Choosing a Generator That's Right for You



Once you've decided to purchase a used diesel generator set, there are several considerations you must keep in mind when choosing which used diesel generator to buy, where to install your generator and how to install it. In this paper we discuss with you some decisions during the selection process. Choosing the right machine is not difficult if you take the time to determine your objectives and facility requirements carefully, and have a fundamental understanding of what to look for in a used diesel generator.  Remember that US Power’s sales engineering staff is available to help you with all steps of the generator selection, sizing and installation process. You will also need to know a few terms and have a basic understanding of the different types of generator sets and their operating principles. Please refer to our glossary of terms as needed.

The first determination you will need to make is whether you will require stand-by or prime power. Simply stated, prime power is required when you have no other source of power. A stand-by diesel generator is a backup to normal utility power. Making this determination helps select the size of the generator as diesel generator kW output is de-rated approximately 20% for prime use.

Generator sets produce either single or three phase power. Choose a single phase set if your motors are 5hp and smaller. Three-phase power is better for larger motor starting and running. Most homeowners will require single phase whereas industrial or commercial applications usually require three phase power. Three phase generators are set up to produce 120/208 or 277/480 volts. Single-phase sets are 120 or 120/240. Use the low voltage to run domestic appliances and the high voltage for your motors, heaters, stoves and dryers.

We recommend diesel generators due to their longevity (30,000 plus hours) and lower operating costs from maintenance and fuel fuel efficiencies.  Is is just a fact that modern diesel generators are more rugged, quiet and normally require much less maintenance than comparably sized gas (natural gas or liquid propane) units.  But one of the most attractive features is that fuel costs per kW produced with diesels are normally thirty to fifty percent less than comparable gas units. One factor that makes diesel generators more rugged is that they run cooler and are built on heavier duty engine blocks. For instance, whereas 1800 rpm water cooled diesel units operate on average 12-30,000 hours before any major maintenance is required, similar 1800 rpm water cooled natural gas generators normally operate 6-10,000 hours before rebuild because they are built on a lighter duty gasoline engine block. One thing to keep in mind is that 3600 rpm air-cooled gas units are normally replaced (not overhauled) at 500 to 1500 hours. And again , because the gas units burn hotter (higher btu of the fuel) you will see significantly shorter lives than the diesel units.

Electric equipment is designed to use power with a fixed frequency: 60 Hertz (Hz) in the United States and Canada, 50 Hertz in Europe and Australia. The frequency output of a generator depends on a fixed engine speed. To produce 60 Hz electricity, most engines operate at 1800 or 3600 RPM. Each has its advantages and drawbacks. 1800 RPM, four pole sets are the most common and least expensive. They offer the best balance of noise, e
fficiency, cost and engine life. 3600 RPM, two pole sets are smaller and lightweight, best suited for portable, light-duty applications.  To give you a word picture, it is similar to driving your car at 120 mph, versus 60mph – at 60mph your car will last longer, is quieter, requires less maintenance and will have a longer life.  Most 3600 rpm units are twin cylinder air cooled lawn mower engines, while the water cooled 1800 rpm units are comparable to those found in forklift and tractor engines. This is why the 1800 rpm water cooled diesel generators are more expensive to purchase, but remember that they will last longer, offer less maintenance problems and be more fuel efficient. Your decision here is typically based on your budget and facility needs.

For long life and quiet operation we recommend four cycle, liquid cooled, industrial duty diesel engines. Whereas air-cooled engines require a tremendous amount of air and may require ducting, they are also noisy. Liquid cooling offers quieter operation and superior temperature control - which means longer life. One way to save time servicing your generator and money for new filters is to purchase a large, integral air cleaner with replaceable filter element and a critical muffler, which is built into the exhaust manifold. This saves the need for an additional muffler.

On a DC electrical system the standard 12 volt system should include a starter motor and battery charging alternator with a solid state voltage regulator. Diesel units should also come with a pre-heat switch and start/stop switch, safety shutdown system to protect the engine in case of oil pressure loss. The generator should also have overspeed, overcrank and high water temperature alarms attached to the DC system circuit breaker.

Your AC generator should have a 4-pole revolving field. An automatic voltage regulator will provide “clean” power which is necessary if you are backing up any computuer equipment or sensitive electronic systems.  Normal utility power is +/- 5% voltage regulation; many 1800-rpm models are even better +/- 2%!  But this is something to confirm when evaluating each diesel generator.

ENGINE ACCESSORIES: Sound Attenuation, Filters and Block Heaters
Upon determining the generator size you will need, make a list of optional and installation equipment you require. For noise abatement, we recommend a residential (not industrial grade) muffler. Many cities require sound attenuated enclosures (70dba or quieter) as well. A good primary fuel filter/water separator is a must to protect your engine's fuel system. Stand-by sets may require a block heater to keep the coolant/water mix at an adequate temperature for easier starting, and a battery charger is always a good idea.

All generator systems require a circuit breaker and a distribution panel.  The circuit breaker protects the generator set from short circuit and unbalanced electrical loads. The distribution panel divides and routes the connected loads and includes circuit breakers to protect these loads. Switchgear can be as simple or complex as you want or can afford. Of course, as complexity increases, so does cost. Most applications will only require a base model automatic transfer switch with a digital panel and weekly exercise options. Some facilities however require soft-load and parelleling switchgear. Balance and a good electrical advisor are the keys here.

Please keep in mind – US POWER & ENVIRONMENT is a turnkey Power Solutions Specialist that can provide for all your generator needs from 3 Mega Watt units for industrial customers seeking heavy duty industrial standby generators to residential models. We can provide you with diesel, natural gas or propane powered units. We offer only top quality products that are priced at or below our competition who offer lesser brands!