AC Generators

Fig. 11-21. A typical inverter monitoring panel, the Xantrex “Link” 1000.

Just as with inverters, advanced troubleshooting and installation procedures for AC generators go way beyond the scope of this book. These things are best left to the ABYC-certified professional marine electrician. However, some general knowledge of these workhorses is still important for the boatowner and will help to ensure that your generator is of the proper size and is performing as it should. The basic troubleshooting checklist at the end of this section will help you to at least point the service technician in the right direction if you do have trouble with your generator.

Rating AC Generators

Inverter Waveform

On the subject of voltage output, there is one additional point that needs to be made regarding inverters. Most (but not all) inverters produce what is known as a modified-square waveform. Shore-based power supplies, on the other hand, produce what is known as a sine waveform. Early inverters produced a standard square waveform.

This technical mumbo-jumbo is all great information for electrical engineers and of little value to boatowners, except for several issues. First, older square-wave inverters had trouble running things like TVs, microwave ovens, and older computers, and could even damage or destroy these appliances. Today’s modified-square-wave and true-sine-wave inverters have pretty much licked that problem, and the new inverters run just about anything.

The second problem caused by waveform is measuring voltage and amperage. As I mentioned earlier in this chapter and in chapter 3, the meter used to measure voltage from an inverter should be of the true RMS variety to give the best results. Typically, modified-sine-wave inverters will show low voltage when measured with an average-responding meter. This is no cause for alarm and not indicative of a fault with the inverter! AC generators are rated the same way inverters are. Wattage is the key here, and the basic AC use-analysis chart for inverters will work just as well for determining your generator requirements.

The essential difference between generator and inverter ratings is that typically, generator manufacturers have rated their units in kilowatts (kW). One kilowatt equals 1,000 watts, so, for example, if your generator is rated at 4.5 kW, it’s a 4,500-watt unit.

As with inverters, boaters are inclined to over- or underrate their needs with generators. Underrating will give poor electrical performance, for what should be obvious reasons by now. But a point that many people don’t realize is that overrating of a generator can wreak havoc with the generator itself. Generators are designed to operate at a very specific rpm to govern and control the AC frequency. They must be able to maintain this rpm over the entire operating range of the unit under all levels of electrical load.

The problem is just that—load. Underworked generators will simply freewheel along, eventually gumming up the cylinders, valves, and rings of the engine. Slight overrating in terms of average combined wattage consumption is OK, but manufacturers recommend that a generator be rated to average 75 percent of its total wattage most of the time. Knowing this, it should be clear that running a TV from even

the smallest generator will damage the unit if that’s the only draw for extended periods. Generators are really only suitable for fairly serious AC loads (such as electric ranges, air conditioners, and hot-water heaters) all running simultaneously and for extended periods of at least and hour or so—for example, during preparation for the evening meal.

Other serious disadvantages of generators include the noise of the engine, those wonderful exhaust fumes permeating into the pre-dinner cocktail hour, and simply having one more engine on board to maintain.

As already mentioned, both the voltage and frequency of AC generators are carefully controlled by an engine-mounted governor that keeps engine rpm stable under all electrical loads. If you have a generator on your boat, your AC panel must have a voltmeter to be in compliance with the ABYC standards. It’s a good idea from time to time to monitor this gauge. Any variation in voltage beyond 10 percent of the normal rated output for the generator that’s indicated by this gauge is an indication of trouble.

Modern generators are commonly regulated to control voltage to as little as plus or minus 2 percent if all is well with the unit. As voltage fluctuates, so too does frequency. Normal frequency here in the United States is 60 Hz. Some of the better multimeters on the market have the ability to measure frequency, and this is not a bad feature to have if your boat has a generator installed or you’re thinking of having one added.

Generator Safety

Over the years, marine AC generators have evolved into quite sophisticated pieces of equipment, and as a result the safety features available today are extensive. Built-in sensors shut down the generator in the event of such things as low oil pressure, engine or exhaustsystem overheating, and even excessive exhaust backpressure on some models. The problem is, different manufacturers use different systems, and features will vary even from one model to another from the same manufacturer. To familiarize yourself with your generator, get out the owner’s manual, and if you intend to do any but the most basic service or troubleshooting on the unit, get the workshop manual as well.

Most inadvertent generator shutdown problems are due to loose connections, low oil pressure (is there oil in the engine?), or an overheating engine. You may have to trace through the cooling-system troubleshooting section of your workshop manual to find the solution and get the generator up and running again.

Measuring Generator Output

Verifying generator voltage and frequency is not difficult, but to ensure that any low-voltage indications on your AC panel are not due to a wiring and voltage-drop problem between the generator and the panel, measurement should be done right at the output terminals on the generator. When checking voltage and amperage at the generator, be careful of moving parts and the hot exhaust on the generator, and be sure to take your readings at the correct location as per the instructions from the generator manufacturer. If in doubt, make sure you have the service manual at hand, and use it! Figure 11-22shows the frequency being verified on a typical marine generator.

Fig. 11-22. Checking frequency with the multimeter.