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Vanagoon posted:Regarding power factor, I've noticed recently that there is a lot of bitching going on about PF in regards to Compact Fluorescent light bulbs. If you have a large building, or a complex of buildings, the power factor could add up if you have mega-VAs worth of lighting.
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Sep 5, 2011 02:47
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Three-Phase posted:A small customer is billed based on volt-amperes (correct me if I'm wrong on this and it's just watts - I'd assume they'd measure VAs to account for reactive power). I know for residential, they only bill for watts. For commercial and industrial, I believe it is still a bill for watts, but a certain PF is required or a fee will be imposed like you stated earlier. I just graduated from college and found myself working in the power industry at a power plant. I found that while my college had done a good job at teaching me about a lot of components, they had completely ignored relays. Unfortunately, relays are in practically every electrical system in the plant. On the job training has done a lot for my understanding of how they work practically, but are relays really so rare outside of industrial facilities that universities don't even bother covering them anymore? I guess semiconductor technology is making them somewhat obsolete?
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Sep 5, 2011 05:37
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Crackpipe posted:What parts of these would be safe to bump into? I've always viewed substations as death mazes where bumping into absolutely anything will kill you instantly. Three-Phase answered this question very well, but another general rule of thumb, although NOT always true, is if you need to step up to touch something, it is NOT SAFE. Usually BUT NOT ALWAYS, everything ground level which you can touch is safe to do so. Energized, uninsulated equipment is suspended high above with the use of insulators, and if it is ground level, it is usually enclosed in fencing. Like Three-Phase said though, the real danger is if you accidentally are carrying something like a ladder or pipe. You must ALWAYS be aware of your clearances between you, any item you are holding, and the energized bus work. quote:SEXY. How many MVA is that? 500? 1000? Actually it is only 327 MVA, believe it or not. ----------------- So I am sure many people wonder when a cable fails in an underground electrical system, how do they find where the damage is? Well here goes..a little background first. In my utility distribution voltages range from 27KV,13.8KV and 4KV. I deal with 27KV. The switchgear is made up of cubicles which look like this when no circuit breaker is in connect:  The 2 red shudders are where the circuit breaker stabs:  connect. The shudders open as the circuit breaker is racked into connect. The top stabs make up with the energized station bus and the bottoms stabs connect to the potheads which are connected to the cables(called Feeders) which go out to the street and will eventually have the voltage stepped down again before it enters your house. Once the breaker is closed, a complete path is made, from the energized station to the feeder. Okay so now back to the scenario I first mentioned. There is a fault somewhere on a feeder. The relays associated with the feeder see a huge spike in current and trip the circuit breaker open. Now the cable is mostly underground and the fault can be anywhere. What we substation operators do, as directed from the energy control center is to "establish a condition". What this means is we put high voltage onto the feeder and it acts as tracing current which goes straight to the damage on the cable. This is done by removing the circuit breaker from the cubicle and inserting a Ground Breaker or a Ground & Test Device. This "breaker" only has stabs which connect to the cable going to the street, it does not have any stabs to connect to the energized station bus. This is the High Voltage Test Set:  It is used to establish a condition (what I am currently describing) and it is also used to test the insulation of feeder cables after repairs are made, to ensure that there are no other faults on the cable. It attaches to the feeder through the ground breaker & test device like so:  (also note the lock out & tag out procedure, Three-Phase mentioned earlier a key interlock system which is hard to see in this photo, but it is there) The black cable is connected to the feeder through the test device it is in, and is connected to the high voltage test set. We turn on the high voltage test set which raises voltage onto the cables until it reaches the fault where the voltage drops off and a huge spike in current occurs. Now crews who are trained in fault locating, go out in trucks with a map of the feeder run in hand and this is what they do; They will usually start in the middle of the feeder run, and go into the manhole where the cable is and place this device, a galvanometer onto the cable:  The red portion is placed onto the cable and the spike in current which I described earlier is read on the meter. If they read that spike, that means the feeder cable is good up until that point. The crew will then go to the next manhole down the line and do the same thing, but now if they do not read any current on the galvanometer, they know the fault is between the last manhole where they read current, and the manhole they are now in. We in the substation then shut down the high voltage test set, and apply a ground via the ground breaker. When the ground breaker is closed, instead of making a complete path from the energized station to the feeder, it makes a path from the station ground grid to the feeder, making it safe for crews to do their repairs. Once the repairs are made, we hook up the high voltage test set again, and this time, raise voltage for a specific amount of time to ensure the feeder cable has been repaired correctly and no other damage has been done. If it is damaged, we will not be able to raise and sustain voltage. We start the process all over again by establishing a condition to find where the other damage is. When we establish a condition however, it is not always required for field crews to open every manhole and read the current via the galvanometer, the reason being, this is what Establishing a Condition often sounds like; http://smg.photobucket.com/albums/v323/somekidfromny/?action=view¤t=0513001048.mp4 The crews will just drive the route of the feeder until they hear that sound coming from a manhole, or we will have customers calling us saying they are hearing something "exploding" from a manhole. That crack is the sound of the current arcing to ground, ie: the fault on the cable grounding out to something in the manhole. That's it for now! Some Guy From NY fucked around with this message at 12:11 on Sep 5, 2011 |
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Sep 5, 2011 12:04
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Vanagoon posted:Regarding power factor, I've noticed recently that there is a lot of bitching going on about PF in regards to Compact Fluorescent light bulbs. Current (above) and Voltage (below) of a CFL:  If the above were an incandescent bulb, the current waveform would be a smooth sine wave identical to the voltage waveform. Tellara posted:A lot of the linked videos and posts in this thread talk about "VA"s which I assume is volts * amperes. Don't we call those watts (W)? Or am I confused? One of the problems with this is that the wires need to be about 50% larger to carry the same amount of power for a .65pf as for a 1.00pf. All this chopped-up "non-linear" has a tendancy to distort the voltage waveform instead. The impact of one CFL won't be noticible, but when a large % of the electrical demand is from stuff like this, instead of a nice clean 120V sine wave, you end up with a really dirty sine wave with lots of peaks and dips (often it looks like a stetson hat) that is simply very hard on power supplies, and causes premature failure. In the old days, they'd slap a power-factor correction capacitor bank to compensate, but while that works with motors (the largest problem in days past), it doesn't work with this type of non-linear load. Specialized electrical harmonic filters need to be designed specific to each piece of equipment with tuned capacitors and inductors smoothing out the spikes back into a smooth sine wave; you see it in high-end servers (which are close to 1.0pf), but not in really really cheap-rear end big-box CFLs. Which is the silver lining, I suppose: there is an easy answer (buy better CFLs with pf of 0.95-0.98), but we pick the one that costs a few cents less instead. grover fucked around with this message at 13:54 on Sep 5, 2011 |
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Sep 5, 2011 13:47
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Simulated fucked around with this message at 23:07 on Sep 14, 2011 |
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Sep 5, 2011 18:11
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Ender.uNF posted:Does this seem like a reasonable layman's explanation? I may have overstated the issue of residential harmonics; yes, CFLs and TVs and computers have cheap power supplies and have an impact, but the sum total is still rather small (a few hundred watts total). All the big loads in a typical home are resistive or motor loads- air conditioners, ovens, refrigerators, hot water heaters, coffee pots, etc. A single coffee pot can draw more power than all the lights and typically running cord & plug stuff in rest of your house combined. grover fucked around with this message at 18:37 on Sep 5, 2011 |
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Sep 5, 2011 18:35
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Ender.uNF posted:You might effectively think of it has these power supplies that turn AC into DC "backflowing" and attempting to force the AC line into a DC one, fighting against the sine wave of the generator. A rectifier that turns AC into DC power does only that, DC to AC is an inverter. I'm not 100% what you mean when you say backflowing. The lift equipment i used to work on, based on the relative load of the car and counterweight would actually allow power stored in capcitors to flow back into the grid as AC (UK). Not only did the voltage and amperage have to be exact but the sinewave of the current had to match exactly aswell. I asked a senior engineer once what would happen if the frequencies were different or if the wavelength was out of alignment, he simply responded it would be bad for all involved. Really enjoying the thread so far, as someone who has had many, many small shocks and detests working on or near live equipment i salute your balls op!
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Sep 6, 2011 02:12
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Been reading through this, I'm an EE student, and admittedly I have trouble wrapping my head around the concept of reactive power. I assumed reactive power was essentially power used to drive a load up to the required voltage, which makes the 'bounce backed to the source' explanation confusing to me. Would someone be able to elaborate? So from there, what effect does a lagging power PF have? and leading for that matter, as a consequence for supplying power? I understand that in both cases, assuming constant apparent power, introducing reactive power will decrease the active power load using the triangle... but what's the actual non-conceptual effect?
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Sep 6, 2011 15:18
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Flash18 posted:Been reading through this, I'm an EE student, and admittedly I have trouble wrapping my head around the concept of reactive power. The best way to put it is that reactive power represents power which is not consumed. If you have a reactive load, it will pull current due to the inductive (for lagging) load, but it will not have a mechanical force applied to the alternator to feed that load. Its like a car with its rear wheels off the ground and the engine being floored. The wheels are turning extremely fast (current), but it represent little load on the engine. Look at it from the perspective of vector analysis. For a motor you will have a fixed vector in the positive j axis that does not change that represents your reactive load (kvar) On the real axis you will have the actual power (kw) consumed, which does change based on load. At zero real load, you will have a resultant vector that equates to only that reactive component and your current will be based upon only that. As you ramp up the mechanical load on the motor, the real vector will increase (somewhat) proportionally and the resultant vector of the triangle will represent your apparent power. Current will always be based on the resultant vector. In physical terms, the reactive power in a motor represents the core losses of the inductor, eddy currents, etc. These losses will generally be constant and unrelated to the loading of the motor. quote:I may have overstated the issue of residential harmonics; yes, CFLs and TVs and computers have cheap power supplies and have an impact, but the sum total is still rather small (a few hundred watts total). All the big loads in a typical home are resistive or motor loads- air conditioners, ovens, refrigerators, hot water heaters, coffee pots, etc. A single coffee pot can draw more power than all the lights and typically running cord & plug stuff in rest of your house combined. Cheesemaster200 fucked around with this message at 16:07 on Sep 6, 2011 |
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Sep 6, 2011 15:59
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Flash18 posted:Been reading through this, I'm an EE student, and admittedly I have trouble wrapping my head around the concept of reactive power. The actual effect is that it makes generators and cabling see higher currents, with less power actually being used to do whatever the load is intended to do. Much of the VAR power is simply reflected to the source, so 1A reactive is 1A you can't use for load power. 1A reactive going down the line with 19A real = 20A apparent, and if that's capacity, well, you get 19A of work instead of 20, despite doing the work of generating 20A. Reactive power takes up capacity in all equipment in the circuit (genset, cabling, motor) and does not contribute to the work being done. Specifically in motors, a low power factor can cause excessive motor heating, since the motor ends up carrying much more current to do the same amount of work. This image is a pretty fun analogy: http://madamenrg.files.wordpress.com/2011/04/beer.jpg?w=435&h=426 KaiserBen fucked around with this message at 16:14 on Sep 6, 2011 |
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Sep 6, 2011 16:07
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quote:The actual effect is that it makes generators and cabling see higher loads, with less power actually being used.
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Sep 6, 2011 16:09
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Ender.uNF posted:For that matter, it makes me wonder how industrial equipment can be switched on and off... or even if it can be switched off internally. I have no idea how you'd make a regulator that could control the motor speed of a 10,000 HP motor. Ooh, a question for me. I'm a VFD commissioning engineer working in the steel industry. First, the AC power is rectified into DC. For a 10k HP motor (7.5MW), we'd typically have 3.3kV incoming power, rectified to a 3 level DC bus, +3400V, 0V, -3400V, then we use an inverter (with enormous transistors, capable of handling up to 6000A) to turn the DC into AC again, but with variable voltage, current waveforms, and frequency. Most VFDs can do at least 0-120hz, some up to 400hz, so you don't have to use 60hz motors (and the limited speeds that restricts you to). I typically see lower frequencies in bigger motors, due to the slow speed and high torque needed. Main drives (7.5-10MW) are typically in the 11-30hz range, 100-300RPM max.
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Sep 6, 2011 16:13
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Cheesemaster200 posted:It makes everything see higher currents. This equates to more losses in transmission and requires beefier alternators, cables, etc. The reactive power doesn't create higher loads by itself. True, fixed.
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Sep 6, 2011 16:14
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Cheese and Kaiser did a nice job explaining reactive power. In my opinion it's one of the hardest to explain topics. (Phasors come in second.) Kaiser, you using SCRs, IGBTs, or IGCTs? (For those wondering what the hell those are, they basically act as big switches to turn current on and off. A "normal" transistor in a circuit can be as small as a grain of sand down to microscopic size. These look like hockey pucks up to the size of dinner plates.) How much THD you get out of those drives? 20%? Three-Phase fucked around with this message at 02:23 on Sep 7, 2011 |
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Sep 7, 2011 02:17
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Electricity can be even scarier: http://www.youtube.com/watch?v=tVJzwG4Ee4c
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Sep 7, 2011 02:38
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Thanks for the info; I'm still trying to understand reactive and PF and revise it into a layman's explanation.
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Sep 7, 2011 02:59
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Three-Phase posted:Electricity can be even scarier: My god it's so full of stars!
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Sep 7, 2011 11:19
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Three-Phase posted:Cheese and Kaiser did a nice job explaining reactive power. In my opinion it's one of the hardest to explain topics. (Phasors come in second.) All of the above. We're using SCRs for rectifiers (our drives can have one large rectifier with several smaller inverters), and IGBTs, IEGTs, and IGCTs for inverters, in increasing order of size. THD is pretty low, a bunch of our new products are IEEE 519 compliant (some need inductors on the output or DC link). Edit: I've been thinking about doing an "ask me about working in a steel mill" thread, if anyone has any interest? Work has me pretty busy right now (12hr x 6 days), but I figured I'd ask. KaiserBen fucked around with this message at 15:06 on Sep 7, 2011 |
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Sep 7, 2011 13:24
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Ender.uNF posted:I'm still trying to understand reactive and PF and revise it into a layman's explanation. Suppose this experiment: Grab a long, relatively narrow piece of paper (say 4 feet by 2 inches or something) and lay it flat on a ground. Put some kind of weight on near one end of it, and grab the other end in your hand. The goal is to lift your end of the paper up and down to make a wave that ripples towards the other end, and the idea is to use the energy in that ripple to push the weight forward, away from you. Chances are that experiment won't work really well, but one way it might work is: you make a wave, it ripples to the weighted end, it pushes the weight forward just a little bit, and a wave reflects back towards you. Unfortunately the paper is lossy, so the reflected wave probably won't make it all the way back to you, but that's kind of what goes on in an AC power system. The apparent power is what you (the generator) put into making the wave. The real power is that consumed by the weight to push it forward, plus the amount lost to the paper. The reactive power is the portion of the wave that is reflected back towards you. As mentioned earlier, reactive power isn't really generated and consumed on average, it's just power that sloshes around between two points in the system. In the worst case, it sloshes between the load and the generator. Now, even though reactive power isn't consumed, its existence cuts into your generation and transmission capacity. So if you have a power generation system that's running at 10% capacity, with a PF of 0.5, it doesn't matter much because you're no where near your capacity limits. If you're running at 95% capacity with a PF of 0.5, you really want to improve your power factor since you're operating dangerously close to capacity, and it's contributing significantly to transmission losses (which increases the closer you get to your transmission capacity).
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Sep 7, 2011 19:00
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KaiserBen posted:Edit: I've been thinking about doing an "ask me about working in a steel mill" thread, if anyone has any interest? Work has me pretty busy right now (12hr x 6 days), but I figured I'd ask. I spend a surprising amount of time around steel mill, and I never got farther than "coal and iron ore go in, steel comes out", so yeah, I'd be interested.
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Sep 7, 2011 21:07
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Ender.uNF posted:Thanks for the info; I'm still trying to understand reactive and PF and revise it into a layman's explanation. I've used this analogy a few times in real life, I hope it work as good on the internet. Imagine a theoretical small village with a single well in the center of town. every resident has a 10 gallon bucket that they fill in the morning. They use water from the bucket all day and at the end of the day they bring their bucket back and return the extra water. An attendant measures how much water they used and charges them accordingly, say 10 cents a gallon, and then dumps the rest back down the well. the person who used 5 gallons pays 50 cents. the person who used all 10 gallons pays 1 dollar. at first this sounds fair. until you realize that it took equal amounts of work to supply each person with their daily water. In fact, it actually took more work to supply the 5 gallon person with water because the well attendant had to handle their return in the evening, yet the 5 gallon person pays less. If theres 100 citizens and each fills a 10 gallon bucket, the well must be able to supply 1000 gallons even if only 500 are actually used. Imagine the above scenario happening 60 times a second and replace the water with electricity and the well with the electric grid. Thats power factor. A customer with low power factor is basically taking more current than needed, using some of its energy, returning the rest, and only being billed for their consumed kilowatts. The utilities compensate by having meters that can read PF and then they add a surcharge for low PF customers. In the above well scenario, the citizens could improve their "power factor" by taking only the amount of water they will need for the day. Then the load on the well its operator would be representative of the actually needs of the community and more customers could be served by the same well.
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Sep 8, 2011 05:13
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The last two explinations made a ton of sense, thanks guys. Dumb question time again: Can someone please explain the difference between a switch, and a load-interupting switch?
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Sep 8, 2011 05:41
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Crackpipe posted:The last two explinations made a ton of sense, thanks guys. A disconnect switch is just a way to lock a disconnect in the system for maintenance, and cannot be opened under load (eg: to stop a runaway machine); you must first turn the load off by another means. A load-interrupting switch can be opened under load, like a circuit breaker, and is much more heavily built to be able to do that.
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Sep 8, 2011 13:39
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An electric thread! I salute all electric engineers and technicians, you guys got real balls to work in that field. I am an electronic engineer myself so I don't work with anything higher than 120v (not that it isn't dangerous). I have a few questions related to residential power so if it is off topic let me know. Why would a power bill increase monthly by 10% in less than 6 months? Basically, our power bill has skyrocketed. I live with my dad and we both work full time so we are less than 12 hours at home each day. Yet we have seen our power bill go up 10~20% each month on this year. I am aware we are running two ACs at night (non-inverters) but what worries me is the climb in our bill. We have 3 PCs running intermittently (they go to sleep mode when we are not using them) but most of the day, there is no one home to use that much power. Recently my brother has come to live with us and as a pilot, his work schedule is kind of irregular and he is a heavy sleeper so my AC is probably being used 12 hours a day. Is that the only cause of the power increase? Is there a way to calculate our power usage? We are thinking of investing about 1200 to get new ACs (those Gen 2 beauties that claim to save up to 60% of a regular AC power usage) but is there something else we should check?
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Sep 8, 2011 16:09
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I'm an EE student (although early in the curriculum) and is there a reason that it is always voltage being described when people talk about power lines and etc and not current? I have a hard time putting a "danger value" on a piece of equipment or whatever without both a current and a voltage, but that may be something I haven't been exposed to yet? Obviously if something is 400kv, it is not going to have an absolutely tiny current running through it, but I'm still curious.
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Sep 8, 2011 16:41
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IratelyBlank posted:I'm an EE student (although early in the curriculum) and is there a reason that it is always voltage being described when people talk about power lines and etc and not current? I have a hard time putting a "danger value" on a piece of equipment or whatever without both a current and a voltage, but that may be something I haven't been exposed to yet? The resistance of a person (or whatever) will stay generally constant. The higher the voltage, the more current can potentially run through you. Current kills, but in most situations current is almost always a function of voltage. Higher voltages are also more likely to arc. They are almost always always closer to the generation source and have higher available incident energy. Transmission also functions a lot different than building distribution in regards to how faults are handled. quote:Why would a power bill increase monthly by 10% in less than 6 months?
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Sep 8, 2011 20:00
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TerryLennox posted:I am aware we are running two ACs at night (non-inverters) but what worries me is the climb in our bill. We have 3 PCs running intermittently (they go to sleep mode when we are not using them) but most of the day, there is no one home to use that much power. Recently my brother has come to live with us and as a pilot, his work schedule is kind of irregular and he is a heavy sleeper so my AC is probably being used 12 hours a day. Is that the only cause of the power increase? Is there a way to calculate our power usage? A smallish portable AC unit (maybe 1 ton or less) is using 5-10x as much electricity while operating as a computer. AC is by far the biggest user of power in homes that have it, and the moderate increase in usage is where the extra is coming from.
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Sep 8, 2011 21:36
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TerryLennox posted:An electric thread! I salute all electric engineers and technicians, you guys got real balls to work in that field. It's not unusual for usage to double in the summer. Can you compare the bills to the same months from last year?
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Sep 8, 2011 21:56
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Do you have an electric hot water heater? With a high-flow shower head? Does your brother take long showers? Extra electricity used for showers, laundry and dishes adds up rather quickly. I think I worked out once that I'm paying about 50 cents per shower in electricity alone using a low-flow shower head and with cheap electricity.
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Sep 8, 2011 22:17
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IratelyBlank posted:I'm an EE student (although early in the curriculum) and is there a reason that it is always voltage being described when people talk about power lines and etc and not current? I have a hard time putting a "danger value" on a piece of equipment or whatever without both a current and a voltage, but that may be something I haven't been exposed to yet? I've seen larger equipment rated in MVAs, where they combine the voltage and current together. I believe that 50-100mA AC at 60hz can kill a person from either suffocation or fibrillation. Boy, did anyone hear about the massive blackout in the SouthWest?!?
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Sep 9, 2011 01:51
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IratelyBlank posted:I'm an EE student (although early in the curriculum) and is there a reason that it is always voltage being described when people talk about power lines and etc and not current? I have a hard time putting a "danger value" on a piece of equipment or whatever without both a current and a voltage, but that may be something I haven't been exposed to yet? Equipment voltage ratings are standardised across the whole power distribution system. Current ratings aren't. At the transmission and distribution levels there are a range of different conductors used with widely varying current capacities but usually* the same voltage rating. The reason is that more current capacity means more aluminium or copper in the wires with bigger towers or poles to support the strain and more land required. Or bigger cables and larger ducts. Even in a substation stuff at the same voltage is rated to different fault current capacities. *Things get operated at lower than their design voltage sometimes.
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Sep 9, 2011 08:59
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slorb posted:Equipment voltage ratings are standardised across the whole power distribution system. Current ratings aren't. At the transmission and distribution levels there are a range of different conductors used with widely varying current capacities but usually* the same voltage rating. Yup, so you want as high a voltage as you can go so you can move the same amount of power (volts * amperes) with less amperes - that means smaller cables. Of course, at higher voltages, you need bigger insulators, and it gets harder to build circuit breakers, transformers, etc. Big HVDC systems need gigantic insulators, like 25 feet long.
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Sep 9, 2011 23:18
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Three-Phase posted:ELECTRIC ARC FURNACE - Turn your speakers DOWN. You need to phone the power company before you start one of these. I'm not joking. I hope by "start one" you mean commissioning. Sure, any mill running an EAF will be working with the local power company to make sure that they'll have enough juice while the thing is being built, but the operators don't call them up every time they start a heat. Even these guys in the middle of a heavily residential Chicago neighborhood don't call up ComEd every time they start melting for the day, although ComEd will tell them to shut down for the afternoon on hot days (bonus: also the first Cubs night game in 1988).
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Sep 10, 2011 00:17
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Three-Phase posted:I've seen larger equipment rated in MVAs, where they combine the voltage and current together. Motors are never rated in MVA's however transformers and other such machinery are. The Official Figuire that I was taught in electrical colledge. Is 160mA @ 240v to induce v-fib. 200mA+ causes severe burns and 120mA+ induces uncontrolable muscle contraction. Thats what the aussie standards say about it at least. On Another note, Hi guys - motor winder/industrial electrican checking in. If anyone has any specific questions about motors of any shape and size feel free to ask away.
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Sep 10, 2011 00:31
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Jows posted:I hope by "start one" you mean commissioning. Sure, any mill running an EAF will be working with the local power company to make sure that they'll have enough juice while the thing is being built, but the operators don't call them up every time they start a heat. What about for power purchace/billing? I thought that was handled on a day-by-day basis at the largest plants. (Maybe that's more plant-wide than just a single furnace.) Aliass posted:Motors are never rated in MVA's however transformers and other such machinery are. Forgot to mention fault ratings on breakers in MVAs. But yeah, I've never seen a motor rated in MVAs, just HP. Aliass, how big does your shop go? Anything bigger than 10,000 HP (synchronous or induction)? Three-Phase fucked around with this message at 01:50 on Sep 10, 2011 |
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Sep 10, 2011 01:46
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A common rumor I've heard from some engineers is that much of the grid is synchronized using GPS signals, and if enough satellites were jammed or disabled, it would wreak havoc. Bullshit, right?
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Sep 10, 2011 02:42
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I believe that to be bullshit. The power grid pre-dates GPS considerably, and what is important is that you're synchronized to the frequency and phase of the grid where you are. I do have a question, why 50/60 Hz? Both why do some places have one vs. the other, and why these two as opposed to 120 Hz or 75 or 400?
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Sep 10, 2011 04:04
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50/60hz is a tradeoff between lighting loads that work better with higher frequency and motor loads that perfer lower frequency. As to the GPS discussion I can tell you that the vast majority of the electrical instrastructure doesnt give a poo poo what time it is. Hell most of the hydro plants in ontario still use mechanical governors because they provide better grid stability than newer digital control systems.
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Sep 10, 2011 04:19
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ANIME AKBAR posted:A common rumor I've heard from some engineers is that much of the grid is synchronized using GPS signals, and if enough satellites were jammed or disabled, it would wreak havoc. Bullshit, right? I believe that GPS signals are used for synchronizing timing across a wide area. That might have to do with carefully synchronizing the frequency and phase offset of many different generating stations. Or for coordinating events so you know exactly when an event in the power system happened. I'll have to ask my co-workers about that. (I'm not sure of the difference between using GPS and, say, using an atomic clock signal.) EDIT: Aha, here's a site that talks about phasor measurement and using GPS signals. It's basically so you can very accurately measure the phasors at different locations on the power grid at the exact same moment. Frozen Horse posted:75 or 400? 400hz is used in aircraft, because you can make much lighter transformers than at 60 or 50hz. An additional nice thing about higher frequencies is that when you have a simple rectifier (converting from AC to DC), you need a filter circuit to smooth out the ripple voltage. If you have a much higher frequency, it gets easier to smooth out the ripple with smaller filtering components like capacitors. (Three-phase rectification has even less ripple than single phase if you have a three-phase full-wave bridge.) Three-Phase fucked around with this message at 04:26 on Sep 10, 2011 |
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Sep 10, 2011 04:20
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Does anyone have a good book or lesson plan for learning about medium voltage switchgears? I've been working on large scale renewable energy systems and I've been subbing out switchgear design and that's an area I'd like to learn more about.
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Sep 10, 2011 04:37
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