If you have a system of stacked FX inverters, it can be setup so that only one inverter is on. Then, when more power is needed, the rest of the inverters will come on to help right away and go off as needed (or not-needed)

That way, you get the low-idle of just one inverter but the power of the whole stack of inverters. This has been shipping for some time now.

Thanks,
boB

I am not too familar with the Outback inverters but based on weight and case config I am guessing they are of high frequency pulse width modulated design.

PWM designs run high frequency chopper (25KHz to 50 KHz) with pulse width modulation to create the 60 Hz sinewave. Similar to modified sinewave, the first thing is to take battery voltage up to high voltage DC. For modified sinewave it is about 155 vdc, for PWM sinewave inverters it is about 175 vdc. The H-bridge output stage chops at high freq with duty cycle matching the average required for instantaneous 60Hz sinewave voltage. A low pass filter removes high freq chopper components of the waveform.

The Trace SW inverters use three low frequency transformer stacking to create 27 voltage steps that approximate a sinewave.

Advantage of PWM design is low weight as the high frequency DC boost is a much smaller/lighter transformer then a 60 Hz low freq transformer. There are some tricks with opto-isolators necessary to get the required isolation for AC main tie-in.

The down side of PWM is no-load power drain because of the power to chop the FET's at high frequency. High current FET's have large gate control input capacitance and power just to turn the FET's off and on is (CVF)^2 where C is total gate capacitance, V is about 12 v for gate turn-on, and F is the chopping frequency (25KHz to 50 KHz). There is also some loss for the required output filtering. Tricks can be performed to cut back on the number of parallel FET's that are actually chopped when load is light but this can cause complications in surge situations.

Because the Trace inverter is chopping the FET's at 60 Hz to 240 Hz, the power just to make the sinewave is much less. This results in much less no-load power drain.

The down side of the Trace design is the transformers are big and heavy because they must work at 60 Hz. Also because of the low freq chopping they are much more noisy. The biggest transformer is getting square wave chopping at 60 Hz which creates the loud buzz.

The high freq PWM sinewave inverters are nearly silent because the chop freq is beyond human hearing range.

I know I am getting in on this late but. George I run a pool pump on stacked SW2512's. The two things that pop in my mind are why not use a balancing transformer to make the pool pump a 120 load. The SW5 should handle that no problem. My second thought is getting a more efficient pump. My main pool circulation pump is 2 speed, 1 hp or .16 hp. On high it pulls about 1400w on low it pulls about 300w. I leave it running on low about 14 hours a day and have crystal clear water. I guess my point is it might be cheaper to look at a more efficient pump for many reasons. Actually a slower pump will filter water better then trying to push it thought the filter faster. Of course it can take longer to "turn over the pool".

Just a thought.

First need to understand what you mean by 2000 watt pump.

That's about 2.5 HP pump. Not that it is out of line but it is a pretty big pump. Most average size residential pools have about a 2 HP pump.

The startup current is what you have to be careful off. The stacked SW5548's together are probably getting close to the limit starting this pump.

Using a single SWplus5548 would start it but you would probably get a voltage slump as the motor started up. I have a similar issue starting a 3.5 ton air conditioning compressor with stacked SWplus5548's (about 3.5 KW run /14.5 amps, but an incredible 110 Amp startup that last 300 msec.)

I don't like the voltage slump the SWplus5548's subjects my refrig to when air cond starts up.

A step up transformer for single inverter might help or hurt. It might create a soft starter effect on the initial starting surge current which would prevent the inverter from slumping in voltage too much. Or is could just saturate the transformer core so the pump and transformer just grunts at you until breaker finally pops or inverter shuts down.

A cheaper solution would be to find the highest HP pump motor that has 120/240 vac wiring option. You can probably find one in the 1.5 to 2.0 HP range. The motor would probably cost you about the same as step up transformer.

To re-evaluate by earlier posting,

I took a look at the disassembled picture on Outback site. It has a single large low frequency transformer in the design. This would steer me to guess they are doing a moderate frequency PWM to a conventional 60 Hz transformer using it for the part of the filtering and providing the isolation.

You can not do too high a frequency PWM to this configuration because of limitations to transformer core material, perhaps something in the several KHz range. I do not see any high freq filtering between the H bridge and the transformer. The PWM would be done at the battery voltage using the transformer to step up, filter, and isolate to 120 vac output.

This lower frequency chopping is inline with the relatively low 20 watt no-load power. This only slightly above the 16 watts of Trace design.

Difference between pushing to grid verses pulling from grid for charging would be slight modification to PWM instantaneous duty cycle.

For a 3600 watt capable unit, the transformer seems a bit size challenged. At <50 lbs total weight also make we think the transformer is a little short changed.

Having the benefit of evolution of power MOSFET's technology, it looks like there are only 12, much beefier FET's in the Outback design verses the 52 lower power FET's of the 7 year old Trace design.

It appears they have only one AC side relay so I guess you are on your own with external relays or manual transfer switch to go between grid and generator input feed. The dual inputs of the Trace/Xantrex unit is a nice feature.

The Outback design appears to be much lower cost unit to manufacture then the older Xantrex/Trace design.

I did a side by side comparison of the specs between a SW4048 and FX3648.

Beside SW4048 having a higher probablility of giving you a hernia, the one spec that concerned me on the FX3648 was the input frequency range.

At 60Hz +/- 2 Hz I would expect trouble with it following generators particularly if some significant loading changes occurs.

I then read the installation manual and it states that it is not designed to work with generators. This is a significant deficiency if you are using FX3648 for power backup purposes. You would have to totally run the AC power from the inverter or transfer switch to totally on generator and any battery recharging would have to be via an independent charger hooked directly to the generator.

Outbacks work fine with a generator. They have several useful settings just for generators