Inverter Dying?

[rwidman]

Originally Posted by Sandero

The actual distance is not what counts... it's the length of the wire... so...new location is maybe 25" from batts... but the wire distance (guessing from memory here) would be 30" from

12v -
24" to neg disconnect switch
4" to neg buss
38" to inverter
66" +/-

12v +
22" to pos buss
12" to fuse
36" to inverter
80" +/-

Best I can do!

Why is there a negative disconnect switch? Standard practice is to switch the positive lead. When you switch it off, everything is still hot except the cable from the switch to the battery or negative buss.

Compare your totals to the installation instructions. My guess is, the cable runs are longer than they are supposed to be but I don't have the instructions.

[Auspicious]

Originally Posted by transmitterdan

These company's marketing teams will be disappointed to hear this as they will have to now rewrite all their sales literature. The current we are discussing is not square wave as no inverter draws square wave current from the battery. Fluke and Agilent claim exactly the opposite as you believe.

I just got off the phone with my Fluke service exec. He said to apply the crest factor adjustment (he says it is in the manual for my 117 and 12 but I haven't looked yet) to 50% duty cycle square waves. For PWM he agrees there is an adjustment but the math is on me. *grin*

Waiting to hear back from Agilent. They do have a white paper on the subject that says 46% adjustment for 50% duty cycle square wave.

[transmitterdan]

Originally Posted by Auspicious

I just got off the phone with my Fluke service exec. He said to apply the crest factor adjustment (he says it is in the manual for my 117 and 12 but I haven't looked yet) to 50% duty cycle square waves. For PWM he agrees there is an adjustment but the math is on me. *grin*

Waiting to hear back from Agilent. They do have a white paper on the subject that says 46% adjustment for 50% duty cycle square wave.

I think you are missing something. There are no square waves in the battery current feeding inverters. It's all low frequency sinusoidal pulses either 100Hz (in Europe) or 120Hz (in US) and harmonics. The crest factor is always less than 2 so no corrections should be required.

Edit to add:

Anyway, these small correction factors are immaterial to the problem. There are greater inaccuracies that this in the measurements. You don't have to have 1% accurate measurements to learn something. 10% accuracy is more than enough.

[Sandero]

Originally Posted by rwidman

Why is there a negative disconnect switch? Standard practice is to switch the positive lead. When you switch it off, everything is still hot except the cable from the switch to the battery or negative buss.

Compare your totals to the installation instructions. My guess is, the cable runs are longer than they are supposed to be but I don't have the instructions.

My boat came with a neg disconnect and kept it. I use a BlueSeas 8080 from the main on off batter switch.

[Ex-Calif]

Originally Posted by Sandero

The actual distance is not what counts... it's the length of the wire... so...new location is maybe 25" from batts... but the wire distance (guessing from memory here) would be 30" from

12v -
24" to neg disconnect switch
4" to neg buss
38" to inverter
66" +/-

12v +
22" to pos buss
12" to fuse
36" to inverter
80" +/-

Best I can do!

That's still a long run 6 feet+ - Also please do take a look at the other wiring anomalies I noted. I'd hate to see you get a chafing short.

[Sandero]

Thank you for your concern. Wiring is well secured and not subject to chafe from vibration.

I can't position any closer to the neg and pos busses. Impossible. Those numbers are guesses and it could be less.

[mitiempo]

Originally Posted by Sandero

The actual distance is not what counts... it's the length of the wire... so...new location is maybe 25" from batts... but the wire distance (guessing from memory here) would be 30" from

12v -
24" to neg disconnect switch
4" to neg buss
38" to inverter
66" +/-

12v +
22" to pos buss
12" to fuse
36" to inverter
80" +/-

Best I can do!

No it's not.

For one you have a negative disconnect switch bot do not apparently have a positive switch between the inverter and the battery.

How close physically can you install the DC end of the inverter from the battery posts? That should be your wire run.

Use a Blue Seas MRBF fuse on the + battery post, there to inline battery switch and then to inverter + input.

Negative from battery - post to inverter.

That may cut your wire lengths in half.

And use at least 2 awg wire.

[Terra Nova]

#2 seems awful light duty.

[mitiempo]

Originally Posted by Terra Nova

#2 seems awful light duty.

I won't argue that - I would probably use 1/0 as I like my high current wiring large.

But the calc I use suggests 4 awg for a total run of 6', which may be possible if it is configured as I posted.

[transmitterdan]

According to Auspicious in a previous post the manufacturer's installation instructions recommends AWG 0 as the minimum wire size. There is a reason they made this recommendation. I would argue that trying to use ordinary ampacity calculators is totally inappropriate for inverter installations. In fact, it can be dangerous. If you can't use the minimum wire size then get a smaller inverter for which the wire available will be appropriate.

[mitiempo]

Originally Posted by transmitterdan

According to Auspicious in a previous post the manufacturer's installation instructions recommends AWG 0 as the minimum wire size. There is a reason they made this recommendation. I would argue that trying to use ordinary ampacity calculators is totally inappropriate for inverter installations. In fact, it can be dangerous. If you can't use the minimum wire size then get a smaller inverter for which the wire available will be appropriate.

Xantrex's wire sizing is not that clear as they are recommending 1/0 wire for inverters from 600 watts to 3000 watts as shown in the chart below. 1/0 is overkill for the 600 watt model and too small for the largest.



The sizing calc I use gives 4 gauge for 100 amps with a wire run of 6' (total of both wires) with 2% voltage drop. My suggestion was 2 gauge to go one step better. My thought was that the wire run could be shortened to this length with straight runs from the battery for the negative eliminating the switch that is not necessary and a straight run from the positive post through a fuse and dedicated switch to the inverter.

I also posted that I would use 1/0 personally. I don't believe penny pinching on wire size makes sense at any time.

I have installed many inverters up to 3000 watts and my customers have not had any problems. The 3000 watt inverter was close to the battery bank and 4/0 wire was used.

I do think a normal wire sizing calc can be used for sizing inverter wires, but I
also err on the large side which I believe is common sense.

[transmitterdan]

Originally Posted by mitiempo

Xantrex's wire sizing is not that clear as they are recommending 1/0 wire for inverters from 600 watts to 3000 watts as shown in the chart below. 1/0 is overkill for the 600 watt model and too small for the largest.

I read the same manual and came to the same conclusion. But there must be another manual floating around. See post #127 in this thread. I was in error in attributing that to Auspicious. It was somebody else. They never posted a link to the manual but he said he found it on the Xantrex web site. I never found it though.

[transmitterdan]

Originally Posted by mitiempo

The sizing calc I use gives 4 gauge for 100 amps with a wire run of 6' (total of both wires) with 2% voltage drop.

But you can't use 100A as the input to the voltage drop calculator. I have been arguing this for days and am just about to give up but will try one more time. The peak current from the battery greatly exceeds 100A. Several people have ridiculed me for my explanation but it is nevertheless a fact that the peak current demand an inverter puts on the battery is 50-100% more than the DC demand. Everyone "thinks" that inverters draw a pure DC current from the battery. But they don't. They have to draw large current surges 120 times per second. That current surge is needed by the inverter on the positive and negative half-cycles of the AC current. The inverter draws the current surges always in the same direction and just reverses them at the output of the inverter to make pure AC current.

Here is how you can estimate the worst case peak 12V battery current demand:

peak current = AC power out * 2 / 12 / efficiency

So for 1250W water heater load it is:

1250 * 2 / 12 / 0.8 = 260A.

But the DC load is only:

1250 / 12 / 0.8 = 130A

Some good inverters have filters inside that reduce the peak demand a little bit. But the best ones only reduce it by about 25%. And it adds a lot of cost to reduce the peak current more than that. The larger the AC current demand the larger the peak current that must come from the battery. At the risk of everyone laughing at me again here it is:

The AC current output by the inverter has to come from somewhere. The largest energy storage device is the battery. Ergo, the battery has to deliver most of this peak AC current.

If you don't believe it put an AC clamp-on RMS current meter on the battery wire while the inverter is running maximum load. There will be AC current there and it is not due to a confused meter. But most meters won't show the peak current. For that you have to use a special wide-band current transformer and an oscilloscope which almost nobody has.

Someone will ask why is the worst case peak current twice the DC current? The answer is simple. AC voltage and current numbers are given typically in RMS because it makes power calculations easy. But the peak voltage and current are both 1.414 times the RMS value. So take a water heater element. We give it 120V RMS and 10A RMS for 1200W. But the peak AC voltage is 120*1.414 and the peak current is 10*1.414 and therefore the peak power is 120*10*1.414*1.414=1200*2=2400W. So 2.4kW is the peak power being delivered to that water heat element when the AC voltage waveform is at its positive or negative peak. That peak happens twice every AC cycle so for 60Hz it happens 120 times per second. Now where do you suppose that 2.4kW is coming from 120 times every second? Most all of it comes from the battery. There is no battery inside the inverter. There are some small capacitors but they don't have near enough storage capacity to supply that 2400W for any length of time. They quickly discharge and the battery has to make up the difference. Thus 120 times every second a big surge of current has to flow out of the battery. Not coincidentally 120 times a second the current flowing out of the battery goes nearly to zero when the AC voltage waveform is crossing zero.

[Ex-Calif]

Originally Posted by transmitterdan

But you can't use 100A as the input to the voltage drop calculator. I have been arguing this for days and am just about to give up but will try one more time.

So Xantrax is wrong?

Take a look at the table in mitiempo's post. Although the "guidelines" are poorly explained they talk predominantly about voltage drop, maximum run of 6 feet and "recommended" wire size of 0.

They are doing a little trick however.

1000W / 12 = 83 amps. They are using 10 as the denominator and nominally rating the circuit at 100amps. In my mind they have already built in an 80% efficiency factor.

They then give this example for the 600W model - 0.006 X 6 X (X2) = .072 voltage drop.

This is ideal at 25DegC. Resistance will go up and be higher with load on the system. Also the battery will discharge and voltage will go down and of course they warn about low voltage cut off at 10.5v

So clearly the minimum cable run and maximum wire size is best for lowest voltage drop.

So if I am inferring what they want, for the 1,000W model I would look at 3/0 cable.

.0063 X 6 X 2 = .075 voltage drop.

Actually if it was mine I'd get the inverter to within 3 feet of the battery bank and go with 0 wire.

.01 X 3 X 2 = .06

Your discussion about AC is interesting but irrelevant and confusing.

[Sandero]

After reading this thread... and derspite the fact that I don't use large or sustained AC loads.. I will relocate and use 1/0 wire... which seems over kill but better safe than sorry.

Thanks to all who contributed their knowledge.