Need help selecting an inverter [RE-wrenches]

[Christopher Freitas - OutBack Power]

In order to say if a specific inverter system can run several motors we 
will need an idea of the start-up current required by each of the 
motors. If the actual start-up current is not available than the motor 
size, construction and application can be used to get a fairly accurate 
idea of what the start amps will be.  

Based on the ability of the two SW5548s being able to sometimes start 
the large 7 hp motor,  I would say that a "QUAD" stack (four) of the 
OutBack FX (sealed) or VFX (vented) series inverters (which version 
depends upon installation issues and ambient temperatures expected). 
The FX and VFX versions have the same motor starting ability, but 
different continuous power ratings (2.5 sealed versus 3.6 kw vented).  
The VFX has a larger power transformer in addition to the additional 
airflow - hence the higher power.  

The X-240 autotransformer will help by keeping the two legs of AC 
balanced and is also required when more than 2 inverters are connected 
in the series/parallel configuration.  This trick with the X-240 
autotransformer is only possible with the OutBack inverters - it will 
not work with the SW series or other models. 

With the four OutBack inverters you will have 100 amps RMS / 140 amp 
peak @ 240 VAC of starting current - this is about 25% more starting 
current than a pair of SWs.  The X-240 autotransformer will also keep 
the 120 VAC motors from overloading one pair of inverters when they kick 
on - the start up current is split up onto all four inverters.  

The OutBack inverters also do not instantly trip off into overload like 
the SW series (especially the SW5548s) but instead will supply the motor 
with reduced AC voltage for up to 5 seconds.  This allows larger motors 
to be started than one might expect from looking at the motor specs.  
The AC voltage will however dip during this "soft start" process so the 
lights will dim etc if they are connected to the same inverter as the 
large motor.  

Another option is to look at a separate soft start control for the 
larger motors.  i have had good luck with the Allen-Bradley STC 
(starting torque controllers - although they changed the model names a 
while ago).  The best ones I found were designed to start conveyor belts 
or similar loads - they have an adjustable ramp time period and current 
feedback which worked well with the SW series in a couple of my more 
extreme installations.     

Christopher Freitas
OutBack Power Systems, Inc.
cfreitas at outbackpower.com  www.outbackpower.com
Tel 360 435 6030  Arlington WA USA

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