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Old 06-08-2014, 07:57   #61
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Re: Are Marine Diesel Generators Pure Sinewave?

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Beyond that all alternators have a fixed (or maximum) field current that sets an upper limit on the maximum current the alternator can produce.

For most small alternators on boats the current is limited to about 2 times the maximum rated current so it can start motor loads.
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Even if the breaker did not trip the field excitation likely would not support this current
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The maximum current that this alternator can carry at 240 V is 6875 VA / 240 V = 28.65A and this is the most important point.
Now if you were to load this genset with a 5kW load with a 0.5 power factor the motor would not be overloaded but the current would be 41.66 A (5000 / 240 / 0.5 = 41.667) and the alternator would be damaged.
Now “2 times the maximum rated current” in the example is 28.65*2 = 57.3 A (calculated by Microsoft Word). 41.66 A as per example above is far less than 57.3 A.

Circuit Breakers tend to be not the first choice in a marine environment on a diesel generator. They tend to rust and are normally rattled to death by the vibration of the set. Thermal overload protections would be more appropriate than fuses or circuit breakers but are rarely seen in small genset because keeping an eye on the load current is all what is needed.

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All generators sold in the US are required to have this
Definitely an improvement since the New York blackout.

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1 megawatt generators stopped cold by a crowbar short but the alternator was undamaged.
A short circuit is not an overload.

But without getting into the detail of why the rotor of an alternator costing more than a Jumbo jet may end up 500 meters from the power station, the same for a diesel fly wheel or what really happen at Chernobyl or Fukushima, lets go back to small capacitor regulated generators.

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The Kill-a-Watt device displays Volt-Amperes or Watts. It does not measure the "angle" of the power factor. This is not a big deal because very few devices in the typical home (or cruising boat) have a leading power factor (current leads voltage).
Can you name the few devices that would have a leading power factor? Definitely an elegant way of correcting a power factor but dangerous.
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Old 07-08-2014, 21:02   #62
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Re: Are Marine Diesel Generators Pure Sinewave?

Your local power distributor uses leading power factor correction (capacitors) and it is quite safe. But on a boat there is no point really. Except for motors most of the devices with poor power factor can't be corrected by a leading device. The power factor is neither leading or lagging, it is caused by distortion products from power supplies on computers, battery chargers and the like. That's why power factor corrected power supplies are finding their way into more devices like battery chargers and laptop supplies.
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Old 13-08-2014, 05:44   #63
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Re: Are Marine Diesel Generators Pure Sinewave?

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Your local power distributor uses leading power factor correction (capacitors) and it is quite safe.
They would not do something unsafe would they? Leaving capacitors on (for power factor correction) when they are no longer required can lead to a power supply cut out.

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But on a boat there is no point really. Except for motors most of the devices with poor power factor can't be corrected by a leading device.
The power factor can be improved by using devices with a power factor closer to unity and corrected as you explain below.

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That's why power factor corrected power supplies are finding their way into more devices like battery chargers and laptop supplies.
As an indication, the voltage of an alternator with constant excitation on a 50% load will be in the order of 93% of the rated voltage at unity (1) power factor, 72% of the rated voltage at a lagging power factor of 0.8, 68% of the rated voltage at a lagging power factor of 0.5, 118% of the rated voltage at a leading power factor of 0.8, 126% of the rated voltage at a leading power factor of 0.5.

Adding a lower power factor device onto a load will cause a voltage dip. If an induction motor (or something similar) is part of that load the current in that induction motor will increase. In a device that contains an in built overload protection, like the compressor motor of a refrigerator or freezer, it is likely that this built in protection will trip if the motor is hot and fully loaded and will take time before the motor restarts annoying if one of the reasons for running the generator is to cool the eutectic freezer.
In the loading of a less tolerant capacitor regulated generator it will be an advantage to load first the set with a resistive load. A minimum 60 W incandescent lamp is most suitable. It also provides an indication that the AC system is energised. By the use of a connection timer the system voltage will not be allowed to rise or fall slowly as the generator is started or stoped. A cheap connection timer can be made of a properly rated relay operated by a fluorescent lamp starter as the timing device. Then the lower power factor devices can be loaded followed by devices near 0.8 pf that require high starting current. Then resistive appliances, which will further improve the power factor of the load.
If the lower power factor devices are still troublesome then connecting them to an inverter may solve the problem.

Note: when revving from 50 Hz to 60Hz it is necessary to check that the alternator is rated for that increase in speed. If not the higher rpm may destroy the alternator, known in the business as “trowing the poles”
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