Spinning into New Green Power Sources

Generator types: Generators have taken on a new importance in this era of brownouts, blackouts, and energy shortages. New wind and solar renewable energy sources have emerged as major sources of local and grid electrical power, and direct-drive generators using permanent magnets are helping, along with advanced power electronics.


Generators have taken on a new importance in this era of brownouts, blackouts, and energy shortages. The growing realization is that new renewable energy sources in the form of wind and solar power have emerged as major sources of local and grid electrical power.

The electrical generator is the major element in supplying electric power in its many forms. The wind power generator is one of the most popular forms currently undergoing many important technology changes.

Windmill generators

While the U.S. government classifies wind turbines under 100 kW as small, windmill turbine generators can be divided into a number of power categories primarily by use:

1. Below 5 kW

2. Between 5 kW and 49.9 kW

3. Between 50 kW and 500 kW

4. Above 500 kW

While there are many overlaps, small rural applications such as remote irrigation pumps would use a smaller than 5 kW windmill driven generator. Residential applications using a windmill generator would fall into the 5 kW to 49.9 kW power range for a block of homes or a small industrial plant. Small local electric grids would fall into the 50 kW to 500 kW power levels depending on their grid size. Anything above 500 kW is commercial power usually located in wind farms.


The emergence of direct-drive generators using permanent magnets (PM) in brushless PM configurations (in combination with power electronics that include force-commutated rectifiers and boost converters) began with smaller turbines. This approach allowed the direct-drive system to handle the variable winds and still develop the needed power levels for smaller dc power systems and ac power for electric grids. The windmill turbine’s use of direct drive is now moving into much larger power levels.

Direct-drive versus geared

The direct-drive wind power generator system is challenging the current ac induction generator and gear train transmission that mechanically changes speeds to match turbine speeds to wind velocities to achieve the needed 60 Hz frequency. The 4-pole induction motors operate at near constant speeds of 1,600 rpm to 1,720 rpm. The massive gear box is typically limited to 2 to 3 years’ operation before it begins to operate improperly. Repair regimens generally cost tens of thousands of dollars and take some months to regain successful operation. While the majority of operating wind generator systems use induction motor and gear train-based systems, the emergence of the direct-drive PM generator plus electronics is gaining market acceptance. Eliminating the gear train improves the direct-drive turbine system’s overall performance and reliability.

Direct-drive power generation has evolved into a smaller sized solution with higher efficiency levels that partially offsets rare earth magnet price increases.


Starting with the smaller 5 kW and 50 kW wind power systems, the direct-drive windmill generators are now moving into the megawatt power range. In June 2011, Siemens Energy announced a new direct-drive 6 megawatt wind power system to be installed in Denmark. Northern Power Systems (NPS), a major supplier of direct-drive 100 kW wind power systems, is building on its success at lower power levels to introduce a direct-drive 2.3 megawatt wind power system.

The direct-drive configuration has evolved into a smaller sized solution with higher efficiency levels that partially offsets the significant rare earth magnet price increases experienced over the last few years. Both new and old mines are being opened in many countries to help alleviate this cost problem.

- Dan Jones is president of Incremotion Associates/Motion Media Group; edited by Mark T. Hoske, content manager CFE Media, Control Engineering, and Plant Engineering, mhoske@cfemedia.com.



Webcast on Critical Power: Emissions Regulations and Technologies - Stationary diesel-powered generator sets. www.csemag.com/webcast 

Rise of the PM machines http://bit.ly/SGUPk3

No shortage of direct-drive wind turbine developments http://bit.ly/MIGBda

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