Many of us own a generator. But how much research did you do before purchasing yours? Generator system integration into you home power design is frequently a series of tradeoffs. I’m going to cover how generators work, potential design features, trade-offs, and strategic considerations. I actually own four generators of various capacities, fuel types, and features, each for slightly different purposes. I also work doing generator fleet maintenance.
I like to break generators down into an alternator, and engine, a DC control system, an AC control system, a fuel system and a cooling system. Generator sets are typically powered with gasoline, diesel, natural gas or propane engines. The engines typically run at 1,800 RPM, 3,600 RPM, or are variable speed. The engines will be air cooled or liquid cooled. Alternators can be permanent magnet, brushless or they cnd bedesigns that use traditional brushes. The AC waveform can be made by the alternator or byan inverter/converter. The AC frequency can be created by the engine/alternator or by the inverter converter. The AC voltage can be controlled by a voltage regulator, the inverter/converter or by a circuit in the alternator. They can be pull-start or electric start. The generator manufacturing industry combines all these to create generator sets with different performance characteristics at different price points.
All the alternators rotate a magnetic field inside a set of wires (windings) to convert the rotating energy of the engine into electricity. The rotor is spun by the motor, and the stator is the stationary copper windings that produce the electricity. The best alternators are brushless with four sets of windings, the exciter stator, exciter rotor, and the rotor and stator windings. The voltage regulator (VR) sends a DC voltage to exciter stator which passes the energy electromagnetically to the rotor to create the rotating magnetic field. These alternators can produce high quality power, they can handle the stress of motor starting surges, and can last for tens of thousands of hours with simple bearing and diode replacement. They tend to be found in commercial generators.
A brush alternator still uses a voltage regulator, but passes the VR DC energy to the rotor via brushes. Brushes wear out, but power quality is still fairly good and brushes can be replaced. Inverter generators typically use an alternator that spins a permanent magnet as the rotor to create the voltage for the inverter. No brushes to wear. PMAs are typically smaller than other alternators, but require the inverter. Power quality is as good as the inverter. Consumer household generators may also contain a simple brushless alternator that uses an extra winding and capacitor in the rotor to somehow energize the rotor. They are reliable, inexpensive, but don’t regulate the voltage very well. My brushless consumer generator produces 129 VAC at zero load and steadily drops down to below 120 VAC by teh time it reaches 2/3 load.
I also have a generator with brushes and a voltage regulator. It produces 128-129 VAC all the time from zero to as much load as I could pull from it. The manufacturer told me they set the voltage high to allow for extension cord voltage line loss. The voltage regulator can be adjusted to a lower voltage, if that is what is wanted. I plan to order a spare pair of brushes. Voltage variability on the inverter will depend on the design of the inverters. I checked the two I own and one dropped about 4 Volts in the transition from zero load to ¾ load and the other dropped less than 2 Volts in the transition from zero load to ½ load.
If you have an alternator preference, then how do you tell what kind of alternator you are looking at? Go online and download the parts breakdown from the manufacturer’s or seller’s website. Find the alternator parts breakdown. If you see brushes in the figure or parts list for the figure you have an external voltage regulator. If you see two sets of windings on both the rotor and the stator, a diode pack on the rotor, and a voltage regulator in the set then you have a commercial brushless design. If you see a capacitor on the rotor and the generator set is very competitively priced, then you probably have a brushless design without the voltage regulator. Permanent magnet alternators are primarily found on the inverter generators. If all this sounds like Greek to you and you are particular find a friend who understands electricity and generators to assist you with investigating your choices. I should mention that I’ve phoned the original equipment manufacturers (OEMs) when sales people couldn’t answer my questions.
Engines and RPM
The higher the revolutions per minute (RPM), then the faster the engine will wear out, on any given fuel. Moving parts = wear with force adding to wear. This is even a good predictor of which parts will wear out in your engine. Most medium size commercial generator sets run at 1,800 RPM. A good 1,800 RPM commercial set with maintenance (especially oil changes) can run for 10,000 hours. There are small diesels typically of less than 20 HP that can run at 3,600 RPM which reduces size and weight, but 3,600 RPM is really pushing a diesel. We typically replace our 3,600 RPM diesel engines at lower hours than in our 1,800 RPM generators.
A good indicator of engine life, independent of fuel, is the cylinder wall material. We have small diesels at work with aluminum blocks and cylinder walls that wear out quickly. Buy cast iron if you can. Our larger diesels have cast iron blocks and/or cast-iron cylinder liners. They are known for their long lives and they can be rebuilt. Engine noise is typically less at 1,800 RPM versus 3,600 RPM reducing sound attenuation cost or requirements. Running at 1,800 RPM does increase the cost of the engine, since a bigger engine is required to produce the required engine HP at the lower RPM. In the 1970s, Onan made RV generators than ran at 1,800 RPM. They were big, heavy, but they just ran no matter what the conditions. They were tough. Their newer ones run at 3,600 RPM, are smaller, lighter, but they aren’t as robust nor tolerant to high temperatures.
Fuel and engine types
If efficiency is your primary consideration, then purchase diesel engines. They operate at higher cylinder pressures that enable them to operate more efficiently than gasoline engines. If upfront cost is the primary consideration, gasoline or one of the gasoline/propane engines will be much less expensive. If life cycle cost is the primary consideration, then you need to determine operating hours over estimated life, estimate fuel costs for different types of fuels, examine specific generator fuel consumption data, and the cost of different engine types from which the cheapest alternative can be calculated. If engine life is the primary consideration, then choose an 1,800 RPM generator, whether it be diesel or propane.
Another consideration for the infrequent generator user is keeping your fuel fresh. What kind of fuel do you have around the house? Can you rotate through the fuel that you have in storage? I have propane for heat and all my vehicles run on gasoline so my generators run on gasoline or propane. My vehicle fuel storage has the dual purpose of running my generators. Gasoline must be rotated, even with preservatives, which limits how much gasoline I can store. Propane can last 30 years in a tank. I chose to purchase two generators that can run on gasoline or propane to increase how much fuel I can store without increasing how much fuel I have to rotate.
I have a 500-gallon propane tank in the ground for house heat. I’ve also accumulated 20-pound tanks and I’ve purchased 100-pound tanks. With a bug out place, I would install a couple thousand gallons of propane storage, probably over time, as funds allow. The portable tanks were purchased, filled and stored. Typically, the larger the propane tank the more cost effective it is to store propane. The problem with big tanks is moving them if you sell your property, though I’ve read that propane companies can pump your tank out. Propane tanks can also be buried reducing the risk of losing your fuel supply from incoming bullets. Propane is less energy dense than gasoline, but it burns cleaner.
(To be concluded tomorrow, in Part 2.)