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  1. 1. Electrical Characteristics of Bicycle DynamosSummaryBicycle Dynamos are alternators equipped with permanent magnets. They are typically claw-pole generators and deliver energy at rather low rpm.The sidewall-running bottle dynamo used to be the most popular type for many years, but todaythe hub dynamo has taken over. There are a few other types (spoke dynamo, roller dynamo) butthose gained limited popularity.This page focuses on the electrical characteristics of a typical bicycle dynamo and the maximumpower that can be extracted from it.Dynamo PowerIn Europe, most bicycle dynamos are rated 3W (that is 500mA at 6V). For sure, the power of adynamo light system depends somewhat on the speed. In a standard light system, the power iszero at standstill, at moderate speed its 3W and at very high speed, its just a little bit more than3W. Some manufacturers include limiting zener diodes or other forms of overvoltage protection,to make sure the voltage doesnt rise to levels that kill the light bulb(s).Test UnitsIve tested 3 dynamos (from left to right): Busch + Müller Dymotec6, AXA HR and a cheap onefrom my local supermarket. 1. The B&M Dymotec6 has the best mechanics. It runs very well on the side of the tire. This dynamo is often found on high-quality touring bikes. I paid EUR 24.90 for this (yr 2004).
  2. 2. 2. The AXA HR has the strongest magnets, it delivers the highest current. It contains 2 zener diodes (BZX 85C 7V5) in series, to limit the output voltage. I had to open the glued plastic enclosure in order to remove these diodes for the measurements. This dynamo is often found on branded bikes. I paid EUR 16.99 for the AXA HR. 3. The cheapy from the local supermarket has a magnetic performance slightly inferiour to the Dymotec6. The mechanics are not designed for heavy use but it received all required approvals. Its held together by two screws and can be completely disassembled. This dynamo is usually stock on supermarket bikes. Riders who rarely ride in the dark may like this unit at a bargain price of just EUR 3.45ToolsFor measuring the electrical characteristics of bicycle dynamos, Ive created this instrument. Itmeasures the speed of a bike based on the rpm of the dynamo: This equiment is built from a Panasonic flashlight and a Trelock FC 404 bicycle computer. It includes additional circuitry (see below) to slice the AC voltage from the dynamo and divide it to a lower frequency that the bicycle computer can handle. Initially, I designed this with a cheap no-name bicycle computer. I soon found that this does some terrible rounding at higher speeds. I switched to a branded item, the Trelock FC 404. It displays the decimals of the speed and it does so correctly. To properly set the wheel circumference in the bicycle computer, its necessary to know the diameter of the dynamo wheel as well as the number of magnet poles. Most sidewall dynamos have 8 poles - you can feel 8 steps per rotation of the wheel (and measure 4 full sine waves per rotation). Refer to the formula below.
  3. 3. Other tools used for the measurements below are multimeters, oscilloscope, a lab power supplyand an adjustable current sink based on the LM317T linear regulator IC.MeasurementsMaximum Power of different DynamosSetup: Dynamo, voltage doubler (Greinacher type using 2x 1N5818 & 2x 1000uF), current sink(variable from 100 to 250 mA). Speedmeter connected to dynamo.Procedure: Run the dynamo at 15 km/h. Measure the voltage across the current sink for currents100, 130, 160, 190, 220, 250 mA. Repeat at 40 km/h. Do this for each of the 3 dynamos.Calculate power from voltage and current. Plot power vs load current.
  4. 4. Observation: AXA HR delivers maximum power at 200 mA load current (note this is aftervoltage doubling !), B&M Dymotec6 at 180 mA, the cheapy at 160 mA. The absolute powerlevels show the AXA HR on top and the cheapy on the bottom at any speed. Conclusion: Themaximum power is achieved at a specific load current. This depends little on the speed, butmostly on the dynamo. In other words: A dynamo is a current source.Power vs Speed of Dymotec6I chose to do this measurement with the Busch & Müller Dymotec6. The nature of the curve issimilar for other dynamos, only they would meet a slightly higher or lower power.Setup: Dymotec6, voltage doubler (Greinacher type using 2x 1N5818 & 2x 1000uF), 180 mAcurrent sink. Speedmeter connected to dynamo.Procedure: Run the dynamo at 4, 5, 7, 9, 12, 15, 19, 24, 31, 40, 50 km/h and measure the voltageacross the current sink. Plot for each speed the calculated power = measured voltage * 180 mAcurrent.
  5. 5. Conclusion: With perfect matching of the load, Dymotec6 delivers 2.7 W at moderate speed, 5W at high speed and 6 W at really high speed. This is what can be achieved without modificationto the dynamo.Question: Why does a standard 3 W light system with the Dymotec6 not burn out the bulbs at 50km/h ? This is because at such high speed, the matching of the load is off (current is too high) sothat it cant drain maximum power.Question: Where goes the lost energy when the load doesnt drain the maximum possible power ?Its not lost. The dynamo spins with less effort. Try and short-circuit the dynamo at full speed,the current of the driving motor will drop a lot.Temperature Performance of Dymotec6As the dynamo runs for extended periods of time, its temperature increases. This test runs theB+M Dymotec6 at 50 km/h and 23ºC ambient temperature. The Greinacher doubler circuit (2x1N5818 & 2x 1000uF) and a 180 mA load is connected. The power into the load is beingmonitored. The experiment is stationary on the workbensch, so theres no draught to cool thedynamo. After around 20 minutes, the dynamo output power has decreased from an inital 100%to 80%. For another 10 minutes, it doesnt drop any further. The temperature on the casing of the
  6. 6. dynamo measures 89ºC at the end of these 30 minutes. Its considerably hotter inside.Continuing this experiment, a standard 80mm computer fan is mounted to provide some coolingas it would be on a moving bike. The power increases and eventually stabilizes at 89% of thestart value. The temperature stabilizes at around 40ºC.The Dymotec6 was chosen for this experiment as it has the best mechanical design of all testeddynamos. It doesnt suffer any noticable deterioration after running at 50 km/h and deliveringwell above 5 W for 2 hours. A lot of dynamos dont withstand this. Bearings and their lubricationsuffer from the high internal temperature, resulting in rapid wear. Once the rotating magnetcomes into contact with the stator, friction and internal temperature dramatically increase, thedynamo body melts and eventually the rotor jams. In my setup, this just tears the couplingbetween motor and dynamo while on a real bike, the dynamo wheel may pop off so that the rotorshaft causes damage to tire, rim or spokes. Bottom line: Avoid using a cheap dynamo to assistthe brakes on long descents. Horsepower Design Equations and Formulas Calculator Automobile - Car - Truck Solving for torque.
  7. 7. This calculation corrects horsepower for normally aspirated internal combustion engine to a standard humidity, pressure and temperature after mesaured on a dynamometer.Voltage-E=pi*z*N*P/60*Ahere pi=magnetic field density/area of magnetic fieldz=no of conductorsN=speed in rpmA=no of parallel path(=2 for wave &=P for lap windings)P=no of polesPower-P=tau*2*Pi*omegawhere tau=torqueand omega=rotational speed rads^-1In case of dynamos ,the output is generally in terms of power.If you calculate output power thenyou can know the energy by multiplying it with time. Calculation of power: In ideal conditions i.e no losses, Output electrical energy=Input mechanical energy.But wemust consider the losses that occur in a dynamo(or generator).The losses that occur are iron orcore losses and copper losses.Iron losses are of two types1)Hysteresis losses(in watts)=khBfv2)Eddy current losses(in watts)=keB2f2t2kh=hysteresis constantke=eddy current constant
  8. 8. B=magnetic flux densityf=frequencyt=thicknessv=volumeCopper losses:Ise2R+Ish2R(Series field copper losses+shunt field copper losses)Apart from these losses there are armature losses and mechanical losses.Armature losses:Ia2RBasically we calculate output power by subtracting the losses from the input power. Input power=output power+losses=> Output power=input power-losses=>Output power=input power - Ia2Ra - Ise2R - Ish2R - mechanical losses-other lossesBut for converting mechanical to electrical energy without using field winding,then the equationwill be: Output power=input power - Ia2Ra - mechanical losses - other lossesCalculate the output power and multiply it with time(considering power independent of time) toget the energy produced.Ia can be calculated from the following equation E=IaRa + VL + BCD (BCD=brush contact drop negligible) VL=load voltage Ra=armature resistance E=emf produced=oNZP/60A o = flux due to rotation N=speed in r.p.m Z=number of armature conductors P=number of poles A=number of parallel paths
  9. 9. Solving for forceHow to calculate how much watts and voltage a crude dynamo will generate?I want to calculate amount of electricity generated by an AC dynamo, given the width andlength or copper wire used as coil and size and power of the magnet.AC is generated by generators, not dynamos; a dynamo is a DC generator. Second problem: itdepends on the speed of rotation of the rotor, among other things. Now to some basic physics:a standard commercial AC generator consists of a rotor, carrying a constant magnetic field; thefield may arise from permanent magnets or from an electromagnet, powered by a separate DCgenerator (called an exciter). The fixed part of the machine has copper windings, in whichvoltage is induced by the turning of the rotor. The voltage is proportional to the speed ofrotation, the strength of the magnetic field, the number of turns of wire, and the area of theloops through which the magnetic field passes. The available current depends on the size of thewire, but is also limited by the torque available to turn the rotor. Some typical values: themagnetic field of the rotor is usually on the order of 0.3 Tesla or so. For AC use for commercialpower generation, 3600 rpm is the usual speed. For 20 amperes out, 12 gauge wire can be usedif you have forced-air cooling. Typical output voltages for commercial use range from 4160 to13,200 or more. The fundamental relationship: a one-turn coil of one square meter, throughwhich passes a magnetic field changing at the rate of one Tesla per second, will show an open-circuit voltage of one volt.Handy Formula to calculate the output of any generator at any given RPM…….First off 3 thing must be known… RPM, Open voltage at that RPM and the Ohmsof the stator coil. 1. Measure the RPM 2. Measure the Open voltage at that RPM
  10. 10. 3. Measure the Ohms of the stator coil.Measured RPM / Open volts = RPM per voltTo find a Desired output the formula is:Volts + ( Amps * Ohms ) = Open Voltage ( necessary to achieve this output )Open voltage * RPM per volt = RPM needed to achieve desired outputExample: Alternator spinning at 1500 RPM delivers an open voltage of 34.8 voltsso….1500 / 34.8 = 43.1 RPM per voltThe stator coil reading is .6 ohmLets say we would like 14.6 volts at 10 amps from our unit14.6 volts + ( 10 amps * .6 ohm ) =20.6 open voltage20.6 * 43 rpm per volt = 885.8 RPMIf you would like to know an output at a certain RPM you simply change theformula to:RPM / RPM per volt =Open Voltage(OpenVoltage-desiredvoltage) /ohms = AmpsExample: 885 RPM at 14.6 volts885 / 43 = 20.58( 20.58 - 14.6 ) / .6 = 9.97 ampsAnd there you have it… since, for the most part, the voltage and rpm are aconstant its easy to calculate the output of any unitDynamo
  11. 11. Main article: DynamoDynamos are no longer used for power generation due to the size and complexity of thecommutator needed for high power applications. This large belt-driven high-current dynamoproduced 310 amperes at 7 volts, or 2,170 watts, when spinning at 1400 RPM.Dynamo Electric Machine [End View, Partly Section] (U.S. Patent 284,110)The dynamo was the first electrical generator capable of delivering power for industry. Thedynamo uses electromagnetic induction to convert mechanical rotation into direct currentthrough the use of a commutator. The first dynamo was built by Hippolyte Pixii in 1832.Through a series of accidental discoveries, the dynamo became the source of many laterinventions, including the DC electric motor, the AC alternator, the AC synchronous motor, andthe rotary converter.A dynamo machine consists of a stationary structure, which provides a constant magnetic field,and a set of rotating windings which turn within that field. On small machines the constantmagnetic field may be provided by one or more permanent magnets; larger machines have theconstant magnetic field provided by one or more electromagnets, which are usually called fieldcoils.
  12. 12. Large power generation dynamos are now rarely seen due to the now nearly universal use ofalternating current for power distribution and solid state electronic AC to DC power conversion.But before the principles of AC were discovered, very large direct-current dynamos were theonly means of power generation and distribution. Now power generation dynamos are mostly acuriosity.[edit] AlternatorWithout a commutator, a dynamo becomes an alternator, which is a synchronous singly fedgenerator. Alternators produce alternating current with a frequency that is based on the rotationalspeed of the rotor and the number of magnetic poles.Automotive alternators produce a varying frequency that changes with engine speed, which isthen converted by a rectifier to DC. By comparison, alternators used to feed an electric powergrid are generally operated at a speed very close to a specific frequency, for the benefit of ACdevices that regulate their speed and performance based on grid frequency. Some devices such asincandescent lamps and ballast-operated fluorescent lamps do not require a constant frequency,but synchronous motors such as in electric wall clocks do require a constant grid frequency.When attached to a larger electric grid with other alternators, an alternator will dynamicallyinteract with the frequency already present on the grid, and operate at a speed that matches thegrid frequency. If no driving power is applied, the alternator will continue to spin at a constantspeed anyway, driven as a synchronous motor by the grid frequency. It is usually necessary foran alternator to be accelerated up to the correct speed and phase alignment before connecting tothe grid, as any mismatch in frequency will cause the alternator to act as a synchronous motor,and suddenly leap to the correct phase alignment as it absorbs a large inrush current from thegrid, which may damage the rotor and other equipment.Typical alternators use a rotating field winding excited with direct current, and a stationary(stator) winding that produces alternating current. Since the rotor field only requires a tinyfraction of the power generated by the machine, the brushes for the field contact can be relativelysmall. In the case of a brushless exciter, no brushes are used at all and the rotor shaft carriesrectifiers to excite the main field winding.Vehicle-mounted generatorsEarly motor vehicles until about the 1960s tended to use DC generators with electromechanicalregulators. These have now been replaced by alternators with built-in rectifier circuits, which areless costly and lighter for equivalent output. Moreover, the power output of a DC generator isproportional to rotational speed, whereas the power output of an alternator is independent ofrotational speed. As a result, the charging output of an alternator at engine idle speed can bemuch greater than that of a DC generator. Automotive alternators power the electrical systems onthe vehicle and recharge the battery after starting. Rated output will typically be in the range 50-100 A at 12 V, depending on the designed electrical load within the vehicle. Some cars now haveelectrically powered steering assistance and air conditioning, which places a high load on theelectrical system. Large commercial vehicles are more likely to use 24 V to give sufficient power
  13. 13. at the starter motor to turn over a large diesel engine. Vehicle alternators do not use permanentmagnets and are typically only 50-60% efficient over a wide speed range.[4] Motorcyclealternators often use permanent magnet stators made with rare earth magnets, since they can bemade smaller and lighter than other types. See also hybrid vehicle.A magneto, like a dynamo, uses permanent magnets but generates alternating current like analternator. Because of the limited field strength of permanent magnets, magnetos are not used forhigh-power production applications, but are very reliable. This reliability is part of why they areused in aviation piston engines.Some of the smallest generators commonly found power bicycle lights. Called a bottle dynamothese tend to be 0.5 ampere, permanent-magnet alternators supplying 3-6 W at 6 V or 12 V.Being powered by the rider, efficiency is at a premium, so these may incorporate rare-earthmagnets and are designed and manufactured with great precision. Nevertheless, the maximumefficiency is only around 80% for the best of these generators—60% is more typical—due in partto the rolling friction at the tire–generator interface from poor alignment, the small size of thegenerator, bearing losses and cheap design. The use of permanent magnets means that efficiencyfalls even further at high speeds because the magnetic field strength cannot be controlled in anyway. Hub dynamos remedy many of these flaws since they are internal to the bicycle hub and donot require an interface between the generator and tire. Until recently, these generators have beenexpensive and hard to find. Major bicycle component manufacturers like Shimano and SRAMhave only just[when?] entered this market. However, significant gains can be expected in future ascycling becomes more mainstream transportation and LED technology allows brighter lighting atthe reduced current these generators are capable of providing.Sailing yachts may use a water or wind powered generator to trickle-charge the batteries. A smallpropeller, wind turbine or impeller is connected to a low-power alternator and rectifier to supplycurrents of up to 12 A at typical cruising speeds.Still smaller generators are used in micropower applications.[edit] Engine-generatorMain article: Engine-generatorThe Caterpillar 3512C Genset is an example of the engine-generator package. This unit produces1225 kilowatts of electric power.
  14. 14. An engine-generator is the combination of an electrical generator and an engine (prime mover)mounted together to form a single piece of self-contained equipment. The engines used areusually piston engines, but gas turbines can also be used. And there are even hybrid diesel-gasunits, called dual-fuel units. Many different versions of engine-generators are available - rangingfrom very small portable petrol powered sets to large turbine installations. The primaryadvantage of engine-generators is the ability to independently supply electricity, allowing theunits to serve as backup power solutions.[5]State the laws of electromagnetic induction.Ans. Laws of electromagnetic induction:(i) A current carrying conductor placed in a magnetic field experiences aforce.(ii) The amount of e.m.f. produced is directly proportional to rate of changeof flux.Explain the principle and working of a D.C. generator?Ans. D.C. generator converts mechanical energy to electrical energy. Itworks on principle of electromagnetic induction. Whenever a current carryingconductor is placed in a magnetic field it experiences a force. In D.C.generator e.m.f. is induced dynamically i.e. flux linkage with the conductorchange due to movment of conductor. The vvorking of a single coil simpleD.C. generator is explained below:A simple D.C. generator consists of a magnetic field set up by permanentmagnet and a conductor is placed in this field. The conductor is made tomove in the magnetic field so that it cuts magnetic lines of force and ane.m.f. is induced . The direction of induced e.m.f. is given by Fleming’s righthand rule.
  15. 15. The e.m.f. so induced produces a current in coil which is collected throughsplit rings and commulator.