The issue is certainly the undersized single inverter. I had an on-grid customer who wanted to be completely self sufficient. His array could supply his annual load and generate enough to recharge his GM EV1 car. The only issue was that his 3 ton air conditioner would not reliably start on his dual SW4048 inverter system. We got it to start about 1 out 10 times, but the other 9 times would shutdown the system on overload. The 3 ton unit used 84A to start and the dual 4048s were rated at 78A max. Running amps was about 18A. We had a professional drives and controls company come out and install a soft-starter on the A/C compressor and pamper it for a couple hours. The soft starter was not just a capacitor. The unit actually limited the current into the load. The current would ramp up and the compressor would start at 84A. No matter how fast or slow the ramp up speed or applied voltage. It seemed that locked rotor amps was truly locked rotor amps. The rotor remained locked until the rated amps on the label plate was reached.
The drives and controls company rep said that most of the time soft start controls don't work on single phase motor loads. They do work most of the time on 3 phase loads, but the effectiveness depends on the type of 3 phase motor. The drives and controls company graciously donated their time as an experiment and we all learned a lesson. I was told by several A/C companies that the SEER rating doesn't make any difference in the starting amps or the efficiency of the motor, only the compressor/evaporator cycle is more efficient. They also said that scroll type compressors tend to have slightly lower starting amp requirement than piston type compressors. My next customer who had paid extra to install an oversized 5 ton A/C unit (to be sure to cool the house down fast) then found that bringing in utility power was too expensive so he lived off-grid on a generator. That 5 ton unit had a LRA of 129A. It really made his 10kW generator groan getting it started, so he didn't use any A/C. After I put in his dual SW5048 battery system, he paid to have an A/C company replace the 5 ton unit with two 1.5 ton compressors in parallel that were set up to start at different times. The dual SW5048's had no problem starting and running the two units with offset start times. The customer found that he only needed one 1.5 ton unit to cool the house adequately so he pulled the fuses from one of the compressors. He lives comfortably off grid in a relatively hot summer climate on 2880W of dual-axis tracking PV, with only minor generator run time in the winter. Aaron Wellendorf Stellar Energy Solutions ________________________________ From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Max Balchowsky Sent: Monday, August 18, 2008 1:01 PM To: RE-wrenches Subject: Re: [RE-wrenches] offgrid system question Dear wrenches, going to look at a system installed by others and would like some input from some of you with more off grid experience than I have ( 4 systems over the years). the system was put on a new home - off grid 20 sharp ND123UJF panels 4 modules in series - 5 parrallel strings, 2460 total array dc voltage. mppt 60-150 charge controller, 12 sun extender sealed AGM batteries, 305 AH at 12 vdc, wired for 915 AH at 48 volt DC. The system is tripping off when the AC unit trys to start - AC is comfortmaker 3.5 ton 13 seer. They say start up inrush is 104 amps (sounds high to me) and they put a DST 10 soft start on the unit (I think that's just a big capacitor). I haven't been on site yet but it sounds to me that they just didn't design enough inverter capacity into the system...... What are your thoughts? Max Balchowsky SEE Systems
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