ABT is a mechanical balancing technology that reduces or eliminates vibration in rotating equipment in many applications. ABT automatically and continuously balances rotating parts in machines producing less vibration, resulting in machines that operates in a more energy efficient manner. Unlike conventional balancing techniques, ABT automatically and continuously adjusts for changes in imbalance of the rotating component and solves the vibration problem at the source.
ABT harnesses rotor displacement energy to move compensating masses within components of the system, which corrects for the imbalance of the rotor.
An embodiment of ABT has been optimized and custom-designed for the Comerton vertical axis wind turbine.
Wind power is one of the fastest growing power generation industries and one of the most sustainable forms of green energy. It is emission-free and infinitely renewable. The recent development of wind turbines in North America has gained a lot of attention from wind turbine manufacturers worldwide.
Wind power is becoming an increasingly important source of renewable energy and is used by many countries as part of their policy to reduce dependence on fossil fuels.
Wind turbines are used to generate electricity. In principle, wind turbine blades are designed to be efficient in transferring wind’s kinetic energy to rotate the blades and blade shaft. A connected gear box increases the rotational speed to drive a generator armature shaft. Armatures usually consist of electrical conductors, coiled copper wires that are tightly wound onto a metal core. As the generator armature shaft rotates inside the stator, which has large magnets, electricity is generated from the magnetism interaction with electrons in the conductor.
A wind turbine has many rotating parts, and balancing them to mitigate harmful vibrations is a considerable challenge. Manufacturing imperfections in wind turbine blades and generator armatures are typical sources of imbalance. The blades are usually made out of composite materials—fibre reinforced polymer (FRP)—through a molding process. The material distribution in the blades is not consistent from one blade to another. Each blade needs to be smoothed out and profiled after manufacturing. Similarly, manufacturing imperfections generate non-uniform mass distribution in the generator armature.
The potential effectiveness of wind energy coupled with Automatic Balancing Technology (ABT) as an alternate to conventional balancing methods is sugnificant.The two most common methods of balancing used in wind turbines are: static and dynamic balancing. Automatic balancing methods are also available, but not used frequently.
Static balancing of rotors is achieved by orienting a rotor with its axle laying across two parallel edges to create a nearly frictionless contact surface between them, as shown in Fig. 1. The rotor is allowed to freely rotate until the heaviest part comes to rest on the bottom. Imbalance is corrected by removing a small portion of material from the heavy side, or by adding material across from it. The process is repeated until the rotor can rest in any orientation without turning.
An alternate method of static balancing is orienting the rotor axle in the vertical direction, with a pivot placed at its centre of rotation. A bubble-level can be used to determine the heavy side of the rotor. Once again, removing material from or adding material to an appropriate location on the rotor enables imbalance corrections. The process is repeated until the rotor balance is acceptable. Static balancing can be used for rotors with a short axial width relative to rotor diameter, where there is minimal chance of a coupled imbalance.
A coupled imbalance occurs when a rotor with a long axial length relative to rotor diameter has imbalances with different magnitudes and angular positions at either end of the axle, as shown in Fig. 2. Such a rotor can appear to be balanced using the static balancing method, however spinning the rotor can cause significant vibration and lead to damage or failure.
Static balancing is mostly suitable for non-critical components and low speed rotating applications where the rotor is not likely to spin above its natural frequency or critical speed. Static balancing machines have advanced in terms of their form and function. Some of them are capable of detecting the heavy spot in a part without having it rotate. This is achieved using a set of sensors to detect the imbalance. The machines can even approximate the amount of mass imbalance that has to be corrected.
Although static balancing is cost effective due to the simplistic setup needed, it is also limited in its ability to accurately balance a rotor and is incapable of detecting or correcting coupled imbalances. Rotors will also have to be balanced again if they become unbalance during operation.
In the manufacturing of wind turbine blades, each blade is inspected after production. Any manufacturing imperfections such as inclusions, thermal cracks, etc., could generate vibration during operation. Blades are assembled on a blade hub. The assembly is then balanced using static balancing methods, which can be sufficient—although not necessarily ideal—because of the relatively small aspect ratio. Fig. 3 shows added weight on a wind turbine blade for static balancing.
Static balancing only achieves balancing on one plane and can be sufficient for a rotor whose aspect ratio—the ratio of its axial length relative to the rotor diameter—is relatively small. However, rotating parts with a greater aspect ratio should be balanced in two or more planes.
Dynamic balancing is needed when there is a possibility of coupled imbalance in a rotating system. This is a more complex process compared to static balancing. For dynamic balancing, the rotor has to be placed between two bearings with vibration sensors to measure vibration magnitude and phase. The rotor is run at a predefined speed, usually lower than the operating speed, and the position and amount of imbalance at both ends is calculated. Instead of correcting for imbalance in only one plane, mass is added or removed at both ends of the rotor to ensure proper balance during rotation.
Dynamic balancing is useful in high-speed applications and for critical components. Depending on the dynamic balancing machine being used, a higher degree of imbalance can be corrected relative to static balancing. Unfortunately, this also comes at an increased cost in terms of the time spent balancing and the equipment required.
Dynamic balancing is the method typically used in balancing generator armatures. The armature is placed on two bearings, which are parts of the dynamic balancing machine. Vibration sensors are placed on the bearings and a tachometer sensor is used to measure rotational speed. The armature shaft is connected to a motor, and it is spun at a defined rotational speed. The machine displays the amount and location of imbalance at both ends of the armature. The operator can remove some material or add weight onto the armature. Removing material or adding weights onto the generator armature has the drawback of reduced power output. Fig. 4 shows material removed from laminations on an armature during dynamic balancing.
The conventional factory balancing processes mentioned above are expensive and time consuming and do nothing to address the imbalance that occurs when a machine is in service. In the case of wind turbines, rotating components often become out of balance because of shrinkage, thermal cracking, debris/ice deposits, corrosion, wear and tear and many more factors. Mass imbalance can create violent vibration, especially at high speeds since the imbalance force is a square of rotational speed. Traditional balancing processes become a bottle-neck in production and act as a band-aid.
Condition monitoring systems are often utilized to monitor vibration. In many cases, most vibration problems are detected too late and condition monitoring systems are not always able to interpret the measured signals precisely. If the vibration is above an allowable limit level, the wind turbines are shut down and may have to be overhauled. The wind turbine would have to be disassembled and the rotating parts rebalanced. Wind turbines could fail unpredictably since the traditional balancing methods cannot compensate for additional imbalance caused by icing, debris deposits, damage from hail or corrosion.
During operation wind turbines are subjected to vibration, essentially wasted energy, that can wear rotating parts and weaken the structure. Ideally, frequent inspections would be required. Inspections become almost impractical because of difficulty accessing rotating components on wind turbines, and the turbines geographical location. Bearing wear is a sign of vibration-related problems and would require replacement. Loose fasteners and a bent rotating shaft also suggest vibration-related issues. If weakening components are not detected early, catastrophic failure could occur, resulting in a significant economic loss.
Comerton has a unique solution to the vibration problems in wind turbines. It is a technology-oriented company that has designed an automatic balancing device, called the Automatic Balancing Technology (ABT). ABT is a mechanical balancing device that is used to automatically balance rotating parts of wind turbines so that they produce less vibration, resulting in more energy efficient operation. The device can be installed on or built into rotating components. It is able to adjust itself to mass imbalance changes exhibited during operation, and also to balance itself if there is no imbalance in the system. The technology could potentially eliminate laborious balancing processes, thus saving balancing costs in the factory and rebalancing and other maintenance costs in situ, and adding a safety feature to the system.
By implementing ABT, wind turbines can remain balanced during operation and several benefits can be realized, including: reduced vibration, improved energy efficiency, reduced noise emissions, greater power output, lower maintenance costs and less mechanical wear. The technology would also improve balancing precision. Downtime can be eliminated and the wind turbine can be allowed to spin faster, thus increasing energy output.
Electronautics believes that ABT would be a breakthrough that could reduce or eliminate laborious and expensive conventional balancing methods in wind turbines while improving their overall performance, reliability and efficiency.
As an unbalanced rotor spins, the imbalance will generate an unbalanced force or centrifugal force that will try to pull the rotating rotor towards the imbalance (deflection), thus creating vibration. An object that travels in a circle behaves as if it were experiencing an outside force. The force, F, varies depending on the mass of the object, m, the speed of rotation, ω, and the distance from the centre of rotation, r. The relationship between these functions is shown by the following formula:
As mass increases, the force increases proportionally. This is also true for increase in the mass’s radial distance. However, when the angular velocity increases, the force increases proportionally to its squared value; this implies vibration can increase drastically with increasing rotational speeds. In theory, a balanced position takes place when a force of the same magnitude, but exact opposite direction is generated to offset the force causing imbalance. Electronautics believes that this could be achieved by adding countermasses to the rotating component that will self-position themselves and generate an equal and opposite force to the force causing imbalance.
At low speed, the vector of maximum displacement will coincide with the vector representing the centrifugal force generated by the unbalanced mass forcing function. As the angular velocity increases, the maximum displacement vector will lag behind the forcing function by some angle, which is known as phase lag, θ. When the phase lag reaches 90°, the displacement reaches a maximum value; and the speed at which this occurs is called the critical speed, or natural frequency, ωn. As the angular velocity increases beyond the natural frequency, the phase angle approaches 180°, and the vibration amplitude decreases, approaching some value asymptotically.
The critical speed of undamped system or undamped natural frequency can be represented as:
Equation 2 shows natural frequency of a system depends on the stiffness (k) and mass (m) of the system. All rotating equipment have a damping coefficient, c. Due to influence of damping, critical speed of damped system or damped natural frequency can be represented as:
It is believed that ABT is a dynamic system that provides a corrective force in response to lateral displacement. The effectiveness of the device will mainly be influenced by the dynamic characteristics of the host machine and any modifications to those characteristics that may be needed to allow the device to operate as desired.
ABT theory proposes to explain how countermasses react to an unbalanced mass at different speeds. Below is a step-by-step explanation of how the ABT system is able to move the countermasses to the opposite side of the imbalance as the angular velocity increases.
At an angular velocity below the natural frequency, the mass imbalance and the countermasses do not offset each other, but align themselves in the same direction, as shown in Fig. 5. In this illustration, the deflection of the shaft has been exaggerated to show the effect of phase shift, where the position of imbalance shifts relative to the shaft deflection while the countermasses remain in the same relative position. The shaft deflection represents the combined contribution of imbalance and countermasses to the centrifugal forces in the system. The direction of shaft deflection is referred to as the high spot, while the direction of the effective imbalance is the heavy spot. At subcritical speeds, the high spot and heavy spot are aligned on the same side of the axis of rotation.
As the angular velocity approaches the natural frequency of the system, the imbalance and countermasses are at right angles to each other, as shown in Fig. 6. This position is referred to as the phase shift. At this point, the countermasses are in the high spot, and the imbalance is in the heavy spot, and the high spot and heavy spot are at right angles to each other. In this illustration, the deflection has decreased, representing that the imbalance and countermasses are no longer exerting force in the same directions.
Finally, at an angular velocity above the natural frequency, the countermasses oppose the imbalance, and the system is in balanced position, as shown in Fig. 7. This shows no deflection in the shaft, indicating that the forces exerted by the imbalance and the countermasses are balanced.
Some clarification is required. At an angular velocity below the natural frequency, the deflection of the shaft is not that much because the rotational speed is low. As the angular velocity approaches the system’s critical speed, the centrifugal force increases, with the imbalance and countermasses pulling at right angles to each other. At the critical speed, the rotor experiences its greatest degree of deflection and ABT uses the energy of the phase shift to align the countermasses to oppose the imbalance.
Perpetual Industries Inc.(PI) has worked on a simple representation of a mathematical model of the ABT system, as shown in Fig. 8. The figure shows a general arrangement of a rotor that is supported by fixed suspensions in the X and Y directions, kX and kY respectively. Damping coefficients are represented by symbols, cX and cY. The centre of rotation, CR is a geometrical centre of rotation of the rotor. The mass of the rotor and imbalance is given by symbol M and located at a distance, ϵ from the geometrical centre of rotation, CR. Movable compensating masses are shown as mass, m at a distance of R from the centre of rotation. Each moveable compensating mass has an angle of θ from the imbalance location. Rotational speed is given by a mathematical symbol of ωt.
Equations of Motion of ABT can be derived from Newton’s Second Laws:
The Equations of Motion of ABT system are given by:
n = number of compensating moveable masses.
x,y = displacement in the X and Y directions.
x,y = velocity in X and Y directions.
x,y = acceleration in X and Y directions.
g = gravity. This only applies if the system is positioned vertically.
Equations 6, 7, and 8 are the Equations of Motion of the ABT system. The equations can be solved numerically to compare the results with and without ABT. A Matlab script in Labview was used to solve the Equations of Motion. Several assumptions were applied to the equations:
Fig. 9 shows a side-by-side comparison of vibration amplitude with and without ABT below critical speed. The vibration amplitude of system with ABT is slightly higher than the one without ABT. Note that we are interested in steady state period where vibration amplitude stabilizes after 6-7 seconds.
Fig. 10 shows a side-by-side comparison of vibration amplitude with and without ABT at critical speed. The average vibration amplitude of system with ABT is slightly lower than the one without ABT. The vibration amplitude shows fluctuations as the countermasses are starting to move to oppose the unbalanced mass.
Fig. 11 shows a side-by-side comparison of vibration amplitude with and without ABT at super critical speed. The countermasses oppose the unbalanced mass. The vibration amplitude of system with ABT is much lower than the one without ABT, closing to zero amplitude. The results are consistent with the theory that was proposed. Testing would be required to validate the simulation results. Test results will be discussed later.
Trolling motors were used to investigate how the ABT system would be able to perform under changing imbalance. Trolling motors were chosen because they represented a simplified wind turbine system. They have comparable vibration characteristics since both systems are suspended in the vertical axis.
The objective of the test was to study the impact that ABT would have on the vibration and power consumption of a trolling motor (Fig. 12).
Specifically, the objectives of the test were to demonstrate the fact that:
Three trolling motors were used for testing purposes:
Two prototype ABT systems were designed to fit on the trolling motor as shown in Fig. 13. Two ABT systems were used to compensate for imbalances in propeller and motor since the trolling motor is long in the axial direction (two-plane balancing). The capacities of the ABT systems were determined by examining the system and calculating linear and moment forces in the unbalanced motors.
The ABT systems were also designed to compensate for imbalance that could be created by damage to the propeller.
Fig. 14. shows damage on the propeller that would create a mass imbalance of 135g-mm, which could lead to severe vibration.
Three test scenarios were used to measure performance improvements using ABT:
The trolling motors were all tested at 1,800 ± 10 RPM to create similar test conditions for comparison purposes.
The following measurements were taken for each test:
Vibration data was measured with single-axis 50g Kistler accelerometers, while DC current readings were obtained using an Extech 80A AC/DC clamp meter.
Power was supplied to the trolling motors using two 12V marine batteries attached in series to produce 24V required to run the trolling motors. The performance of the unbalanced motors running with a prototype ABT system was compared to the factory balanced motor to demonstrate the viability of using the ABT system.
Testing was performed without a propeller installed on the trolling motors to establish a baseline for performance. Figs. 19 to 20 and Table 1 show vibration and current draw values of the two unbalanced motors running with the ABT system and compare them to the data for a factory balanced motor. The prototype ABT system had a great impact on the performance of unbalanced motors and reduced vibration levels by up to 45% when compared to a factory balanced motor. This is a significant achievement because it means the ABT system balances the armature of the electrical motor inside the trolling motor better than the current dynamic balancing method. Another advantage of using the prototype ABT system was that it decreased power consumption of the unbalanced motors by 17% when compared to a factory balanced motor.
Testing with a propeller installed on the trolling motors was completed to show the improvements expected with the ABT system while operating under typical conditions. Figs. 21 to 22 and Table 2 show vibration and current draw values of the two unbalanced motors running with the ABT system and compare them to the data for a factory balanced motor. The propellers designed for these motors are not balanced during manufacturing; this means that the propeller can add an unknown quantity of mass imbalance to the system, and it is not compensated for by the balancing process currently used on the motor armatures. The prototype ABT system had a great impact on the performance of unbalanced motors and reduced vibration levels by up to 29% when compared to a factory balanced motor. The prototype ABT system also decreased power consumption of the unbalanced motors by 19% when compared to a factory balanced motor.
A damaged propeller can cause severe vibration and make it difficult to operate a trolling motor. We simulated damage to the propeller by attaching a mass to the propeller blade to create a mass imbalance of 135g-mm; this is proportional to an imbalance created by damaging a portion of the blade. Figs. 23 to 24 and Table 3 show vibration and current draw values of the two unbalanced motors running with the ABT system and compare them to the data for a factory balanced motor. Vibration levels increased dramatically for these tests due to the simulated damage on the propeller; however, the prototype ABT system was very effective and reduced vibration levels by up to 43%. ABT system also decreased power consumption by up to 17%.
The ABT system has been successfully implemented on a small scale system that represents a simplified version of a wind turbine. A prototype ABT system running on unbalanced motors reduced vibration levels by up to 45% when compared to a factory balanced motor. The results show that trolling motors do not have to be balanced using conventional methods if they are running with the ABT system. This may hold true for wind turbines as well.
The ABT system contains a movable compensating mass that can dynamically adjust itself in situations with changing mass imbalances; this behaviour will provide the following possible benefits for wind turbines application:
The ABT system has a huge potential in this market and can benefit both manufactures and wind turbine operators. Manufacturers can lower costs by reducing the time and effort to balance their wind turbines using conventional methods. Wind turbine operators will gain the benefits of a superior performance, reliability and energy efficiency.