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Watch that Voltage! This page is under construction and includes musings and ramblings, in my typical style. Most Generators sold have no means to set the voltage independent of the frequency (RPM). This is true of the majority of Hardware Store Generators. Although they usually have a field, the field current is preset at the factory to give a satisfactory voltage at 50 or 60hz depending on your standard. Typical hardware store generators and the ST generator design have quite a bit in common, the ST is bigger, heavier, and uses a salient type rotor, but the simple excitation system is very similar to my old Coleman Briggs and Stratton Generator I bought at Home Depot some years ago. One of the handiest and cheapest tools a generator owner can buy is the 'Kill A Watt', a quick Google search will get you more info about it, I consider it a 'must have' and for little more than $20, you'll have a handy reference for voltage, Current, and frequency. If you think this handy little device is limited by it's inability to read high current, wait till your Read Bill Roger's Simple modification to extend the current capability to cover most of our needs! First a story: Years ago, I bought a Coleman Generator, and I used it camping for about three or four years, I even managed to run a small skill saw off this 1850 watt unit. I kept the blade sharp, and went slow, and used it only when the need was high. There was that day that I became real curious about the governor speed, and I made the effort to borrow a frequency meter from work. To my surprise, this little unit was running at about 55hz, and I'll bet it left the factory that way. Yes, the voltage was a little low, but I ran TVs, entertainment systems, and all kinds of stuff without a problem. I adjusted the frequency to nearly 61HZ (no load), which is a typical trick, as this gives some room for the governor to react, and it usually ends up around 60hz with a medium load. I found a more favorable voltage, and that little skill saw ran a lot better. I thought back at some of the loads I had run, and remembered a pellet stove that I ran at home during an outage, the pellet stove worked, but I noticed that the glass had smudged up fairly quickly, and all of a sudden it dawned on me as to why that was. Induction motors are sensitive to the frequency, their RPM is directly related to the frequency of the generator. In this particular pellet stove, an induction motor was used to bring in air for combustion, and a restriction was tuned at the factory to deliver just the right amount of air. Since I was running 55hz instead of the expected 60hz, the fan turned slower, and delivered less air, this caused the pellet stove to run with less efficiency. This whole issue of motors can be confusing at first, what is an induction motor, and what's not? there are exceptions to the rule, but most vacuum cleaners are not induction motors, skill saws, compact table saws, jig saws, kitchen blinders and other Kitchen appliances, plus lots of shop tools >often< makes use of universal series wound motors, this type of motor doesn't really care about frequency, and some will even run on DC of the proper voltage. Series motors have brushes, induction motors don't as a rule. Induction motors are typically found in Fans, well pumps, Air compressors, some of the bigger table saws, drill presses, and more. these loads might run quite well between 58 and 62 hz, but the RPM of the motor will change a little as the frequency of the generator changes. Most of the time this is not a big deal. If you run a universal motor at the end of a long extension cord, (skill saw is a good example) you will sometimes overheat it and literally 'smoke it', this is easy to do when you run this type of motor off a generator delivering low voltage as well, so I'd say the 'kill A Watt' is one of the first investments you make! Low voltage is not good, but some loads are far more forgiving than others. If you are running skill saws, use a high quality heavy gauge extension cord , and use the shorter cord. those 100 foot cheap extension cords have ruined a lot of equipment, and some folks have no clue why their saw or compressor went up in smoke! Got a compressor for a nail gun? Use a short power cord, and a long compressor hose! High voltage can be another problem, and it is best we pay attention to our standards. In North American, the IEEE Standard says the max allowed voltage is 127volts. Some of us call this 110 volts, and we call the higher voltage found on our household electrical grid 220 volts, but in most places it's really more like 120/240, and we might do well by calling it that. The upper limit for 120/240 is 127/254. The reason you don't want to exceed the Max IEEE figures, is that appliance and tool makers engineer their products to tolerate this voltage, exceed it, and all bets are off, you might find a short appliance or tool life, or a fire hazard. I have a small plastic cased heater in my shop I use for a load, I wrote on the top of the heater that it was carrying 1292 watts at 114 volts, when I was testing something in the past. Tonight, I was running a prototype generator at 130 volts and noted the power reading of the 'Kill A Watt' was 1736 watts. The tag on the heater claims it's a 1500 watt max unit, but I've noted many actually deliver less than the tag says on our commercial power here at the shop (which is 120-123 volts typically. Would it eventually catch fire being fed 130 or more volts? Why take a chance? If we burn something up, or worse yet, start a fire operating above the IEEE max voltage standard, we could find ourselves liable for something. If someone asks you, what's most important, having the right voltage, or having the right frequency? The right answer might be, always run at less than the IEEE maximum voltage, everything else may be secondary. Having the right frequency is 'more' important when you have induction motors for loads, but again, do not exceed the IEEE max voltage. I have found a cycle or two one way or the other is not all that important with most loads like air compressors or well pumps. Equipment that relies on a more precise RPM of the motor are rare, and my example of the pellet stove blower motor is a far example. If your generator has a simple field, there are ways to adjust the voltage if it is too hot at 60hz, and in some cases, you can even increase the voltage by simple means. If you have a PMG (Permanent Magnet Generator), you will pay attention to the voltage, and you will lower the prime mover governor speed to keep the voltage below the IEEE max. Voltage regulation is an interesting topic, and in many; if not most cases, I think manufacturers of generator sets take the wrong approach to regulating the voltage. Typically a Generator set is designed to operate at a voltage of plus or minus a percentage of 120VAC, here in North America. If we had a generator that was designed to operate at plus or minus 5%, then we'd expect to see around 126 volts at no load, and no less than 114 volts at the full rated output of the Generator. Depending on the generator and the prime mover, the generator may carry more than full rated load, but the voltage >might< drop (droop) lower than the 114 volts. There is no such thing as a free lunch, 99% of all generators sold do NOT have voltage regulators, and this often means they are more reliable. I can not tell you how many emails I have received from people looking for voltage regulators for their 'out of service' generators, and often the cost of a replacement exceeds the value of the generator. If you retrofit your generator with something other than an exact replacement Voltage Regulator, you will note that it is possible to exceed the current rating of your field, and literally burn it up. If you are designing your own VR, you will NOT be the first to 'SMOKE' your field winding, and you should place a fuse in series with your field or take other measures to assure you don't exceed the maximum current it can handle. A typical voltage regulator works to regulate the field Current, as the generator load increases, the frequency often drops off a little, and this is totally dependent of the Governor and it's ability to regulate the RPM, RPM and frequency have a direct correlation. Sometimes a poor governor design, poor maintenance, or poor setup of the governor can cause an excessive drop in frequency when the generator is loaded. Most voltage regulators work independently of the Governor, and in the case of a malfunctioning or poorly designed governor, the Voltage regulator will attempt to keep boosting the field current, and attempt to raise or maintain the voltage even if the frequency has fallen below a reasonable figure. There are many ways to design a voltage regulator, and no doubt, there may be some regulators that might monitor the frequency and have some built in provision to disconnect or open the field when the frequency has dipped below a certain preset. Don't assume anything, and learn as much as you can about how your particular Voltage Regulator works. As this time, I think it would be just as easy to regulate the governor as it is to regulate the current in the field, and I see this as a more appropriate means to control the typical voltage 'droop' (drop). In my mind, poor governor performance is often the source of the problem, and boosting the field current to make up for RPM drop, (as the voltage Regulator often does) is just applying a Band-Aid to the real problem. More precise control over the governor can give us all the benefits of the 'KISS' generator design and more. Imagine having a generator system like my friend has in his $275,000.00 diesel pusher motor home, the generator control board burns up, and you have no way to manually start it, or make power. Yes, according to Murphy, this will happen during the storm of the century, and your neighbor will be running his bargain KISS generator next door, making ice, blending Margaritas, and inviting the neighbors in to enjoy the Air Conditioning during the extra humid 95+F days. You might be sweating like a pig, and eyeballing those funny looking fried components with 'in house' part numbers on them. You call 1-800-help, and someone in India answers, he can understand you, but you can't understand him... Possibly, the best setup for the DIYer is to optimize the mechanical governor, and then fit it with an electronic means to 'BIAS' the governor for more precise control. In this way, if the electronic bias fails, you still have your "KISS' system, and you still function. Adding this control is simpler than some might think, and my prototype uses a simple inexpensive model airplane servo. This is by no means an original idea, Briggs and Stratton even offered an electronic governor making use of the airplane servo, and might still sell them for all I know. Here's how it works (basic overview) Plug in a transformer (often called a wall wart), 2nd hand stores have at least 100 different ones, and they're often 99 cents each. You'll be looking for a unit that produces low voltage AC. At the output of this transformer, you will wire up a Schmidt trigger and send the output to an I/O pin of a simple controller like the PIC, or an all too easy to implement Stamp. Next you'll place a full wave bridge rectifier and then a 7805 volatge regulator to provide a supply voltage for your Microcontroller. If you have never played with a Stamp before, you'll be amazed as to how easy and fun it is, and soon you will realize all the other things you might have the controller monitor and how easy it is to do it! As part of your design, you now have a voltage reference, and you can compare this to the voltage output of the Gen Head, and send an order to open your generator contactor under the abnormal conditions you specify. The generator is started in it's normal manner, the transformer delivers juice to the processor, it boots up, counts the cross overs of the AC sine wave from the gen head, and determines the frequency. From this, it decides where it will position the arm of the servo to 'BIAS' the governor, and more accurately adjust the frequency. This is done thru a set of push pull springs between the servo arm. and the throttle arm. since, the controller can observe both frequency and voltage, you can set the parameters, and you can open the generator contactor when there's trouble. What makes it so easy to drive this servo is a built in PWM (Pulse Width Modulation) function inside the controller. You can simply write to an 8 bit address and have up to 255 different positions you can set that servo arm to. You can even use the extreme closed position to perform an emergency shut down function. By now, you've looked thru the simple to understand tutorial that came with the STAMP, and you realize you can easily read high coolant temp, low oil pressure, high gen head temp, and you could easily fit a strain gauge to the shelf in a nearby fridge to alert you to the fact that you are nearly out of your favorite beverage! It is only your imagination that will limit the possibilities, and properly implemented, flipping off the switch, leaves you with a 'KISS' system. It it works fine, if something goes wrong, you can still function in the kiss mode. Ordering the Stamp and the tutorial might be the most fun you've had in years. Now it's time to mention the PMG, (Permanent Magnet Generator), or PMA (Permanent Magnet Alternator), I believe these will become far more popular, and since there is no field winding to fail or regulate, we need another approach to regulate the output. It is simple as I see it, and that is to use the microcontroller to 'BIAS' the governor as mentioned above. In 'KISS' mode, we adjust the Gen Set to provide 127 volts max, (NO load), and let the generator set droop as it may. In enhanced mode, we 'BIAS' the governor to provide the voltage or frequency per our stored parameters. More info on Stamps http://geocities.com/SiliconValley/Orchard/6633/started.html There are many others.
Got feedback? send me a message All the best, George B
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