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Old 22-07-2014, 01:58   #1
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Are Marine Diesel Generators Pure Sinewave?

I don't seem to be able to find any information on the output of diesel gensets. Are they typically a pure sinewave output like that found on many portable generators? thanks in advance.
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Old 22-07-2014, 02:38   #2
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Re: are marine diesel generators pure sinewave

Any good one will be but you'd have to double check the model in question.
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Old 22-07-2014, 04:56   #3
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Re: Are Marine Diesel Generators Pure Sinewave?

Any AC electricity made by a rotating "generator", will have a "pure" sign wave.
Inverters can have a square sign wave, or modified sign wave, or pure sign wave.
Some portable generators have inverters, and a few marine generators have inverters.

If it's not an inverter type diesel generator (of which there are few), it will produce a pure sign wave.

If your generator runs at 1800 or 3600 rpm (60hz) it's going to produce a pure sign wave. Inverter generators, typically, run at slightly lower rpm than one of these previously mentioned speeds (Usually between 2000 and 3000 rpm).

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Old 22-07-2014, 05:38   #4
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Re: Are Marine Diesel Generators Pure Sinewave?

Then to confuse things there are even a few "geared" generators that the engine operates at an RPM lower than 3600, but the generator is spun at 3600, so RPM alone may not tell the whole story.
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Old 22-07-2014, 05:50   #5
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Re: Are Marine Diesel Generators Pure Sinewave?

They are all sine wave, but the 3600 rpm units usually have a fair amount of distortion in the wave.
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Old 22-07-2014, 06:03   #6
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Re: Are Marine Diesel Generators Pure Sinewave?

A64 pilot has a good point. Although uncommon, there are units with "gear drive" which will lower the engine rpm to something a little more peaceful.

So to keep things simple: If your genset runs at 1800 or 3600 rpm (60hz) you can safely assume it has a pure sinewave.

If it runs at any other rpm, you'll have to research it's specifications to be sure.

*** I just re-read the OP.
The answer is YES, they are typically Pure sign wave, like that found on typical generators.

********

I apologize for getting too deep into this subject, as often happens here.

Cheers, and good luck.
Paul.
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Old 22-07-2014, 06:07   #7
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Re: Are Marine Diesel Generators Pure Sinewave?

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They are all sine wave, but the 3600 rpm units usually have a fair amount of distortion in the wave.
That's right, they are not all created equal.

There would be no need to make small petrol inverter generators if all generators created a clean sine wave. It would simply be an added complication.

Many people have bought marine diesel generators without doing their homework to find them less than satisfactory.

For example, the battery charger produces close to its rated 100amps on shore power but on generator only 30 amps. The reason for this is a substandard wave.

As long as you ensure the manufacturer says its clean you should be safe.

Onan, Northern Lights, Kohler for example are all very high quality brands that understand marine requirements.
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Old 22-07-2014, 06:16   #8
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Re: Are Marine Diesel Generators Pure Sinewave?

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For example, the battery charger produces close to its rated 100amps on shore power but on generator only 30 amps. The reason for this is a substandard wave.
That is way beyond substandard. It is common for them to lose 10-15% with dirty power (lower peak voltage) from a generator, but the 70% in your example means something is really wrong.

Here is the output from a nominally 3600rpm 5.5kw generator. This generator's engine actually runs at 2800rpm and pulley ratios are used to run the generator head at 3600rpm.

NextGen generator output

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Old 22-07-2014, 06:50   #9
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Re: Are Marine Diesel Generators Pure Sinewave?

All non-inverter generators produce a pure sine wave (even the cheap lawn mower motor ones).

The problem is any time the motor is not running smoothly at the prescrived RPM to achieve appropriate Hz & Volts (such as when you apply heavy loads and they bog down), that creates distortions in the wave form which can be as bad (though different) than a cheap modified sine wave inverter.

If you have a non-inverter generator that is coughing and sputtering, expect to have low quality electricity.

As with most things, a good quality non-inverter generator sized appropriately to the load and in proper running order will provide good quality electricity.
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Old 22-07-2014, 07:04   #10
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Re: Are Marine Diesel Generators Pure Sinewave?

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That is way beyond substandard. It is common for them to lose 10-15% with dirty power (lower peak voltage) from a generator, but the 70% in your example means something is really wrong.

Here is the output from a nominally 3600rpm 5.5kw generator. This generator's engine actually runs at 2800rpm and pulley ratios are used to run the generator head at 3600rpm.

NextGen generator output

Mark
Yes, thats true, I made up that example.

The real example that I have in mind was around 70 amps on shore power and around 40amps on generator.
Still a big loss. The generator manufacturer suggested that we replace a capacitor.
This changed things only very slightly.
It was not possible to fix it in the 2 days before the boat sailed.
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Old 22-07-2014, 07:54   #11
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Re: Are Marine Diesel Generators Pure Sinewave?

To answer the OP's question...

And, there is allot of pretty good advice on this thread, and some, well...

First, the post about engine RPM stability is spot on. If the engine varies in RPM with loads then the output frequency and voltage will vary as well. Remember that mass = frequency stability somewhat. Small, lightweight engines will not be as frequency stable when a sudden load is applied or removed than larger heavier engines.

Second, the problem with "dirty power" from a generator set is either related to engine RPM stability or it is related to the method of voltage regulation. It can also be related to wether the generator is a old "brush type" or a "brushless" design.

Voltage regulation is generally one of two types. The cheap generator ends use one or more capacitors to filter some of the output, back into the generators excitor winding. This method produces a output that has for the lack of a better word allot of "noise".

Better generator ends utilize either a transformer type regulator or a solid state regulator to provide current for the excitor winding. This type of generator end will produce a clean pure sine wave.

Another issue is generators that use brushes to couple excitor current back to the rotor. This design works great in theory, and was the standard untill the 1970's but in practice the brushes wear, and their movement produces "noise" in the excitor current, which produces noise in the output.
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Old 22-07-2014, 08:03   #12
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Re: Are Marine Diesel Generators Pure Sinewave?

Yes, the generator output above is from a brushless capacitor-excited unit. They offer an AVR controlled head, but that uses brushes and also produces dirty power.

The rpm's are dead steady - the issue lies in the generator head and is common for these generators.

Again, the "dirtiness" isn't an operating problem, although things tend to draw ~10% more power than their ratings due to the lower peak output voltages.

It is a tradeoff, of course, with these generators. A low-rpm 4-pole genset is physically larger and weighs 50% more. For us, our usage is occasional for short time periods and we do not have the space for a larger unit. If one needs to run continually, a low-rpm unit is the way to go.

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Old 22-07-2014, 08:28   #13
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Re: Are Marine Diesel Generators Pure Sinewave?

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Originally Posted by colemj View Post
Yes, the generator output above is from a brushless capacitor-excited unit. They offer an AVR controlled head, but that uses brushes and also produces dirty power.

The rpm's are dead steady - the issue lies in the generator head and is common for these generators.

Again, the "dirtiness" isn't an operating problem, although things tend to draw ~10% more power than their ratings due to the lower peak output voltages.

It is a tradeoff, of course, with these generators. A low-rpm 4-pole genset is physically larger and weighs 50% more. For us, our usage is occasional for short time periods and we do not have the space for a larger unit. If one needs to run continually, a low-rpm unit is the way to go.

Mark

Just so readers do not get confused...

Brushless, AVR regulated generator ends are the industry standard, and have been the industry standard for 30+ years in industrial and fixed marine installations.

There are manufacturers that try to offer lower priced units, and there always have been, and there always will be.
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Old 22-07-2014, 08:42   #14
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Re: Are Marine Diesel Generators Pure Sinewave?

Everything you ever wanted to know about marine diesel generators is here:

Marine Generator Test - Victron Energy
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Old 22-07-2014, 08:50   #15
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Re: Are Marine Diesel Generators Pure Sinewave?

I recently learned about pure and modified sine wave. Never heard of good and bad electricity until now. This is a great thread.
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