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History of Wind Power

January 26, 2022
Paul Gipe

When is a 1,250 kW Wind Turbine Only 1,000 kW? Setting the Smith-Putnam Record Straight

French colleague Philippe Bruyerre called my attention to an error I’ve been promulgating for at least four decades. He noted that I’d described the famed 1940’s era Smith-Putnam wind turbine as rated at 1,250 kW. It was not he wrote. It was really only a 1,000 kW wind turbine.

Bruyerre first raised this question in his recent book.[1] (See La Puissance du Vent (The Power of Wind)--A Review.) His claim caught my eye, but I didn’t act on it. I’d read Palmer Putnam’s book decades ago and Putnam himself calls his great experiment a 1,250 kW wind turbine in the first page of his Power from the Wind.[2]

So, what was it?

To avoid another long journey down the historical rabbit hole, I contacted Jim Manwell at the University of Massachusetts—UMass for short. Manwell is not only an engineer, he teaches engineering, specifically wind engineering. Manwell is also an expert on the Smith-Putnam machine, becoming intrigued by it years ago when he was a student. As a result of his interest, UMass houses some papers and artifacts from the project. He also spoke recently on the topic for the York County History Center, where the remainder of the Smith-Putnam papers are archived.

Manwell dived in and found that indeed Bruyerre was correct. Fig. 71 on page 119, noted Manwell, clearly shows the wind turbine reaching a capacity of 1,000 kW during tests. It then regulated output at 1,000 kW at wind speeds from 25 mph to 60 mph.

Digging a little further there is an image in Fig. 72 of strip charts recording performance on page 121. And there it is again, Putnam’s turbine regulating power at 1,000 kW.

Ouch. How had we gotten it wrong all these years? How did Putnam get it wrong?

The key is Putnam’s description of his choice of a generator for his proposed wind turbine: "The generator was a General Electric synchronous machine, rated at 1250 kilovolt-amperes at 2400 volts, operating at 600 revolutions per minute, and with a direct connected exciter." [3]

And, therein, is the problem: kilovolt-amperes. From high school physics everyone knows that power in a direct current system is voltage times current in amperes. Simply, its kilovolts x amps = kW. For an AC system, however, the story is more complicated.  In that case kV x amps = kVA, and is known as "apparent power".  How much real power that results in depends on the power factor.

Putnam, as in common parlance, equated kVA to kW. But that’s not the case for synchronous generators where there’s this little problem with power factor. The difference is the ratio between real and apparent power. When that relationship is unity (1), kVA = kW. But that’s seldom the case in the real world and the power factor is assumed to be 0.8 as it was in the case of the Smith-Putnam generator. Thus, a 1,250 kVA generator produces 1,000 kW.

Is this distinction important?

Yes, it’s more than a mere academic argument—though it is that too. In the early days of the Great Wind Revival in the 1970s and 1980s many wind turbine manufacturers made outlandish claims about what their wind turbines were capable of. They would deliberate inflate the “rated capacity” of their wind turbines to fool unsuspecting investors and charge more for their wind turbines than competitors who didn’t inflate their “rated capacity” as much. One example I like to use is one wind turbine rated at 100 kW and another at 25 kW, yet both wind turbines were basically the same size—10 meters in diameter—and would generate about the same amount of electricity!

Note: I've made corrections to my pages on the Smith-Putnam project. If there are any pages that I've missed please let me know.

I’ve railed against the “rated capacity” at “rated wind speed” game for decades. (See Generator Ratings & Capacity Factors: Why You Should Avoid Them.) The rating system and inflated ratings did a lot of damage to the wind industry that has taken us decades to overcome. So it behooves us to call a wind turbine that delivered only 1,000 kW a 1,000 kW wind turbine and not something else.

There is also a little matter of national pride. Americans have been bragging about the 1,250 kW Smith-Putnam turbine for three-quarters of a century. Taking one of our showcase technological achievements down a notch is something that will rankle some in the trade. So be it.

The French dealt with the same problem themselves with their 800 kVA BEST-Romani experimental turbine in the 1960s. Despite the director of EDF’s bureau of research describing the wind turbine as 800 kW, the people who actually worked on the wind turbine said it was rated at 640 kW.[4]

Another experimental French turbine of the period, the Neyrpic, was rated at 1,000 kW at 17 m/s. It did produce 800 kW at a wind speed of 15 m/s, but it also reached a peak power of 1,085 kW, justifying its rating of 1,000 kW in the historical record.[5] Today we'd be more specific and say it's "peak power" was 1,085 kW.

There may have been an intent on the part of the French program to match if not exceed the size the Smith-Putnam turbine had reached two decades before.

In this, EDF, the client behind the experimental French turbines, chose the wrong parameters for comparison: rated power. It is the area swept by a wind turbine’s rotor that determines how much electricity it will generate, not the size of the generator. This is fundamental to wind energy though many get this wrong still to this day.

The Smith-Putnam wind turbine used a rotor 53.3 meters in diameter, sweeping 2,231 m² of the wind stream. BEST-Romani used a 30-meter diameter rotor, Neyrpic’s 1,000 kW model used a 35-meter diameter rotor. The Smith-Putnam turbine was far larger than both, sweeping more than twice the area of the wind stream as Neyrpic’s 35 meter wind turbine.

In the end, both the Neyrpic and the Smith-Putnam turbines suffered the same fate. They both lost a blade ending their respective development programs as well as wind energy in both France and America for three decades until the Great Wind Revival.

More on Smith-Putnam

Smith-Putnam Wind Turbine Virtual Exhibition Goes Live

80th Anniversary of Smith-Putnam Turbine Going Online

New Pages Added on Smith-Putnam Turbine: Patents and Movie Clip

New Smith-Putnam Records Uncovered Include Log Books

Smith-Putnam Industrial Photos


[1] Bruyerre, Philippe. La Puissance du vent: Des moulins à vent aux éoliennes modernes. Les collections - Histoire - Histoire et Techniques. Toulouse: Presses Universitaires du Midi - Toulouse, 2020.

[2] Putnam, Palmer Cosslet. Power from the Wind. Reprinted 1974. Van Nostrand Reinhold, 1948, p. 1.

[3] Putnam, p. 118.

[4] Bonnefille, R. “Les réalisations d’Electricté de France concernant l’énergie éolienne.” La Houille Blanche, no. 1 (January 1975): 45–66, p. 50. Bonnefille writes 800 kW. “EXPERIMENTAL AEROGENERATOR TYPE B.E.S.T. - ROMANI DESCRIPTION, ASSEMBLY, TEST PROGRAM,” NASA TT F-15,037, May 1958, 54, notes it was rated at 640 kW.

[5] Bonnefille, R. “Les réalisations d’Electricté de France concernant l’énergie éolienne.” La Houille Blanche, no. 1 (January 1975): 45–66, p. 53.

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