20 kw st head and 6/1 engine

[MeanListerGreen]

I know I read a post about this earlier but I can't find it with the search.   What was the reason you shouldn't use a larger kw gen head with a 6/1?  Those 20 kw st heads are much cheaper then a 5 or a 7 and they could be used with larger engines down the road.

[Ray C]

I asked the same question not long ago ( http://listerengine.com/smf/index.php?topic=1326.0 )...

Answer:  No Problem.  Go for it.   It's over-kill without deleterious effect.

[MeanListerGreen]

Thanks Ray.  That answers my question.  I am considering the 20 kw because I have some money down on a 50 hp diesel engine which I will use for those big jobs. 

[rcavictim]

Hey MeanLG,

What kind of `big` engine are you invested in?

[Tom]

I'm sure someone will correct me if I am wrong here  , but my understanding is that an over size gen head will use more power for the field coils and be less efficient.

[rcavictim]

I'm sure someone will correct me if I am wrong here  , but my understanding is that an over size gen head will use more power for the field coils and be less efficient.

I think the larger machine will have less leakage flux and the heavier wire will have less `aye squared arrrr` losses.  My only concern is the windage losses in a big head will be higher.

[Jim Mc]

The 20 kW head, loaded at only 3 kW will be less efficient than the 3 kW head at the same loading.  Two reasons:  Bigger fan and bearings causing more friction, and a larger magnetic 'circuit' requiring more field excitation.  The efficiency of these machines will usually peak at maximum load.

Another, not-so-obvious drawback is the larger machine's rotational inertia will put higher peak loads on the belt.  The reason is that the Listeroid speed varies throughout its 4 stroke cycle, even at zero electrical load.  So as the engine slows during the compression stroke and speeds up during the power stroke, the belt is being alternately slackened and tightened.  The more mass you are spinning in the alternator, the worse this problem becomes.  It has been the cause of some frustrating belt issues in my power plant project of late.

I like the idea of oversizing the generator a bit, but it does come at a cost.

[rcavictim]

The 20 kW head, loaded at only 3 kW will be less efficient than the 3 kW head at the same loading.  Two reasons:  Bigger fan and bearings causing more friction, and a larger magnetic 'circuit' requiring more field excitation.  The efficiency of these machines will usually peak at maximum load.

Another, not-so-obvious drawback is the larger machine's rotational inertia will put higher peak loads on the belt.  The reason is that the Listeroid speed varies throughout its 4 stroke cycle, even at zero electrical load.  So as the engine slows during the compression stroke and speeds up during the power stroke, the belt is being alternately slackened and tightened.  The more mass you are spinning in the alternator, the worse this problem becomes.  It has been the cause of some frustrating belt issues in my power plant project of late.

I like the idea of oversizing the generator a bit, but it does come at a cost.

The extra inertia of the larger ST head armature will give you a benefit of less flicker, especially when driven with a single cylinder diesel.  I vote go oversized if you can, unless the power is rectified and buffered through a storage battery system.  In that case the smaller alternator armature mass will be easier on belts or drive components.

[CD in BC]

MY CD 8hp @ 1200rpm was driving a BIG gen when the previous owner had it.  He said the lights would go up and down as the engine fired - he may have had it governed down as at 5 ignitions per second at 1200rpm I rather doubt this would have been noticeable.

Whatever the case, the head it was driving was at least 7.5Kw and maybe as much as 12Kw - There was no plate on it to tell - but he had 'flickering' problems for some reason.

Perhaps too much draw?