dynamo v alternator

[skyflyer]

First post here so forgive my ignorance! I did however do a search to see what had been posted up before and culdnt find much, so here goes...

Why convert? I know alternators are less speeed sensitive, ie produce a charge at almost any revs, but unless idling very slowly the red genny light doesnt come on.

As i understand it, the gen light actually means that the battery is discharging, so no light (unless its bust of course!!) means the battery should be getting a charge - yes?

I ask becauise the battery wont hold its charge - after a long drive its fine for a few hours, but by next morning (say) - almost flat again.

I think this is a battery problem not a charging problem, but not sure. If my above thinking is wrong, maybe the genny doesnt produce enough charge to recharge a battery, unless revving really high? (well - not pootling round town, anyway)

Obviously a new battery is the first thing to try, but if thats nbot the problem, thenmaybe the genny is on the way out. But am i better off fixing/replacing genny or doing a conversion to alternator - as I said - whats the plus side?


By the way - in seraching the forum i found a few queries about other reasons for the genny light not going out. I found that when i misconnected my side lights with ground to live and live to ground, the fuse blew (perictably enoiugh) and the red gennty light then came on. IT glowed stronger if you used other services, eg lights etec. Once thye fuse was replaced and sidelight rewired properly, the red light went out again. Might help someone else someday?

 

[Pete B]

Fundamentals. Check/top up battery level. Clean battery terminals. Connect volt meter to battery when engine running, should have between 12.7 and 13.3 volts (if memory serves correctly).Could have up to 14.5 if the battery is flat/faulty. If it's charging o.k. then either the battery is faulty or something is draining it. A dynamo system in good order works fine. Your local garage can check this and will have a 'drop tester' and hydrometer so can see if the battery is faulty. Hope this helps.

 

[noddy]

Had a similar problem about 2 months ago, get the battery check first to see if its holding a charge if battery is ok, its going to be the rectifier.

Put a new one on mine sorted this type of problem strait away, also worth checking the state of the brushes in your genny.

 

[skyflyer]

genny bushes worn but not 'expired'! I am pretty certain that genny is providing power, else the battery wouldnt even temporarily charge up, ie push start when nearly flat, drive for 20 minutes, starts ten minutes later, drive for 20 minutes but wait a few hours and battery wont turn over engine - so battery is getting a charge somewhere along the line.

Thus it either isnt holding the charge or its being drained by a power consuming source.

Where is the rectifier? Could a faulty rectifier create a short that means the battery is being drained?

Thanks for advice so far

 

[noddy]

The rectifier is like a one way valve it stop the battery discharging back through the genny, if its shot your battery will only partial charge which is why its ok once your up and running.

Rectifier is usual located near the fire wall on the same side as the batter, looks like a small tin box about 100mm tall by 60 wide, trace the wires from the genny they will go to it, and then from this to the battery.

New ones cost about 40-50 quid just make sure you get the correct one for your generators out put, some are 30 amp and some are 38 amp, if you get the part number of your genny any good supplier should be able to tell you which one you need, got mine from coolairvw, and its been sweet since.

 

[mjknight71]

The rectifier is like a one way valve

hmmm rectifier converts AC into DC.....1/2 wave or full wave.....diodes more like a valve..or indeed a valve/transistor :?:

A generator is a DC motor...split ring commutators and all......

 

[noddy]

Yes MJ so whats your suggestion then to fix his problem :?: and a rectifier/regulator does contain diodes will post picture of my old voltage regulator/rectifier showing the cracked diode.

 

[Araon]

all your gen lamp indicates is that the gen o/p and battery are equal potential a fault either way and the lamp will glow.....

so not a lot really, you'll need to crack out the volt meter and see whats happening across batt terminals, dynamo and regulator....

 

[Pete B]

Sorry to sound like a condicending know all ('cos i don't), but too much pontification, not enough basic checking! Primary contender, the battery. Check levels, charge, test. (yourself or garage. easy either way) My money say it's the fault. If it is ok, then is it being charged? Volt meter, 12.7 rising to 14.5 when reved, it's charging. If not, more involved to identify dynamo or regulator box, but still easy! It is a simple but effective system, doesn't take much sorting. Worst case, take it to Kwik Fit, they will sort it in a few minutes!

 

[mjknight71]

Yes MJ so whats your suggestion then to fix his problem :?: and a rectifier/regulator does contain diodes will post picture of my old voltage regulator/rectifier showing the cracked diode.

I think Pete has covered a logical fault finding 8) ....comment wasn’t to offend but a rectifier and regulator are totally different things and when you are talking about a dynamo and alternator this is fundamental to replacing parts ......…..

........In very simple terms, generators work because any coil of wire rotated in a magnetic field produces an electric current. The conventional generator consists of a rotating shaft, on which is wound copper wire, called an armature. The armature rotates between fixed coils of wire called the field windings. The field windings produce a magnetic field which in turn generates an alternating current in the armature winding. It should be noted that conventional generators can only produce alternating current in the armature. The alternating current is converted to direct current by the commutator. The commutator consists of a series of copper segments separated by insulating material on the end of the armature on which two fixed brushes conduct the current away from the armature. However, the voltage produced by the generator increases as the engine rpm increases. Voltages can be produced that are sufficiently high to cause lead-acid batteries to overcharge, damage accessories and even generate sufficiently large currents to damage the armature winding. In addition, when the engine (and generator) are stopped the current flows in reverse from the battery through the armature winding causing the battery to flatten and possibly damage the armature winding by turning it into a motor.The generator has two field windings that produce a magnetic field through which the armature rotates. The armature produces the electrical power to recharge the battery and operate the various accessories such as the lights and ignition coil. A voltage is applied across the field windings to produce the magnetic field. The regulator controls the voltage and current delivered to the battery and car accessories. The voltage regulator keeps the voltage at a constant value and therefore controls the output in accordance with the requirements of the battery and any accessories operating. When the battery is low or power consuming items such as headlights are on, the generator output is near maximum. But when the demand for power is very low, the voltage regulator limits the generator output so as to protect the battery from overcharging and protect the electrical system from high damaging voltages. When the generator output voltage is low (at low engine rpm), the full output voltage is applied across the field windings of the generator. When the generator voltage reaches the value at which the voltage regulator is set, the generator field current then passes through the resistor. This reduces the current flow through the generator field windings and so decreases the generator voltage and current output. The resulting reduction in the output of the generator causes an increase the generator field (reverse of above) and output increase. This cycle is then repeated and an oscillation of the armature contacts is maintained. The average value of the output of the generator remains the same even though the speed of the generator may be increased. The cut-out is simply a switch connected in the charging circuit between the battery and the generator. When the speed of the generator increases, the voltage output increases until it reaches a pre-set value, then the current from the generator flows........on older regulators you could adjust the cutout voltage levels etc. but the bus has a sealed regulator.....in an alternator, the rectifier diodes (a alternator is AC) act as the cut off switch as well as rectifying the output to enable the DC battery to use it...regulation uses a similar feedback loop as described above............. modern alternators have internal regulators 8)

 

[skyflyer]

Thanks for the info so far which with the less than comprehensive explanation in Haynes has just about got me to understand what was going on.

So - today I got the voltmeter going...

First surprise was that the van started first time, despite not having been used for several days, but then the weather has been a bit warmer of late and I know batteries tend to fail when its cold.

Voltage when not running was 11.77.Once started it stabilised at about 13.6 to 13.8V. Slight increase if you rev the engine, but I have tickover set quite high in any case during winter, so difficult to see much variation.

So, I think we can say with certainty that the genny is producing adequate voltage to charge the battery. On the face of it there is no 'back circuit' discharging through the genny whilst the car is parked or else we wouldnt have started today.

Without getting into the more complex tests advocated by haynes to check the current output etc., I think theres enough evidence here so far to risk spending out on a new battery.

Cheers