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.

 

[waross]

Or buy an air starter añd an off the shelf, gas engjne driven air compressor and a large air storage tank.
This may be cheaper and mire dependable.

 

[RoarkS]

Or buy an air starter añd an off the shelf, gas engjne driven air compressor and a large air storage tank.
This may be cheaper and mire dependable.

I promise we started with basic physics and worked up. Gas driven generator is the only way. A turbine would be arguably better, but development costs are way too high.

 

[mfgenggear]

OP What heck are you doing. The whole mess doesn't make sense.
What is the generating requirement mega watts, kilowatts?
How much is you budget?
Is a diesel or turbine required.
3 phase or 2 phase , 240 or 440 volts
What does the generator needed for
Back up or off grid

 

[LittleInch]

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.

RoarkS

I think the main question for me is whether you actually have the right data to determine anything.

I've had a quick look at the world of aircraft 24V 40Ah Ni-Cad batteries and they either don't state much or many of the ordinary batteries seem to max out at about 350-400A for not very long and many are less than this. The fact you pulled a nominal 24V battery down to 15V tells me you exceeded it ability to give you current and lost power also as the voltage dropped. Rather surprised it didn't burst into flames.

For me you either need a "super starter" battery which can give you 1200A for 15 seconds or you need to get a bigger generator. Or get some 12V lead acid batteries and plug them in when you start the bigger engine.

Anything else is just ignoring the issue.
And generally under volt relays and other protection devices are there for a reason.
Sort out the problem (not enough amps), not the symptom.

 

[dtimberlake]

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

I think separate ballast resistors were a Mopar/Chrysler technique.
Ford and GM often used Special wire between coils and points.

ignition coil resistor - The 1947 - Present Chevrolet & GMC Truck Message Board Network

ignition coil resistor Electrical

67-72chevytrucks.com

Ballast resistor needed?? - The Ford Barn

Ballast resistor needed?? Late V8 (1954+)

www.fordbarn.com

 

[RoarkS]

OP What heck are you doing. The whole mess doesn't make sense.
What is the generating requirement mega watts, kilowatts?
How much is you budget?
Is a diesel or turbine required.
3 phase or 2 phase , 240 or 440 volts
What does the generator needed for
Back up or off grid

300 amp 28v DC starter generator. Last I checked that's 8.4kw... DC.
Off grid as it gets.

--------------------

RoarkS

I think the main question for me is whether you actually have the right data to determine anything.

I've had a quick look at the world of aircraft 24V 40Ah Ni-Cad batteries and they either don't state much or many of the ordinary batteries seem to max out at about 350-400A for not very long and many are less than this. The fact you pulled a nominal 24V battery down to 15V tells me you exceeded it ability to give you current and lost power also as the voltage dropped. Rather surprised it didn't burst into flames.

For me you either need a "super starter" battery which can give you 1200A for 15 seconds or you need to get a bigger generator. Or get some 12V lead acid batteries and plug them in when you start the bigger engine.

Anything else is just ignoring the issue.
And generally under volt relays and other protection devices are there for a reason.
Sort out the problem (not enough amps), not the symptom.

I'm fairly impressed too. There are sensors in the battery to help monitor if they do decide to catch on fire. But I promise I watched it hit 15.2 volts at the peak inrush... then stabilize out about 21 volts. These are the top of the line super starter batteries and even then as hard as they are getting hit they don't last as long as they should.

I did come across some systems that are used in multiengine turbine aircraft that use DC starters, where once they get one engine up and running, the gen on that engine helps the other. Just curious how they do it. The manuals don't explain what's happening inside the magic box.

They have what's called a 'gen start assist'... the manual I'm reading at the moment, takes the battery off the system bus, dedicates it to starting the engine, but allows the generator to provide some current to help the battery. Wish I knew the specifics of the matter. Reached out to some vendors who make the controllers but see what happens.

 

[mfgenggear]

Found this white paper from NASA
Starter generator for turbine engines

 

[mfgenggear]

They reference 12 v and 24 v batteries.
Once the starter starts the engine the system no longer requires the battery. But does recharge it.
I guess I get confused easier these days
I built these gears in the gear boxes with AGMA Q 12 or higher.
Some of the starters were for APU, and turbine engines with pneumatic starters. Why I got confused.