RPM wide-speed-range Generators & Wind Turbines


Main Features of  Generators and Turbines:

DC (direct current) generated from wind power is a good combination with PV (photo-voltaic) solar panel building-integral on-site installations. Together, solar and wind power is less variable, and thus can reduce needed battery storage. Installations in structures that increase and limit wind speed at turbines integral with wide-speed-range generator shafts, enable high energy yields with high reliability and low cost, in locations where low average ambient winds are not practical for conventional wind-powered generators.  

RPM Generators include power electronics that protect loads from over-voltage. Their turbines and generators, in structures that increase and limit wind, are protected from severe weather, and bird kills are prevented. Integral turbine-generator can harvest power at low wind speeds, in contrast to conventional high friction+cogging+loss generators coupled to turbines on towers, that harvest power at needed voltage only at high wind speeds.     

RPM Generator efficiency exceeds 95% over a 10-to-1 speed range, with DC load current and voltage control by integral switch-mode boost-regulation power interface electronics.

Modular disk assembly affords wide power range selection, with less production parts inventory, to maximize yield from various rotary power drive sources.

Zero cogging torque and no gearing, plus boost-regulation integral DC power interface electronics, provides useful current and voltage regulated generator output DC power over a very wide speed and torque range.  Their energy yields from wind can be 100 times more than a conventional generator with its shaft coupled to the same size turbine mounted on a tower ! !   

That's in addition to delivering far higher power quality (current and voltage control protects loads; never draws power from loads; supplies steady power during low winds and over a very broad speed range; supplies DC current with no ripple). High energy yields described here are clearly measured and documented. Current and voltage control, without power disruptions, provide highest quality power from generators that protect batteries and loads from damage.

RPM wind turbine and generator service life without maintenance would be far greater than conventional generators on towers.   

No gears so no gear friction or cogging, no periodic maintenance, no output disconnect switchgear, and no cooling systems for gear lubricants or generator coolants.

DC output current regulation facilitates parallel connection of like generator power interface electronics.  And it facilitates parallel electronics power interfaces, connected to stacks of stator disks of the same generator assembly, thus accommodating a wide power range with a small inventory of different parts.

Its DC output power is compatible with RPM flywheel batteries and all chemical batteries.


RPM Generator Versions: 


Left: Vertical axis version, mainly for coupling to vertical axis wind turbines, over a wide range of power ratings to optimize yield from a wide variety of wind turbine sizes. We built and tested 2 prototypes that each generate 500 watts at about 1000 rpm, with useful power down to about 80 rpm, charging a 48 vdc battery pack.



Horizontal Axis ModelLeft: Horizontal axis version, mainly for coupling to horizontal axis wind turbines.

Another version, with foot pedals attached to the shaft at each side, can be installed in ultra-light electric vehicles (EVs), to augment onboard PV power by generating electric power from a recumbent cyclist driver and/or passenger. This would extend the EV capabilities while affording a healthy and convenient exercise option.




Testing the RPM wide-speed-range Generator output current and voltage over variable selected shaft speed and torque is shown in the photo below.



Left:  RPM generator DC electric output power and efficiency vs. shaft speed.

Dashed part of Efficiency curve is zero until generated power reaches a few watts needed for its integral electronics.

For wind power above these few watts, DC output power increases with turbine speed3.

This generates maximum DC electric power, with optimum turbine torque load varying as speed2.







Left:  PWM (about 100 kHz switch-mode Pulse Width Modulation) boost-regulated integral electronics output DC voltage and current as a function of speed is shown at left.

Note that output voltage equals load voltage, over entire broad-speed power generation range.

Also note that DC load is protected from over-voltage. 








Left:  Rectified voltage and current vs. speed, of a conventional (common alternator type) generator. 

Note that generated power is zero until its rectified voltage exceeds voltage of its battery load. No power is generated at over the first half of this speed range.

Also note that loads are not protected from over-voltage, unless voltage regulation electronics is added between this conventional generator and its load.






Left:  Power to 3-phase grid from a typical wind-farm tower-mount induction generator.

No output voltage or power is generated, unless the induction generator is connected to the grid, and its speed is above minimum to synchronize with the grid. 

When first connected to the grid, brief negative power is drawn; and blade pitch and wind angle control, plus gear oil and coolant pumps, consume power even when its circuit breakers disconnect it from the grid.






Wind Power and Yield Calculations:

Electric power from a wind powered generator = (.005)(windspeed)3 (swept area) (wind capture percent / 100)
where power is watts, and wind-speed is miles per hour MPH
swept area is square feet swept by wind turbine blades
wind capture percent is for wind turbine and generator.

Since wind is so variable, average wind-speed is used to predict duration at each speed, which follows the Rayleigh statistical distribution:
Probable time at each wind-speed V varies according to  V / Vavg2 * exp(-.785 * V2 / Vavg2)
where total time is 720 hours/month
Vavg is the average wind speed at the wind turbine.

Probable time at each speed, in hours, for winds from 0 to 50 MPH is plotted below, with probable incremental power KWH (obtained by multiplying power generated in KW at each speed V by mean hours at V) as mean KWH at MPH for 10 MPH average. Probable energy yield in a 1 month period is the area under the curve Mean KWH at MPH.  

The curve showing Mean Hours at MPH for a 10 MPH average wind speed is determined for a representative location by compiling anemometer and/or Pitot tube recordings, typically available for many locations over decades. If a specific location has a different average wind speed from the 10 MPH average shown above, its Wind Speed axis for the above graph would accordingly reflect the Average Wind Speed for that location, with the same ratios of Mean Hours at MPH to Average Wind Speed, and following the same statistical distribution except for different Wind Speed parameters.

The curve of KW at MPH shows available shaft power from a wind turbine as a function of wind speed. Shaft power varies as the third power of wind speed. Hence the RPM Wide-speed-range generator power interface electronics control output current so that power generated is also proportional to the third power of wind speed. This attribute extracts and produces maximum available power from the turbine shaft over a far wider wind speed range than other generators.

For a wind power installation where average ambient wind-speed is 10 mph that is channeled so 25 mph average wind-speed drives a 40 sq.ft. wind turbine and generator with overall wind capture efficiency of 35%, power generated at 25 mph wind would be about 1 kw, for a simplified average yield of about 720 kwh per month. Probable energy yield, by adding KWH at each wind-speed from winds covering the total statistical distribution, but with power limited at the wind turbine to 8 kw, would be about 1500 kwh per month. Very little energy yield is lost, by limiting the maximum wind-speed at the turbine to about double its average, because wind-speed above that will be so infrequent. Besides protecting the wind turbine, this speed limiting accommodates efficient broad range current and voltage control, and lower required peak power ratings. For example, by limiting wind-speed at the turbine to 50 mph (double the average wind speed at this wind turbine), the generator and power electronics maximum power rating would need to be 8 kw. All power ratings would need to be substantially higher without a speed limit: Power electronics would need to have higher voltage and current limits. Efficiency at low wind speeds would also be less if generator speed is not limited, and buck regulators would be needed to protect battery loads from over-voltage.  

US Patent 7646178  "Broad-Speed-Range Generator" and published application US20120256422A1 plus patents pending fully describe and illustrate the RPM generator's many details.

Custom wind turbines mounted integral with the generator rotor assembly afford substantial advantages over usual installations with turbines having separate bearings. Integral turbine-generator assembly circumvents need for turbine bearings that also need flexible shaft couplings and critical shaft alignment of generator to turbine.    



Conventional Permanent-magnet Generators:

Alternator output voltages vary proportional to shaft speed. Hence they cannot reach output voltage levels high enough to charge batteries or drive DC-to-AC power inverters, until wind speed is relatively high. And if connected to loads with no current regulation, internal losses may cause generator overheating and/or excessive load currents if shaft speed is not limited.

Most have iron poles that tend to align at rotor angles where magnetic reluctance is minimum, thus causing magnetic cogging torques.
Shaft alignment common practice is laborious and expensive. Despite that high labor and expense, the coupled shafts always incur high friction and mechanical cogging torques, because the coupled turbine and generator bearings inevitably can't be aligned and maintained in adequate alignment, even by the most skilled installers.

Friction and cogging torques prevent startup at low wind speeds. No output power is delivered to DC battery loads until stator voltage plus rectifier forward voltage drops plus (when included) series voltage regulator circuit voltage drops exceed the DC battery voltage. So useful generator power at low wind speeds is forfeited. As the above Rayleigh chart shows, conventional generators that can't produce useful power at low wind speeds harvest under half the average energy over time, compared to RPM generators. They also don't produce power when it is most needed, compared to RPM generators. DC power to loads, without ripple (particularly if for flywheel batteries), generated when most needed, plus load current and voltage control, are RPM generator power quality attributes that are not available from conventional generators. 


Conventional Induction Generators:

Widely used induction generators can only generate power when frequency at their shaft speed exceeds the synchronous frequency of the power grid connected to them, to augment grid power. At shaft speeds below synchronous frequency, they would consume grid power, and so must be disconnected as wind speed fluctuates. At shaft speeds where induction generator losses are very high, the grid connection must also be disconnected, because power exchanged fluctuates excessively and internal generator losses can cause generator overheating. Although these shortcomings are widely recognized, induction generators are widely used in wind farms, directly connected to 3-phase power grids with no power interface electronics.

Utilities limit numbers of induction generators connected to them, because they have caused grids to fail due to inherent generator disruptions, power reversals, and uncontrolled phase. Also, wind farms where induction generators are usually installed require very windy locations. 


RPM Generator Versions with Integral Wind Turbine and Increased/limited Wind-speed:

This version circumvents existing high installation costs and shaft coupling problems, because its bearing pair serves as both generator and turbine bearings. Also, its generator speed limiting and broad wind speed range is optimized as a complete system that harvests maximum energy with highest power quality.  

A VAWT (Vertical Axis Wind Turbine) in a structure that can increase wind-speed at the turbine to nominally 3x ambient, and can limit turbine wind-speed, is illustrated below. For example, in a location where average ambient wind speed is 10 miles/hour, average wind speed at the turbine blades can be increased to 30 miles/hour and limited to 60 miles/hour.

Its custom 4-blade Savonius turbine is integrally attached to the generator's rotor shaft. This  avoids in-line shaft alignment problems incurred by the conventional practice of coupling generator shafts to wind turbines that have turbine bearings. 

That common conventional practice incurs inevitable high mechanical cogging torque and friction loss, high installation labor to align generator and turbine shaft, and need for flexible shaft coupling.  

Conventional practice does not normally include turbine and (more importantly, in many cases) generator speed limiting.

The wind diverters shown increase wind-speed driving the turbine blades moving in the same direction as the ambient wind, regardless of ambient wind direction. 

The wind diverters also prevent reverse torque incurred in contentional practice, from wind on blades moving in a direction opposite the ambient wind, which normally causes Savonius turbines to harvest only 15% of power intercepted by their blades.

Increasing wind-speed at the turbine to 3x ambient will increase wind power 27x. And preventing reverse torque from wind on the opposite blade side can increase conventional 15% harvest of wind power reaching the Savonius turbine through the opening area to over 40%.  

The wind diverter structure also includes shutter vanes at its 4 openings that automatically limit wind-speed at the turbine. And the structure can protect the turbine, generator, and associated electronics from rain, snow, and sun, which cause damage to conventional exposed wind-powered turbines and generators.

Savonius turbines have a rectangular wind-intercept area, which facilitates a matching rectangular wind opening. Thus, most of the wind through the opening is caught by the turbine blades. 

A Savonius turbine with more than 3 blades will incur considerably less torque fluctuations as it rotates, compared to those having 2 blades. Those with only 2 blades may not start rotating unless wind is at angles that produce higher torque than their 2 "dead spot angles". This problem is compounded by cogging, friction and stiction, in common wind power generators. Conversely, the RPM integral design 4-blade turbine spin startup will be reliable and prompt, with blade speed almost equal to wind speed. 



Left:  Savonius turbine cutaway view of integral generator shaft coupling.

The turbine does not have conventional separate bearings and a shaft coupled to a conventional generator shaft.

Main advantage of the integral shaft shown: No mechanical cogging torque, nor shaft alignment, nor flexible coupling required.  

So performance would be far better than conventional. 

And installation would cost far less.





An example of minimum energy harvest ratio, from this integral turbine-generator in a diverter as shown in these images, which funnels winds to 3x ambient velocity, compared to an identical size conventional Savonius turbine with shaft coupled to an alternator type generator: 

RPM KWH compared to conventional = (RPM Efficiency / Conventional) x (RPM Wind Speed / Conventional)3  x (~2x KWH speed range)

RPM KWH compared to conventional  =  (40% / 15%) x (3 / 1)3 x (2) = 144  ! ! !

Total land area needed for conventional small-scale wind turbine towers that can generate as much total power exceeds the roof-top area of this wind diverter installation. Conventional towers are costly. Conventional turbines and generators on towers exposed to storms incur damage. Long power cables from conventional towers are costly. Conventional wind power is poor quality and unreliable. So conventional small-scale turbines don't have wide market acceptance. 

An on-site building-integral solar plus wind powered installation that can produce far more reliable, far lower cost per kwh generated, better power quality, is illustrated below. 

Its dependable, zero-maintenance, low service lifetime electric power cost, and abundant on-site generated electric power, could make connection to central grid power optional. 


Installations that channel wind from only 2 directions (compared to the 4 directions possible in the figures shown above) would need 2 openings with wind limiting (compared to the 4 openings shown above). A wind power installation between 2 high buildings would have wind from either of 2 opposite directions. Axial wind turbines are a good choice for buildings compatible with 2-directional wind installations, because their rotational speed can be high compared to Savonius turbines, so the generators they drive can use widely available low cost rotor magnets and so are lower cost.

A HAWT (Horizontal Axis Wind Turbine) may be preferable to a VAWT for this environment. One HAWT can be  mounted on the RPM generator shaft extending from one end of the generator assembly, plus a second HAWT on the opposite end, as illustrated below.

This integral 2-HAWT and generator assembly can (like the above VAWT) be in an enclosure that increases wind-speed  in it to about 3x ambient and limits wind-speed at the turbine to about double its average in this installation.

HAWT rotational speed is higher than VAWT speed, so the HAWT generator can have less rotor pole magnets, and thus would be lower cost.

This integral HAWT design requires less space than common axial turbines requiring a tail so their turbine axis follows wind direction.

This HAWT design does not require slip-rings because its turbine and generator assembly do not need a horizontal rotation axis to follow wind direction. Slip-rings are needed for conventional HAWT towers, to prevent generator output conductor damage, by excessive twisting as its angle changes to follow wind direction.

The above 2 HAWT turbines are identical to each other. Both turbines harvest power from wind, to drive the generator shaft. The turbine facing the wind can harvest about 45% of wind  power passing through its spinning blade area. The turbine at the other side can harvest about 30%. So total power from the generator shaft can be about 75% of wind power passing through this installation.  Wind passing the first turbine incurs a rotational speed component opposite the turbine spin, which would reduce power harvested by the second turbine. The rotational wind speed component is reduced by fin shaped generator mounts at its bottom, top, and 2 sides.  

Each HAWT has blades with a gradually varying optimum pitch angle vs. distance from their spin axis.  

For example, pitch angle at blade tips may typically be about 15o from wind direction; whereas pitch angle where blades are attached to their hub may typically be about 45o, with a gradual pitch angle transition from tip to hub. Blade width is greater near the hub, compared to the tip. So maximum power yield is delivered to the generator, and blade stress at hubs is very low  compared to common available axial wind turbines. We have not found any commercially available wind turbines with enclosures as described here. So they are subjected to wind storms. Most don't have variable blade pitch to limit blade speed and stress in high winds. High blade shaft stress at hubs is responsible for many turbine failures of conventional axial wind turbines.

An example of energy harvest ratio, from our integral turbine-generator in a wind funnel and limit installation as shown in these images, which increases winds to 3x ambient velocity, compared to an identical size conventional tower-mounted horizontal-axis turbine with shaft coupled to an alternator type generator: 

RPM KWH compared to conventional  =  (RPM Efficiency / Conventional) x (RPM Wind Speed / Conventional)3  x (~2x KWH speed range)

RPM KWH compared to conventional  =  (75% / 45%) x (3 / 1)3 x (2) = 90 !!

Both the custom VAWT and HAWT illustrated above should be installed within enclosures providing wind-speed limiting that protect the turbines, generators, and their loads. This wind-speed limiting also enables generator assemblies and their PWM boost-regulated electronics to be built with components having lower maximum power and voltage ratings. This results in lower cost, broader speed range of power delivery to (for example) 48vdc or 170vdc loads, and higher efficiency (for higher energy yields). 

Some wind turbine and generator brochures quote their maximum power ratings as if those ratings were the power yield. Actual power and energy yield from such devices is far lower than what they imply. The RPM generator's output, over a typical wind speed range, can be readily demonstrated by mounting its turbine-generator on a truck and recording power to batteries it charges vs. truck speed, when ambient wind speed is essentially zero. This demo can prove actual performance, of its turbine and generator, and of its turbine-generator in wind funneling and limiting enclosures.

Electric motors that automatically control vane closing/opening make wind-limit adjustments only when wind is high and generator power needs to be limited; so no electric power yield is lost driving the motors. 


The wind-funneling diverters and protective wind-turbine housings shown above are mainly for existing structures. New functional architecture buildings can have exterior walls that funnel wind, with average speed at wind turbines 3x ambient, as shown below: 


RPM Generator and Flywheel Battery in Solar/Wind-Powered Building

Left: Building-integral solar and wind power generation with flywheel battery power storage and regeneration to provide uninterrupted electric power as needed.

Advantages of solar+wind power building-integral installations are:

The building exterior walls channel wind to the turbines driving the generators, which increases wind speed 3x to 5x at the turbines. Doubling wind speed increases generated power 8x. Tripling wind speed increases power 27x.

The generators and wind turbines do not need towers to support them. 

Screens around the generators and wind turbines can prevent birds from colliding with turbine blades.

Motors powered by our generators can limit wind speed at the turbines by vane angle control, to provide steady and regulated generator output power during wind storms and to prevent turbine damage. Motor vane angle control is needed only in high winds, when generator output is highest, so no generator power yield is lost to drive the motors.

The building can also protect the wind turbines and generators from rain and sun. Achieving these exceptional goals requires a dedicated team of engineers, physicists, and architects, plus resources to manufacture and implement the technologies described. 

Combining wind and solar power reduces energy storage requirements, compared to power from only one option. Flywheel batteries offer reliable, low-loss, long-term energy storage:   


RPM flywheel battery that stores and regenerates electric power as needed, as kinetic energy of its spinning rotor, from broad-speed-range generator and solar panel power maximizer electronics

Left: 1st version flywheel battery prototype RPM built and tested, having a non-contacting rotor whose axial and radial position is stabilized by servos. 

It is described in U.S. Patent 6,794,777. 

While its rotor balancing is not critical, its size is scalable, and its probable service life is long, it needs many rotor position and rate sensors and close proximity magnetic bearing servo electronics, due to ground loop problems and signal interference.

Its magnetic bearing servos are difficult to stabilize, and need over 2kw to position a 40-pound rotor.

It has challenging problems that we learned to circumvent with 2nd and 3rd version flywheel batteries. 







Left: Prototype tests, with descriptions of main elements, for our 2nd version flywheel battery. It has repel magnet lift, radially stabilized by ceramic ball bearings.


Its integral regenerative motor is inside the rim and its top and bottom rim holders.


The motor stator is fixed to the center shaft. Its four 2-phase stator wires and four connections to two aligned Hall sensors are accessed by a center bore that can be seen at the top of the center shaft.


Main rotor axial lift is provided by the ring magnet shown and an identical ring magnet that it repels upward, in the bottom rim holder. The rotor is radially aligned by a ceramic ball bearing in the top rim holder and another in the bottom rim holder. 


An axial preload spring under the top ball bearing and another under the bottom ball bearing prevent ball skipping and sliding, and provide consistent additional rotor lift force to each inner race. Main rotor axial lift is provided by the magnets.


The 4 power (from 2-phase regenerative motor stator windings) and 4 sensor (from 2 Hall-effect devices each aligned to a respective stator winding, that sense rotor spin angle) conductors of the assembly, connect to power interface electronics, which exchanges DC current with a 48vdc power bus.


U.S. Patent 8,242,649  "Low-Cost Minimal-Loss Flywheel Battery" describes and illustrates its details.





 Horizontal Axis Broad-speed-range Generator Version and Motor-wheel for Ultra-light EV with Onboard PV:

Left:  A 4-wheel ultra-light EV that seats 2.  PV would cover all top surfaces, that would collect more than 1500 watts in sun.  Thin-film amorphous PV would cover windows.

With power electronics, its 2 rear wheels are each driven by a brushless regenerative motor-in-wheel, a special version of the motor described in US Patent 4520300.   Instead of conventional connection to tire rims, it has  springs between the motor housing and rear wheel  rim, and between the front wheel hubs and rims.  So unsprung mass (only its tires and rims) is very low, and its motor-in-wheel is cushioned from road shock.

This EV weighs about 700 pounds.  Its regenerative motors have cruise control for any speed from zero to maximum.  It also controls downhill speed, and regenerates power to charge the battery whenever braking or decelerating.

Optional pedal power from a driver in a recumbent position (where we output the most power without tiring) to this EV's generator (shown in red) can augment solar power.  An athlete can generate 370 watts almost indefinitely, a physically fit person 180 watts.  So a driver, pedaling  in daylight with 2000 watts from onboard PV, could travel indefinitely at over 50 mph average speed, carrying a few hundred pounds of cargo, without discharging onboard batteries. This EV would be capable of traveling at speeds up to 75 mph, on mostly battery power,  recharged by plugging into a garage power outlet, by its onboard PV, and by pedal power.



Left: Block diagram of  EV with onboard battery power, charged by ac or dc plug-in sources, probably in owner's garage.

Batteries are essential for regenerative braking, whenever the EV's 2 motor-in-wheel brushless regenerative motors decelerate the EV.

This EV can be driven for extended durations at speeds averaging 50 mph, without discharging onboard batteries. 

If provided in-transit power, via the 2 red extendable contacts shown, on electrified highways, it could maintain 75 mph indefinitely. 

Main motor and braking effort may be applied to the two rear wheels, by regenerative motor drive and braking, plus a friction brake (as a parking brake, and backup mechanical brake).  No motor clutch or gearshift or differential gear is needed.  With 2 large diameter motors in the 2 rear wheels, no speed reducer is needed.  It could be driven at  0-15 mph from onboard generator pedal power only.

Recumbent cycling data is shown in the nest chart:

Its time scale is logarithmic. A 160-pound athlete can output 1.5 horsepower for several seconds; a physically fit person about 1 hp.

The athlete can sustain about 0.5 hp for over an hour; the fit person about 0.25 hp.

This lightweight EV might have only 5 kwh onboard battery capacity.  Its aero drag coefficient could be 0.1 and its frontal area could be 12 square feet. 

With 20-kw peak motor power, this EV with full cargo load, can accelerate to 15 mph in 2 seconds and 50 mph in 10 seconds (mostly on battery power with less than 1% momentary discharge). 


Images below show the EV and 1 of its 2 rear motor-wheels.:


RPM web links describe more proposed sustainable technology; to improve our environment; lessen global dependence on fossil fuels and nuclear energy; provide far more convenient safe low-cost road travel; and flywheel batteries.

Dual-mode Electric Highway Vehicles

Flywheel Battery

Building-integral Solar and Wind Powered Buildings

Flywheel Basics Tutorial

Comparison of  RPM flywheel battery with others

Flywheel Facts and Fallacies

Technology: Public and Business Policy

Future distributed on-site solar/wind power, and more

Motor-wheels, PV, batteries, pedal-power generator of ultra-light EV 

If you have comments or suggestions, email    fradella@earthlink.net


Aug 2015