Mech guy needs help... 2

[RoarkS]

Hey so I'm building a generator system with an existing 300 amp 28v DC starter generator. For most of it's use it's going to be driven by gas engine to make about 200 amps. But I need it for starting a large engine, that pulls north of 500 amps spike on inrush. The system currently uses a large 40ah battey for start that gets kicked in the teeth.

The electrical guys who designed it made it so the generator chases 28volts. That's fine for most of the operation, but when I need to hit that big starter I think I want the system to follow the battery voltage sag so the battery and generator share the current load for the start sequence.

The first system test, the EE guys thought the battery would just share the load... maybe even attempt to clamp the voltage a little but it didn't. It was like the batteries were not even there. Generator took the brunt of it as it tried to pin 28v, and the battery sagged out to about 15 volts. Batt internal resistance went high, generator impedance was low... boom we hit our 300 amp max and the start sequence took WAY too long.

Really I'm curious is this a normal problem? Is there a simple discrete device way to handle this? Cause I'm thinking of having two manually selected modes... 28v normal run, then a battery voltage follower mode that ya switch to "arm" for starting.

Thoughts... again, sorry mech guy trying to get into electronics and a little over my head.

/I wish I had good scope data on the inrush but I don't. the starter is rated to 1400amps peak, but 600a continuous. I'd say we dropped below 600a in 5 seconds, then slowly tapered off about 200 amps after 20 seconds.

 

[RedSnake]

But I need it for starting a large engine, that pulls north of 500 amps spike on inrush. The system currently uses a large 40ah battey for start that gets kicked in the teeth.

Well theoretically your 40Ah battery could deliver 500A for 4,8 seconds fully loaded.
I am assuming this batteri is a led acid one made for starting things and not for slow continuous outtake.
Easiest solution would be to buy another 40Ah one connect them in parallel, then you would have twice the time but each battery would only have to take half the load or just a battery with higher Ah.

Maybe not the solution you were looking for, but a simple one.

There are solid state relays or thyristor you could use in combination with some time relay but it's not easy finding anything that can handle so high DC currents, not sure its worth the time, money and effort either.

 

[RoarkS]

You're not wrong.
but... that would require an infrastructure to support loaner batteries that doesn't exist. These batteries are about $5k.

 

[RedSnake]

loaner batteries ??
$5k hmm..
Must be some very special batteries of course they are 28 V which is a bit special..
But not far from 2 serie coupled car batteries which would give something like 24-25,4V and 80 to100Ah.
Here that would cost around 280 USD.

 

[RoarkS]

I know.
Has to be a gas generator.
That part of the design is above my pay grade.

 

[mfgenggear]

OP
I am totally confused.
$5 k battery ?
40Ah ?

What does the manufacture of starter recommend.
Car battery is 100 Ah cost at $150 for AGM battery. What am I missing.
How many cylinders , horse powered
Engine compression, may be the starter is Mal functioning or inadequate.

 

[mfgenggear]

Two 12v batteries on series is 24v
Why 28v this is wierd

 

[waross]

Two 12v batteries on series is 24v
Why 28v this is wierd

12 Volt nominal. Actual open circuit voltage = 27.6 Volts to 28 Volts.
Drops to 15 Volts under heavy load?
I haven't done automotive work for many years but 15 Volts under heavy load would be close to 8 Volts during the starting of an old school 12 Volt auto engine.
Before the inception of electronic ignition on car engines it was common to use 8 Volt ignition coils.
There was a series resistor to drop the coil voltage to 8 Volts during starting so that you got full spark while cranking.
BUT
Are you sure you have your numbers right?
I can buy 2, 12 Volt nominal 40 Ah batteries for about $150 each. In series that would give about 28 Volts actual voltage.

Your generator:
Not enough information.

 

[mfgenggear]

Waross
Worked on old school cars for years. 6 and 12 v systems. Maybe I have forgotten some of it. Don't recall the resister between points and coil.
Back to OP generator does his battery appear correct have seen 28 v system.

 

[waross]

300 amp 28v DC starter generator.

What type of field control does the starter-generator use?
What type of generator is it?
Starter generators were once common on small riding tractors.
Those units used one field winding for generating and another for starting.
There was about a 10:1 ratio between the starting current rating and the generating current rating.

What is the HP of the motor driving the generator?
Calculations say that you will need about 25 real HP driving the generator.
In the real world, that will probably be a small engine rated at 50 HP or more.

Your description of the sharing of the starting current is almost what I would expect, but I am surprised that you get that much out of a 40 Ah battery with that load.
I see two issues;
1. Starting the large engine.
2. Starting the large engine with your starter-generator.
Issue #1.
Use two 8D batteries in series. You will find 8D batteries with CCA (Cold Starting Rating) of from 1100 Amps to 1450 Amps.
You may connect them in series for a simple circuit,
or
You may add a "Series Parallel Switch" and charge with 12 Volts nominal and support 12 Volt loads. This was common in large diesel freight trucks for many years.
Issue *2.
It may be possible to start the large engine with the equipment that you have, but I can't help you until I know what you actually have.
Pictures will help.
Legible pictures of name plates will help even more.

 

[waross]

Don't recall the resister between points and coil.

Did you ever wonder what this was?
Found on the fire wall of big three cars up until the mid 60s.

Customer;
"My car starts right up, but as soon as I stop cranking the engine stops."
Mechanic;
"Your coil resistor has failed. You may short it out to drive to the shop, but if you don't repair it within a couple of days the coil will burn out."
A common complaint and solution.
(I once gave my Dad that advice when he had that problem. It worked so well that he didn't bother to get it fixed. Four days later he needed a tow to the shop and a new coil as well as a new resistor.)
BTDT

 

[mfgenggear]

I have an have RV , one 12v AGMA battery 100 Ah to start my V10 engine .
My old1953 chevy hot rod. 350 high per formance had so much compression a standard 12v starter could not crank it.
I had to purchase a hotrod starter.
High capacity cranking.
With a higher Ah 12v battery.

 

[mfgenggear]

Did you ever wonder what this was?
Found on the fire wall of big three cars up until the mid 60s.
View attachment 4651
Customer;
"My car starts right up, but as soon as I stop cranking the engine stops."
Mechanic;
"Your coil resistor has failed. You may short it out to drive to the shop, but if you don't repair it within a couple of days the coil will burn out."
A common complaint and solution.
(I once gave my Dad that advice when he had that problem. It worked so well that he didn't bother to get it fixed. Four days later he needed a tow to the shop and a new coil as well as a new resistor.)
BTDT

Waross lol yes I remember it now. Lol

 

[waross]

does his battery appear correct have seen 28 v system.

Seen many 24 Volt start systems on highway trucks.
Put a meter on it and see 28 Volts.
There may have been other fields such as telecommunication back ups where battery systems were rated at 28 Volts.
In the early days they needed 24 Volt systems to get the big diesels started.
But then they found out that the filaments of the 24 Volt lights would not stand up to the vibration encountered in a freight truck.
(If you double the voltage, the cross sectional area of the filament drops to 1/4.)
Hence the series parallel switch.
Everything including battery charging was 12 Volts nominal. Starting was 24 Volts nominal.

 

[waross]

With a higher Ah 12v battery

Do you remember the CCA ratings?
Two batteries with similar Ah rating may have a 2:1 ratio between CCA ratings.
This is related to internal resistance and voltage drop under heavy loads.
For starting, CCA is more important than Ah.

 

[mfgenggear]

Do you remember the CCA ratings?
Two batteries with similar Ah rating may have a 2:1 ratio between CCA ratings.
This is related to internal resistance and voltage drop under heavy loads.
For starting, CCA is more important than Ah.

I don't remember I sold my hotrod last year.
But good to know. It did have a high cold cranking amperage. Paid extra for it.
Reliability was a big issue as well.
I generally purchase as high as possible and in stock. Ugg

 

[mfgenggear]

So I will start a new post as not to take over
OP post. I engineering hobbies. How to rewire a 1947 cj2 jeep.

 

[TugboatEng]

We don't know the condition or chemistry or the batteries. Let's let OP start with that before we speculate. For all we know these batteries are past their expiration date.

In my experience, 24V describes lead acid batteries that are actually 25.2 volts open circuit and charge between 26.4 and 29 volts. Meanwhile, 28V describes NiCad batteries and I don't know anything about their charging specifics.

 

[LittleInch]

Hey so I'm building a generator system with an existing 300 amp 28v DC starter generator. For most of it's use it's going to be driven by gas engine to make about 200 amps. But I need it for starting a large engine, that pulls north of 500 amps spike on inrush. The system currently uses a large 40ah battey for start that gets kicked in the teeth.

The electrical guys who designed it made it so the generator chases 28volts. That's fine for most of the operation, but when I need to hit that big starter I think I want the system to follow the battery voltage sag so the battery and generator share the current load for the start sequence.

The first system test, the EE guys thought the battery would just share the load... maybe even attempt to clamp the voltage a little but it didn't. It was like the batteries were not even there. Generator took the brunt of it as it tried to pin 28v, and the battery sagged out to about 15 volts. Batt internal resistance went high, generator impedance was low... boom we hit our 300 amp max and the start sequence took WAY too long.

Really I'm curious is this a normal problem? Is there a simple discrete device way to handle this? Cause I'm thinking of having two manually selected modes... 28v normal run, then a battery voltage follower mode that ya switch to "arm" for starting.

Thoughts... again, sorry mech guy trying to get into electronics and a little over my head.

/I wish I had good scope data on the inrush but I don't. the starter is rated to 1400amps peak, but 600a continuous. I'd say we dropped below 600a in 5 seconds, then slowly tapered off about 200 amps after 20 seconds.

You're confusing the living daylights out of me.

This gas engine powered generator seems to be being used as generator only or do you use it to start this gas (do you mean gasoline?) engine?

Either way I think that is irrelevant to what you are then using your generator for.

So this bigger engine starter takes up to 1400A at 28V or 24V or what?

This mysterious rather small looking very expensive battery is getting "kicked in the teeth" Care to explain a bit are what that means? Does it start this bigger engine or not? If not why not? But the key is what this battery is, what it's voltage is and max instantaneous current and current after say 5 seconds.

How is this all connected? If the voltage drops to 15V for both battery and generator then that's a pretty big voltage drop for sure. And even at 300A, at 15V that's less power than your 200A @ 28V.

Is this a normal problem? No one knows as ther is lack of understanding about your components and how they work. It's a normal problem in respect that the generator is underpowered to start this bigger engine and hence needs back up from the battery which isn't happening.

So as usual, you need to supply more details for anyone to give you a better way to do it.

 

[RoarkS]

Okay I wrote this as a forum post, but ignore that. Lets sus out the main question at the end. "Sorry to leave you hanging... took a minute to come back to this project.

So I just looked... the batteries are closer to $10k. But like I said We’re making an engine driven generator. I was joking earlier today that I just need to make an “engine driven battery”. LOL

Sorry for the confusion, my typo. 28volt system, uses nominal 24volt batteries. This is a new system design, not a one-off project. The system is being designed around well-maintained NiCad batteries.

Waross:

-The starter generators of the family I’m playing with have a series field for starting, then for generator mode, shut wound field and interpole. Externally excited, driven by a voltage regulator, then on top of it, is the generator control which takes care of making sure the power is clean.

-You nailed it on the horsepower. It started out rated at 40hp, but significantly derated. The original generator we were using was fantastically efficient and was hoping for about 15hp continuous... but we’ll see as we dumb the system down for this more off the shelf starter-generator.

-Really the design is to prevent from using up the batteries running other initialization systems (environmental and electrical controls) so that when we hit the start button, the batteries are at full charge and can handle the anticipated 1400amp inrush spike. You’re absolutely right... the newer systems use 2 batts tied together, and actually some pretty cool sequencing circuitry to basically kill the first one, then bring the second online to finish the job. ***but*** what if I could use the generator with the batts to “soften the blow”

----

As far as name plates... unless you work for the shops that build my stuff it would be meaningless. It’s all custom... but basically a Series starter, and “A” circuit generator.

Littleinch:

Small gas engine with a starter generator installed... starter to start small gas engine, and small gas engine drives the generator. That whole unit is used to ideally just make sure the overall system batteries are fully charged and available for the massive load hit of starting a **much** larger gas engine. It would be nice if the generator could share the load with the batteries on start. This much larger engine is typically started with the generator isolated (if it has one, usually not), using only 24v nominally rated batteries and yes, the inrush on start can hit 1400amps for a few ms, but it goes down to 500a real quick, especially when the batt voltage sags down to 15.1V... below that more batteries are brought online... until it starts.

So is this a normal problem... We set out on the project to keep the batteries topped off for starting so we can run stuff to “warm up”. But it has now begged the question, can we use the generator to help the batteries during big engine start? Basically, the field engineer doing the testing decided to test this unit on start-up “cause it’s high load”... and tripped every safety we had onboard. It’s kinda a miracle a few million dollars of equipment didn’t fry because he had to jumper the relay that cuts out if the system sees any undervolt/overvolt/surge/ripple... all that.

So with more details... Back to the main question: Does it make sense to make a mode in this unit, to stop seeking the 28v charging mode, and have it follow the battery voltage sag... or slightly ratio the battery voltage so we can take the load off the batteries.