3000 watt inverter

This is the place for any magazine-related discussions that don't fit in any of the column discussion boards below.
Post Reply
paulrevelcet
Posts: 177
Joined: Fri Mar 05, 2004 1:01 am
Location: Kentucky
Contact:

3000 watt inverter

Post by paulrevelcet »

I have a maxx sst 3000 watt inverter, I cant seem to find the full load specs, the book says its 90% efficient, and can put out 27 amps on the ac side, does this mean that it will draw 30.4 amps( 10% more than 27 amps) on the dc side?
I also need to measure the amp draw of the inverter while in use, would a cheep clamp meter be sufficient? Its ok if its within an amp or 2 I just need to make sure its working.
rshayes
Posts: 1286
Joined: Tue Mar 04, 2003 1:01 am
Contact:

Re: 3000 watt inverter

Post by rshayes »

Unfortunately not. The efficiency is the output power divided by the input power. If the output power is 3000 watts, the input power at 90 percent efficiency would be 3333 watts. If the input voltage is 12 volts, the input current would be 278 amps. At this load, the battery voltage will probably drop. The inverter is specified to operate down to a 10 volt input. At this point, the input current would be over 333 amps.

Plan on using big wire. You also want a switch and fuse. If the battery and wiring drops are less than 2 volts under a 333 amp load, the short circuit fault current will be over 2000 amps. You want some way of interrupting this if you have to.

Cheap clamp-on ammeters (and not so cheap ones) measure AC current. The clamp forms the core of a current transformer with a 1 turn primary winding. A few clamp-on meters are made with Hall-effect sensors. These will measure DC current.

Another alternative is to use a current shunt with a suitable meter. A current shunt is basically a low value resistor connected in series with the circuit. The voltage drop across the resistor is usually 100 millivolts at full current (sometimes 50 or 25 millivolts). A 500 amp shunt would have a resistance of .0002 ohms. There are usually four connections. The big two are for the load current current to flow through. The small ones are the meter connections. This avoids having the contact resistance interfering with the calibration. The "resistor" will be several wide strips of metal between two copper blocks. You might find one of these on the surplus market.

A compass held near the wire will deflect when current is being drawn. This won't be very accurate. Some automotive ammeters are built this way, but they arn't very well calibrated and they won't read 300 amps. A nulling measurement might be possible with a compass by using a 100 turn coil to buck out the field from the wire. One amp in the 100 turn coil would equal 100 amps in the wire.

<small>[ November 23, 2005, 11:44 PM: Message edited by: stephen ]</small>
paulrevelcet
Posts: 177
Joined: Fri Mar 05, 2004 1:01 am
Location: Kentucky
Contact:

Re: 3000 watt inverter

Post by paulrevelcet »

Thanks for the reply, This whole thing started when I ordered a battery isolator and the sales man got to talking about the load on the aux batteries, and how the isolator was only good for about 90 amps so to keep the isolator safe he recomends 25 feet of #8 wire from the isolator to the aux batteries, he says that will provide enough resistance to keep the current down to within specs because the aux batteries will have a normal load of about 100 amps. I think I am on the right track now. thanks alot.
dyarker
Posts: 1917
Joined: Fri Aug 22, 2003 1:01 am
Location: Izmir, Turkiye; from Rochester, NY
Contact:

Re: 3000 watt inverter

Post by dyarker »

Doesn't the isolator go between main battery and aux battery so you can start the engine in the morning, even if you leave the aux load on all night? And, the aux load (including inverter) runs direct from aux battery?

If so, then the isolator only handles aux battery recharge current while engine is running. I guess 25ft. of 8AWG would also limit over-load current while engine is running, protecting the isolator and the alternator.

Will you ever even get close to needing 27A of AC?

Later,
Dale Y
User avatar
Crowbar
Posts: 67
Joined: Thu Feb 21, 2002 1:01 am
Location: Pittsburgh, Pennsylvania
Contact:

Re: 3000 watt inverter

Post by Crowbar »

I don't see how an 8AWG wire will limit current. The 2002 NEC lists the DC resistance at .76 to .8 ohms per kfoot. That would make 25 feet a resistor of about 0.00924 ohms. At 27 amps that would only be a voltage drop of 0.24948 in the wire.
Keep Prying...

Crowbar
dyarker
Posts: 1917
Joined: Fri Aug 22, 2003 1:01 am
Location: Izmir, Turkiye; from Rochester, NY
Contact:

Re: 3000 watt inverter

Post by dyarker »

But at 100A it is 0.924V drop. Plus drops in main battery system, isolator and the connections. The 27A number is for the 120VAC side of the invertor; the current on the 12V side will be over 250A at full load, as stephen pointed out already.
Dale Y
User avatar
Crowbar
Posts: 67
Joined: Thu Feb 21, 2002 1:01 am
Location: Pittsburgh, Pennsylvania
Contact:

Re: 3000 watt inverter

Post by Crowbar »

I see that I erred on the current figure, however the connections should not offer a voltage drop, if they did they would have to be poorly made to be significant. 250 amps on an 8AWG will for any amount of time melt the insulation off regardless. 8AWG bare in free air is rated for 98 amps, that would have to be some high temp insulation for essentially a 577 Watt heater at full load.
Keep Prying...

Crowbar
Post Reply

Who is online

Users browsing this forum: No registered users and 29 guests