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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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Three-Phase posted:Aliass, how big does your shop go? Anything bigger than 10,000 HP (synchronous or induction)? My Current shop doesnt do anything over 2MW. But my previous overhauled/rewound up to 10MW. However i have done HV testing on larger salient pole motors. (Tan delta testing is an awesome bludge). As for arc furnaces and whatnot, extremely large loads are either run at nightime to benefit from the lower tarifs to save money. Or if its nessicary for production then its usually run 24/7 no matter the size. The state i live in (queensland australia) has 1/3 of its power used alone by one aluminum smelter. Let alone the masses of industry and any mines that are connected to the grid. The only time people need to talk to the supply authority is if they have to start a shitload of large inductive devices up. Mainly beacuse of the power factor correction required. I asked my farther (he works for the supply authority in distrobution management) if they had ever asked people responsible for larger loads to switch off during peak demand. But they never had to thanks to the "smart grid technology" making the grid that much more efficient. However what i do find interesting is often times at night they have to make the power factor of the grid worse using switchgear and whatnot. Just so they don't have the shut the base loads down. AFAIK it takes about 2 weeks to restart a coal turbine.
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Sep 14, 2011 22:23
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helno posted:That sounds like bull to me. The coal plants in these parts are used for load foallowing and are up and down on a daily basis. Im talking from complete cold start. That was the figure i was told but most of ours are pretty old. We normally use gas turbines to deal with the excess demand and only use coal fired power stations to provide the baseload power. My understand is that you can get much faster starting coal power plants but they arnt nearly as efficient, for some reason (ill have to ask my electrical engineer friend about that). Unfortunatly my state is retarded and wont allow nuclear power for some reason. Hence coal as the baseline.
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Sep 15, 2011 10:00
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modig posted:Great thread. I'm a physicist and we use some equipment that runs on 408V, or three phase 210V, also a coworker is currently designing an experiment that will use like 12kV and 200A. But you guys work with way crazier poo poo than that. I have two questions: They are "ribbed" so if theres dirt/carbon buildup current doesnt "track" aka flow over the outside causing a fault. By "ribbing" it it allows much more time for a buildup of contaminents and also increases the distance/surface area by up to 600%. This combined with the exterior coating of the material and the angled surfaces helps prevent short circuits/earth faults in the hv switchgear.
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Sep 18, 2011 04:17
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grover posted:I always understood it to be more related to water; that a solid conductor would be apt to get long streams of water, whereas the ribs break up the streamlet of water into drops and reduce the risk of corona discharge. I think its a little of both tbh. Alot of people don't realise just how well high voltage can track till they have played around with a 33kv Insulation resistance Tester (aka megga) some leads and a dirty floor.
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Sep 18, 2011 14:20
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Pretty much wrong on all counts fellas. (at least with motors) Proper commutation in a dc motor produces NO sparking at all. If there is sparking, something is drastically wrong! Normally its a bad connection, underloading or incorrect brush grade. However dc motors are almost never used for pumping as its not only more expensive to run/maintain its a complete waste as 3phase ac will do the job just fine. (at least for pumping water/oil/tar/slurry. As for closed circuit hydrogen cooling , you have to understand that you have different types of enclosures and different heat ratings for winding insulations. There are motors designed to run at 260degrees C (i have a pair on my workbench at the moment) When you have a totally enclosed motor the only thing thats cooling it is either an extieror fan and/or a interior fan. The exterior one blows air over the stator fins thus helping to cool, the interior ones normally don't do much good unless theres a heat exchanger. However they still function but run a little hotter which is why people will often upsize the ac motor to prevent as much heat being generated. When you have a closed circuit hydrogen system the internal fan just circulates the hydrogen through air to water heat exchanger or air to air (most common) to increase the effectiveness of the system. This really isnt a problem as unless theres a pretty drastic fault in the windings theres NEVER any sparking inside these motors and a pure hydrogen environment means theres no flames. For it to explode you would have to have something amazing happen like the internal fan shattering into the windings and cracking the endsheild. When a motor is water cooled (relativly rare) unless its a pump the rotor never touches the water , pumping it would add way to much load to the motor. As is with a fan if you replace it with one designed for a slower speed motor you can overload it / induce premature failure. So, Basically the stator is designed so instead of the back iron being one soild unit, its much thicker and has a bunch of tubes through the entire length of it allowing water to circulate. There is no need to immerse the windings as copper > iron>iron > water is a fairly effective method of transferring heat. Like Transformers the water can either be circulated with convection or forced with a secondary pump (fairly uncommon for stuff under the 10megawatt range) I have never seen nor heard of a motor being cooled with mineral oil , that would not only be worse then water it would cost more. Perhaps you were thinking of transformers (which are often immeresed in oil). "God drat I had no idea this was done. If someone ever suggested to me using hydrogen gas for cooling electronics, then I would think they were insane. I've actually known of some large accidents that have happened in chemistry and combustion labs where people used hydrogen in the vicinity of brushed motors (which were driving the pumps, loving idiots" I find this very hard to beleive i have never EVER seen a dc or a 3phase slipring motor used to drive a pump (under 100kw). They are far more expensive then a generic 3phase ac motor. As thus i cannot understand why someone would use them in a laboratory situation. Hazardous/flammable environements are why explosion proof motors were designed.
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Sep 19, 2011 11:25
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ANIME AKBAR posted:I'm talking about something a bunch of PhD chem students rigged up without supervision. I'm pretty sure jobs were lost over it. Gotta love people playing with electricity. Many a smart person has got a belt from tinkering around with stuff they shouldnt have.
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Sep 19, 2011 13:28
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DuckConference posted:Doesn't hydrogen gas have a tendency to be absorbed into metals and make them brittle? Or is that only at very high pressures? Im not sure about that , but i will say its never been a problem for me. Bad casting on internal fans/balance weights on rotors are much more likely to cause premature failure.
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Sep 19, 2011 21:54
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Big Steveo posted:I would recommend "Electrical Trade Principles" by Hampson or "Electrical Principles" By Jim Jenneson. Both books are highly recommended to apprentices. I used both of these at tafe during my electrical apprenticeship. I would recommend Jenneson over hampson as hampson is more of a straight up reference book where as jenneson is much better for learning the material in the 1st place. Also how many other motor winders are here/what countries? I used to do motor winding in Australia.
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Jan 24, 2014 00:25
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