| Author | Message | ||
     Robert Noel Reddington Grand Master Username: bob_uk Post Number: 663 Registered: 5-2015 |
The main reason alternators took over from dynammos is that at low rpm the dynamo cannot charge and must cut out else the battery will try to motor the dynamo which will burn out the dynamo and cause a heavy discharge. So why not run the dynamo faster. This would work but at revs the armature windings would be shed due to centrifugal force and the brushes would have trouble. The same problem is starting afflict alternators. Not enough electricity at low revs. So to get round this the alternator pulley is also a constant speed device. The alternator runs fast at low revs and the same speed at high revs. One aftermarket parts maker is tryimg to develop a retro fit constant speed alternator pulley. Rolls Royces are equipped with high quality charging systems which are derated for reliably so constant speed pulleys are not needed. I have seen this alternator on trucks and buses running at 24v x 75 amps. (Shadows are 12v x 60 amps) These engines have a max revs of 2500 rpm so the alternator is geared to run fast at idle and not to fast at 2500 rpm, the rev range is about 1/2 that of a car engine. My car has no extras that are power hungry apart from what Crewe fitted so providing one doesn't fit power hungry stuff the standard system is more than adequate. I saw a 12v kettle. It takes 20 minutes to boil a cup of water. 20 mins of that sort of drain will flatten the battery making restarting may be marginal. I shan't be getting one. | ||
| Robert Noel Reddington Grand Master Username: bob_uk Post Number: 818 Registered: 5-2015 |
I have been inventing again. The water pump, AC compressor , alternator and pas pump all run at variable speed. Not so much the water pump but the alternator AC and pas would all benefit from running faster at lower engine rpm. So fit a constant speed pulley in place of the crankshaft pulley. I have come up with a system like DAF CVT using centrifugal weights to move the pulleys apart. More and more designs are using electric motor driven ancillaries. This is not as efficient as it it first appears because car alternators are not that efficient 70% at best. And changing a turning motion into electric and back again to a turning motion introduces more losses. All of this is just thinking about the possibilities and not actually making one. | ||
| Richard Treacy Grand Master Username: richard_treacy Post Number: 3245 Registered: 4-2003 |
Then reason that dynamos/generators disappeared is that very cheap and reliable power diodes replace those high-maintenance carbon brushed for the armature current.That a diode works fine at low engine RPM while brushes struggle is a bonus. Given that our motors are running mostly at 580 RPM on idle and just 1500 (late SZ)-2200RPM (Silver Shadow and eary SZ) at 110 km/h depending on the specification of the final drive, concern about variable gearing of the alternator must be somewhere deep below the first rung on the totem pole. | ||
| Robert Noel Reddington Grand Master Username: bob_uk Post Number: 819 Registered: 5-2015 |
Agreed. But a dynamo when producing less than battery voltage must be disconnected from the battery by the cut out. Also to get say 60 amps from a dynamo means a big dynamo and big brushes. This has a knock on effect which further narrows the speed range. This also means that any change in rpm also has to accelerate or deaccelarate a heavy armature which means better drive belts. No wonder they ditched dynamos. The Lucas 22 amp 12v dynamo was quite reliable ,brushes lasted about 50k miles and cost 2 quid a pair. So maintenance isn't that high. My " invention" is aimed at new car designs not retro fit to old stuff. Also if an alternator is geared to twice engine rpm. Then at idle of 700 rpm the alternator is 1400 rpm. And at 6000 rpm, 12000 rpm. A speed range 10600 rpm. My car at 26 mph per 1000 rpm means I very rarely go above 3000 rpm. The alternator easily copes with this range. All of this is hypothetical. I am not going to ever make a constant speed thingy. I am just putting the idea out there. | ||
| Bob Reynolds Grand Master Username: bobreynolds Post Number: 354 Registered: 8-2012 |
What about a 2-speed gearbox inside the alternator? At low speeds the high speed is selected and at high speeds the low speed is selected. The gear could be changed by a centrifugal mechanism. Just thinking aloud, same as you!  | ||
| Robert Noel Reddington Grand Master Username: bob_uk Post Number: 820 Registered: 5-2015 |
That would work. But an abrupt gear change and the mass of the rotor may cause a problem. Sturmy Archer used to make hub gears for push bikes. |
