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\Generator speed vs frequency


eric freedman
 

Hi Craig,

The frequency of AC is determined by the

RPM of the genset.

It would not be feasible to change the speed .


Generators consist of an engine directly connected to an alternator to produce electricity. One of the most common ways of changing the output frequency of a generator is to change the rotation speed of the engine.

The two factors are related as per the following formula –
Generator Frequency (f) =
Number of revolutions per minute of the engine (N) * Number of magnetic poles (P) / 120
Conversely, P = 120*f/N

As per the above formulae, a 2-pole generator producing an output frequency of 60 Hz has an engine speed of 3,600 rpm. To change the output frequency to 50 Hz for the same generator configuration, the engine speed needs to be reduced to 3,000 rpm. Similarly, for a 4-pole generator, an engine speed of 1,800 rpm produces output of 60 Hz. Reducing the engine speed to 1,500 rpm yields an output of 50 Hz.

Fair Winds
Eric
Kimberlite Amel Super Maramu #376

 

 

 

From: amelyachtowners@... [mailto:amelyachtowners@...]
Sent: Saturday, March 17, 2018 12:51 PM
To: amelyachtowners@...
Subject: RE: [Amel Yacht Owners] Generator load, carbon buildup, etc.

 

 

Hi Eric,

I googled the spec sheet on your MDKAV and it says it runs at 1500 rpm. It's the newer model that runs at 2900 rpm. I should think a variable speed would be great at it would always present a full load to the engine, regardless of rpm.  Oh well, next boat.

Cheers, Craig



---In amelyachtowners@..., <kimberlite@...> wrote :

Craig,

My Onan is a MDKAV model and I don’t have the slightest idea how many rpm the 3 cylinder Kubota is running at. However it puts out 220-230 volts at 50 cycles.

Fair Winds

Eric

Kimberlite Amel Super Maramu #376

  


Craig Briggs
 

Hi Eric,
    While you're totally correct as far as you go, you're speaking only of the legacy style generators like you and I have, where, obviously, it's 1500/1800 or 3000/3600 rpm. 
    However, the new ONAN's run at 2400 and 2900 rpm and there are variable speed generators out there. ONAN talks about 2/3 winding's and stuff like that and the variable speeds use magic. 
     If any of us are considering replacements it would certainly be worth some research to update our knowledge and learn about the modern products. 
     For example, a variable speed generator that, by design, always runs at an optimum rpm for whatever load is presented sounds super - plus it seems like it would be at a lower/quieter rpm if its only got a light battery charging load, which many of us may be doing much of the time. The newer ONAN's running at 2400 or 2900 may be at a better spot in the engine's power curve than 1500 or 1800.  Maybe even why ONAN changed the design.
Cheers,
Craig, SN#68 Sangaris


---In amelyachtowners@..., <kimberlite@...> wrote :

Hi Craig,

The frequency of AC is determined by the

RPM of the genset.

It would not be feasible to change the speed .


Generators consist of an engine directly connected to an alternator to produce electricity. One of the most common ways of changing the output frequency of a generator is to change the rotation speed of the engine.

The two factors are related as per the following formula –
Generator Frequency (f) =
Number of revolutions per minute of the engine (N) * Number of magnetic poles (P) / 120
Conversely, P = 120*f/N

As per the above formulae, a 2-pole generator producing an output frequency of 60 Hz has an engine speed of 3,600 rpm. To change the output frequency to 50 Hz for the same generator configuration, the engine speed needs to be reduced to 3,000 rpm. Similarly, for a 4-pole generator, an engine speed of 1,800 rpm produces output of 60 Hz. Reducing the engine speed to 1,500 rpm yields an output of 50 Hz.

Fair Winds
Eric
Kimberlite Amel Super Maramu #376

 

 

 

From: amelyachtowners@... [mailto:amelyachtowners@...]
Sent: Saturday, March 17, 2018 12:51 PM
To: amelyachtowners@...
Subject: RE: [Amel Yacht Owners] Generator load, carbon buildup, etc.

 

 

Hi Eric,

I googled the spec sheet on your MDKAV and it says it runs at 1500 rpm. It's the newer model that runs at 2900 rpm. I should think a variable speed would be great at it would always present a full load to the engine, regardless of rpm.  Oh well, next boat.

Cheers, Craig



---In amelyachtowners@..., <kimberlite@...> wrote :

Craig,

My Onan is a MDKAV model and I don’t have the slightest idea how many rpm the 3 cylinder Kubota is running at. However it puts out 220-230 volts at 50 cycles.

Fair Winds

Eric

Kimberlite Amel Super Maramu #376

  


Herbert Lackner
 

Craig, 

we have an Fischer Panda 4000i with a 1 cylinder kubota engine, 4kw.  Initial it had fixed RPM ~2900 but two years ago I upgraded it with a variable speed option (step motor that pushes the throttle depending on the load), now it turns between 2400 and 2900, depending on the load.  normally it runs with low speed because normal load is 1 to 2 kw, so it runs with ~2500. It runs more silent now and only uses ~0,5l diesel per hour compared to ~0.8l we used before.

herbert
SN120, KALI MERA, on the hard in Shelter Bay


Craig Briggs
 

Wow, Herbert, that sounds very good, indeed. Yes, there are many options to the old legacy designs running at fixed (and inefficient) rpm that give quiet(er) operation, better fuel economy, cleaner emissions and weight savings.
Kali Spera, Craig SN#68 Sangaris


---In amelyachtowners@..., <herbert@...> wrote :

Craig, 

we have an Fischer Panda 4000i with a 1 cylinder kubota engine, 4kw.  Initial it had fixed RPM ~2900 but two years ago I upgraded it with a variable speed option (step motor that pushes the throttle depending on the load), now it turns between 2400 and 2900, depending on the load.  normally it runs with low speed because normal load is 1 to 2 kw, so it runs with ~2500. It runs more silent now and only uses ~0,5l diesel per hour compared to ~0.8l we used before.

herbert
SN120, KALI MERA, on the hard in Shelter Bay