O&S wrote:
As I think about what has been my experience in operating the various items, makes me wonder if any and all of these various stated required wattages are actual or not.
Floyd (Old & Slow), it is time for class to begin again. Take out your notebook, pencil and calculator and listen carefully.
Before we get started with all the fun stuff, take a minute to look at this
site for specifications related to a particular brand of generators that has an extremely wide selection of models.
You will note there is a big difference in the ratio of running watts to surge watts in the majority of models. Some only have a 50 spare watts for surge, while others may have several thousand. Larger gensets have a higher ratio of surge to run watts โ we will explore more on this later in the lesson.
Why the difference in the ratios?
One, of course, is the method used to determine run wattage.
Typically, run wattage is a rating that will allow a generator to operate for a period of 30 minutes at an ambient temperature of 78 degrees F without exceeding 190 degrees F within the generator core. Notice I used the word "typically". Various other standards are often applied. I do not believe the standard I just quoted is carved in stone for the USA. Canada, on the other hand, does have some more specific standards for wattage ratings. Sorry, but I have not investigated what they specifically are.
According to data left by our ancestoral fathers of electricity; Ohm, Kirchhoff, Volta, and Watt, 746 watts of electrical power can accomplish one horse power of energy (provided there is no waste). When using a gasoline engine to produce electricity in a generator or alternator (wattage/power) typical efficiencies for the Chinese built 3,000 watt class gensets rarely exceed 70%. The other 30% is lost due to the conversion of chemical energy (fuel) into heat energy then into mechanical energy and finally into electrical energy. Note: The 70% figure is not for every Chinese or competive alternator (generator). The type of design will determine what the true efficiency will be. I am of the opinion that the alternator design introduced last Spring by CPE on their UltraFab, Home Depot and CSA models may indeed be higher than competing "conventional" designs in the small Chinese Generator Market. For the newer CPE gensets, 73% to even 75% efficiency may be a realistic expectation...... and this is NOT advertising or bias. Inverter models using Pulse Wave Modulation on their output signal are apparently more efficient than their all mechanical cousins as well.
One of the tricks used in advertising for generator output is to โjuggleโ numbers. For example:
In an electric motor wattage is the amount of power the motor actually consumes.
But, horse power is the amount of work (power) the motor produces.
(Read the above several times and think about it.)
Going further with the concept, an electric motor using 1,000 watts to produce one horse power is actually many times more efficient than the motor (or generator) that claims 1,000 watts as its power, but only produces ยพ horse power. It is much more costly to produce a motor with high efficiency that one with lower efficiency.
Part of the difference in published ratings is related to the efficiency game and how a company advertises, which can vary considerably โ and still be within the Truth in Advertising Guidelines. Both motors just cited can be advertised as 1,000 watt motors but the less efficient cannot be advertised as having 1 hp output. Still, in the numbers game one might find the less efficient motor advertised as having an
input power equivalent to 1-1/3 hp. On the container the motor is packaged in you will most likely find the 1-1/3 HP in big, bold letters that catch the eye, and the words input or equivalent input in very small letters. Not many people catch on to the game.
More and more appliances are being required to publish standardized efficiency ratings related to energy used compared to energy produced to reduce consumer misunderstand. An example would be the SEER ratings used in air condition systems. The higher the SEER number, the more efficient the system is regardless of wattage or motor horse power ratings.
Another factor effecting run wattage vs. surge wattage is the amount of energy stored in the electromagnetic field surrounding the genset power producing coils. In a high surge situation a generator will actually use energy from this electromagnetic field (also known as electromagnetic induction) as its own internal energy producing capacity is reduced by an attached load.
Then, as I pointed out in a comment made a day or so ago, โplaying with wordsโ can also confuse the consumer. Peak wattage will be 1.414 times the RMS wattage. It is easy to say a generator with 2,800 continuous watts has 4,000 peak watts. Others may rate their equipment using average wattage (average power) which is approximately 1.274 times the RMS value. That translates to a 2,800 continuous watt unit being advertised as a 3,567 watt generator. You may be lucky enough to find the rating method in small print elsewhere in the literature.
If the RMS term is not normally in your vocabulary, it stands for โroot mean squareโ (Vrms = Vo (1/2)1/2). More simply explained and understood: If we make a complete circuit using X resistance (X will be equal to 10 ohms for this example) and apply a DC voltage equal to Y (Y will be equal to 100 volts for this example) we will produce heat in the resistor. If we measure the heat produced we should find that it is 3,410 BTUโs (100 volts/10 ohms = 10 amps, 100 volts x 10 amps = 1,000 watts, 3.41 watts will produce 1 BTU, 3.41 BTU x 1,000 watts = 3,410 BTU).
The alternating voltage and alternating current required to produce the same amount of heat in the same 10 ohm resistor as the DC source did will be equal to the RMS voltage.
Want some more confusion?
Honda has a model EU2000i that it advertises at 2,000 watts. Then read the fine print and you discover the continuous rating is 1,800 watts.
Buy an Onan 3500 Homesite, but the running wattage is 2,500. Same sort of confusion with their 5500 which is 4,000 watts continious. If that sounds odd, consider that their MicroLite 2500 really is 2,500 watts and the PX4800 has 4,800 surge watts and 4,100 running watts. Seems to me consistency is really lacking in model number designation vs. true wattage. But, then again, there is no regulation that they must be equal.
Letโs get back to SURGE wattage. Most folks realize it takes more force to get an object moving than to keep it moving on a level plane. A Tow Vehicle (TV) for a travel trailer is an example. Much more power is needed to get a 7,000# trailer rolling to 25 mph than it is to sustain that 25 mph on a smooth, level road. Same is true for most electrical loads, especially electric motors. A motor that will run efficiently at 1,000 watts may need twice that wattage to get it moving. Where does the extra 1,000 watts or โsurgeโ of power needed come from? As stated earlier in this commentary, from a large inductive field or reserve horsepower from the gasoline engine and a conservative operating or run rating on the genset to begin with.
Whatโs the bottom line?
Efficiency will โnormallyโ fall in the 70% range for our small gasoline gensets. Thus an engine that really produces 6.5 HP at 3,600 rpm and is 100% efficient should deliver 4,850 watts. But the efficiency of our example genset is 70%, not 100%. So, 70% of 4,850 is going to yield 3,395 watts. At 60% efficiency we will be looking at 2,910 watts. We might get a smidgeon over 70%, but it takes a lot of work and money to design and build a small engine genset that will do that.
The second issue is the advertised HP rating. Most of the gas engines we have on our gennys are rated 6.5 HP from around 196 cc. But that is a
gross HP rating, not a net rating. In real life these engines do not deliver 6.5 HP at the end of the crankshaft. So, what do they deliver at the crankshaft with mufflers, flywheel fans and air cleaners installed? How about 5.5 HP? Using that HP figure we now whittle the actual, sustained wattage down to between 2,500 and 2,800.
Let's go back and look at that Onan 3500 Homesite I used as an example earlier. When it was first introduced to the market, it was advertised as 5.5 HP and 2,800 watts (there is a previous discussion on this). Later the engine HP became 6.5. I now strongly suspect the original rating was a fair and accurate rating. But, due to competive advertising Onan elected to get on the train, so to speak, and show their engine as being identical in power to the gross rating used by the competition. I don't think the engine changed at all, just the numbers.
None of this number juggling or tricky advertising is new. It has been around for years. You just have to go with the flow or educate yourself on all the variables so you may interpret the numbers as meaningful.
Now, who said all of this rating stuff was confusing?
Floyd - class is dismissed. Pop quiz can be expected next class meeting!
Professor Randy T. Agee & Nancy Agee. Also Oscar, the totally ruined Dachshund.
2009 Cedar Creek 5th Wheel - 2004 Volvo VNL670 class 8 MotorHome conversion as toter.
Turbocharged, 12L, 465 HP and 1,800 ft. Lbs. of torque.
