A gearbox is often used in a wind turbine to increase the rotational speed from a low-speed main shaft to a high-speed shaft connecting with an electrical generator.
Gears in wind turbine gearbox are subjected to severe cyclic loading due to variable wind loads that are stochastic in nature.
Thus, the failure rate of gearbox system is reported to be relatively higher than the other wind turbine components.
It is known in wind energy industry that improving reliability of gearbox designs is one of the key points to reduce wind turbine downtime and to make wind energy competitive as compared to fossil fuels.
However, a wind turbine is a complex multi-physics system involving random wind loads, rotor blade aerodynamics, gear dynamics, electrical generator, and control systems.
How to get an accurate prediction of the gearbox lifetime is a challenging issue.
Furthermore, although some studies about wind turbine gear failure modes are carried out, limited studies have been carried out regarding design optimization including the reliability-based design optimization (RBDO) of the gear system considering wind load and manufacturing uncertainties.
In order to address the essential and challenging issue on design optimization of wind turbine gearbox under wind load and gear manufacturing uncertainties, three contributions have been made in this chapter:
(1) development of an efficient numerical procedure for gear dynamics simulation of complex multibody gear system based on the multivariable tabular contact search algorithm to account for detailed gear tooth contact geometry with profile modifications or surface imperfections;
(2) development of an integrated multibody dynamics computational framework for deterministic design optimization (DDO) and RBDO of the wind turbine gearbox using the gear dynamics simulation method developed in (1) and incorporating pitting gear tooth contact fatigue model, a dynamic wind load uncertainty model, and a wind turbine aerodynamic model using FAST;
and (3) development of the DDO and RBDO of a wind turbine gearbox to minimize total weight while ensuring 20-year lifetime considering dynamic wind load and gear manufacturing uncertainties.
References
Alemayehu FM, Osire SE (2015) Probabilistic performance of helical compound planetary system in wind turbine. ASME J Comput Nonlinear Dyn 10:1–12Google Scholar
Aydin E (2013) Determination of best drive train technology for future onshore wind turbines as a function of the output power. Master Thesis, Eindhoven Technical UniversityGoogle Scholar
Barbieri M, Scagliarini G, Bonori G, Pellicano F, Bertccchi G (2008) Optimization methods for spur gear dynamics. In: Proceedings of EUROMECH nonlinear dynamics conference, Saint PetersburgGoogle Scholar
Budynas R, Nisbett K (2008) Shigley’s mechanical engineering design, 9th edn. McGraw-Hill, New YorkGoogle Scholar
Cardona A (1997) Three-dimensional gears modeling in multibody systems analysis. Int J Numer Methods Eng 40:357–381CrossRefGoogle Scholar
Choi Y, Liu CR (2006a) Rolling contact fatigue life of finish hard machined surfaces, part 1. Model development. Wear 261:485–491CrossRefGoogle Scholar
Choi Y, Liu CR (2006b) Rolling contact fatigue life of finish hard machined surfaces, part 2. Experimental verification. Wear 261:492–499CrossRefGoogle Scholar
Choi Y, Liu CR (2006c) Rolling contact fatigue life of finish hard machined surfaces. Wear 261:485–491CrossRefGoogle Scholar
Cornell RW (1981) Compliance and stress sensitivity of spur gear teeth. ASME J Mech Des 103:447–459CrossRefGoogle Scholar
Crossland B (1970) The effect of pressure on the fatigue of metals. In: Pugh H (ed) Mechanical Behaviour of Materials Under Pressure. Elsevier, London, pp 299–354Google Scholar
Dang VK (1973) Sur La Resistance a La Fatigue des Metaux. Sci Tech Armem 47:647–722Google Scholar
Dong W, Xing Y, Moan T, Gao Z (2013) Time domain-based gear contact fatigue analysis of a wind turbine drivetrain under dynamic conditions. Int J Fatigue 48:133–146CrossRefGoogle Scholar
Ebrahimi S, Eberhard P (2006) Rigid-elastic modeling of meshing gear wheels in multibody systems. Multibody Sys Dyn 16:55–71CrossRefGoogle Scholar
Ghribi D, Bruyere J, Velex P, Octrue M, Mohamed H (2012) Robust optimization of gear tooth modifications using a genetic algorithm. In: Condition monitoring of machinery in non-stationary operations. Springer, Berlin, pp 589–597Google Scholar
Glodez G, Flasker J, Ren Z (1997) A new model for the numerical determination of pitting resistance of gear teeth flanks. Fatigue Fract Eng Mater Struct 20(1):71–83CrossRefGoogle Scholar
Guo Y, Keller J, LaCava W (2012a) Combined effects of gravity, bending moment, bearing clearance, and input torque on wind turbine planetary gear load sharing. NREL/CP-5000-55968Google Scholar
Guo Y, LaCava W, Xing Y, Moan T (2012b) Determining wind turbine gearbox model complexity using measurement validation and cost comparison. NREL/CP-5000-54545Google Scholar
Guo Y, Bergua R, Dam JV, Jove J, Campbell J (2015) Improving wind turbine drivetrain designs to minimize the impacts of non-torque loads. Wind Energy 18:2199–2222CrossRefGoogle Scholar
Haug EJ (1989) Computer aided kinematics and dynamics of mechanical systems. Ally and Bacon, BostonGoogle Scholar
Helsen J, Vanhollebeke F, Marrantb B, Vandepitte D, Desmet W (2011) Multibody modelling of varying complexity for modal behaviour analysis of wind turbine gearboxes. Renew Energy 36:3098–3113CrossRefGoogle Scholar
Houser DR, Bolze VM, Graber JM (1996) A comparison of predicted and measured dynamic and static transmission error for spur and helical gear sets. In: SEM 14th international modal analysis conference, DearbornGoogle Scholar
Hu W, Choi KK, Zhupanska O, Buchholz J (2016a) Integrating variable wind load, aerodynamic, and structural analyses towards accurate fatigue life prediction in composite wind turbine blades. Struct Multidiscip Optim 53:375–394MathSciNetCrossRefGoogle Scholar
Hu W, Choi KK, Cho H (2016b) Reliability-based design optimization of wind turbine blades for fatigue life under dynamic wind load uncertainty. Struct Multidiscip Optim 54:953–970CrossRefGoogle Scholar
International Organization for Standardization (2005) Wind turbines part 1: design requirements, IEC 61400-1:2005. ISO, GenevaGoogle Scholar
Jiang B, Zheng X, Wang M (1993) Calculation for rolling contact fatigue life and strength of case-hardened gear materials by computer. J Test Eval 21:9–13CrossRefGoogle Scholar
Jonkman JM, Buhl ML (2005) FAST user’s guide. Technical Report NREL/TP-500-38230Google Scholar
Kahraman A (1994) Planetary gear train dynamics. ASME J Mech Des 116:71–720Google Scholar
Kapelevich A, Shekhtman Y (2009) Gear tooth fillet profile optimization. Gear Solut 9:63–69Google Scholar
Karagiannis I, Theodossiades S, Rahnejat H (2012) On the dynamics of lubricated hypoid gears. Mech Mach Theory 48:94–120CrossRefGoogle Scholar
Kato M, Deng G, Inoue K, Takatsu N (1993) Evaluation of the strength of carburized spur gear teeth based on fracture mechanics. Bull Japan Soc Mech Eng Ser C 36:233–234Google Scholar
Keer LM, Bryant MD (1983) A pitting model for rolling contact fatigue. ASME Tribol 105:198–205Google Scholar
Kin V (1994) Computerized analysis of gear meshing based on coordinate measurement data. ASME J Mech Des 116:738–744CrossRefGoogle Scholar
Lee CH, Bae DS, Song JS (2012) Multibody approach of gear transmission error dynamics. In: Proceedings of Asian conference on multibody dynamics, ShanghaiGoogle Scholar
Lesmerises A, Crowley D (2013) Effect of different Workscope strategies on wind turbine gearbox life cycle repair costs. Int J Prognos Health Manag 17:1–7Google Scholar
Li H, Terao A, Sugiyama H (2015) Application of tabular contact search method to multibody gear dynamics simulation with tooth surface imperfections. IMechE J Multibody Dyn 229:274–290Google Scholar
Li H, Sugiyama H, Cho H, Choi KK, Gaul NJ (2016) Numerical procedure for design optimization of wind turbine drivetrain using multibody gear dynamics simulation considering wind load uncertainty. In: Proceedings of the ASME 2016 IDETC & CIE conference, Charlotte, DETC2016-59654Google Scholar
Li H, Cho H, Sugiyama H, Choi KK, Gaul NJ (2017) Reliability-based design optimization of wind turbine drivetrain with integrated multibody gear dynamics simulation considering wind load uncertainty. Struct Multidiscip Optim 56:183–201CrossRefGoogle Scholar
Litvin FL, Fuentes A (2004) Gear geometry and applied theory, 2nd edn. Cambridge University Press, CambridgeCrossRefGoogle Scholar
Liu J, Zenner H (2003) Fatigue limit of ductile metals under multiaxial loading. Biaxial Multiaxial Fatigue Fract 31:147–164Google Scholar
Maatar M, Velex P (1997) Quasi-static and dynamic analysis of narrow-faced helical gears with profile and lead modifications. ASME Mech Des 119:474–480CrossRefGoogle Scholar
Madhusekhar D, Madhava Reddy K (2014) Reliability based optimum design of a gear box. Int J Eng Res Appl 4(10):1–8Google Scholar
Melchers RE (1999) Structural reliability analysis and prediction. Wiley, ChichesterGoogle Scholar
Milburn A (2011) Wind turbine gearbox wear and failure modes and detection methods. NREL Wind Turbine Condition Monitoring WorkshopGoogle Scholar
Mohammadpour M, Theodossiades S, Rahnejat H (2014) Multiphysics investigations on the dynamics of differential hypoid gears. ASME J Vib Acoust 136:1–3CrossRefGoogle Scholar
Newman CJ (1992) Small-crack test method. ASTM STP 1149:6–33Google Scholar
Osman T, Velex P (2011) A model for the simulation of the interactions between dynamic tooth loads and contact fatigue in spur gears. Tribol Int 46:84–96CrossRefGoogle Scholar
Oyague F (2009) Gearbox modeling and load simulation of a baseline 750-kW wind turbine using State-of-the-Art simulation code. NREL/TP-500-41160Google Scholar
Ozguven HN, Houser DR (1988) Mathematical models used in gear dynamics – a review. J Sound Vib 121:383–411CrossRefGoogle Scholar
Palermo A, Mundo D, Hadjit R, Desmet W (2013) Multibody element for spur and helical gear meshing based on detailed three-dimensional contact calculations. Mech Mach Theory 62:13–30CrossRefGoogle Scholar
Parker RG, Agashe V, Vijayakar SM (2000) Dynamic response of a planetary gear system using a finite element/contact mechanics model. ASME J Mech Des 122:305–311CrossRefGoogle Scholar
Peeters JLM, Vandepitte D, Sas P (2005) Analysis of internal drive train dynamics in a wind turbine. Wind Energy 9:141–161CrossRefGoogle Scholar
Piegl LA, Tiller W (1996) The NURBS book. Springer, New YorkzbMATHGoogle Scholar
Qin D, Wang J, Lin TC (2009) Flexible multibody dynamics modeling of a horizontal wind turbine drivetrain system. ASME J Comput Nonlinear Dyn 131:1–8Google Scholar
Sainsot P, Velex P (2004) Contribution of gear body to tooth deflections- a new bi-dimensional analytical formula. ASME J Mech Des 126:748–752CrossRefGoogle Scholar
Sansalvador RL, Jauregui JC (1993) Practical optimization of helical gears using computer software. Gear Technol 10:16–21Google Scholar
Shabana AA (2010) Computational dynamics, 3rd edn. Wiley, ChichesterCrossRefGoogle Scholar
Sheng S (2012) Wind turbine gearbox condition monitoring round robin study. NREL/TP-5000-54530Google Scholar
Sheng S, McDade M, Errichello R (2011) Wind turbine gearbox failure modes-a brief. In: Proceedings of ASME/STLE 2011 international joint tribology conference, Los AngelesGoogle Scholar
Shikin EV, Plis AI (1995) Handbook on splines for the user. CRC Press, Boca RatonzbMATHGoogle Scholar
Spitas V, Spitas C (2007) Optimizing involute gear design for maximum bending strength and equivalent pitting resistance. Proc IMechE 221:479–488CrossRefGoogle Scholar
Straffelini G, Molinari A, Marcupuscas T (2000) Identification of rolling-sliding damage mechanisms in porous alloys. Metall Mater Trans A 31:3091–3099CrossRefGoogle Scholar
Sundaresan S, Ishii K, Houser DR (1991) A procedure using manufacturing variance to design gears with minimum transmission error. ASME J Mech Des 113:318–324CrossRefGoogle Scholar
Tallian TE (1983) Rolling contact fatigue. SKF Ball Bear J 217:5–13Google Scholar
Tavakoli MS, Houser DR (1986) Optimum profile modifications for the minimization of static transmission errors of spur gears. ASME J Mech Trans Auto Des 108:86–94CrossRefGoogle Scholar
Vanhollebeke F, Peeters P, Helsen J, Lorenzo ED, Manzato S, Peeters J, Vandepitte D, Desmet W (2015) Large scale validation of a flexible multibody wind turbine gearbox model. ASME J Comput Nonlinear Dyn 10:1–12Google Scholar
Veers PS, Winterstein SR (1998) Application of measured loads to wind turbine fatigue and reliability analysis. ASME J Solar Energy Eng 120:233–239CrossRefGoogle Scholar
Velex P, Bruyere J, Houser DR (2011) Some analytical results on transmission errors in narrow-faced spur and helical gears: influence of profile modifications. ASME J Mech Des 133:1–11CrossRefGoogle Scholar
Vijayakar S (1991) A combined surface integral and finite element solution for a three-dimensional contact problem. Int J Numer Methods Eng 31:525–545CrossRefGoogle Scholar
Wang J, Li R, Peng X (2003) Survey of nonlinear vibration of gear transmission systems. ASME Appl Mech Rev 56:309–329CrossRefGoogle Scholar
Yu CJ (1998) Design optimization for robustness using quadrature factorial models. Eng Optim 30:203–225CrossRefGoogle Scholar
Zhang Y, Litvin FL, Maruyama N, Takeda R, Sugimoto M (1994) Computerized analysis of meshing and contact of gear real tooth surfaces. ASME J Mech Des 116:738–744CrossRefGoogle Scholar
Ziegler P, Eberhard P (2009) An elastic multibody model for the simulation of impacts on gear wheels. In: Proceedings of ECCOMAS thematic conference on multibody dynamics, WarsawGoogle Scholar
Holding on at 300 feet: A day in the life of a wind turbine worker
Fixing wind turbines isn’t an easy job. Being up in the air at higher than 300 feet, changing filters or greasing bearings on a wind turbine can bring about challenges. This job isn’t for the faint of heart.
Cameron Short, maintenance technician at Enbridge Inc., loves his job, because it isn’t the same thing every day. “Even though you’re doing the same job, you’re basically in a different office every day,” he says. He adds that maintaining these turbines is an essential part of their lifespan. “It’s just like a vehicle, if you don’t change the oil or filters in your car – just doing preventative maintenance – it’s not going to run good,” Cameron says. “It’s the same thing with the wind turbines. As long as you’re taking care of it properly, it’s going to last.”
In 2014, wind energy was responsible for about 24 percent of the new sources of electricity generated in the U.S., according to a U.S. Department of Energy report. Curious about how much power these turbines bring to the table? If you had a one megawatt turbine on land, it could potentially power about 300 homes, according to the Wind Energy Foundation. 3M provides wind turbine owners with a variety of product options. These products are designed to enhance reliability, improve performance and provide protection against weathering and harsh environments. For Cameron, that’s a big part of the job, but not the only reason he loves what he does.
“The biggest thing I like about working on wind turbines is being able to be outside at 300 feet and being able to take a quick break to sit back and take in the scenery.”
CAMERON SHORT -- MAINTENANCE TECHNICIAN, ENBRIDGE, INC.
how an electric motor works
https://www.youtube.com/watch?v=CWulQ1ZSE3c < How does an Electric Motor work? (DC Motor) < VIDEO Jared owen animator
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How Electric Motors Work
December 1, 2002 for QuietFlyer Magazine
Much has been written about choosing the right motor, estimating performance, installing the motor in your plane, and so on.
This month, I’ve decided to go back to basics and describe how the motor actually works. Do you need to know this to fly electric models? Probably not, but a good understanding of the functioning of a motor can help you diagnose problems. And some people, myself included, like to know how everything works. So, if you’re interested, read on!
I’m going to start with the very basics, so if you already know some of it, feel free to skip ahead. I won’t be offended.
Magnets
The fundamental driving force behind all electric motors, whether brushed or brushless, AC or DC, is magnetism. We’ve probably all played with magnets at some time or other, and have learned about them in science class in elementary school.
Recall that any magnet has a north pole and a south pole (it just so happens that the earth is a magnet whose poles happen to correspond very roughly to the geographical poles, hence the names for the magnet’s poles). If you take two bar shaped magnets and line them up, they will be attracted to one another if one’s north pole is next to the other’s south pole. If you line them up north to north or south to south, they will repel each other. Opposites attract.
Consider an assembly of three magnets, as shown in Figure 1. The left and right hand magnets are fixed to some surface, and the center magnet is free to rotate about its center.
Figure 1. The central rotating magnet will turn until it is aligned with the two fixed magnets, north pole to south pole.
Because of the attraction of opposite poles, the center magnet will rotate until it is aligned as in Figure 2.
Figure 2. Once aligned, it will resist being turned further.
Because the magnet has weight, and thus momentum, it would actually overshoot slightly, and then come back, overshoot again, and so on a few times before settling down.
Now, imagine we could work some magnetic magic and swap the center magnet’s north and south poles just as it overshoots the first time, as shown in Figure 3.
Figure 3. If we magically reverse the poles of the central magnet just before it comes to rest, it will keep turning.
Instead of coming back, it would now be repelled by the fixed magnets, and keep turning so it can align itself in the other direction. Eventually, it would reach the state in Figure 4, which looks suspiciously like Figure 1.
Figure 4. Eventually, it will get back into the position it started from in Figure 1.
If we perform this pole-swapping every time the center magnet just finishes overshooting the aligned position, it would keep turning forever.
The problem is how to perform this feat of magnetic motion.
Electromagnets
The magnets we play with are called permanent magnets. These objects have a fixed magnetic field that’s always there. The poles are fixed relative to one another and relative to the physical magnet.
Another kind of magnet is the electromagnet. In its simplest form, this consists of an iron bar, wrapped in a coil of wire, as in Figure 5.
Figure 5. An electromagnet is just a piece of iron or other magnetic metal with a wire coil wrapped around it.
By itself it does nothing. However, if you pass an electric current through the wire, a magnetic field is formed in the iron bar, and it becomes a magnet, as in Figure 6.
Figure 6. Applying current in one direction will produce a magnet.
If you turn off the current, it stops being a magnet (that’s a bit of a simplification, since in reality, it ends up remaining a weak magnet, but we needn’t concern ourselves with that for the moment).
So far, the electromagnet already seems quite useful, since we can use it to pick up iron, steel, or nickel objects, carry them somewhere, and then drop them by just turning off the power (wrecking yard cranes do this with entire automobiles).
The really interesting thing about an electromagnet is that its polarity (the location of the north and south poles) depends on the direction of current flow. If we pass the current through in the opposite direction, the electromagnet’s poles will be reversed, as shown in Figure 7.
Figure 7. Applying current in the opposite direction will produce a magnet with opposite polarity.
Eureka!
If we replace the central magnet in our set of three magnets with an electromagnet, as in Figure 8, we have the beginnings of an electric motor.
Figure 8. Replacing the central magnet in Figure 1 with an electromagnet gives us the beginnings of a motor.
Now we have two problems to solve: feeding the current to the rotating electromagnet without the wires getting twisted, and changing the direction of the current at the appropriate time.
Both of these problems are solved using two devices: a split-ring commutator, and a pair of brushes. Figure 9 illustrates these.
Figure 9. By adding a commutator (the semi-circular arcs) and brushes (the wide arrows), we can change the polarity of the electromagnet as it turns.
The two semicircles are the commutator, and the two arrows are the brushes. The current is applied to the brushes, indicated by the "+" and "-" signs.
With the current as shown, the electromagnet will be repelled by the two permanent magnets, and it will turn clockwise. After it has turned almost half way around, it will be in the state shown in Figure 10.
Figure 10. The magnets are almost aligned, but soon, the polarity will reverse, sending the rotating electromagnet on its way around once again.
Then, just as the magnet reaches the aligned state, the split in the commutator passes under the brushes, and then the current through the electromagnet reverses, which takes us back to the condition in Figure 9. As a result, the magnet keeps turning. We have a motor!
Some Terminology
The discussion above has culminated in the design of a simple two-pole, two-slot, permanent magnet, brushed, direct-current (DC) motor.
The term two-pole refers to the fact that there are two permanent magnet poles involved in the operation of the motor, the south pole of the left hand magnet and the north pole of the right hand magnet. The motor would actually work with only one fixed magnet (for example, only the left hand magnet), but would be less powerful and efficient.
The rotating electromagnet is known as the armature. Two-slot means that the armature consists of a single coil of wire around a single bar with only two ends (the term "slot" refers to the gap between the armature ends, since the armature is not typically bar shaped, but has a wider end).
Real Motors
In a real two-pole motor, the two poles are often the two ends of the same magnet. Although the motor may appear to contain two separate magnets, the steel motor case ties them together to act as a single magnet. It’s really as if our motor were built like in Figure 11, with the rotating electromagnet inside a hole in the permanent magnet.
Figure 11. In many motors, the two fixed magnets are really one the two poles of what is effectively one magnet (although it may be made up of two separate magnets connected by the motor housing).
Practical real motors usually have at least a three-slot armature, and a commutator with three segments. There are however still only two brushes. Higher voltage and higher efficiency motors have even more slots (an odd number) and more segments on the commutator (the same as the number of slots), and more brushes (always an even number). Photos 1 and 2 show the armature, commutator, and brushes from a typical low-cost three-slot motor.
Photo 1. This is a three-slot armature from an inexpensive 540-sized ferrite "can" motor.
Photo 2. The brushes in a "can" motor are held in place by alloy leaf springs that also serve to carry current. The commutator has been simulated with a piece of dowel with some markings on it to better show how it mates with the brushes.
Figure 12 illustrates a three-slot motor in conceptual form. Notice that the brush is now wider, contacting the commutator segments over a wider area, and actually spanning two segments sometimes.
Figure 12. This is a schematic representation of a typical three-slot two-pole brushed motor. The armature has three electromagnets, and three commutator segments. The brushes sometimes contact more than one segment.
Also notice that both ends of electromagnet number 2 are contacting the "-" brush at the particular point in time captured by Figure 12. This means that no current is flowing through electromagnet 2, and only number 1 and 3 are on.
Effectively, the armature is now a pair of electromagnets; number 3 is being attracted by the north pole of the right hand permanent magnet, and number 1 is being repelled.
One twelfth of a turn later, as in Figure 13, all three electromagnets have current flowing through them.
Figure 13. The same motor as in Figure 12, one twelfth of a rotation (30 degrees) later.
Now, electromagnet number 1 is being both repelled by the right hand permanent magnet, and attracted by the left hand one. Number 2 is being repelled by the left magnet, and number 3 is still being attracted by the right magnet.
Another twelfth of a turn later, in Figure 14, electromagnet 1 is being attracted to the left hand magnet, and number 2 is still being repelled.
Figure 14. The motor from Figure 12, one sixth of a rotation (60 degrees) later.
Electromagnet 3 is turned off. This progression of electromagnets switching on and off continues as the motor turns, eventually returning to the state of Figure 12.
The Brushless Motor
There are a number of drawbacks to the brush and commutator mechanism used in a brushed motor: the brushes cause friction, there is some electrical resistance in the brush-to-commutator interface, and the mechanical switching of the armature current results in sparking, which can cause radio interference. Brushless motors do away with the brushes and commutator to get around these problems. The result is greater efficiency (more output power for a given amount of input power), and less electrical interference.
The basic principles by which a brushless motor operates are exactly the same as those of a brushed motor. Figures 15 and 16 show two stages in the operation of a simple brushless motor.
Figure 15. This is the brushless motor equivalent of Figure 9. The electromagnets are fixed, and the permanent magnet rotates.
Notice that Figure 15 is almost identical to Figure 9, except that there are no brushes and no commutator, and the types of the magnets have been exchanged. The permanent magnets have become electromagnets, and vice versa. The rotating permanent magnet is being repelled by the two electromagnets.
Figure 16. The motor from Figure 15, almost a full turn later. Notice that the electromagnets have changed their polarity.
In Figure 16, almost a full turn later, the polarity of the left and right hand magnets has changed. The rotating magnet is now being pulled into alignment.
The problem to be solved here is how to cause the electromagnets to reverse their polarity at the right time. One could devise some sort of mechanical scheme controlled by the rotating permanent magnet, but this would nullify the main benefits of brushless motors.
Instead, the electromagnets are controlled by external circuitry. This circuitry monitors the current position of the rotating magnet, and energizes the external magnets appropriately to keep the motor turning. This circuitry is part of the brushless electronic speed control (ESC).
There are two ways for a brushless ESC to monitor the position of the rotating magnet. One is by way of magnetic sensors (based on the Hall-effect). These sensors report back to the ESC through a separate set of wires. The other method is known as "sensorless". Roughly, in this method the ESC monitors the three motor power wires for fluctuations caused by the spinning magnets.
Brushless Terminology
Since the electromagnet assembly in a brushless motor remains stationary, it is called a stator instead of an armature. The rotating magnet assembly is called the rotor.
Real Brushless Motors
Just as a real brushed motor rarely has only two poles and a two-slot armature, a real brushless motor rarely has only a two-pole rotor and a two-slot stator. Most commercially available brushless motors have at least four poles, and a nine or more slot stator. However, for purposes of comparison, Figure 17 illustrates a hypothetical two-pole three-slot brushless motor, corresponding to our two-pole three-slot brushed motor.
Figure 17. This is a schematic representation of a hypothetical three-slot two-pole brushless motor. The rotor has one permanent magnet (two poles), and the stator has three electromagnets (three slots) and three connection points.
Notice there are three connection points to receive power from the brushless ESC (a motor with more than three stators has them wired in three groups, so there are still only three power leads).
Photo 3. The components of an Aveox 36/30/1.5 brushless motor. www.Aveox.com
In the state represented by Figure 17, power is being applied to the two leads labeled "+" and "-", which energizes the electromagnets as shown. The upper left electromagnet is attracting the rotor’s north pole, the lower left one is repelling it, and the right hand electromagnet is repelling the rotor’s south pole. As the rotor turns, the ESC will change which leads have power applied to them. Sometimes only two leads will, as in Figure 17, and at other times all three leads will (just like in Figure 13 for a brushed motor).
Real World Issues
The theory of motor operation described here is correct, but somewhat simplified. If you examine the diagrams closely, you’ll notice situations where the polarity might reverse too soon, apparently causing the motor to stop. Because of a number of factors, such as the time it takes for the magnetic field to collapse, and the momentum of the armature, a real motor won’t necessarily stop in this situation.
The relationship between the position of the armature (or rotor) and magnets (or stator), and the time that the electromagnets change their polarity, is known as "timing". In a brushed motor, it is adjusted by repositioning the brushes relative to the permanent magnets. In a Hall-effect sensored brushless motor, it is the sensors that are repositioned. In a sensorless motor, the ESC adjusts the timing automatically based on the feedback it is getting from the motor.
The optimal timing depends on motor speed and current, and for maximum efficiency, should be adjusted for the particular operating condition of the motor.
If you are familiar with internal combustion engines, this is similar to setting optimal spark plug timing. Theoretically, the plug should fire when the piston reaches the top of the cylinder (top dead center), but due to engine momentum and the time it takes for the fuel to actually burn, the plug must fire sooner. Modern car engines adjust this electronically to precisely suit the conditions; older car engines used a vacuum driven advance mechanism to adjust it according to engine load.
Other Motors
There are many other types of electric motors, such as AC induction motors, AC synchronous motors, stepper motors (really a specialized form of brushless motor), and so on. All of these motors operate on variations of the principles we’ve looked at. They differ only in how they perform the job of the commutator. Currently, none of these other types of motors are used in electric flight.
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