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Three-Phase Alternatives

December 8, 2010
By: Dan Anderson, Farm Journal Columnist
DSC 9506
When upgrading his grain drying and handling system to meet National Electric Code standards and provide three-phase power, Mark Ellerman of Dallas Center, Iowa, hired professional electricians to design and wire.  
 
 

One of the hidden complications of upgrading grain drying and handling systems is providing enough electricity to power the new larger electric motors.

Electric motors that are larger than 15 hp require a three-phase power supply. Since most farmsteads operate on single-phase electric power, farmers must upgrade their electrical systems to three-phase capability when they incorporate high-horsepower electric motors in a grain system expansion.

There are two basic options and one unconventional option to provide three-phase power to a farmstead: have the power company run a three-phase line to the farmstead; install a rotary phase converter to convert single-phase service to three-phase service; or rent or purchase a three-phase diesel generator. Each option has advantages and disadvantages.

Three-phase electrical service. Three-phase electrical service from a power company is seamless once installed. There is no need to activate converters or generators and no extra maintenance. The system operates invisibly to power high-horsepower electric motors.

This service comes at a cost, how-ever. Three-phase electrical service is billed at a different, often higher rate than standard residential and farm single-phase electrical service. Power companies also often charge a minimum monthly fee for three-phase service, even when it is not used.

The cost of upgrading single-phase service to three-phase service is currently $7 to $10 per linear foot, from the nearest existing three-phase power line in the neighborhood. Installation can be expensive and is most often borne by the customer.

“If a customer isn’t going to use three-phase year-round, it’s not cost-efficient for a power company to pay to install the line,” explains Bruce Keeney, operations manager at
Midland Power Cooperative in Boone, Iowa. “We project our potential net revenues from the new line over three years and base the installation costs the customer pays on those projections. A customer who uses three-phase to power a grain system only a couple months each year could end up paying most of the cost of installing a three-phase line.”

Rotary phase converter. The cost of installing a three-phase line to his farmstead swayed Mark Ellerman of Dallas Center, Iowa, to install a rotary phase converter to power his new grain drying and handling system.

“It was going to cost $67,000 to have three-phase run the half-mile to our farm,” Ellerman says. “We bought an $8,000 phase converter, paid $7,500 to put in a second 1,000-amp transformer and trenched in underground lines, all for less than $16,000.”

Rotary phase converters are essentially specially designed single-phase electric motors that generate three-phase power. Ellerman activates his Rotoverter-brand rotary phase converter only when he runs the 30- and 40-hp motors on his grain system.

“I run the farmstead most of the year on single-phase,” he says. “I only use the converter and three-phase power when I need it.”

Three-phase diesel generator. Multiple large electric motors on their 900,000-bu. grain drying and handling system led Bob and Rob Manning of Granger, Iowa, to rent, and eventually buy, a 425-hp, diesel-powered three-phase generator.

“We looked at [phase] converters, but they can’t put out the power we need for the size and number of electric motors we have,” Rob says. “You have to add up the kilowatts needed by all the motors you will have running at the same time, and get something that can put out that much total power.”

The power limitations of rotary phase converters (maximum capacity is about 225 kW) and the $80,000 estimated cost of running a three-phase line to their bin site led the Mannings to find a third option.

“We found we could rent a 400-kW diesel generator for $2,500 a year, run $3,100 of fuel through it during an average harvest and have all the power we needed,” Bob says.

They bought a three-phase generator after several years of renting one. Selecting the proper type, and looking for a used unit, saved them money.

“Hospitals and other places that absolutely have to have emergency power trade in their standby generators every few years,” Rob says. “A new standby generator costs about $60,000, but we found a unit with 420 hours on the diesel engine for $15,000.”

The Mannings view their generator as “another tractor on the farm,” Rob says. “We service the diesel motor ourselves and have the generator serviced once a year by an electrical guy.”


The Hidden Costs of Supplying Power

“When expanding grain drying and handling systems, the electrical components to power the system can cost almost as much as a drying bin,” says Dallas Center, Iowa, farmer Mark Ellerman. He discovered this when he upgraded his own grain drying and handling system.

“With big electric motors, you have to figure out how to get three-phase power, which is expensive,” Ellerman says. “Then there’s the cost of installation, along with the cost of upgrading your other electrical services to blend with the new electrical system.”

In Iowa and many other states, any new electrical installation has to meet the National Electric Code and be inspected before the power company can hook up to it. That means industrial-duty switches and controls and fully enclosed wires—no do-it-yourself wiring with taped connections.

“It’s nice to have heavy-duty components that are safe,” Ellerman says. “But you have to factor in more money for the commercial-duty electrical components when you’re estimating what a new grain system will cost.”


Generators Must Be Off the Grid

"Connecting any [gas- or diesel-powered] generator without a double-pole, double-throw switch to isolate it from the main power grid is a violation of the National Electric Code,” says Doug Luellen, co-owner of Luellen Brothers Inc. in Dallas Center, Iowa.

“In some places it’s against the law and subject to fines, and anywhere in the U.S. it leaves the owner and whoever installed it open to lawsuits.”

The old farmer’s trick of throwing the main circuit breaker or pulling the primary fuse before connecting a generator to a farmstead’s electrical system does not meet National Electric Code standards.

“We are required to disconnect service if we discover a non-Code connection while repairing problems with a power outage,” says Bruce Keeney, operations manager at Midland Power Cooperative in Boone, Iowa. “We can’t reconnect until a legal, Code-approved connection is in place.”


Static Versus Rotary Phase Converter

There are two types of phase converters to convert single-phase electric power to three-phase electric power:

Static phase converters are mounted on individual electric motors and use electronic circuitry to briefly create three-phase power to start motors. Once the motor is running, the circuitry switches and runs the motor on single-phase power.

The disadvantage of this method is that a three-phase motor with a static phase converter operates on single-phase power once it is started, and therefore cannot operate at more than two-thirds of its rated three-phase capacity. Static phase converters are inherently inefficient and may risk damage to three-phase motors exposed to variable loads while they are running on single-phase power.

Rotary phase converters use specialized electronic circuitry with an “idler” or “donkey” electric motor operating on single-phase power to generate true three-phase power. Three-phase electric motors powered by rotary phase converters operate at or very near their rated capacity. Three-phase electric motors down-circuit from quality rotary phase converters receive consistent, “clean” three-phase power equal to that which power companies provide.

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FEATURED IN: Farm Journal - December 2010
RELATED TOPICS: Technology, Energy, Farmstead

 
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COMMENTS (1 Comments)

RTfarmer
I have installed Rotary phase converters up to 300HP worth and don't see that there is a limit due to the converter, just the single phase current available.

Also, for grain driers I have been using American Rotary's T3-transformer converter. They make them up to 30 hp and are solid state, more cost effective than a rotary, and balance the current and voltage just about perfect, definitely better than a typical rotary. These converters have come along way now with digital controls, boosters etc. Stay away from statics for anything less more than about 7.5 hp.
8:32 PM Dec 9th
 



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