cs 6/1 gen size

[needenginerunnin]

evening all
another newbie here.
i am a cs owner, and are just about to start and build a gen set for my self. i was lucky enuf to buy an origanal 6/1 with som flywheels with no grooves in.
i have read with interest the thread about gen sizeing, and lots of people seem to go for the ST5. what i am wondering is it worth going one better and get a ST7.5 instead. i understand that at low load levels the electric made with a ST7.5 can be poor. but is the extra top end power worth it, as i have som flywheels etc.
does anybody out there run a ST7.5 head on a 6/1?
cheers for now

[sailawayrb]

6 HP times 0.746 equals 4.5 KW assuming perfect mechanical/electrical conversion.  With actual conversions losses, you are likely to only obtain 3.5 KW.

[needenginerunnin]

"6 HP times 0.746 equals 4.5 KW assuming perfect mechanical/electrical conversion.  With actual conversions losses, you are likely to only obtain 3.5 KW"

yes i am well aware of the maths it has been posted many times.
i am not after more kw's than the engine can generate as it's not going to happen.
i am thinking for larger motor start up, as you run a bigger motor than you can start from a set generator size.

[oliver90owner]

welcome to the forum.

i am thinking for larger motor start up, as you run a bigger motor than you can start from a set generator size.

Then tell us what size motor starting you would require.  it helps if you ask the right questions in the first place.  We would not know 'whether it is worth it' without some more information. 

My immediate answer would be no, it is not 'one better' at all, unless the gen was really cheap or you are continually starting heavy loads. 

Also depends on your run times. 8000 hours run-time each year at lower eficiency can add up quite quickly to the cost of a bigger prime mover, depending on what fuel you are using, of course (ie.cost). 

You will only get 3 1/2 kW for short periods as that represents overload on a 6/1.  Expect around 3kW as a normal maximum load.  Yours may well be less than that if you choose to get an inefficient machine running at less than 50% of its plated rating.

Regards, RAB

[ronmar]

What RAB said...  Anything that would have a load that would exceed the momentary startup capacity of a ST-5, yet still be within the 3-3.5KW of electric load that the engine is capable of sustaining once the load is up and running, would really kick the engine in the teeth big flywheels or not.  I don't think anything larger than a ST-5 on a 6/1 is of any real benefit, and efficiency losses will then start to add up.  If you have something that you think will be marginal for a ST-5 to start, then perhaps you need a larger prime mover...

Good Luck with your heavy wheeled 6/1 

[M61hops]

I have an ST7.5 belted to my 6-1 right now because the bearings in my ST5 got really noisy; and it seems just like the ST5 as far as I can tell.  An ST5 is an oversize head for the 6-1 in general and it would be hard to see a need to go larger.  Someday I might try a ST10 and do some fuel consumption tests to see if there is any noticable loss from turning the oversize heads.  I'm seriously considering bringing all the stator leads out of the case and connecting a full wave bridge to each winding and making a switching setup so I could charge a 48V battery bank direct from the ST windings, or flip a switch and run AC loads.  I think an ST5 is probably the best size to use with a 6-1 but if I want to use the stator windings to charge batteries directly I think the larger wire size of the 7.5  or even the 10 might be of some help.  In theory the ST5 should be able charge my batteries at 80A when wired the way I'm thinking but the 6-1 can probably only push around 60A.  I'm a little concerned the windings of an ST5 would run hot using them this way and if so that would be a reason to use a way oversize head.  Tinkering will tell the tale eventually .          Leland

[xyzer]

You will only get 3 1/2 kW for short periods as that represents overload on a 6/1.  Expect around 3kW as a normal maximum load.  Yours may well be less than that if you choose to get an inefficient machine running at less than 50% of its plated rating.
Regards, RAB

And then there is the altitude factor to enter into the issue. I have 2 ST5 6/1 setups and never have problems with it starting a motor it is capable of running continiously. My load limits are right in the numbers RAB mentioned.
Dave

[needenginerunnin]

quote
welcome to the forum.

i am thinking for larger motor start up, as you run a bigger motor than you can start from a set generator size. [/b]

Then tell us what size motor starting you would require.  it helps if you ask the right questions in the first place.  We would not know 'whether it is worth it' without some more information. 

My immediate answer would be no, it is not 'one better' at all, unless the gen was really cheap or you are continually starting heavy loads. 

Also depends on your run times. 8000 hours run-time each year at lower eficiency can add up quite quickly to the cost of a bigger prime mover, depending on what fuel you are using, of course (ie.cost). 

You will only get 3 1/2 kW for short periods as that represents overload on a 6/1.  Expect around 3kW as a normal maximum load.  Yours may well be less than that if you choose to get an inefficient machine running at less than 50% of its plated rating.

Regards, RAB



thanks for the welcome rab

i was thinking of hopefully starting a 3hp 2.25ish kw motor hence the reason why a ST5 wouldnt be big enuf working at 3x ish motor start up.
my normal load will be 2-3kw the rest of the time, and running upto 1000hrs a year or maybe a lot more if electric prices increase.
fueling on a mix of waste oils and others.
what are the down sides of running a ST7.5 at less than half load ?
cheers

[oliver90owner]

needenginerunnin,

what are the down sides of running a ST7.5 at less than half load ?

Efficiency.  You are running a bigger cooling fan at rated speed for every hour you run it.  Costs Watts which, in turn, means fuel.  You are requiring excitation for the windings all the time, which is likely more than a smaller machine.  Small extra drag driving the machine (mechanical losses).

Machine efficiency (if it is even given) will be less than the quoted figures.  Manufacturers rarely give an efficiency rating apart from the highest they can achieve.  If you consider when running your set while taking no load at all, efficiency is zero  (useful energy*100/ energy input).  It has to rise to maximum somehow but that graph is rarely ever touted to the user.  That graph might rise rapidly to the maximum or, more likely, rise gently to it's maximum at rated output.

All machines should give an indication for transient back-up for starting loads.  Many do not.  Why?  Because they have little back-up reserve.  The heavy rolled magnet frame machines with rotating armature are much better at starting heavy loads than stator-wound machines with rotating field, as a general rule I believe.

Starting a 3 HP motor on any sort of load drains my mains (lights go dim) at start-up.  I presume there are 'soft start' options to reduce the power required at start-up these days but I am not really an electrician, so I always used to ask someone at work, for help and guidance, in the distant past.  Lead size and cabling lengths to the motor may well have a noticeable effect on  starting ability as well.

Regards, RAB

[atimperson]

needenginerunnin

The 5kw and the 7.5 are basicaly the same head. The 7.5 weighs 10 to 12 pounds more ,mostly copper. The fans are the same size.  I've run a 5kw for over 4800 hrs and a 7.5 over 9000 hours on my 6/1 and you can't tell the difference.
Tim

[needenginerunnin]

quote

The 5kw and the 7.5 are basicaly the same head. The 7.5 weighs 10 to 12 pounds more ,mostly copper. The fans are the same size.  I've run a 5kw for over 4800 hrs and a 7.5 over 9000 hours on my 6/1 and you can't tell the difference.
Tim


thanks for that Tim,
is there anybody else out there running a ST7.5 who can say one way or the other if a 6/1 is a good match.
cheers

[Ratman]

Am I right in thinking that running a larger gen set than the engine is capable of is not such a bad idea, to supply initial start up load demand for things like motors and microwaves, so long as the continuous load is within the engines capability?
Good point Needenginerunnin.

Rob.........waiting for the right CS to come along.

[listerdiesel]

As a generalisation, the matching of engine and generator on a factory-built set is more an economics question that anything else.

A Lister 6/1 at 650rpm would run a lot of alternators that are beyond its maximum power, and the one thing in support of this is that a 4-pole 1500rpm alternator at say 10kVA output would give a better sine wave than a smaller machine. (Inverter output types not included)

Avoid a 2-pole machine like the Black death as it has an awfully distorted waveform which gets worse on low power factor loads.

We did some tests on our big rail chargers, running them off various portable generators for the rail industry, and the best match for a 5.5kW 110V output single-phase input charger was a 10kVA 4-pole alternator. The low power factor of the charger transformer affected the output as expected, but the sine wave distortion was nowhere near as bad as the 10kVA 2-pole 3000rpm set.

We are just looking at a 'large' battery charger for a customer, 300kW.... :-))

Peter

[snail]

Avoid a 2-pole machine like the Black death

At least as far as battery chargers go, I second what Peter says. I used a Forklift charger (60A, 24v) to charge my solar batteries. On a 2 pole Honda (5 KvA) I was lucky to see 40A. Immediately hit the changeover switch to the ST10 and voila, 60A.
Sorry  if this is wandering off topic a bit .

Brian

[ronmar]

Am I right in thinking that running a larger gen set than the engine is capable of is not such a bad idea, to supply initial start up load demand for things like motors and microwaves, so long as the continuous load is within the engines capability?
Good point Needenginerunnin.

Rob.........waiting for the right CS to come along.

In the case of a listeroid yes, as the large spinning flywheel mass can get larger loads started, so having a larger reserve capacity is a good thing within reason.  That is probably why I would say that the majority of 6/1 users opt for the ST-5 generator head.  There are ST-3 and I believe ST-3.5's out there, and these are a good match for the 6/1, but I think more people opt for the 5KW gen head.   Regardless of generator attached, the 6/1 is only going to handle a little over 3KW as a sustained load.  The ST-5 rated at 5KW continous is approaching 2 times that already.  I don't think I have ever seen a published surge rateing for a ST-5, but looking at the wiring compared to other generators I have peeked inside, it appears reasonably stout, so I would take a guess at a 7-7.5KW surge capacity.