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Old 15-02-2015, 22:37   #1
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Inverter / Alternator

Hi Guys,

I have a 1000W inverter and a quick calculation suggests 1000W/12V = 83Amps draw at full power. My alternator is rated at 80amps.

What would be the best way to get these guys together? The alternator would obviously be charging the batteries but would this reduce the power available to the inverter? Is it OK to wire the inverter straight on to the alternator output?

TIA

Steve
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Old 15-02-2015, 22:50   #2
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Re: Inverter / Alternator

You'd want to wire the alternator to the batteries and then have another hard wire run from the batteries to the inverter.

Depending on the physical arrangement you can have the alternator tied into this run in between the alternator and the batteries, but the main thing is to have a nice low resistance run between the inverter and the battery.
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Old 15-02-2015, 23:31   #3
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Re: Inverter / Alternator

Without knowing what your running from your inverter...

It's unlikely you're drawing full power all the time. Your alternator charges at higher amp rates as battery voltage drops. So when you start using power the alternator is just cruising along until it senses a voltage drop. The lower the drop the greater charging rate. After the inverter use goes down the alternator starts dropping amps as it nears full voltage. It sounds like you want to use the alternator as a stand alone generator for the inverter. Neither are designed to be used that way. The alternator probably doesn't have a 100% duty cycle and would overheat continuously outputting max amps. You need a battery as a kind of buffer and to trigger the alternator into outputting. The alternator belt needs to handle about 25hp at full charge. You may need dual belts or some flat belt.
I also run an inverter when running to save running a generator. Mine is a pure sine wave with 2 120 volt legs that either supply 120 or 240 volts. I rarely run close to maximum, but can run any ac on the boat including a 50 gallon water heater. I use a 48 volt bank (smaller wires to the battery-smaller wire from alt.) and run a 48 volt alternator on one of the mains to keep the batteries up. Everything runs thru the inverter. When on shore or generator power, the ac passes thru the inverter but keeps the batteries up. I have yet to install solar cells and a wind generator that is my plan for anchoring.
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Old 16-02-2015, 19:31   #4
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Re: Inverter / Alternator

Hi Guys,

Thanks for the replies. My question is about not breaking anything. The alternator is wired into a full battery charging circuit, house + starter batteries, joining relay, switches, etc. I wired the inverter in to the alternator common point before it went off to the batteries. Started the motor, turned everything on (inverter & microwave on standby) and it seemed fine. However, running the microwave for a few seconds, and the inverter alarm went off - though it seemed to continue to provide enough power. The thing that concerns me is that the engine tacho stopped working at this time. This uses a signal off the alternator winding and I'm concerned that I have damaged the alternator by drawing too much current. It's probably not helped by the fact that the house batteries are not holding a charge very well and will be replaced shortly.

The question is, should I move the inverter connection point closer to the battery and should I be doing something else to protect the alternator from over-current?

I'll run some tests on the alternator on Saturday next time I'm out on the boat.

Cheers

Steve.
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Old 16-02-2015, 19:40   #5
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Re: Inverter / Alternator

Steve, there's a pretty good chance that you have damaged the alternator. At the very least, blowing out the diodes. I would suggest you have someone with some experience do your electrical work until you have a good understanding of what your doing. To save yourself some money for unnecessary repairs. Your inverter should be connected ONLY to the house battery bank and be properly fused. Chuck
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Old 16-02-2015, 21:32   #6
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Re: Inverter / Alternator

Quote:
Originally Posted by steve_roach View Post
Hi Guys,

I have a 1000W inverter and a quick calculation suggests 1000W/12V = 83Amps draw at full power. My alternator is rated at 80amps.

What would be the best way to get these guys together? The alternator would obviously be charging the batteries but would this reduce the power available to the inverter? Is it OK to wire the inverter straight on to the alternator output?

TIA

Steve
Steve,

Inverters are not 100% efficient. Typical efficiency is 85% plus or minus. A typical 1000W inverter draws about 100A DC. But inverters are not constant loads. The battery current into the inverter varies with the AC current drawn by the load. So the peak current is well over 100A. For this reason you must connect the inverter directly to a battery with very short cables of the correct size. Read the manual that comes with the inverter and follow their instructions and it should work. If they tell you to limit the length of the battery cable they are not kidding.

The alternator will not work unless it has a battery connected. The alternator should have its own wires to the battery and the inverter should have its own separate wires. Operating an alternator without a battery connected will almost always destroy its internal diodes.
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