Years ago I adjusted a neighbor's generator so that it put out 60 cycles at 120 volts. I used a TEK 465B 'scope to look at the frequency. I saw the major 60 cycle waveform and then a lot of smaller waveforms at higher frequencies. I was told that these harmonics were typical of small generators. Today I am replacing the voltage regulator on a small generator and adjusting the governor. I expect I will see the same thing as with my neighbor's generator. Looking inside the generator all I see is rotating coils inside a stationary coil. The rotating coils are energized with DC from the voltage regulator. Looking at old power plant pictures and video it looks as if those giant generators were built the same way. And many of them are still in use today. But I know that the sine wave from the wall has been very good for almost since, if not since, the first generator installation at Niagara Falls. So how do they get the good clean AC from these big generators? Or maybe it's not so clean and is instead filtered. If so how is this done? And can I do this myself without too much effort? Is the effort even worth it? Until the voltage regulator failed on my generator it ran my stuff at home just fine. The fridge, freezer, lights, phone and router. I didn't try any computers. So maybe I don't need to even consider filtering the generator output. But I do see a lot of generators that advertise "pure sine wave output" from an inverter that is part of the generator electronics. So maybe a pure sine wave is important for some electric or electronic stuff. Clocks for example. Anyway, thanks for reading. And for any answers too. Cheers, Eric
Shameless stolen from a forum about these things: To begin with, most people, including many equipment manufacturer's, many electricians, and just about all the average Joe's in the world, haven't got a damn clue about power quality. And many who do, have no idea what the effects on power systems, equipment like generators, UPS, 'sensitive electronics equipment', etc., cause. In fact, if the average person were to take an oscilloscope and look at what they think is a 'pure sine wave' coming from their source, they would be absolutely horrified; and probably wouldn't be sure if they've made a mistake in hooking it up. The terms 'dirty power' and 'clean power', generally mean absolutely nothing in the power industry, and are used mainly by people who don't know anything about power quality issues; and snake oil salesmen trying to sell their overpriced products to these very people. So, let's get some fact's straight.
First, most electronics equipment that people think of with bogus terms like 'sensitive electronics equipment' aren't sensitive at all. In fact, for the most part; this equipment cares the least about the quality of its power input. Bear in mind, that most of this equipment first converts its power source to DC, before using any of it. It will be the equipment itself, including its filters and voltage regulators that determines the quality of its DC power - not the source. Yes, there could be some ripple transmitted to the DC power; but that is primarily a function of how crappy its power supply is. Most of the buzz about the need to 'clean up your power' actually comes from manufacturer's selling equipment that supposedly does just that. And more often than not, it's pure BS.
Computers, PCs, and anything with a switch mode power supply (SMPS), draws its power in two large bursts of current during each cycle of a sine wave. It basically doesn't give a crap what the source looks like as long as there is power to draw. And oppositely, SMPSs do more to damage the quality of the input power from their source, than anything else. If you want clean power, don't put SMPSs on circuits with other equipment. This equipment is designed to function as long as its input source is within the CBEMA (ITIC) curve. Basically, its designed to function without rebooting with a very crappy source. This means that some 90% of equipment people think are 'sensitive electronics', in fact are not; and also damage the quality of the source horrendously by loading it up with harmonics.
Now, lets talk about UPS. The majority of UPS - especially ones that cost under $20,000 - simply present a large non-linear power supply to their source. Just like the SMPSs, they wreak havoc on the power quality of their source. Someone mentioned 'double-conversion' UPS. Nice; but clueless. (Sorry.) The output of a double-conversion UPS is almost always much, much poorer than the source sine wave entering. They use PWM or some other method to convert the DC back into AC, resulting in one of the most choppy 'sine waves' one could ever see. Yeah - they ensure the voltage doesn't drop when their input does; and they filter transients. However, that's about all they do. Aside from that, the power quality of their output is horrible. Again, anything under $20,000 and the output is going to look more like a square wave than anything. (Add capacitance of the circuit, and its much poorer than that.) The majority of low cost (under $20k) UPS, chop their output into three squarish looking chunks each half cycle. You have to pay big bucks to get a 12 or 18 pulse UPS, with adequate input and output filtering. But its not a problem, because the SMPS equipment that is connected to UPS systems 99% of the time, doesn't care one bit.
Now lets talk generators. Even cheap, low quality ones. Mostly the power quality problems from cheap generators fall into two issues - varying input voltage and frequency; and harmonics. Not big deal, because most of the equipment you load onto a generator, doesn't care about any of this. Certainly, none of the equipment with SMPSs cares at all. Take a look at their nameplate. Typically 90 to 240V, 50 or 60 Hz. They will work at a frequency of probably 30 to 100. As long as your generator doesn't drop below 90V for more than a second, they will operate just fine as well. SMPS's generating harmonics into the source may actually cause more issues for your generator (overheating), than the other way around.
Now for the big surprise. What types of loads have the most concern from harmonics, voltage and frequency? Motor loads. Harmonics causes the windings of cheap motors (and generator windings) to overheat. Motors that deal well with harmonics are made to be run on variable frequency drives. They cost more money, because they have larger windings, and often special treatment to their bearings. Obviously, dropping voltages can cause motors not to start. Minor frequency issues won't upset them; but should the frequency of your source drift severely, your motors will have problems. Trying to use a UPS to help this, will only make it worse
- mega harmonics.
The generator itself may actually be the most concerned about issues being generated by its loads. Loads with leading power factor (such as can happen with loads with high capacitance, large filters or high harmonics) can wreak havoc on the generator, cause it not to run, and actually physically damage it. Harmonics being generated by the loads can add to the harmonics being produced by the generator and cause over-heating. The only solution for this is to over-size the generator (or de-rate the one you have). Leading power factor problems are generally solved by introducing isolation transformers between the generator and its loads. Isolation transformers go a long way towards cleaning up the power that a generator or motor is seeing; but to really work wonders, it needs to be a three-phase system. (3-phase delta-wye transformers filter out triplen harmonics.) Electrostatic shields between the windings will improve the transformer's filtering of transients.
In addition to motors, there certainly are some other loads that don't like AC sources with rich harmonics. Could be televisions; not really sure. Probably, after the issues I've discussed; the most concerning issue to loads, especially electronics, is transients. These are much more prevalent from your utility source, than your generator. Switching transients from the utility opening and closing circuits; as well as large industrial loads being switched in and out, are the primary culprits. Nearby lighting strikes are also to blame. The best way to protect your equipment from these utility power quality problems, is via high quality surge suppression. So, if you want to waste money in an attempt to 'clean up your power'; go ahead - plunk down as much as you can afford and buy an expensive UPS. Chances are probably 95% or better, you've just made your power quality worse. (FYI - UPS are for having 'uninterrupted' power - not quality power.) If you are concerned about power quality, the first thing to spend your money on is high-quality surge suppression. That is likely all you need. The only thing more, in addition to cleaning up transients (an uninterrupted power) that an expensive double-conversion UPS will give you is a steady output voltage
- at the cost of high harmonic content. Most of your equipment doesn't care.
Seriously, what you are trying to do is akin to replacing the carburetor on a car that is running fine; because you think it may not be running fine. Don't waste your money trying to fix something that has no symptoms and is not broken. First, you'll probably make it worse. Second, you'll be out of a lot of money, that you could have used usefully for something else. If you really have money burning a hole in your pocket; hire someone with the equipment and know-how to perform a power quality study, and provide a proper interpretation. Then spend money fixing anything that is truly a problem.
Those big power house alternators have more slots in the stator. The winding pattern is set up such that the middle of the magnetic pole has more turns, due to ovelapping sets of coils, than the ends of the pole. This creates the proper wave shape. Smaller alternators can't have as many slots, and thus separate coils, so the induced waveform has some "jumps" anf "flat spots" in it.
Right, the inductance of typical motors smooths out the small imperfections in the voltage wavem and you pretty much get a sine-wave current.
Well, some inverter generators produce horrible square waves or stepped approximations of sine waves, that might cause problems for a variety of equipment.
Running clocks off the typical generator will be a waste of time, the frequency control will be awful.
Everything made since about 1890 had some scheme to control the harmonic problem. The first alternators had "salient poles" on the stator, which were just blocks of stacked steel laminations with coils wound on them. But, even with these, it was possible to shape the rotor flux field to minimize the harmonics. Later, they went to slotted stator lamination sections that made it possible to have coils that spanned several slots, and this made harmonics much smaller.
Also, in the early days, they ran the alternators off steam engines, and the AC had a definite pulsation with each stroke of the piston.
We had a power plant on site with huge diesel generators, low speed (300 rp m). They did have huge flywheels. I never knew how clean the power was. They were built to power an entire block of buildings, but by this time the y were peak shaving only, and have since been scrapped. Very impressive, t he floor shook when they ran.
Greetings Jon, Do you know how the harmonics were measured? Did they use an oscillograph? I am amazed by how much good science was done with such relatively simple instruments. And by how much was discovered and figured out when many measurements took so long and then doing the math that also took so long. Eric
The alternator, itself, was a big flywheel. But, with a several hundred HP steam engine with just one cylinder per pressure stage, the power pulses would be pretty strong. They did often put an additional flywheel in the system, but they were NOT infinite.
No, and I'm wondering if even oscillographs were available back in the
1890's. But, I can easily imagine some improvised methods, like mounting a small motor on Prony brake-like structure. The motor's stator is mounted on bearings, and allowed to swivel around the shaft. An arm rests on a scale to measure motor torque. You could replace the scale with a spring and watch the arm wiggle.
Anyway, the guys who were DEEP into the physics of this stuff, like Steinmetz and Tesla, could analyze the magnetic fields and the shapes of the machine's poles and figure out what the harmonics would be. So, they didn't do it ad-hoc, they analyzed what they needed to do to get harmonics (waveform distortion) to manageable levels, and built the machines that way, I do have some electrical engineering texts from 1910 or so that descibe how this was done.
Another thing that was well-understood way back then was power factor. It was typical to have one real big machine in a plant that had a wound-rotor synchronous motor driving it. By adjusting the rotor field, you cound turn it into a source of leading power factor, correcting the power factor of the whole plant. These were called synchronous condensers, ie. a rotary replacement for capacitor banks. I actually found a pair of these in a
1950's vintage facory built for the Korean war. They had a pair of HUUUUUGE Ingersoll-Rand air compressors of 50 - 100 Hp each. They had salient-pole synchronous motors that were about 8 feet diameter and one foot length. The piston was a foot in diameter and had a stroke of several feet, horizontally. On the wall, there were power factor meters that adjusted the rotor field strength to keep the plant power factor near 1.0
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