2000 - Volume #24, Issue #4, Page #07[ Sample Stories From This Issue | List of All Stories In This Issue | Print this story | Read this issue]
He Built A Farm-Size Standby Generator For Under $750
Olson, a Madison, Minnesota corn, soybean and wheat grower, says Meredith called him last winter to tell him about a great bargain he found at Harris Machinery Co., a surplus store in St. Paul, Minn. It was a 100 hp, 3-phase synchronous electric motor.
Olson had no use for an electric motor that big, but Ulstad, a retired electrical engineer, offered to help convert the motor into something Olson did need - a backup generator. Olson bought the motor and then built a trailer on which to mount it. He credits Meredith, though, with the entire motor-to-generator conversion.
"If you set the wiring right and apply power to the output shaft, a synchronous motor becomes a generator," Olson says.
The generator puts out 3-phase 220-volt alternating current. Ulstad located a center-tapped transformer the kind used on farms from the main power feed from the electric company - and then connected it between two of the output lines to provide single-phase 110 volts.
"You really need to know what you're doing with this or you could cause a lot of damage or worse," Olson cautions.
He made the generator trailer from the front axle of a 3/4-ton pickup. "It needs to be a fairly substantial trailer, since the motor is rather heavy. We mounted a spline on the end of the motor shaft. I cut the pto shaft from a Deere 7 ft. rotary mower to the right length and attached that to the spline shaft on the motor. We added shields, too, to reduce the danger around the shaft. I built the trailer with a shelf on the back and put a 220 outlet on it, so I can load up my welder and take it anywhere I can get with the trailer," Olson says.
He powers the generator with a 3020 Deere. The pto is set to run at about 720 rpm's, which assures him that the current coming from the generator is 60 Hertz, like the power feed he gets from the electric company.
"To determine the rpm's necessary for an AC generator to produce 60 Hertz AC power, you divide 7,200 by the number of poles on the generator," Ulstad explains. A typical pto-powered generator might have only 4 poles, meaning it would need to turn at 1800 rpm's.
"One of the greatest benefits of this particular motor is that it is a 10-pole machine. This means it needs to be turned at only 720 rpm's, so it can be hooked directly to the tractor's pto shaft," he says.
Ulstad says most pto-powered generators require higher rpm's or a gearbox or V-belt arrangement to increase the speed, in order to generate 60-Hertz alternating current like the feed you get from the power company.
Ulstad says the generator is capable of producing more than 65 kilowatts of sustained output when operating as a single-phase power source. Olson says that's much more than is needed to power his farm.
"Olson figures the total cost was between $700 and $750.
For more information, contact: FARM SHOW Followup, Ellsworth Olson, Rt. 2, Box 24, Madison,
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