The boat generator comprises the diesel engine and the attached AC alternator. AC alternator rating selection must consider a number of factors. A total expected load analysis must also be undertaken to calculate the peak loads that might be encountered. Starting current values may be as high as 5–9 times that of the actual normal running current. The in-rush current at starting causes these high currents, and is the energy required to overcome bearing and load inertia in electric motors, winding magnetization and so on. The actual duration of the peaks is typically less than one second, and most alternators are designed to withstand 250% overloads for nominal periods.
Boat Generator Rating Calculation. From a load analysis, the following calculations can be performed to estimate required minimum alternator size:
a. Generator to run at 80% maximum load with 20% reserve.
Largest Single Load Value = 2400 watts
Max. Start Current (Ir) = Power (watts) = 2400 = 12.50 amps
Volts x Pf = 240 x 0.8
Current (Is) = Ir x 4 = 50 Amps
b. If alternator can withstand overloads of 250% for 10 sec., the starting current (Is) must be divided by 2.5 = 20 Amps.
c. Rating is therefore 20 x 240 = 4.8 kVA or 0.8 x 22.8 x 240 = 3.8kW. Adding a 20% minimum margin to operate at 80% = 4.5kW.
In selecting a boat generator output rating, an estimate must be made of the maximum load likely to be applied. Some loads are resistive and do not have large starting currents, such as kettles and heaters. If the device having the startup current is to operate simultaneously with other equipment, the other loads need to be added. So if an air conditioner with 3.8 kW rating is to be run with hot water, then add 2.4kW, and for a kettle 1.8 kW then a figure of 7 kW, plus a margin for other small loads such as lights, TV etc you require 8.4kW. The higher the rating, the higher the initial capital cost, the greater the weight and space required. A decision must be made about how the equipment will be used in order to reduce the boat generator to the lowest suitable size. A generator should be loaded to at least 35% and ideally operate at 75–80%. The running of boat generator diesels on light loads causing cylinder glazing will increase maintenance costs and is to be avoided. What you need to do is work out a load schedule, what you can power simultaneously together in order of priority.
Voltage. The typical rated output voltages are 115/230, or 220/240 volts for single-phase machines. The Automatic Voltage Regulator (AVR) maintains the nominal voltage output. The AVR is an electronic regulator that senses output terminal voltage and varies the field strength to maintain the correct voltage value. Regulation is typically within 2% of nominal rating. The AVR must be able to control the output rapidly in response to large load fluctuations. Recovery in most boat generators is typically 3% of rated output within 0.25 seconds when full load is applied. Voltage is not a function of the engine speed or the frequency when an alternator is running at or near rated speed.
Frequency. Frequency is specified in hertz, the number of alternating cycles per second. Output frequency is a direct function of speed and varies in response to speed fluctuations. Stability depends on the ability of the machine to maintain nominal frequency over the complete power output range; this is typically within 1%. The engine governor controls engine speed. When a large load is applied, such as a motor starting, the generator loads the engine, causing it to slow momentarily. The governor reacts by increasing fuel flow and speeding the engine up. When the load is removed, the reverse occurs. Stability depends on response time, and governors are factory set. A small time lag is inherent in the system and helps minimize hunting, caused by continual alterations based on small load fluctuations. The boat generator has many issues when selecting. More boat diesel generator information.