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Archive for the ‘AC’ Category
Thursday, April 2nd, 2009
I’ve been looking at different generators after reading some of your articles. My goal is to build a home that uses generator power while I slowly shift to solar as it becomes financially effective. That being said I’m looking at using a propane generator so I can have a propane stove, and a propane water heater for radiant floor heating. Using this much propane I figured that a propane generator just made sense in order to stick with one fuel source.
Here’s the problem – all the Kohler generators I look at all output AC power and some output it in single or triple phase. I thought it would be better to get a generator that outputs DC to the batteries then use an inverter to output to AC.
With most examples I would have a generator that outputs AC which would have to get converted to DC which would get reconverted to AC when I use it. Isn’t this an awful lot of energy loss?
Isn’t there a propane generator out there that runs at 1800 rpm and will output DC? I mean after all, a propane motor can have different kinds of generators hooked up to it can’t it?
Up until around 1995 what you are trying to do was very common as most DC to AC inverters were not as reliable and could only produce a modified wave form output which was not very good to power any type of battery charger. There were several manufacturers making a generator that consisted of a gas or propane engine driving a heavy duty truck DC alternator for direct charging large batteries. There are a few still being made, but since todays higher quality inverters now have perfect grid quality (or better!) sinewave output and very high capacity battery chargers built in that are designed to operate from either grid power or a generator, we now design everything around normal 120 or 120/240 home wiring systems.
Since these new inverters are so efficient, the system losses are almost the same when using a high quality inverter to charge a battery bank with a generator to power the inverter, as it would be to use an engine driven alternator to charge the same battery bank. Also, since your generator would be supplying 120/240 volt AC, you can supply larger AC equipment and power tools directly that you could not do with a DC output generator. Unless you are planning a very small battery system and are trying to avoid the cost of an inverter, I would go for a high quality generator like the Kohler which we prefer since it uses a very simple 2-wire start/stop circuit. This makes it very easy to remote control from almost any inverter, while many other generator brands require complex 3 or 5 wire start circuits which sometimes require buying a separate control box to allow remote start/stop control with an inverter.
Tuesday, March 31st, 2009
I have a 7500 Watt AC Generator and have a output voltage problem. If I set speed to 60 cycles my AC voltage climbs to 165 volts. Can you direct me to points of this problem that may help me correct same?
The normal rated output should be 60 cycles and 120 volts AC
There are many possible reasons you are having this problem and each generator brand and model have different designs that could cause this. First, what type voltage meter are you using? If you are using a $20 Radio Shack model, then your meter is giving you a good indication that there is a problem. If you are using a high dollar RMS volt meter, then this may be telling you something else. The actual “peak” voltage of the utility grid and a 120 volt nominal generator is actually 169 volts. This is because the voltage is changing every 1/60 of a second from a high of +169 volts to -169 volts and passing through 0 volts twice, so 120 volts is the “average” of this sinewave curve.
As you start adding loads to a lower cost generator, the internal voltage regulator allows the voltage to drop as it tries to maintain current for the load. This means the peak to peak voltage of 169 volts will start to drop. Since all battery chargers use only the “peak” part of the sinewave, as soon as this peak voltage drops, most battery chargers will stop charging, which is why you should not use a low cost generator to charge a battery bank like you would find in a solar home.
Some generator designs are based on a rotating coil that is supplied a DC current, and by varying this DC current the generator can control the AC voltage output from the fixed coil. Sometimes the voltage regulator device used to vary this voltage includes a control that you can adjust.
Some higher cost generators include a special “inverter” electronic circuit connected to the generator output to stabilize this voltage and maintain the 169 volts peak to peak at a perfect 60 cycles per second. For example, most Honda generators add a “i” to the end of the model number if they include this option.
Odds are your generator has a circuit board that is used to maintain the voltage output and something has gone wrong with this control board. Even lower cost generators have some type of control board to regulate the voltage output even if they are not as accurate as the inverter models.
Sorry, but sounds like this is going to cost,
Monday, March 23rd, 2009
Is there a UL product out there similar to a DC grid tie in inverter that would work for an AC generator?
I am aware of how the DC is converted with an inverter to AC and the phase angle is adjusted as well as the phase voltage to maintain proper alignment of electric main power. I have seen wind generators plugged directly into a breaker in the main panel as a grid tie in and initially was confused on how this was causing a dead short. However, I understand how it is possible now (there is an inverter in the wind generator head which adjusts Phase angle and Phase voltage to allow simultaneous feeding), but, haven’t been able to find anything for AC generators.
What I am looking for is something that will go between my AC generator and the main panel that will allow me to directly connect to the panel without a dead short, and allowing back feeding to the main line. Also an automatic shut off or transfer switch that would turn off the generator’s power supply from pumping electric back into the power grid when the main power goes out.
I am looking for an AC grid tie in device that will not cause a dead short.
Your thoughts would be appreciated,
I can’t imagine why anyone in the world would want to do what your are suggesting, although it is technically possible. Yes, there are special “in phase” monitors that will sync a generator with the grid, but this is only dome with very larger generators like you would find in hospitals or military bases. The main reason they do this is they are on a time of day rate and if they are approaching a peak demand period, by running their generators in sync with the grid they can save hundreds of thousands of dollars per year because these rates usually carry over the high penalty for the next 11 months. In addition, this switchgear to sync the generator with the grid costs more than your house and requires all kinds of coordination between the utility engineers and your installer. You will also be required to carry a huge liability insurance policy listing the utility as the insured in case something goes wrong and your power feeds back into a down line and kills a lineman.
Inverters for solar and wind systems have special circuits that make sure power is not fed back into the grid during a power outage, and the inverters must supply this power within a very narrow window of voltage and cycles. Most smaller generators are not that easy to maintain a constant output under varying loads.
Now that you know the legal issues involved with doing this, I will give you the simple reason why nobody in their right mind would ever do this:
If you count the cost of fuel, generator maintenance, repair parts, added oil changes, and annual service, you will be paid about 20% of what your actual costs will be, because you can never ever generate electricity cheaper than a utility. The only reason many of these solar and wind systems are doing this is either they are receiving some type of grant or tax credit, or they have been given a special feed-in tariff rate.
Many people still install solar systems as some systems also offer emergency backup, or at least do not need constant fueling or make noise like a generator. The national average cost today to generate electricity from a solar system is 35 cents/kWh. The national average for grid electricity is 9 cents per kWh. You figure it out.
Thanks for the insight and your knowledge on these subjects is amazing. Everyone at times has an idea and can’t figure out why the situation is the way it is; until they ask an experienced individual.
Thank you again. So if I wanted to have some sort of “economical” unit (and I use that term loosely), we would be best off using a UL device DC based unit (solar, wind, hydro) with an inverter monitoring the phase angle and voltage back to the main panel which would shut off when the main power goes out? If we wanted any sort of power when the main power goes out we would be using a battery system or a separate backup AC generator/transfer switch which would not back feed to the main power until main power comes back on. We would then turn off the generator and manually switch the transfer switch back to main power?
I guess I am in the same boat as most people trying to figure out what will work best for my individual circumstance. Oh, yeah I’m not a millionaire so the in phase monitoring for an AC generator would be out of the question.
I will probably be investing in a solar or wind powered DC unit with an inverter for my needs. Would you recommend any companies that you have had a positive experience working with for these devices. Probably, a 2KW-5KW max output would be what my financial situation will allow.
Jeff I can’t thank you enough,
Any inverter designed for grid tie in the US should have the automatic transfer function built in. If you select a battery based inverter, it will also include a built-in transfer switch to disconnect from the grid. Some battery based inverters include a “second” transfer switch to allow also connecting a generator and the grid, and will switch to the generator when the grid is down and the battery charge is low. If you select an inverter that does not have the second transfer switch, then you will need a generator that includes its own transfer switch panel.
Friday, January 23rd, 2009
I just read your article on solar powered refrigeration. I agree that efficiency is very important as being off the grid, you have a limited amount of power. We are generating our own solar power but we run 120vAC from an inverter. Your article was for 12vDC fridges and freezers. I was wondering about 120vAC fridges instead. I would think they would be much cheaper but perhaps really efficient ones are not available as generally power availability is not a problem. If you have any thoughts on 12ovAC fridges vs 12 vDC fridges, I would appreciate hearing what you have to say.
Dwight Yachuk and Lucy Willemsen
Dwight and Lucy:
You said you had a solar powered inverter but did not say if you were actually using it to power your refreigerator, or you were just considering adding a refrigerator to your system and were not sure which to buy.
I will admit that refrigerators and freezers have really improved in the past 3 or 4 years, and they are now putting much more insulatation in the walls and using more efficient compressors. However, if you have a small solar system like I described for an off-grid cabin or weekend retreat, any refrigerator or freezer will be a really big load on the system.
The bottom line is, I have not in the past 20 years found any 120 volt AC refrigerator or freezer that was even close to the low energy usage of the solar DC refrigerators and freezers made for off-grid solar power systems.
For example, a SunDanzer 8 cubic foot freezer is rated to draw 0.36 kWh per day at 70 degree room temperature, and a SunDanzer 8 cubic foot refrigerator is rated to draw 0.09 kWh per day at 70 degree room temperature.
The 12 cu.ft. SunFrost refrigerator/freezer requires 0.29 kWh at 70 degrees room temperature, and the 16 cu. ft. SunFrost refrigerator/freezer requires only 0.48 kWh per day at 70 degree room temperature.
Now compare this with the Energy-Stare rated super-efficient General Electric 14.3 cubic foot refrigerator/freezer which draws 1.4 kWh per day, or 3 times as much as the same size SunFrost.
If I had a very small solar system and an off-grid weekend cabin, the much higher energy usage for a standard 120 VAC refrigerator, plus adding in the energy conversion of about 10% for the inverter, means your you may not have the power or batteries to power it.
If you live in a larger solar home with 4 to 6 kW of solar array, then buying a high efficiency 120 VAC refrigerator may not be a problem for your system, but it still will be the highest energy user in your solar home.
Thanks for the very informative reply. I had been thinking about adding an electric powered fridge to our new “winter” home we are designing for across the lake where our present “year round” and future “summer” home is located. We are on the east side of a narrow lake and only get 4-5 hours of sunlight a day now, with no sun before noon, so we are very solar challenged, hence building a new home to take advantage of the winter sun.
I had only been thinking of 120 VAC systems, but since writing you we thought that we could run dual 120VAC and 12 VDC systems off the same battery bank, even to the point of wiring the house for both. In that case, putting in a 12 VDC fridge, especially a very efficient one, would be practical, and economical powerwise.
We are still in our investigative phase but running a dual system is starting to make sense. The alternative would to use a propane fridge just as we are using now in the cabin. Thanks again for the info.
Dwight and Lucy
Dwight and Lucy:
I do not think what you are planning as far as the wiring you describe is the way to go, and there are too many technical reasons why to list here.
I strongly suggest that you enlist the help of a solar power system designer to guide you before you build. Otherwise, it may be costly to make the needed wiring corrections after you close in the walls.
Friday, November 14th, 2008
We recently had our home completely remodeled from top to bottom. As part of the construction we had all new wiring, panels, and associated electrical work done brand new. Additionally we had a 3.2 KW solar array mounted onto our new metal roof. This solar array is grid inter tied with a 8 gel battery backup connecting 3 circuits in our home. We moved in on December 15th 2007 and have lived without any issues until June 7th, 2008.
On June 4th, 2008 we had a violent wind storm that dropped a tree limb on the power line serving our street. The power went out, our battery back up kicked on and all was fine. On June 6th the power company restored power to the street and we were back running on AC power from the power company with the solar array supplement.
Starting from June 7th through the date of this letter my wife and I have been experiencing physical discomforts such as intermittent “electrical shocks” and what feels like a mild electrical current running through our bodies. Additionally we have been getting recurring dull headaches where neither of us have had them before.
We have had our electrician, solar company, city inspector and power company out to check things out and all have said that they cannot find anything wrong.
The only time that we find complete relief from these physical symptoms is when we shut down the power at the utility junction breaker.
I am reaching out to you to see if you have any ideas of what may be causing these physical symptoms. We are both sane individuals in our early 40’s with no history of mental or physical ailments of this type.
I hope to hear from you soon.
I am not a doctor and don’t play one on TV either, but I can provide some information that may at least rule out some things.
I won’t insult you about how many of these type problems can be psycho-somatic, as I am sure you already know that and still feel there is something physical involved.
You did not describe your system but said it was grid-tie and battery based. From this I can only guess that it is a 48 volt DC system which would hardly give you a shock even if you touched the battery terminals unless you were standing barefoot in water! Of course many grid-tied solar arrays are wired for over 400 volts DC which can give you a severe shock.
You said the solar array was installed on a metal roof and you had a recent strong storm, and all this started after the storm. I have seen bad wind and rain storms cause one or more of the array wires to short out on the array frame or rub a bare spot on a array wire touching the metal roof. Normally this would trip the array ground fault breaker, but you may not have one. Also, if the metal roof has a poor ground or the ground fault circuit was wired wrong, if the array is now energizing a metal this may not trip the ground fault breaker.
I am not sure how even a metal roof could cause the problems you are experiencing even if it is energized, but this is easy to check. Did the service installer do a “meg-ohm” test of each array down lead in reference to ground? We do this for every installation to make sure there is no dead short or electrical “leakage” to ground or to the metal roofing.
Are you near high voltage power lines and could your metal roof be acting like an antenna? Again, making sure the solar array frame and metal roof are grounded to an earth ground would keep this from happening. Does your solar array have lightning arresters installed? These can fail after a severe lightning storm and may allow electrical path to the metal roof since they are wired to both the positive, negative, and ground wires. These are also easy to dis-connect and check for shorts.
Some inverters can develop a very high frequency “hum” that can be very annoying and can act like a dog whistle that most people cannot hear. This type of high frequency noise has caused headaches in some test subjects, but I have not heard anything specific related to an inverter. This can be checked with a sound pressure meter which can be purchased at Radio Shack. It can check sound levels in many different frequencies, and I would expect you are concerned with those frequencies just above normal hearing range or ultra-low.
If these checks prove negative, then shut the system down for a few weeks and see if anything changes.
Hang in there,