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Old 02-21-2020, 05:19 PM   #1
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run inverter off alternator?

I've never really understood alternators... like if/how they determine what goes where when there are multiple draws on them, and how their output relates to engine rpm. lol, i dont really believe they can target their output.

i have a 320 amp alternator wich i intend to power an inverter to run the a/c while driving, then use shore or gen power when camped.

my bus lacks ammeter (and tach), so i'm in the dark here. in reality i guess house batteries would be isolated and hooked to inverter and would 'buffer' when alternator is strained, but for now lets assume only starting battery, accessories and inverter.

does alternator output vary on, rpm, or both or either? would inverter take precedence over charging the battery? would starting battery drain at high idle, then inverter shut down hot/hard?

thanks

evan
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Old 02-21-2020, 07:36 PM   #2
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The batteries is what does all the work for the inverter. You can charge batteries with solar, shore power, alternator, or generator. As long as the battery voltage does not drop too low the inverter will be fine.

The only way to hurt the alternator is if the house batteries are very dead and you try to power up any loads before they are charged back up.
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Old 02-21-2020, 08:18 PM   #3
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In my first bus, I had a small residential window A/C unit. The bus had a D-4 truck battery as the house battery, a 3000 watt inverter and I had a A/B switch so I could charge either the house battery or the bus battery as I drove. Once, when driving, we tried to run the A/C with the house battery selected for the alternator. When doing this, the amp meter would peg to the high side so I was afraid that it would fry the alternator.
Alternators are really designed to maintain the charge of your battery, they are not designed to carry the max rated load for extended periods of time. At least, that is what I was told by my dad (mechanic) many years ago.
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Old 02-21-2020, 10:27 PM   #4
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First, a bit of information about alternators:
Alternators vary their output current in an attempt at keeping a set output voltage. As the load changes or the engine speed changes, the alternator adjusts to try and maintain that output voltage. Alternators are rated at maximum output at a certain *alternator shaft* speed. At speeds less than the rated speed, the output current is less than the rated current. The "minimum" current output at the "minumum" alternator rotational speed varies from alternator to alternator. Some are designed to output a lot of current at low speeds (those are expensive and are usually associated with special tasks, like powering a fire engine equipment bay) but most have relatively low power output at slow alternator rotational speeds ... usually about half that of rated power.


The chassis designers select alternators that will provide enough current to maintain the chassis systems at idle and not much more. At higher than idle speeds, the excess current capacity is used to charge the batteries. Once the chassis power requirements are met and the batteries are charged, the alternator cuts back the current thus reducing the load on the alternator.



We have a second alternator which is connected to our house batteries. It is a 350A unit that puts out about 200A at engine idle. This is enough current to power our air conditioner through our inverter. When the air conditioner is running at maximum, it pulls about 120A (DC 12V nominal) through the inverter. So as long as the engine is running, the alternator produces enough power to run the air conditioner.


Our house batteries and chassis batteries are isolated from one another, with a battery isolation relay between them so that I can connect them together if needed. Most of the time, they are run independent with the chassis alternator taking care of the chassis systems, and the house alternator taking care of the house systems. I designed the systems this way so that the house systems would not put additional loads on the chassis systems and vice versa.


Bear this in mind when extending the loads and functions of your chassis systems.
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Old 02-21-2020, 10:32 PM   #5
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Quote:
Originally Posted by Native View Post
First, a bit of information about alternators:
Alternators vary their output current in an attempt at keeping a set output voltage. As the load changes or the engine speed changes, the alternator adjusts to try and maintain that output voltage. Alternators are rated at maximum output at a certain *alternator shaft* speed. At speeds less than the rated speed, the output current is less than the rated current. The "minimum" current output at the "minumum" alternator rotational speed varies from alternator to alternator. Some are designed to output a lot of current at low speeds (those are expensive and are usually associated with special tasks, like powering a fire engine equipment bay) but most have relatively low power output at slow alternator rotational speeds ... usually about half that of rated power.


The chassis designers select alternators that will provide enough current to maintain the chassis systems at idle and not much more. At higher than idle speeds, the excess current capacity is used to charge the batteries. Once the chassis power requirements are met and the batteries are charged, the alternator cuts back the current thus reducing the load on the alternator.



We have a second alternator which is connected to our house batteries. It is a 350A unit that puts out about 200A at engine idle. This is enough current to power our air conditioner through our inverter. When the air conditioner is running at maximum, it pulls about 120A (DC 12V nominal) through the inverter. So as long as the engine is running, the alternator produces enough power to run the air conditioner.


Our house batteries and chassis batteries are isolated from one another, with a battery isolation relay between them so that I can connect them together if needed. Most of the time, they are run independent with the chassis alternator taking care of the chassis systems, and the house alternator taking care of the house systems. I designed the systems this way so that the house systems would not put additional loads on the chassis systems and vice versa.


Bear this in mind when extending the loads and functions of your chassis systems.
Where did you find yours auxiliary alternator bracket? I have been searching for one for my Cummins for some time and have not had any success.
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Old 02-22-2020, 12:07 AM   #6
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Where did you find yours auxiliary alternator bracket? I have been searching for one for my Cummins for some time and have not had any success.
CWMill.com ... they had made a bracket for the CAT 3126 for ann air conditioner compressorprior to my request for the alternator. So basically, it is a compressor bracket modified for my alternator.
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Old 02-22-2020, 07:01 AM   #7
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Yes you really want a high-quality and largish capacity House bank there to **buffer** between the aircon load draw rate and the alternator input.

So long as the latter averages higher than the former, the battery SoC won't be drawn down (the bank will last a very long time).

With any energy source not just alternators, there is no concept of "directing" current "to" the various loads. Loads are actively drawing what they need, and the voltage regulator is striving to keep the current sufficient to keep the voltage within its specified range, no "knowledge" of where the electrons are going.

There are fancy external VRs you can fit that will prevent very high draw loads (e.g. LFP bank without a DCDC fronting its CAR) from overheating the alternator.

And resizing your pulley ratio can tune the rpm required to get up to a decent current rate.

But selecting a robust large-frame high output alternator designed to power first responder rigs' auxiliary circuits in the first place is the best foundation, and usually the cheapest component out of those others enhancements.

Use a clamp ammeter and DMM to analyse how things work under various conditions, and then mount meter displays with their shunts located to show the flow rates at specific points in the system if you want to track how things are going "live" while in use.
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Old 02-22-2020, 09:42 AM   #8
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Quote:
Originally Posted by Native View Post
CWMill.com ... they had made a bracket for the CAT 3126 for ann air conditioner compressorprior to my request for the alternator. So basically, it is a compressor bracket modified for my alternator.
Thanks Native.

Unfortunately, I have already tried them. No solution for my 8.3 .

My quest continues
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Old 02-22-2020, 10:38 AM   #9
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Thanks for the thoughtful responses, i have a much better understanding of where to go from here!
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Old 02-27-2020, 08:19 AM   #10
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I have seen crankshaft pulleys on bue bird transit busses that run a/c units. Priced one thru blue bird was $400 they wouldn’t give me part number. Called Cummins gave them the serial number of engine didn’t come from them
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Old 02-27-2020, 08:54 AM   #11
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Originally Posted by BarnYardCamp View Post
In my first bus, I had a small residential window A/C unit. The bus had a D-4 truck battery as the house battery, a 3000 watt inverter and I had a A/B switch so I could charge either the house battery or the bus battery as I drove. Once, when driving, we tried to run the A/C with the house battery selected for the alternator. When doing this, the amp meter would peg to the high side so I was afraid that it would fry the alternator.
Alternators are really designed to maintain the charge of your battery, they are not designed to carry the max rated load for extended periods of time. At least, that is what I was told by my dad (mechanic) many years ago.
Your dad is correct. Most alternators aren't built to run full out for long periods of time. Proper ventilation for the alternator is also important for it's life.

How'd you have the voltage regulator hooked up in your scenario?

Unless you also switched which system the voltage regulator was sensing, your system would have been messed up because it would be charging the house batteries based off the voltage of the bus batteries. Which would very easily peg the ammeter.

Or you were using a one wire alternator, but I don't think any md/hd alternators are one wire.
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Old 02-27-2020, 08:58 PM   #12
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Most alternators aren't built to run full out for long periods of time. Proper ventilation for the alternator is also important for it's life.
So, if you need that functionality, buy the right type of alt, designed for that purpose, set it up properly and avoid the problems that result from not doing so.
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Old 02-28-2020, 07:29 AM   #13
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So, if you need that functionality, buy the right type of alt, designed for that purpose, set it up properly and avoid the problems that result from not doing so.
100%

And note, those alternators aren't typically cheap, or are readily available at any ol parts store.
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Old 02-28-2020, 08:24 AM   #14
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disagree, the external VR is the pricey bit, also if you need to pay for labor going to a multi-vee (serpentine) setup, or want a custom pulley size to increase "high idle" alt rpm

the alt itself is cheap on eBay, can pick up top notch units in great shape for well under $200

or get a junkyard one

Leece-Neville off a fire truck or ambulance

and have it reconditioned locally, not expensive
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Old 02-28-2020, 10:19 AM   #15
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disagree, the external VR is the pricey bit, also if you need to pay for labor going to a multi-vee (serpentine) setup, or want a custom pulley size to increase "high idle" alt rpm

the alt itself is cheap on eBay, can pick up top notch units in great shape for well under $200

or get a junkyard one

Leece-Neville off a fire truck or ambulance

and have it reconditioned locally, not expensive
Now I'm wondering about picking up a "not running gas powered generator" for cheap just for the ac generator -- then build brackets for serpentine belt mounting ,or use a pto to operate it.
I guess that notion is really time for a different thread...
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Old 02-28-2020, 10:55 AM   #16
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disagree, the external VR is the pricey bit, also if you need to pay for labor going to a multi-vee (serpentine) setup, or want a custom pulley size to increase "high idle" alt rpm

the alt itself is cheap on eBay, can pick up top notch units in great shape for well under $200

or get a junkyard one

Leece-Neville off a fire truck or ambulance

and have it reconditioned locally, not expensive
When I was looking to replace mine(stock leece neville in 160 amp), I couldn't find one in the J180 mount(what mine has stock) that was over 200 amps for less then 500 bucks. Couldn't find anything at the local junkyards either. Local shop wanted 150 to rebuild, so I had it rebuilt. Granted it's self exciting so maybe that's the cost increase/rarity.

Quote:
Originally Posted by banman View Post
Now I'm wondering about picking up a "not running gas powered generator" for cheap just for the ac generator -- then build brackets for serpentine belt mounting ,or use a pto to operate it.
I guess that notion is really time for a different thread...
Looked at that too. Most gas generator are lacking a shaft bearing on the side toward the engine. So make sure the generator head is dual bearing. Also IDK how you'll get the frequency correct for ac power while going down the road. Maybe find large enough diodes and make your own alternator.

I was also looking into retrofitting one of those inverter generators as well, but good luck finding one of those cheap enough to tear apart for experimentation.
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Old 02-28-2020, 11:05 AM   #17
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My default for off grid power is DC.

Home style appliances designed to feed off mains power are way too thirsty.

Even standalone at a fixed site, I'd go "DC generator" using an HO alternator and whatever engine suited.

Sure for portable quiet usage, can't beat the little inverter suitcase style, but then I get a matched mains charger (AC-DC rectifier) so the output is mostly used to quickly recharge the big lithium House bank, and then turn off.

A big alt in a moving vehicle does the same job, can give a full week's energy from just one trip into town for supplies, water, propane etc.

Powering an AC genset head from a bus engine would not at all fit into my way of thinking.
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Old 03-05-2020, 06:26 AM   #18
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Thanks Native.

Unfortunately, I have already tried them. No solution for my 8.3 .

My quest continues
There is a guy on Youtube that has a channel called Transcend Existence bus conversion. He has a bus with a CAT3126 that he made a bracket for a 24V alternator for his setup. It was pretty descriptive of the making of the bracket. You could do the same kind of setup.

If you are worried about AMP draw, you can always change your house system to 24V so you have less current for the same watts draw. I know thats expensive but the higher your battery voltage, the more watt capable you can get with your inverter and your efficiency goes up.
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Old 03-06-2020, 12:44 AM   #19
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Quote:
Originally Posted by Native View Post
First, a bit of information about alternators:
Alternators vary their output current in an attempt at keeping a set output voltage. As the load changes or the engine speed changes, the alternator adjusts to try and maintain that output voltage. Alternators are rated at maximum output at a certain *alternator shaft* speed. At speeds less than the rated speed, the output current is less than the rated current. The "minimum" current output at the "minumum" alternator rotational speed varies from alternator to alternator. Some are designed to output a lot of current at low speeds (those are expensive and are usually associated with special tasks, like powering a fire engine equipment bay) but most have relatively low power output at slow alternator rotational speeds ... usually about half that of rated power.


The chassis designers select alternators that will provide enough current to maintain the chassis systems at idle and not much more. At higher than idle speeds, the excess current capacity is used to charge the batteries. Once the chassis power requirements are met and the batteries are charged, the alternator cuts back the current thus reducing the load on the alternator.



We have a second alternator which is connected to our house batteries. It is a 350A unit that puts out about 200A at engine idle. This is enough current to power our air conditioner through our inverter. When the air conditioner is running at maximum, it pulls about 120A (DC 12V nominal) through the inverter. So as long as the engine is running, the alternator produces enough power to run the air conditioner.


Our house batteries and chassis batteries are isolated from one another, with a battery isolation relay between them so that I can connect them together if needed. Most of the time, they are run independent with the chassis alternator taking care of the chassis systems, and the house alternator taking care of the house systems. I designed the systems this way so that the house systems would not put additional loads on the chassis systems and vice versa.


Bear this in mind when extending the loads and functions of your chassis systems.



Why would you do this? Wouldn't it be easier and cheaper to buy an inverter generator to run the a/c. This would work both with the bus running and parked. Besides using the bus engine fuel, what is the advantage of this huge alternator/ battery/inverter setup to run the a/c? Space saving?



It is my understanding that an inverter generator is basically a gas engine running an alternator, then converting to dc to run thru the inverter to make ac electricity. The alternator being able to ramp up the engine speed and therefore current due to demand allows the engine to slow down when current needs are less and reduces noise and fuel consumption.


It does not have to maintain a set speed like a generator does to maintain the frequency requirements (60hz in the US) for ac power, the inverter takes care of that.
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Old 03-06-2020, 07:14 AM   #20
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thanks for the ideas/explanation everyone.

i have an onan qd5500 which is not mounted yet, and this will run the a/c units while camping and if needed while driving.

I was just trying to find a way to (if this makes sense) utilize the excess power off the alternator while behind the wheel, my thinking being 'ive got it so i might as well use it'.

is this faulty thinking?
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