So your customer is interested in purchasing a generator set. There are several items to consider when choosing which set to buy, where to install it and how to install it. This guide will help you make informed decisions during the selection process.

Choosing the right gen-set is not difficult if time is taken to analyze your customer’s requirements carefully.  The manufacturer’s sales professionals are well trained to assist with all steps of the generator selection and sizing process.

Familiarizing yourself with a few terms and having a basic understanding of the different types of generator sets and their operating principles, is key. Installation requires expert assistance and a strict adherence to local codes and regulations. You, as the contractor or electrical consultant, are the first step in managing the installation .

Stand-by or Prime Power, and Right Sizing

The first determination necessary is to determine whether your customer requires stand-by or prime power. Simply stated, prime power is required when no other source of power is available. Stand-by power is the backup to normal utility power.

Next, size the future generator system to your customer’s requirements. Take a look at the alternator options. Choosing too small an alternator will require you to automatically or manually shed some loads to use the system without tripping breakers. Too large an alternator will not allow for the best efficiencies and can cause a loss of money and possible maintenance issues, especially with a diesel-powered generator. There are many ways to calculate size and it’s best to have a full audit done of the home or business by a power and/or electrical professional first, to assure you have fully captured the customer’s power requirements.

Generator Phases

Generator sets produce either single- or three-phase power. An easy rule of thumb is to choose a single-phase set if there are no motors above five horsepower. Three-phase power is a better option for motor starting and running. However, there are smaller three-phased motors so be sure to choose the right power for the application. Most residential applications will require single-phase power, whereas industrial or commercial applications usually require three-phase power. Typical three-phase generators are designed 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 the motors, heaters, stoves and dryers. Again, make sure your customer consults with you and/or the manufacturer’s power professional to assure compatibility with the customer’s current electrical system.

Natural Gas, LP, Gasoline or Diesel?

For longevity and lower operating costs, diesel gen-sets are much better suited for industrial or large towable applications.  Today’s modern diesels are quiet and normally require much less maintenance than comparably sized gas (natural gas or propane) units.  Fuel costs per kW produced with diesels are normally 30 to 50 percent less than gas units. 

The 1800 rpm water-cooled diesel units operate on average 12,000-30,000 hours before major maintenance is required.  The 1800 rpm water-cooled gas units normally operate 6,000-10,000 hours because they are built on a lighter duty gasoline engine block.  The 3600 rpm air-cooled gas units are normally replaced – not overhauled - after 500 to 1500 hours of use.  Because the gas units burn hotter (higher btu of the fuel) you will see significantly shorter lives than the diesel units.

Operating Speed

Electric equipment is designed to use power with a fixed frequency. The United States, Canada, and a few other countries operate on 60 Hertz (Hz); 50 Hertz is predominate in Europe and the bulk of the world. Frequency is measured by the amount of times-per-second a voltage goes from 0 volts to the positive maximum and crosses 0 again as it moves towards the maximum negative voltage. This is considered one cycle. 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 speed has its advantages and drawbacks. At 1800 rpm, four-pole sets are the most common and least expensive because they offer the best balance of noise, efficiency, cost and engine life. At 3600 rpm, two-pole sets are smaller and lightweight, and therefore best suited for portable, light-duty applications. 

In simple terms, it’s like operating your car at 90 mph, versus 45 mph. At the slower speed, your car will last longer, run quieter and require less maintenance.  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. The 1800 rpm water cooled units will last longer, offer less maintenance problems and be more fuel efficient.

Engine Considerations

Engines are available in many different types for use in generator power. For long life and quiet operation, consider a four-cycle, liquid-cooled, industrial-duty diesel engine.

Air or liquid cooling? Air-cooled engines require a tremendous amount of air and may require ducting. They're noisy too, so a quiet sound housing may be required to abide by the local sound ordinances. Sound levels vary greatly in each local municipality so it pays to check what your ordinances require for any generator.

Liquid-cooled engines can offer numerous benefits: quieter operation, and improved and more even temperature control. Intake and exhaust are key to engine performance. If you’re looking to save installation time and money, consider a large, integral air cleaner with a replaceable filter element and a residential muffler, which is built into the exhaust manifold. The lubrication system should have a full flow, spin-on oil filter with bypass. With recent EPA emissions changing all the time, be sure to choose an engine that meets the latest requirements. Many off-brand engines or those built by Chinese manufacturers may not meet these EPA emission requirements and you may not be able to obtain the necessary follow-on parts and service support when you need it.

DC electrical system A standard, 12-volt system should include a battery charging alternator with a solid state voltage regulator and a secondary back-up battery charger. These are required due to the long intervals the generator may run and the possible cold weather that a stand-by generator experiences. Diesel units should include a pre-heat system and meet all UL and NFPA requirements. A safety shutdown system should be standard in the control system. This will protect the engine from oil pressure loss, generator over-speed or over-crank, and high water temperature and over current. The DC system circuit breaker on the generator should also meet UL requirements and is optional on most generators.

Generator, the Business E nd

An AC generator should have a 4-pole revolving field. An automatic voltage regulator will provide “clean” power.  Normal utility power is +/- 5 percent voltage regulation. Kohler’s 1800-rpm and 3600-rpm models are designed at +/- 2% voltage regulation and all Kohler models offer AVR – Automatic Voltage Regulation.

There are many competing technologies for alternator design. The main types are:

• 4-pole synchronous alternator;
• Gearbox-driven, synchronous alternator;
• Variable speed inverter output;
• Variable speed AC field alternator;
• 2-pole synchronous alternator;
• Induction alternator; and
• Permanent magnet alternator.

Which type you use depends greatly upon the application. As the electrical consulting engineering, you, along with the manufacturer’s power professional or dealer, can determine which technologies best suit your customer’s application.

Engine Controls and Accessories

Upon determining the generator size, make a list of optional and installation equipment you require. For noise abatement, consider a residential (not industrial grade) muffler. A good primary fuel filter/water separator is also a must to protect the 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 in cold-weather climates.

 AC Transfer Switches and Switchgear

Switchgear can be simple or complex, depending on your customer’s needs and budget. Of course, as complexity increases, so does cost. Balance and good electrical advice are key here. Stand-by systems require a main circuit breaker between the utility source and the transfer panel. The transfer panel switches power from the utility to the gen-set and back, so that both aren't on at the same time. A few types of transfer switches are:

• Open Transition : In phase monitor (Synch check relay);
• Programmed Transition : Has an “off” position - Specify if shedding required;
• Bypass Isolation : Can bypass contactor; and
• Neutral switching : Integral, switched and overlapping.

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.

Motor Starting Capability

Generator sets are typically evaluated on the basis of their ability to start electric motors and accelerate them to full speed. Regardless of whether the motor has a load on it or not, starting it requires kV•A far in excess of the motor’s normal running kV•A demand. The motor starting capability, measured in terms of starting kilovolt-amperes (SKVA), is an indication of a generator set’s ability to start motors with minimal drop in voltage. When a generator is coupled to a synchronous driver (i.e., a non-slip motor tied to utility power), a generator voltage dip of 30 percent maximum is generally acceptable. The voltage will recover to within ±2% of normal voltage as soon as the motor has accelerated to full speed — normally about one second. Due to engine lug, diesel-powered recovery is longer but still occurs within seconds. The higher a generator’s SKVA for a given voltage dip, the better the generator for motor starting.

There is no consistent industry standard on how motor starting capability is measured. That said, some manufacturers refer to a " sustained voltage dip"value, which they explain as being the under-voltage that occurs for some period after motor starting commences. However, this value fails to quantify the absolute instantaneous voltage dip experienced during motor starting. Further, the method of calculating the value of sustained dip is largely undefined and does not account for the fact modern voltage regulators will most always return a generator to rated voltage after motor starting (assuming the engine can overcome the load).

So in conclusion, the best advice is to be sure your customer includes you in the generator selection and sizing process, and consult with the generator manufacturer’s power professional or dealer to help meet your specific application needs. This will save you time and money in the long run, and help maintain a safe and cost efficient supply of power when and where your customer needs it.