Charging systems seem to be a common topic, so I thought I would put down what I know of permanent magnet alternators in the hope it may help someone. Feel free to make any corrections or clarifications.
Permanent Magnet Alternators:
Electricity is generated in a conductor when a magnetic field passes through the conductor (Faraday's Law) . The amount of power generated increases:
- with an increase in the strength of the magnetic field
- with an increase in the speed of the magnetic field (flux) passing through the wire
- with an increase in the number of turns of wire the magnetic flux is cutting.
The VStrom charging system is comprised of three parts: magnets (flywheel/rotor), wire coils (stator), and a regulator/rectifier.
The magnetic field could be increased with stronger magnets, or by a reduction in the air gap between the magnets and the coils. This flux is concentrated by putting a ferrous material inside the coil of wire. The magnets are placed in the flywheel with alternate poles facing the coils, so the coils are subjected to alternating (North -> South -> North...) poles as the flywheel spins.
The number of turns of wire have to fit in the available space, so an increase in wire size (diameter) will decrease the number of turns. Less turns will reduce voltage at near idle speeds, while conversely, more turns will produce too much voltage at higher speeds that will have to be absorbed by the stator and regulator. Thicker insulation on the wire adds to it's diameter, and generally thin / better insulation is more expensive. The wire is subjected to engine vibration as well as vibration from the rotating magnetic fields.
As the metal core the coil is wound on is also an electrical conductor , some current is generated inside the metal – these currents are called “eddy currents” (similar to eddy (swirling) currents in a river). These currents cause heat, so the stator cores are made of laminated plates that are insulated from each other to reduce these parasitic losses. As the laminated plates switch magnetic polarity as the magnets pass, the laminations tend to heat from the constant reversal, as well as lag the change slightly ( hysteresis). Above a certain speed, the laminations cannot switch magnetic polarity fast enough (saturation), and available power output will start to drop. (see http://www.protolam.com/page3.html
for the complexity of lamination choices). Thinner laminations and/or better metals both reduce core losses, but increase cost. Picture of a stator showing fine stacked laminations on the pole pieces:
As the copper wire has some direct current resistance, (0.2 to 0.5 ohms between B1,B2,B3 on attached diagram), heat will also be generated in the windings from the current flowing through it. Some alternators are loose wound (the wire coils are not potted in epoxy), so engine oil can get to each wire too help cool it – but vibration and corrosion become more of an issue. (used oil can become corrosive). Wire wound into a coil shape also has a resistance to alternating current (inductance), that increases it's losses with increasing frequency (RPM)
A graph of stator power output vs RPM should show a fairly smooth rise to the maximum output (5000 rpm on the VStrom), and then flatten or even reduce slightly with increasing RPM. The increased power output due to the higher speed of the magnetic flux cutting the wire is offset by the increased magnetic core and copper losses at higher RPMs. Output graph shows less voltage at low rpm on the ElectroSport stator, but more at higher RPM – likely done with less windings of larger diameter and better core material to reduce the higher RPM losses (?) - (from ElectroSport site) “is built with the highest grade lamination materials and the copper windings are triple insulated for maximum reliability.”
The Regulator / Rectifier:
The alternator (stator / flywheel) puts out AC (Alternating Current) that alternates from positive to negative. The rectifier steers the positive going pulses to the Red battery lead, and the negative going pulses to the Black battery lead (sort of like one way valves for water). The diode symbol is an arrowhead with a vertical line - the direction of the arrow is the direction current will flow from positive to negative. When current is flowing through a diode, it will drop ~0.6 volts across it, in the reverse direction no current should flow. At 30 amps out, each diode is dropping 18 watts (30 amps x 0.6 volts) peak.
The regulator portion is an integrated circuit (IC) that senses the output voltage, and should it go high - the IC turns on a SCR (silicon controlled rectifier). The SCR is a diode that doesn't conduct until the third terminal (the gate) is pulsed by the IC. The SCR then shorts the stator winding (i.e. B1 to B2 through the SCR and diode 5), converting the excess power to heat in the stator and rectifier/regulator.
The alternator system creates a lot of heat - each 25 amps generated requires ~1 horsepower (HP) to spin it. (1 HP equals 746 watts). Permanent magnet alternators are generally only 50->60% efficient, (Efficiency = Output / Input ) with the losses converted to heat in the stator and.or at the regulator/rectifier. Regardless of the load, because the regulator shorts the stator on over-voltages, the alternator runs at peak output at all times.