Car Alternators vs Motorcycle Alternators - Page 1
Forum section: electronics / projects

author: quanttrom
author_rank: Regular Contributor
timestamp: June 17, 2014, 07:29:10 pm
content: I have had a few questions regarding the charging systems of cars vs motorcycle and I have never been able to get a solid answer.I have a basic understanding but I am by no means familiar with motors/generators so please explain as best as you canQuestion: Does the feedback loop in a motorcycle alternator just dump the excess power or does it change the properties of the stator windings resulting is less torque required?This is the extend of my understanding so far. Please correct me if I am wrong.Most Car Alternators:http://www.c-realevents.demon.co.uk/altgen/hraltdiag.jpgReferencing the diagram above my understanding is that those are 3-phase AC generators(alternators). The stator has 3 windings in Y formation which generates 3 AC waves 120 degress apart, that get rectified and provide a stable DC voltage. The feedback loop is through the excitor winding (field coil). The magnetic field is varied by applying a different voltage/current. The varied magnetic field produces a different voltage on the stator coils and regulator is achieved. Any transients are absorbed by the battery and electrical system of the car.Most Motorcycle Alternators:http://i444.photobucket.com/albums/qq167/rjdoles/regulatorschematicdiagram.jpgReferencing the diagram above. Similar design except this time there is no field coil. The magnetic field is always there generated by permanent magnets. The regulation seems to be achieved by shorting the alternator output. And here comes the question...Does shorting the output simply dump the excess energy in the form of heat, or does the short change the behavior of the coils and the rotor rotates more freely( less torque required) ?

author: oPossum
author_rank: Super Contributor
timestamp: June 17, 2014, 08:17:10 pm
content: The SCRs in the motorcyle generator allow the windings to be used as sort of a boost converter as needed. It is not dumping power anywhere.Regenerative braking on an EV can work in a similar way. The motor inductance is used to boost it's output above the battery voltage.

author: quanttrom
author_rank: Regular Contributor
timestamp: June 17, 2014, 08:55:41 pm
content: I am looking at Kawasaki 650R(ER-6F) documentation. It says that the open output of the alternator (no rectifier connected) will produce 42V AC (measured across any of the wingdings) when the engine is at 4200 RPM. This would be around the cruising RPM of this type of bike.I assume there will be a dip in voltage when loaded but I am not sure why they would need to make a boost converter.Can you explain please?

author: G7PSK
author_rank: Super Contributor
timestamp: June 18, 2014, 08:32:59 am
content: That type of alternator regulates by dumping part of the waveform, They produce a higher voltage than required at all times, the torque or power to drive them goes up with the speed as the higher the speed the higher the out put voltage is and therefore the greater the angle over which dumping occurs.http://www.electrosport.com/technical-resources/technical-articles/how-motorcycle-charging-system-worksBut some of the very old units just used a very lossy iron core to the coil, and as the engine RPM rose the lights would get brighter.

author: richard.cs
author_rank: Super Contributor
timestamp: June 18, 2014, 09:37:58 am
content: Three generations of motorcycle alternators:1) Lighting only.These are wound for high leakage inductance, both output voltage and impedance increase proportional to engine RPM so the output is constant current a.c. with RPM. This works ok with lightbulbs so long as the correct type are fitted. They are always single-phase.2) Early battery charging.These are current-limited as for 1) but to a higher value and the output is rectified. The battery is shunted by a large zener that dissipates quite a bit of power in daytime running and very little when all the lights are on. Because the current limiting in the alternator is reactive not resistive the dissipation in the windings is still low. These can have single or three-phase stators.3) Modern systems.The rectified output of the alternator drives a switch mode power supply that charges the battery and runs the bike electrics. Sometimes these work in boost mode when the engine is idling and a the voltage would otherwise be too low to charge, they usually work in buck mode at higher engine revs. These do not waste as much power. There are many varients on this general principle including some that short the stator for some fraction of time as described by G7PSK. Note that this does not waste as much energy as might be thought because with the output shorted the alternator sees its own inductance with little resistance and this reactive load does not draw much mechanical power from the rotor.

author: woodchips
author_rank: Frequent Contributor
timestamp: June 18, 2014, 06:33:14 pm
content: Umm, are you sure?Lighting only is like an old moped, and if you went downhill you could easily blow the bulbs.Early type as used on UK bikes in the 60s and 70s, the zener just limited the system voltage to 15V.Modern just like a car with a field controlled machine sensed alternator.The whole point of an alternator was that it would charge at idling, so stuck in a traffic jam in winter didn't mean the battery ended up flat. I have never seen any buck mode on any vehicle alternator, but anything newer than 15 years is a closed book to me. With the alternator output shorted what you will see is the magic blue smoke, do not short a running alternator!That motorcycle charging article is a bit suspect a well. Using the term 'short-circuits a certain amount of power that is produced by the stator to ground' is very poor technical English. It implies a short circuit, what it does is trim the voltage using a zener to a maximum of 15V. This is quite a different operation. Quite what the description about shorting the stator windings beats me, just seems a good way to let the magic blue smoke out. The concept of the extra wire having a reduced voltage drop is irrelevant. Most alternators are machine sensed, that is they measure their own output voltage and regulate that to be 14.4V. This works fine, if there is a voltage drop in the external cables then it should be just millivolts, this is why the alternator cable is large cross section.The field controlled alternator is the most common, simply because it is the easiest to control. Either a variable mark space ratio sequence of current pulses or an analogue varying current to the field. The generated voltage depends on both magnetic field strength and the rate of cutting of the magnetic field. Don't forget though that at high engine speeds the generated AC can be several kHz, and the consequent losses in the laminations can be several times the actual power output. The same losses occur in the lighting and permanent magnet alternator as above, so this does in fact also give power limiting. This isn't impedance limiting, more magnetic hysterisis losses.

author: richard.cs
author_rank: Super Contributor
timestamp: June 18, 2014, 06:48:58 pm
content: I've never seen a field controlled alternator on a bike but most of the ones I've played with have been pre 90s. Yes my "early" was before about 1960 when zeners started to become available. 3phase permanant magnet + zener or black box covers most I've worked on.

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 18, 2014, 08:18:18 pm
content: Quote from: woodchips on June 18, 2014, 06:33:14 pmThe (classic) Honda Insight used a 144V motor/alternator and a switching converter to step it down to 12V. It came out right at 15 years ago, so you just missed it. It ends up more efficient since large loads (e.g. HVAC) can be run directly from the high voltage supply and the 12V converter makes use of very efficient synchronous rectification.It appears that for the most part, motorcycles are designed to be cheap, not to achieve extremely high MPG. Even the Elio, with its impressive sounding 84 MPG rating, does nowhere near what is practically attainable (a few hundred MPG). The Aptera did an impressive 300 MPG, but didn't sell because it was too expensive.

author: mikerj
author_rank: Super Contributor
timestamp: June 18, 2014, 08:46:44 pm
content: Rectifier/regulators for the permanent magnet alternators are mostly shunt type*, and simply dump the wasted energy as heat. There is nothing clever about these systems, no buck/boost operation etc. They work a somewhat like an AC dimmer switch, but instead of conducting and allowing part of an AC cycle pass to the load, they conduct and divert it to ground.Most of the wasted heat is dumped into the stator which is surrounded by engine oil in larger engines. However some of the energy gets dissipated by the SCR's due to the fairly large voltage drop these introduce under load. More modern regulators have replaced the SCRs with MOSFETs, making the regulators run much cooler and are more reliable because of this.There is a company making a linear series pass regulator, with an enormous heatsink on it. I've often thought a switching regulator would be a far more elegant solution, but making them as reliable and cheap as the simple shunt system would be tricky IMO.

author: Circlotron
author_rank: Super Contributor
timestamp: August 25, 2015, 06:01:56 am
content: Quote from: oPossum on June 17, 2014, 08:17:10 pmOnce the SCR is triggered on it will stay on until the generator winding current decreases to zero at the end of the AC cycle. It would need to turn off while there is still current flowing in the winding for it to function like a boost converter.