessing they were just to compensate for variations in the delivered voltage. These transformers were about 1 meter wide and 2.5 meters high, each (3 of them). I did not see any reference to a kVA rating. They were also very old looking (pre-WWII). They were humming.
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On 15 May 2008 05:20:27 GMT, snipped-for-privacy@ipal.net wrote:

All distribution transformers, sometimes called "pole pigs", that I have seen had some sort of voltage adjusting system, usually referred to as taps. Usually they are an actual bolted "tap" and you open the transformer and set the output voltage by making the proper tap connection when the transformer is installed and frankly it is usually ignored thereafter.
The other "cans" you often see on poles are capacitors used to adjust the power factor on some secondaries.
Bruce-in-Bangkok (correct Address is bpaige125atgmaildotcom)
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wrote:

Or disconnect switches, plain or with high-voltage fuses.

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What I was talking about appear to be used to adjust for supplied voltage (they're often used right after a stepdown transformer bank) or long runs, which may produce somewhat variable voltages that need adjustment at times.

Around here, capacitors for power factor compensation are rectangular boxes with two bushings on top, on poles, in banks of 3, 6 or sometimes 9.
Like the ones Phil mentioned, the cans I talked about hum.
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The pole pigs here [7200v in/120-240 out] are fixed tap, I'm told. Saves money. I think they are fused at 10A in. Older ones may have settable taps.

Capacitors are in various places but we also have three 7200V line regulators a block away, one on each primary phase. They are auto-transformers, with allegedly auto-controlled tap changers, much as the other poster described. [But his description is more complex than I recall from the class covering same. The essential aspect was you CAN short two taps together while switching; the inductance limits the current change while you do..]
I say "allegedly" as twice now, the regulators have stuck and my UPS woke me up at 2:30AM with notices it was disconnecting from the now-128v+ line. I solved the issue that night by putting a Variac in the line ahead of it, and cranking it down.
It took multiple calls and finally PSC [oversight agency] complaints to get PEPCO to fix the damn thing.
I envy EU houses. If we had regular 240V/30A+ outlets, I'd be able to buy a snowblower with real guts. The 120v@15A ones are wimpy.
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wrote:

Don't generalize too much. Multi-tap transformers are the normal for rural co-ops. I have a couple sitting in my shop right now. It's cheaper to change taps on the transformer than it is to deal with the various voltages that happen around a large geographic area.

You can have as many of those outlets as you want. In fact, I did just that in my restaurant. Every place there was an outlet I installed a double ganged box and included one 20 amp 120 volt outlet and one 30 amp 240 volt outlet. I'm doing the same thing here in my cabin as I slowly rewire it.
UK appliances are available on the net. A 4kW tea kettle beats the hell out of a puny 1700 watt 120 volt version. Same with a commercial 240 volt toaster, coffee maker, etc. All I have to do is change the plug, getting rid of that UK abomination. Some of the e-stores sell appliances sans plug, aimed at the european market. The customer installs whatever plug is used in his country.
For a snow blower, why not make your own? Find one with a blown engine or buy a new one and sell the engine. Install a suitable electric motor and away you go. You can figure about 2/3s the HP of the gas engine is necessary for a normal high torque farm-duty motor.
If I were going to do that, I'd probably go a step further and use a 480 volt motor along with a 2:1 autotransformer at the house. That way the cord can be much lighter, something to think about when you're slaving away out in the white stuff.
Depending on what I could find and at what cost, I might even go with a 3 phase motor and VFD. The VFD will take single phase 240 as input and generate 480 three phase output and at whatever frequency you desire. Considering the cold operating environment, you could spin a smaller lighter motor faster and get more power than with a straight 60 hz motor. You could even have a "throttle" (a potentiometer) on the snowblower. Small VFDs (10 hp and less) are fairly easy to find used. For that matter, they're not all that expensive new.
John
-- John De Armond See my website for my current email address http://www.neon-john.com http://www.johndearmond.com <-- best little blog on the net! Tellico Plains, Occupied TN No one can be right all of the time but I'm getting close.
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| | |>All distribution transformers, sometimes called "pole pigs", that I |>have seen had some sort of voltage adjusting system, usually referred |>to as taps. Usually they are an actual bolted "tap" and you open the |>transformer and set the output voltage by making the proper tap |>connection when the transformer is installed and frankly it is usually |>ignored thereafter. | | The pole pigs here [7200v in/120-240 out] are fixed tap, I'm told. Saves | money. I think they are fused at 10A in. Older ones may have settable taps. | |>The other "cans" you often see on poles are capacitors used to adjust |>the power factor on some secondaries. | | Capacitors are in various places but we also have three 7200V line | regulators a block away, one on each primary phase. They are | auto-transformers, with allegedly auto-controlled tap changers, much as | the other poster described. [But his description is more complex than | I recall from the class covering same. The essential aspect was you | CAN short two taps together while switching; the inductance limits the | current change while you do..] | | I say "allegedly" as twice now, the regulators have stuck and my UPS | woke me up at 2:30AM with notices it was disconnecting from the now-128v+ | line. I solved the issue that night by putting a Variac in the line | ahead of it, and cranking it down. | | It took multiple calls and finally PSC [oversight agency] complaints | to get PEPCO to fix the damn thing. | | I envy EU houses. If we had regular 240V/30A+ outlets, I'd be able to | buy a snowblower with real guts. The 120v@15A ones are wimpy.
So put one in.
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snipped-for-privacy@ipal.net writes:

The issue is not the outlets available in my house [but I sometimes wish for 3 phase..].
Rather, it's the ready market of consumer appliances that would take advantage of them. That would require many houses to have them.
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David Lesher wrote:

Get UK/commercial appliances, they're out there.
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ones are wimpy.

Yes like my Amana commercial RadarRange which is 4KW in 2.2KW out and has 3 HV magnetrons along with 3 each of the other necessary items (cap, diode, etc.). It even has a current transformer that tells the control board via current draw when the magnetrons are warmed up so that the timer doesn't start counting down until it is actually cooking. It has a standard NEMA 6-20 plug on it now and will pop a bag of popcorn in roughly 75 seconds without scorching it. I can tell you it sure beats the hell out of regular microwave ovens for most things. The only thing I still use the regular one for are items that involve liquids as the Amana tends to make them either boil over or boils out all of the water before the food is cooked.
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Daniel Who Wants to Know wrote:
<snip>

Does this oven somehow injection-lock the magnetrons? Can you describe the (RF) plumbing?
Michael
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Can't comment on the microwave oven, though I suspect that it works the same as my gadget. A couple of years ago I built an EWF (electronic warfare) device to solve a particularly obnoxious boom-boom stereo problem. This guy would drive by my restaurant every evening on the way home from work. His stereo was loud enough to rattle things off my dining room shelves. Talking to him didn't work soooo...
My gadget used 4 1kw microwave oven magnetrons placed in a suitable waveguide one wavelength apart, the magnetron antennae simply protruding into the waveguide. With suitable use of tuning stubs, they phase-locked and the power added nicely. A quite large rectangular horn terminated the waveguide and matched it to the ether. I didn't bother with pulsed operation, as the first CW test was successful :-) Simon and Garfunkel's "The Sound of Silence" song came to mind.
I had the thing positioned in my dining room, aimed through the plate glass window at the area behind the stop sign. When he pulled up to the stop sign, the stereo a-thumping away, I touched the plate supply push button. Instant silence. Permanent silence. It killed his engine too, but it restarted.
John -- John De Armond See my website for my current email address http://www.neon-john.com http://www.johndearmond.com <-- best little blog on the net! Tellico Plains, Occupied TN I don't suffer from insanity, I enjoy every minute of it.
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