Air Cooled Generators
Welcome to our air-cooled generators page. I have been involved with small air-cooled generators for the past 35 years. Much of my experience involves Onan generators, having worked for two Onan distributors. I have taken and passed their Level ll RV exam, having worked on many RV gensets over the years. In addition to my Onan experience I have passed the OPEESA technicians certification exam on generators. These credentials have helped me grow as a generator technician over the years.
There are many different brands of small generators in the market place today. The Import units continue to be popular with Honda leading the way with very reliable units. The USA has many units in the market place using popular air cooled engines like Briggs & Stratton, Tecumseh, Onan and Kohler.
Most of these units run at 3600 rpm to maintain 60 hertz. Keep in mind these high speed units were not designed for prime power, but are great for backup power and construction work. They will normally be quite noisy, because they run at high speed and most do not have a sound deadening enclosure. For more information on this click here. So if you are purchasing a genset to run in a residential area or campground, keep this in mind when making a decision on what model to buy. Not everyone likes listening to a genset run at 3600 rpm's while they are trying to sleep. Some of the new inverter type gensets vary the engine speed depending on load. The new Honda Inverter genset is a good example of this in a portable unit if noise is a factor for you.
Some models have features like automatic idlers that slow the unit down when no power demand is on and low oil shutdown, in case you forget to check the oil. Commercial grade engines like the Briggs & Stratton Vanguard, shown in a Gillette Gen-Pro at right, are now appearing on many gensets. A genset is a generator with an engine attached to it. Some generators are available as separate units that require a drive source. A genset is only as good as its weakest part, so look at the total package when you buy.
Most small gensets are rotating field generators. These units are self exciting and rely on residual magnetism in the rotor core to get things started. This resdidual creates a voltage which is fed into an exciter winding and then through a diode bridge converting it to DC (direct current). This DC voltage supplies the field which rotates in the center of the unit and induces voltage into the stator windings, located in the generator case. This induced voltage comes out as AC (alternating current) at 60 cycles per second if the engine speed is correct. Here in the USA it is usually 120 or 240 volts, primarily single phase in small units. Generators are commonly rated in watts or KW (1000 watts). A 5 KW generator would be 5000 watts. Many of the units now use voltage regulators or special capacitors to maintain a closer voltage under load. Keep in mind these units produce the same voltage as your commercial power company and have the power to seriously harm you if you are not careful.
Home Standby Systems
One of the popular things to do with a genset these days is to have a home standby system in the event of power outages. My email load increases on this during hurricane season and now Y2K. The most important thing to remember when installing such a system is that you must never have a situation where the genset can connect into the commercial power. Their are three ways to accomplish this.
I'm including a diagram to illustrate how numbers 2 or 3 would be assembled. It may be best to install an outside power inlet box from the genset to the transfer switch, so you can wheel the genset near the box and plug it in without having to use windows or doors. You may also need to install the additional auxiliary breaker panel for the emergency circuits, depending on what type of transfer switch you purchase. The newer systems have the emergency breaker panel built into the transfer switch.
Manual transfer switches are reasonably priced and therefore best suited for the average home user that has a portable genset that he wants to interface with his home. These newer systems make installation simple and a safe way to avoid any crossover of power. You must interface with your present breaker box installation and depending on your installation and your ability, it is recommended that you check with an electrician to make sure your completed system will meet electrical codes in your area.
The Automatic transfer switch is more costly and requires a genset to be setup for auto start. This system is the only one that will start and take over your emergency load circuits while you are away. A fully automatic genset package is a top of the line system that maintains itself and comes online with no owner interaction.
Propane or natural gas is a popular fuel option for a home standby unit, since it will not get stale like gasoline setting for long periods of time. Keep in mind that the KW rating is de-rated a little for these fuels, with propane being about 5% less and natural gas 25% less than gasoline. Also, if you are having long outages, remember to check the oil periodically. You should exercise the unit at least once a month, preferably with load for about 30 minutes. Many commercial standby units exercise weekly. If the unit is electric start, you may want to have a small battery maintainer. The system is worthless if it won't start when the lights go out. Last but not least, when designing your system, start by making a list of everything in your house that you will need in an emergency. Keep the load as light as possible and remember to allow for starting loads of electric motors. Most 240 volt loads will be too much for the average home genset (water heater, electric clothes dryer, electric range, etc). More information on generator sizing can be found by clicking here.
In order to troubleshoot a generator you must know what the unit is doing with both no load and full load applied to it. You will need four things to properly make this test.
The rated load to be applied to the unit is usually on the name plate on the genset. It will be listed in amps. A generator in good condition will usually show 125 to 135 volts and 61 to 62 hertz with no load applied. Some of the units with out voltage regulators may go slightly higher on the no load voltage. With name plate load applied the generator should then hold at least 110 volts and 58 hertz. If it will not hold 58 hertz, suspected a weak engine, possible burnt valves, worn rings or governor problems. The engine could be out of tune or the exhaust system could be obstructed or plugged.
If the engine meets name plate load requirements, but will not handle the load you are trying to put on it, then you are exceeding the capability of the generator. An example of this would be starting an air compressor. Starting requirements can be four or five times what the nameplate amperage is on the compressor label. Keep this in mind when sizing a genset.
If your generator will not put out any voltage at all then check for blown fuses or circuit breakers. I would not recommend disassembling the generator unless you are experienced at this and have the proper test equipment to accurately test the rotor and stator windings, rectifiers and capacitors. In closing, I would recommend working closely with the generator retailer on sizing the genset to the load you will be using. I have seen many people who have purchased gensets and then found out they were too small for the starting loads of the equipment they wanted to use. Here is a link to the Briggs & Stratton Power Generator website for a wattage calculator that may help you decide what size unit you need. I hope this information has been helpful to you - Bruce Perrault
The Precise Engine Repair notebook is maintained by Bruce Perrault