Permanent magnet alternators?

Ah, so electromagnets have a larger magnetic field per unit volume?

The output voltage can be regulated much easier.

No, but by the time you finish surrounding a permanent field alternator with its supporting circuitry it may be bigger than current technology.

An alternator that uses DC magnets will have an output voltage that is very strongly coupled to it's speed and that cannot be adjusted. Instead, you'd need to have some sort of a switching power supply circuit to take whatever happened to come off the alternator and feed it to the battery.

Normal car alternators are regulated by adjusting the current to their field coils; in effect their motor constants are changed by the regulator circuit. This makes them fairly easy to control with very little, and efficient, circuitry.

--
Tim Wescott
Control system and signal processing consulting

cost, heat, and size ?

Because they use the dc magnetizing current to adjust the output voltage.

Some motorcycle alternators do, spinning a PM field and using fixed output windings. The regulators generally short the output to keep the charging current down. That seems to work for the fairly low power needs of a motorcycle.

John

Wow, thanks! I'll have to look into those.

Thanks to all for the replies

Michael

| car alternators are regulated by adjusting the current to their | field coils; in effect their motor constants are changed by the | regulator circuit. This makes them fairly easy to control with very | little, and efficient, circuitry.

I loath the amperes wasted by alternator's excitor circuit before engines are started successfully. Does a brushless alternator likewise waste amperes before an engine is started successfully ?

They do. Motorcycles have been PM'd for many years now. Modern ones can compete with car alternators for power produced. You're familiar with outboards? Tractors? Even the lowly Briggs added an extra coil to their early electric start engines using the magneto magnet to charge the battery.

For some reason cars are often the last vehicles to apply technological improvements.

You must hate 99.8 percent of all cars, or more likely you are trolling.

have you measured it? typically it is exactly enough to light the 'altenator warning' lamp.

no, but it could make the engine harder to start.

I assume you mean a permanent magnet altenator, the are brushless electromagnet field altenators.

--- news://freenews.netfront.net/ - complaints: snipped-for-privacy@netfront.net ---

| > I loath the amperes wasted by alternator's excitor circuit before | > engines are started successfully. | have you measured it? No.

| typically it is exactly enough to light the 'altenator warning' lamp. Wikipedia says a 70 amp alternator uses 2 amp as exciter, so I think exciter should be activated only after engine starts.

| > Does a brushless alternator | > likewise waste amperes before an engine is started successfully ? | no, but it could make the engine harder to start. How ?

| I assume you mean a permanent magnet altenator No, see

, in 4-09 1 Toyota salesman said new Camry ( but not lower models ) has this. I suspect rare earth magnets can't be used ; heat will ruin these magnets.

the last altenator I looked closely at an altenator it took the exciter current from separate (positive side) rectifier diodes (D+ terminal) which was fed back into the altenator exitation (DF terminal) by the regulator

with it's stopped or at low RPM the exciter current coming from the lamp circuit is is only a few hundered millianps, once the speed gets up the rectifier kicks in to supply the power.

Those things still need power for the exiters stationary winding. if that current is't taken mostly from the rectifiers then it's going to cost the same,

if you're worried you could rig a horn relay (form C automotiver relay) to cut the feed to the altenator when you activate the starter.

--- news://freenews.netfront.net/ - complaints: snipped-for-privacy@netfront.net ---