Not to be argumentative or anything, but I disagree about the safety.
Assuming that you will have the motor reasonably close to the blades which will be in the water, and that ton of batteries in a sealed box will have to have cables to the motor, you will still have a potentially large amount of power right next to the mixer, in the slurry slinging zone. The motor is electrically coupled to the water through the shaft and blades unless you are using a belt system, but you want to bond everything anyway. With DC you generally need large cables which should be as short as possible. My generator probably weighs less than batteries for a forklift motor, and I can place that right next to the mixer as well (if I wanted to) if AC is not available at the site, but I would rather have a safe "rough usage" waterproof cord that is small enough (less current = smaller wire) to run a distance away from the wet area. I put my barrel next to the forms, place the motor/blade frame into the drum, go flip on a breaker, walk back to the mixer and push a start button. No batteries to charge or lug around. Hey, the motor and frame are heavy enough by themselves. I'm happy to have a HF wagon : )
While small batteries like AA, AAA, C, etc. are low enough voltage to be mostly safe, even 12v batteries can hurt. Try wearing shorts and leaning your sweaty legs against a grounded metal bumper as you grab a positive lead on a car battery and you'll see for yourself. 12v batteries can be used as welders. Multiply that 12v three or four times, and you can fry something pretty quickly - especially with the amperage that is available from a forklift battery or two. DC is not safer than AC. AC motors have a specific running current and you protect them with proper sized fuses. If they draw more than the design amperage, the fuses disconnect the power. DC motors have a variable current all the way from zero up to the maximum that will burn out the coils, and there is no way to protect them at anything less than that maximum current. That means that much higher current may be available in a fault situation.
Another concern is the fact that shunt wound motors will accelerate to a speed where they self destruct, if they are run with no load. You would need to ensure that your mix is thick enough to provide the torque to prevent that self destruction, and some sort of interlock to prevent the motor from starting without the tank being filled.
With all these factors, I believe it would be easier to make and use a safe AC mixer than it would be to use a forklift motor and associated requirements. Probably a lot cheaper, too, even if you have to buy a generator.
In either case you want a switch right at the mixer for emergencies, of course waterproof and with all the power isolated and contained in a grounded enclosure. This is easily done with AC since it is a common type of installation and all the parts are easy to get even at a big box store. DC controllers and parts would be a bit harder to find. Add to that your charging system for the batteries, which will need to be fairly large if you plan to mix for very long and often. You might use a bank of solar panels as a shade for your work area. A normal forklift charger is large and heavy and has heavy power requirements of its own, which admittedly doesn't need to be near the mixer but still is a requirement.
Any metal parts of the mixer should be bonded together and grounded, including the motor. If you do it right, according to proven practices which are written in that nasty old code book, then any rare fault will be cleared without harm to the equipment or the operator. Like you, I do not put much faith in GFI protection. They are set in the range that is an average amount of current which is enough to stop many hearts. Best to never get connected to a hot wire while you are grounded, but if it does happen then GFI is better than nothing.
On 3/14/2012 3:46 PM, JayH wrote:
It's a lot safer because I can build a completely contained unit that has no power cord to worry about. One can have the batteries in a sealed battery box. All the connections can be run inside sealed areas. Then roll the mixer, batteries, motor, mixing tank, and all, right up close to the worksite. Configure an isolated on off switching system were the operator never gets close to the electricity, and neither does the water. I don't see that being done nearly as easily with an AC unit. The idea of dragging a power cord around through puddles of slurry runoff sounds to me like a death wish. I think GFI protection is a great invention, but I don't want to count on it as a primary protection for my heart, only as a last resort emergency backup when things go terribly wrong.
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