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Pv Powered Inverters

Eric Said:

Is anybody running 3 ph motors as on a CNC mill from inverted battery stored power? Any primers or how to's?

We Answered:

The first thing is to determine the power rating for the CNC mill and its other bits like air supply, lighting. This could be 12 to 40KW in my experience. That is between 100 and 333A for a single phase supply at 120V, half that for a 240V supply, so you tend to need three phase. There are inverters that convert single phase to three phase, but this load is tending towards too big for the typical home solar system or home single phase grid connection. The ratings are the maximum load, and what the average is depends on your usage of the machine over a 24h period. You can get better figures by looking at your electric bill with the machine in operation, to work out the average usage in KWh/day.

The following example will give you some idea (with your own figures inserted), but you need to locate a local consultant to go ahead with this, in view of the cost. This person needs to know how to resolve the three phase issue at your power level (most solar systems are single phase) and limited to 5 to 10KW for the panels and inverter. Lets say you have a 12KW load that averages 12KW x 3 per day, so 36KWh/d.

Equivalent full sun per day...
A system that could supply 10KW or 40KW with solar panels and batteries is quite big and expensive using current technology silicon panels. The first question to answer is how many "equivalent full sun" hours a day do you have in your location. This is likely to be from 2 to 5 hours in temperate to tropical regions. Look it up on the internet for your area.

Decide what power you will get from solar...
To get the full power at 36KWh and 2 hours a day equivalent full sun would need 18KW of solar panels. If you use batteries, that would need 1.4 times to allow for charge losses, so 25KW, considering that most of the power would come from batteries as the full sun equivalent is 2 hours a day in this example. Larger panels are rated at 200W each, and you would need 25KW/200W of panels. That is at least 125 panels at about $1200 each in Australia. If the panels are connected in series to get 500V absolute maximum, the number of panels in each series string (for so called 12V panels with 36 cells) is something like 500V/22V = 22 panels per string. These will generate about 410V at 10A per string, so call it 4KW per string, considering likely performance. You decide how many strings you need. In this case for the battery stand alone operation, it is 7 strings at least = 28KW. That allows a bit of margin for dirty panels and aging. The actual number of panels is 154. You can hope this will last for 20-25 years if all done properly. You could try different voltage strings with lower than 500V (calculate at the O/C voltage of 22V per panel). I believe there are also higher voltage systems around, up to 800V. This depends on the charger/inverter rating, and also gets beyond the 600V "Low Voltage" system regulations.

These are only required for a stand alone system. How big, is the question. The first approximation is 36KWh per day in our example. That assumes that every day is a good day and delivers two hours of full sun equivalent. My experience is to allow for 3-5 days, and have a backup power supply for longer periods. This is a decision to be made by you. Lets go with a minimalistic 3 days, and say you hire a generator or use the grid power when the batteries are down. Now we need 3 x 36KWh = 108KWh battery capacity. Lets say the batteries are 240V, they will need a capacity of 108KWh / 240V = 450Ah. The standard battery size is 500Ah. The maximum load current at 12KW is 50A, so that works out well, no need to go bigger to accomodate the load.

Connecting to the grid...
This is a better option. You don't need to get the total power if the panels can be grid connected. Consider them as supplementing the bill. That means you can have a lower number of strings, at 4KW each, and it will help, even if not sufficient for stand alone operation. No batteries are needed, and that is a very worthwhile improvement in maintenance and capital costs. This is done by a specialised inverter, a grid connect or grid tie inverter. You make your own mind up about how many panels, and just call it a system to provide a maximum power of the number of KW, e.g. 2 strings is 8 KW. You could have less panels per string and more strings to get other total numbers of panels.

Grid connect inverter...
It converts the DC from the solar panels. The type I know is for domestic use. It accepts a range of voltages, maybe 200V to 500V, and converts that to the exact frequency and voltage required to export excess single phase up to 5KW to the grid. I don't know about three phase versions. The consultant should be able to advise on the best way to do this. These cost a few thousand dollars each at the 5KW size.

An inverter for a stand alone system needs to be able to convert the battery supply to deliver the 12KW three phase maximum demand with additional surge capability to start the motors on the mill. It also needs a KVA rating to allow fo

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