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Author Topic: Charging System  (Read 7103 times)
Capo
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« on: November 02, 2008, 07:52:08 AM »

The following is based on the charging system as fitted to a 04 Desmoquattro S4R, the system is similar on other models.

Charging System.
Electrical power is generated by a 3 phase 520W (43A) permanent magnet alternator driven directly at engine speed from the left hand end of the crank shaft. The magnetic rotor is bolted to the flywheel and round outside the stator which is fixed to the left hand engine cover (the alternator cover). The alternating current voltage output is proportional to engine speed producing a regulated DC output of 14.5V at 3000 RPM. The frequency of the alternating current also varies with the engine speed,
The maximum system load is well within the capacity of the alternator, up to 28A being available for battery charging at normal loads.

Engine RPM                               2000     6000
Output Voltage (Phase/Phase)   27±10   78 ±10

Alternating Current (AC) is rectified to Direct Current (DC) and regulated according to the battery voltage,
AC flows alternately in one direction then the other in cycles that are dependant on the speed of the alternator rotor but typically could be hundreds of times per second. Diodes rectify the AC into DC, which flows in only one direction. Three pairs of diodes rectify both directions of the AC output of each phase.
The regulator' reduces the average current through the rectifier to the amount that is necessary to satisfy the operating loads at any given time, the regulator senses the battery voltage and regulates the output voltage to 14.5Vdc by shorting the alternator phase outputs to ground, limiting the power input to the rectifier. A 40A fuse is fitted in series with the regulator DC output to the battery. Ducati specs the output at 14.5V @ 3000 RPM. When the load is in excess of the alternator output, the battery is discharging to maintain the load.
The ground in the charging system is the negative output from the rectifier. The AC-part of the three phase system is floating from ground, measurement of the AC-output should be from phase to phase, and not from one phase to ground..
In this system, the alternator is producing full power all of the time, the excess power is converted to heat which is dissipated by a finned heat sink surrounding the Rectifier/Regulator (RR).

Charging System Schematic.


Operational Issues.

Heat is main cause of RR failures, as noted the alternator produces full power at all times, excess power being shorted to ground, with a fully charged battery over 330W of excess power is converted into heat, additional heat is generated from the rectifier. A flow of cool air over the RR heat sink is essential if problems are to be avoided.

Loose and corroded connections are another source of failures, with up to 40A flowing through the alternator wires, high resistance connections will heat up and have been known to melt the insulation on the wires. All connections should be periodically checked to ensure they are clean and secure. A coating of Corrosion Block Grease (A Lear product) is recommended.

Extensive city use where engine speeds are lower than 3K, will cause the battery to discharge, in such cases the battery should be charged overnight to ensure adequate power availability.

It is recommended that a battery tender be used to maintain the battery charge during periods of no use.
« Last Edit: November 02, 2008, 07:54:16 AM by Capo » Logged



Capo de tuti capi
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« Reply #1 on: November 30, 2008, 07:40:24 PM »

Nice work Capo,  excellent as always.   Re: the charging system, this design really sucks.  So they are using a shunt regulator (read shorting). This type of regulator design is as old as vacuum tubes  Tongue  Seems like they are dumping a significant amount of engine power down the drain. 330W at nominal RPMS but I bet at high revs its over 749W or 1hp getting pissed away.  Not to mention the thermal stress on the active components. This explains why some people have noticed the heatsink melting the plastic on the seat.   

SHIT!!



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Howie
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« Reply #2 on: November 30, 2008, 10:05:24 PM »

Nice work Capo,  excellent as always.   Re: the charging system, this design really sucks.  So they are using a shunt regulator (read shorting). This type of regulator design is as old as vacuum tubes  Tongue  Seems like they are dumping a significant amount of engine power down the drain. 330W at nominal RPMS but I bet at high revs its over 749W or 1hp getting pissed away.  Not to mention the thermal stress on the active components. This explains why some people have noticed the heatsink melting the plastic on the seat.   

SHIT!!





This type of system is common on most motorcycles, particularly sport bikes.  The alternative would be to control field strength through windings in the rotor, adding complexity and weight.
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Capo
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« Reply #3 on: December 02, 2008, 03:03:55 PM »

Nice work Capo,  excellent as always.   Re: the charging system, this design really sucks.  So they are using a shunt regulator (read shorting). This type of regulator design is as old as vacuum tubes  Tongue  Seems like they are dumping a significant amount of engine power down the drain. 330W at nominal RPMS but I bet at high revs its over 749W or 1hp getting pissed away.  Not to mention the thermal stress on the active components. This explains why some people have noticed the heatsink melting the plastic on the seat.   

SHIT!!


Thank you,  I have also come to value your posts.

With regard to the heat issue, have you noticed that on the sportsbikes, the regulator is mounted on a big metal heat sink and is oriented with the cooling fins parallel to the airflow, whereas on the monster (S*R) it's mounted under the seat out of the airflow on the plastic breather box with the cooling fins at right angle to the (meagre heated) airflow. Could be done better. It's probably a good practice to run with as much load as possible as alternative to letting the regulator burn it up.

I have the lightweight 290W vs the std 540W alternator, got led's everywhere and a euro switch so I can have some control over the load.
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Capo de tuti capi
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