RPM Wide-speed-range DC Generators --
Description & Specifications
Main Features of RPM Generators
DC (direct current) generated from wind power is a good combination with photo-voltaic solar panel installations. Including both power sources can reduce need for battery storage because wind or solar power is usually present. Installations that limit generator speed enable maximum energy yields with high reliability and low cost. Loads are thus protected from over-voltage, and generators with lower peak power cost less. Our generator harvests power at low wind speeds, in contrast to existing high friction/cogging/loss generators that harvest power at needed voltage only at high wind speeds.
Power generation efficiency exceeds 95% over a 10-to-1 speed range, with current and voltage control by integral switch-mode boost-regulation power interface electronics.
Modular disk assembly affords wide power range selection, to maximize yield from various rotary power drive sources.
Zero cogging torque and no gearing, plus boost-regulation integral power interface electronics, provides useful current and voltage regulated generator output DC power over a very wide speed and torque range. So energy yields are 2 to 10 times more than other generators, in addition to delivering far higher power quality. Our higher energy yields are clearly measured and documented. Our current and voltage control, without power disruptions, provide high quality power from our generator that protects batteries and loads from damage.
No gears and hence no gear friction or cogging and far less periodic maintenance, no output disconnect switchgear, and no cooling systems for gear lubricants or generator coolants.
The RPM Generator's 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 uniquely compatible with RPM's flywheel batteries as well as chemical batteries.
Our Other Generator Versions
Another version installed in electric vehicles (EVs) or electric power water-craft would augment onboard battery power while affording a healthy exercise option. This version could also produce electric power from health club exercise equipment.
Customized Models for specific applications
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 have built and tested 2 prototypes that each
generate 500 watts at about 1000 rpm, with useful power down to about 85 rpm,
that charge a 48 vdc battery pack.
The vertical axis version has a relatively large diameter and large number of axial-field poles, to accommodate direct shaft connection by a flexible coupling to relatively low speed vertical axis wind turbines, and does not need the normal speed-up gearing of conventional generators (which would include drawbacks like friction and stiction and need for periodic maintenance, lubricants and additional bearings).
Left: Horizontal axis version, mainly for coupling to horizontal axis wind
turbines, over a wide range of power ratings to optimize
yield from a wide variety of wind turbine sizes.
Another version, with foot pedals attached to the shaft at each side, can be installed in an ultra-light electric vehicle, to augment onboard battery charge by generating electric power from a recumbent cyclist driver and/or passenger. This would extend the range of the EV while affording a healthy and convenient exercise option.
Both the vertical and horizontal axis versions provide regulated DC current and voltage to their loads through boost regulators in their power interface electronics. Both are self-starting, and never need to be disconnected from their loads. So they deliver far better power quality, at regulated voltage, optimized for the normally varying mechanical power driving their shaft.
Early applications for the Broad-speed-range RPM Generators, with mechanical shaft input power supplied by wind turbines, will supply regulated DC current and voltage to a 48-volt DC power bus, connected to 48vdc chemical batteries.
Later version Broad-speed-range RPM Generator versions will accommodate higher DC voltages. For example, generator versions that will supply DC power to poly-phase AC inverters, can provide relatively very high quality power synchronized to poly-phase power grids.
Test Setup to Demonstrate Controlled DC Output Current and Voltage over a Wide Speed Range
Testing the RPM Wide-speed-range Generator output current and voltage over variable selected shaft speed and torque is illustrated by the photo below.
PWM boost-regulated integral electronics DC generator output voltage as a function of speed is shown below, and compared with existing self-starting (common alternator type) generators.
DC electric output power and electromechanical power conversion efficiency as a function of shaft speed, for a representative RPM Wide-speed-range DC generator for use with wind turbines, is shown in the figure below.
Its extraordinarily high DC electric energy yield from wind turbines, and the high power quality (well regulated and with no disconnect switchgear), of the RPM Wide-speed-range DC generator, is best understood by examining the Rayleigh Statistical Distribution illustrated below.
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.
The curve of Mean KWH at MPH is plotted from the product of Mean Hours at MPH and KW at MPH. Energy yield available from a wind turbine is thus the area under this curve.
Fradella's US Patent 7646178 "Broad-Speed-Range Generator" and patents pending fully describe and illustrate our 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 flexible shaft couplings and critical shaft alignment of generator to turbine. Generator bearings of this version serve also as turbine bearings.
Conventional Induction Generator Drawbacks
Widely used induction generators can only generate power when their shaft speed exceeds the synchronous speed of the power grid connected to them, to augment grid power. At shaft speeds below synchronous, 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.
Conventional Permanent-magnet Generator Drawbacks
Common alternator output voltages substantially 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.
RPM Generator Versions with Integral Wind Turbine and Enhanced/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 custom 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 our generator's rotor
shaft. This avoids in-line shaft alignment problems incurred by the common
existing practice of coupling generator shafts to standard turbines that include
turbine bearings.
That common existing 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.
And common practice does not normally include turbine and (more importantly) 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 normal 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 normal 15% harvest of wind power reaching a 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 otherwise cause damage to normally exposed wind powered systems.
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.
Our 4-blade Savonius turbine will incur considerably less torque fluctuations that those having fewer blades. In addition, our 4-blade spin startup will be reliable and immediate, with blade speed almost equal to wind speed.
Conversely, 2-blade turbines, particularly with common mechanical and magnetic cogging torque, will not start when wind at the turbine is near 900 from its maximum torque angle, until wind direction shifts and provides a startup torque.
Installations that channel wind from only 2 directions (compared to the 4 directions possible in the figure 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.
A custom HAWT (Horizontal Axis Wind Turbine) may be preferable to a VAWT for this environment. One HAWT can be mounted on our generator shaft extending from one end of our 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 60 miles/hour.
HAWT rotational speed is considerably higher than VAWT speed, so our HAWT driven generator can have less rotor pole magnets, and thus correspondingly 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 horizontal rotation axis does not need to follow wind direction. Slip-rings are needed for common HAWT towers, to prevent possible generator output conductor damage, by excessive twisting over its service life-time.
Our 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 40% of wind power passing through its spinning blade area. The turbine at the other side can harvest about 20%.
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. High blade shaft stress at hubs is responsible for many turbine failures of common axial wind turbines.
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 loads, and higher efficiency (resulting in 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. Our generator's output, over a typical wind speed range, can be readily demonstrated by mounting our 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 our turbine-generator, and of our turbine-generator in wind funneling and limiting enclosures.
A reliable, practical and simple design, to limit wind speed at our turbines, is illustrated below.
Since wind funneling and limiting, also protecting wind turbine and generator, are not common practice, educating customers about its advantages will be challenging. But customers might be easier to convince, whose traditional turbines were destroyed by strong winds, or damaged by normal weather, or caused frequent battery failures, or had turbine blades damaged by the sun, or did not harvest even a small fraction of energy levels that vendor brochures imply.
Advantages of this integral design approach include: Far higher service life and reliability for all UPS elements, with far less maintenance. Lower total cost of turbine and generator -- facilitating molded plastic turbines, lower generator power ratings, etc. And our wind power systems will deliver our specified far higher energy yields, with far better power quality, than all others.
RPM Flywheel Battery, to store power as kinetic energy of its spinning rotor, from the RPM Broad-speed-range Generator and other power sources, and regenerate electric power as needed
A photo that illustrates early laboratory prototype testing, with descriptions
of main elements of RPM's prototype flywheel battery that has magnetic bearings
stabilized by ceramic ball bearings, is shown below.
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 in the bottom rim holder. These magnets are radially aligned to axially repel each other, by a ceramic ball bearing in the top rim holder and another in the bottom rim holder. Their axially free inner races are centered by the center shaft.
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. The main rotor axial support is provided by the magnets.
The 4 power (from 2-phase stator windings) and 4 sensor (from 2 Hall-effect devices each aligned to a respective stator winding, that sense regenerative motor magnetic field) conductors of the assembly shown in the above photo, connects to power interface electronics, which exchanges DC current with a 48vdc power bus.
Follow-up development tasks include: Replacing the aluminum rotor rim with a carbon fiber composite rim (that will provide over 4 times the energy storage capacity for the same weight flywheel battery); mounting the completed flywheel assembly, centered in a vacuum enclosure; purging the enclosure at elevated temperature under high vacuum, to remove contaminants that may otherwise be released during the flywheel service life; sealing the vacuum enclosure; mounting the flywheel in its vacuum enclosure within a self-leveling structure; and finally installing the complete flywheel battery system preferably in an underground site that can safely absorb the stored flywheel energy in the unlikely event it may explode.
Fradella's US Patent Pending 12/463,275 "Low-Cost Minimal-Loss Flywheel Battery" fully describes and illustrates its many details.
A flywheel battery prototype we built and tested, having a non-contacting rotor whose axial and radial position is stabilized by servos, is shown below. It is described in Fradella's U.S. Patent 6,794,777. Its manufacturing cost would be considerably more than the flywheel battery shown above. While its rotor balancing is not critical, and its probable service life is virtually unlimited, its many rotor position and rate sensors and need for close proximity magnetic bearing servo electronics with high sensitivity to ground loops, difficult to stabilize magnetic bearing servos, and high servo startup power, are challenging problems that we learned how to circumvent in a successor flywheel battery we are now developing.
The flywheel battery we are now developing, that we expect will supersede the very complex flywheel shown in the photo below, has passive magnetic bearings that inherently maintain radial centering for the rotor and provide rotor lift force that is stabilized by a single axial servo. Startup power for the successor flywheel battery is a small fraction of that needed for the flywheel prototype shown below. Axial forces needed from the new flywheel battery top and bottom electromagnets are a fraction of the axial forces needed to position the rotor for spinning, and can achieve desired rotor position in about one second. The successor flywheel battery is expected to cost about the same as the flywheel shown above, and will not have its size, speed, and service lifetime limitations.
Status of the RPM Broad-speed-range Generator and Flywheel Battery
RPM (Regenerative Power & Motion) and EES (Excellent Energy Solutions, LLC) jointly developed the generator and flywheel battery prototypes shown above. Prototype tests and demonstrations have been conducted for 2 generator versions and 2 flywheel versions.
The technology shown and described above is covered by Fradella's U.S. Patents
4085355, 4520300, 6566775, 6794777, and 7646178, plus 2 pending patents.
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 building-integral RPM Generator installations are:
The building exterior walls channel wind to the turbines driving the generators, which increases wind speed. Doubling wind speed increases generated power 8x.
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.
Movable louvers around the generators and wind turbines can limit wind speed at the turbines, to provide steady and regulated generator output power during wind storms and to prevent turbine damage.
The building can also protect the wind turbines and
generators from rain and sun.
Horizontal Axis Broad-speed-range Generator Version for Ultra-light Solar/Fitness EV
Left:
A "see-through" view of a personal, 4-wheel ultra-light "Fitness EV" that
seats 2. PV can be applied on all top surfaces, that would collect
about 500 watts for several hours daily. Thin-film amorphous PV in
window glass can reduce glare and interior heat load from sunlight, comparable
to conventional tinted glass or reflective coatings that don't provide
electric power.
Intelligent power electronics can enhance this EV, by providing infinitely variable speed control, with synchronized non-conflicting proportional regenerative braking.
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 S-shape 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 the motor-in-wheel is cushioned from road shock.
This EV weighs 800 pounds or so. Its ultra-efficient motor has 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 supplied by a driver in a recumbent position (where we output the most power without tiring) to the RPM EV version generator (shown in red) can augment solar power. Effort level is selectable, like cardio workout gym equipment. As can be seen from the graphs below, a champion athlete can generate 370 watts almost indefinitely, a physically fit person 180 watts. So a driver, pedaling in daylight with 500 watts from PV, could travel indefinitely at about 35 mph. This EV would be capable of traveling at speeds up to 60 mph, on mostly battery power, recharged by plugging into a garage power outlet.
Left:
Block diagram of ultralight 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.
While driving, power can be augmented by about 500 watts from thin-film amorphous photovoltaics on all upper EV exterior surfaces, including its front and rear windows. Also, power can be augmented at any speed, by the pedal-powered generator. A second generator can be easily included for a passenger who might also want the exercise it affords, while extending the EV's range. Pedal effort level can be selected by the user. Depending on the user's fitness level, each generator can output up to 1.5 hp (1100 watts) for brief periods and 0.5 hp (370 watts) for over an hour, as can be seen in the graph below. Total sustained power, from 1 generator and the EV's PV, can thus average about 870 watts. So the EV can thus be driven for extended durations at speeds averaging 35 mph, without discharging the batteries. At such speeds, aero drag would be negligible, even with open windows, for ventilation.
If provided in-transit power, via the 2 red extendable contacts shown, on electrified highways, it could maintain 60 mph indefinitely. We need to make the public aware of this simple, clean, very low-cost option, so politicians will come onboard, and permit the highway infrastructure for it.
Main motor and braking effort may be applied to the two rear wheels, by regenerative bi-directional 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 rear wheels, no speed reducer is needed. If the batteries ever fail (and must be disconnected), the EV may be driven powered only by PV and/or pedal power. It can be driven forward or reverse at 0-15 mph on pedal power only.
Two red stripes are shown at the EV's rear left side. Early versions may have only an extension cord, to plug into 60 Hz outlets. Until we have electrified highways for EVs, the 2 red extendable contacts shown could be used as charging contacts, automatically extending to engage recessed electrified conductive charging strips, in a future home's garage.
PV and sustained pedaling power can sustain ~35 mph without discharging
the batteries. Considerable data from cyclists is available. It's compiled
in the chart at right:
Note that the time scale is logarithmic. Also note that a champion 160-pound athlete can output 1.5-hp for several seconds, while a physically fit person can output about 1-hp.
The athlete can sustain about 0.5-hp for well over an hour, while the fit person can sustain about 0.25-hp. A driver wanting to power his vehicle more from his pedaling will probably choose to have 4 onboard 600 watt-hour batteries or less.
This lightweight "fitness EV" might have only 2.5 kwh onboard battery capacity. Its aero drag coefficient could be 0.1 (large area, sloped PV windows, and narrow large-diameter tires, help achieve this), but aero drag is higher when side windows are open for ventilation. Its frontal area could be 12 square feet (with a bit less head-room, and a bit more reclining recumbent driver sitting position than shown in the image at the top of this page). With less batteries, there would be more dependence on PV power. Nickel-metal-hydride, lithium-ion batteries, and ultracaps may soon cost less, and higher efficiency PV with 800 watts output may be worth the higher cost for this market segment.
On battery power only, its cruising range would be about 70 miles at 45 mph -- and 55 miles at 60 mph. This range is not reduced much, for night driving, with ultra-efficient LED head-lights and tail-lights. In daylight, on PV and pedal power only, a fit driver could maintain 35 mph, and achieve occasional 45 mph bursts.
With 10-kw peak motor power, this EV can accelerate to 15 mph in 2 seconds, 30 mph in 7 seconds, and 45 mph in 20 seconds (mostly on battery power).
Aero drag will increase when interior ventilation is needed, during high driver pedal effort. But that's no problem at speeds up to about 35 mph (where rolling friction considerably exceeds aero drag).
The images below are from 3D CAD models. Analysis shows RPM's generator, with electronics that includes selectable effort level; plus a motor-wheel version of RPM's regenerative motor having a tubular non-rotating shaft that supports its motor-wheel ball bearings and is a conduit for its 8 electrical connections to its power interface electronics, can enable a very low cost no-gas "solar/fitness-EV". Spring connections between the motor and wheel rim enable very low unsprung mass; and with its large diameter/width ratio tires, incur low rolling friction (inverse to tire diameter) and smooth ride.
While applications for the "cleantech" electric power products we hope to manufacture and distribute shown above may seem diverse, their technologies are related. And the generator, flywheel battery, plus the ultra-light EV shown above are very compatible with each other and with solar power.
RPM's other 11 webpages also cover sustainable technology we are developing; to improve our environment; increase building and vehicle safety; lessen global dependence on fossil fuels and nuclear energy (and their serious negative consequences); and provide far more convenient and reliable UPS (Uninterruptible Power Supplies). To view them, please click on any of the links below.
Comparison of RPM's flywheel battery with others -- a somewhat detailed study
Brief Summary of RPM's Business Plan -- what we've done and plan to do for the future
RPM's Resources -- our people, tangible properties, office and lab facilities, etc.
Technology: Public and Business Policy
RPM's UPS can enable future distributed on-site solar/wind power, and more
RPM's brushless regenerative
motor and generator in ultralight EVs
RPM greatly values your interest in this exciting venture, and welcomes your participation.
If you have comments or suggestions, email Dick Fradella at fradella@earthlink.net
Edited October 2011
