Dynamo queston

It doesn't have permanent magnets. They are just iron cores which are magnetised by the field coils. In this way, varying the current in the field coils also varies the output of the dynamo. A permanent magnet dynamo would have an output that simply varied with engine speed - like a bicycle one feeding the lights. So would end up overcharging the battery on long runs with little other load, like in daylight.

Early systems had a simple low high charge switch on the dashboard. Later ones an automatic regulator.

It doesnt have a permanat magnet. The residual magnetisim in the iron starts the exciting process. If the dynamo has lost this due to standing then you need to flash it by putting it across a 12v battery, (or 6v it it's a 6v dynamo) in the polarisation that it will be used in the car. The dynamo will motor up when connected to a battery, which also tells you if it's working alright. It should pick up quickly and level off at a moderate to fast speed.

The only degredation a dynamo will get over the years is breakdown in the laquer in the windings, which will result in some windings, stator or rotor becoming short. In this case it will draw huge amounts of current when being used as a motor, and will fail to charge efficiently when used as a dynamo

Alex

Go for Cords ;-)

Rich

Made me laugh though..

I wouldn't recommend trying it as a motor. A 12V battery will spin it fast enough to destroy the commutator by centrifugal force.

BTDTGTTS

Jim

Great answers thanks chaps. I will "motor" it then spin it up on a lathe, carefully monitoring the current at all times.

Thanks again, I am hopeful! :-)

Pete W

Bad example, I'm afraid. Bicycle "dynamos" are actually alternatos, and that's precisely because the output doesn't vary directly with speed. The increasing reactance of the coils as speed and frequency increases serves to choke off the output. That's why the lights work at 5mph but don't burn out instantly at 30!

Yours pedantically,

Ian

Hi, We also have a 1936 MG (mmm.) My hubby asks me what car have you ? If it is a 2 brush dynamo with a regulator, the best way to test it would be to put it on the car . Pat.

He'll be round your place in the morning with his dynamo and a set of spanners.......

Alex

:-) Pat

Phwoar! On my way! :-)

Pete W

That's simply rubbish. Some types may be alternators but others are dynamos.

A dynamo generates DC ie: it is always flowing in one direction even if it isn't entirely regular.

An alternator generates AC ie: it flows one way then the other but averages out to 0VA or thereabouts.

A dynamo has a segmented commutator (it's been a while, correct me if I'm wrong) which is basically a rapidly rotating switch. At high currents this is not a good thing. Arc welders are designed on the principle of being very bad switches. Good switches are designed to avoid being arc welders.

An alternator has just a simple slip ring with no segments and so no switching. Because there is no switching the current can be a lot higher than a dynamo of a similar size.

A bicycle dynamo does indeed increase the output with speed but most decent set-ups include either a regulator or a Zener diode which goes a long way to preventing the voltage from rising to a killer level.

The traditional variety are alternators, for precisely the reason I give. And also becuase spinning permanent magnet past a fixed coil is the easiest way (one moving part, no slip rings/commutator) to make a generator.

Ian

Perhaps modern posh ones do, but good old traditional "dynamos" rely entirely on the self-limiting nature of alternators.

For those who like the maths ...

At angular velocity / frequency w

EMF output E = k w (the fast it goes the higher the voltage)

Internal impedance Z = j w L (inductance dominate resistance)

Load R has voltage V = E . R / (R + Z) = k w R / (R + j w L)

At high speeds, V -> k R / j L, which is a fixed 90 degree lagging value.

Ian

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember "Ian Johnston" saying something like:

As a point of interest; are hub 'dynamos' (whether traditional or modern, dynamos or alts? I've never had one apart for a look.

They contain a multi-segment soft iron core onto which a coil is wound forming the part that doesn't rotate, and the two ends of the coil are the two contacts from which the wires from the lights run. The part that rotates with the wheel contains a multi-pole magnet - I think it was 12 segments, but the last time I had one apart was over 40 years ago, and I can't be sure now. A magnet running inside a fixed coil gives you an alternator, I believe.

At slow speeds, the lights used to pulse visibly, but over about 6mph the pulses merged to give a continuous light.

Jim

The only one I've looked at was an old Sturmey-Archer, and I'm pretty sure that was an alternator.

Ian

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember "Ian Johnston" saying something like:

Yes, it makes sense that they'd go for something that would produce a usable amount of output at slow speeds; something with a number of poles on it.

No, the reason an alternator has small brushes relative to its output, compared to a dynamo, is that they're only carrying the field current not the output current.

Because a dynamo needs mechanical commutation to rectify the output, the output is on the rotor (and is thus switched by the brushgear). An alternator uses diodes to rectify the output, so this allows it to be "turned inside out" relative to a dynamo and the low-current section mounted on the rotor.

As there's obviously more volume in the machine around the rotor than inside it, windings of a given size (set by the max current) are thus easier to fit in the stator than in the rotor. This also tends to make alternators smaller than dynamos, for the same rating.