Curious loss of charge

PeterZRH

Well-Known Member
#22
Likely there is no "standard size" as these are used in loads of cars so the speed would depend on the engine pully too. I don't recall them being too different. Alternators and dynamos are fundamentally different in that the magnetic side is done with field coils which can vary to govern the output. This means they don't generally need to spin as fast at lower engine speeds as the give a much higher output. So they are just lots better sitting in traffic idling on a cold winter commute.
 

sdibbers

Well-Known Member
#23
Yep, I've had a dyno equipped car in the past. TBH it was much more reliable than the alt'. I think its dwn to their relative simplicity. I'll measure the two and decide after some calculations re: old vs new pulley.
 

roverp480

Active Member
#24
As understand things, one advantage of an alternator is that it can spin faster than a dynamo without it disintegrating so a smaller pulley can be used relative to a dynamo and thus generate more output at all speeds, as its spinning faster . I have an original Lucas technical bulletin that suggests a Dynamo running at about 1.8 times engine speed could have an alternator at twice engine speed . This obviously depend also on the speed range of the engine. Rover's are relatively low revving in the grand scheme of things . I know at Rover we fitted smaller pulleys when alternators became general fit which then raised issues with belts failing both by slipping and delaminating on the small pulleys . This resulted in the notched V belt , so it can go round a smaller radius , and with what is referred to as a "raw edge", the cords being exposed on the faces of the V , that grip the pulley better.
 
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sdibbers

Well-Known Member
#25
Alternators are just more efficient than Dynamos through the use of coils to create the magnetic field (as Peter mentioned).
As for RPM you don’t want it spinning faster than needed, partly for the sake of the alternator’s lifespan and partly because you’re just sapping engine power for no reason.
 
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PeterZRH

Well-Known Member
#26
Yes. the point being when the alternator is spinning slowly, the field coils are more highly energized to get output i.e. more mechanical work is done against the field (increased rate of cutting of flux if you remember that from school). As the engine spins the alternator faster, this magnetic field drops off to avoid overloading - the feedback in the old 11AC being in the 4TR but integrated in later designs (this also feedbacks the battery voltage). With the dynamo with fixed magnets, you engineer it for the "worst case scenario" i.e. maximum rpm plus a margin. Therefore at low speeds the output is tiny.

My alternator came with a test sheet showing the output. This reached a maximum on the bench at somewhere around 2800rpm - shaft speed. Now as the pulley is smaller than the engine one by at least 1:2, so likely that this equates to quite a low engine speed - possibly in the range idle to 2x idle. So for most of the rev range the output can be up to or at maximum output if needed.

If a dynamo has a linear relationship then given idle is maybe 1/10th the full engine speed, then the output will be similarly be lower the 10% of the maximum.

Massive difference if you are sitting in traffic with your lights, rear screen heater and blower running for a few hours.... If you were around in the 1970s, an alternator was a popular upgrade for older cars. With very good reason for an everyday driver.
 
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sdibbers

Well-Known Member
#27
Well, I fitted the new A127 today. Smaller pulley as other have mentioned but seems fine. I had to shim the pulley and the pivot for the adjustment strap slightly so the new belt would be aligned with the other pulleys (I have the long nose water pump for ref). Belt was 930mm and is now 900mm in length, it could be 910 for a perfect fit I think.

The new alternator has studs instead of spade connectors, so I added ring terminals with heat shrink to make sure they stayed clean. I was alble to use the existing 10 gauge wire to the + terminal of the solenoid and the AL (ign light) wire that used to go to the 3AW unit and bridged the wire from the alternator to the ign light with a small tail with two insulated male spades, again heat shrieked to ensure they were sealed.
I’m glad to report that everything works and the charge continues after ten mins of driving. (Old alternator would charge fine for 10-15 mins then stop-off way to fail).
 

sdibbers

Well-Known Member
#29
Have seen this failure mode before - year or two back - 10-15 mins charging ok, then nothing.
That’s really interesting, the other thing was the ign light never came on to warn of no charge.

a long running issue was if the car was left for five days or more without being driven the ign light would stay on even when charging for about ten mins.
 

PeterZRH

Well-Known Member
#30
Well, I fitted the new A127 today. Smaller pulley as other have mentioned but seems fine. I had to shim the pulley and the pivot for the adjustment strap slightly so the new belt would be aligned with the other pulleys (I have the long nose water pump for ref). Belt was 930mm and is now 900mm in length, it could be 910 for a perfect fit I think.

The new alternator has studs instead of spade connectors, so I added ring terminals with heat shrink to make sure they stayed clean. I was alble to use the existing 10 gauge wire to the + terminal of the solenoid and the AL (ign light) wire that used to go to the 3AW unit and bridged the wire from the alternator to the ign light with a small tail with two insulated male spades, again heat shrieked to ensure they were sealed.
I’m glad to report that everything works and the charge continues after ten mins of driving. (Old alternator would charge fine for 10-15 mins then stop-off way to fail).
Great result. The ring terminals suggest that yours was the Ford variant of the A127.
 

sdibbers

Well-Known Member
#31
Great result. The ring terminals suggest that yours was the Ford variant of the A127.
Interesting, I actually prefer the ring terminals as they provide a more reliable joint for the charging circuit. The old spade connector on the AC 11 showed signs of heat from poor connections in the past (even some arc erosion).
 

PeterZRH

Well-Known Member
#32
Yes that solution is better. I speculate that the kludge of having a 2 power connector block was to maintain compatibility. In the original installation it is of course safer with a single wire because in the (ok unlikely) event of one part of the connector or the wire failing, you have all the output potentially overloading one wire.
 
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