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Here is a picture of the solar panels in use on this project. Currently this consists of one Solarex MSX-120 panel connected in series to one Solarex MSX-60 panel. The panels are on a home-made rack and are adjustable for summer and winter angling. |
The Electric Tractor Solar project
Houston, we have the green light!
Well, it's official: After a weekend of work, I connected the last wire, threw the disconnect switch, and I now have a solar powered lawn tractor!
If you're wondering why I built this system, please click here for the explanation.
If you have read my other pages, you might know that my original plan was to install a third Solarex MSX-120 panel and connect it in series with the existing two panels (configured for 24 volts each) to provide a 36 volt feed for the tractor. However Atlantic Solar has been back-ordered on the MSX-120 panels for a few weeks now, and I really wanted to get this project going.
So instead I bought a MSX-60 panel, and wired it in series with one of the two MSX-120 panels I currently have. Due to my use of blocking diodes on the larger panels, I don't have to worry about the impact of current back-feeding off the other panel. Output will be only 50% of full, but that's still about 10 amp/hours a day; enough to handle the tractor in the early spring.
The panels went up on Saturday without much incident. I am running a bit tight on space on the roof of the shed; need to consider building another shed really soon if I want to continue this. There is a moral here: If you're installing on a roof, go for high power panels...
The next step was to wire up the disconnects. After a fair bit of debating, I went with a second 30 amp Cutler and Hammer disconnect panel (HP rated of course). This will protect the panels, and is technically required by NEC codes. The second disconnect pipes into a watertight box which contains the 36 volt Solarex regulator.
A word on regulators.
Elec-Trak tractors require 36 volts to operate. From a solar standpoint this can be a bit problematic, since most solar systems are either 12 volt, 24 volt, or 48 volt systems. Thus there aren't many regulators out there for 36 volts.
By chance I found a 36 volt model from a company called thelastplaceyoulooked.com. They apparently bought a lot of solar equipment from Australia, and in Australia they use 36 volt systems much in the way we use 48 volt systems here in the states. The Solarex SC-18 controller fit the bill perfectly: It could be configured for 36 volt operation by changing a solder-pad setting, and it was a two stage charger to boot.
Changing it to 36 volts is not a completely simple task. In order to do this you not only need to know how to solder, you need to know how to *remove* solder, since you will take the solder off the 12 volt pad while putting a new solder joint on the 36 volt pad.
Once the regulator was installed, I ran a 25 foot extension cord (outdoor rated) out to the tractor. I terminated it with a 20amp Hubbell twist lock connector, which plugs into the Elec-Trak's option power supply.
As soon as the tractor was plugged in, the charge light came on, and the tractor began to accept a charge. In checking with my Ammeter, I noted that the tractor was pulling almost the same amount of current that would be pulled from the panels in a direct short to ground, indicating that the system was functioning at maximum performance. Initial charging rates were up to 4 amp/hours.
Issues
The first thing I need to do at this point is install a third MSX-120 panel. By doing this I will be able to harness the output power of all three main panels on the roof, and should be able to charge at a rate of 20 amps a day. With 100amps a week of current, I will be able to recharge the tractor from a 50% level of charge to full in about a week.
Second, I need to install a grounded plug on the Elec-Trak. When GE built the ET series tractors, they really didn't think too much about things like grounds, and as a result the E20 is not bonded to the shed's ground while charging. With low charge currents this is not too much of a problem, however as I add more panels a ground fault could be an issue.
One solution would be to replace the 2 prong Hubbell plug with a three prong plug. It might be possible that Hubbell makes a 3 prong verson of the midget locking plug. If so, that could allow one to retrofit the tractor while allowing the use of ungrounded things like the chainsaw.
The final issue I need to work on is some sort of interlock to keep me from driving the tractor off with it plugged into the array. Since the plugs are twist lock, a drive-off would probably destroy the cables....
Pictures
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Here is a picture of the solar panels in use on this project. Currently this consists of one Solarex MSX-120 panel connected in series to one Solarex MSX-60 panel. The panels are on a home-made rack and are adjustable for summer and winter angling. |
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The charger for the ET project is a Solarex SC-18 unit. The diconnect is an off-the-shelf Cutler-Hammer 30 amp unit rated for outdoor installs. They are connected with conduit; the ground wire will be installed in conduit soon. |