Can you damage a dynamo?

[Kylora]

I know that you can damage an alternator by running it with no connection to to the output, but can you do the same to a dynamo (which is self excited)

I have a Volvo MD6A, with an alternator supplying the domestic battery, and a dynastart connected to the engine battery.

I noticed that the dynastart warning light was on, and on investigation found that the (charging) output cable had broken off the terminal.

I've replaced the cable, but no output, only 0.5 volt. The unit still works OK as a starter.

I have a vague memory from my time on a ship with DC mains, that the cure for a Genny which wouldn't self excite, was to give the casing a mighty hit with a big hammer!

Is it likely that the shunt coils have burnt out? The dynamo would have been pruducing volts which would have been supplied internally to the shunt coils, but the regulator would not have been receiving volts from the output cable, and so wouldn't regulate. I don't know how high the volts might have risen, certainly the engine wasn't running very fast.

Any thoughts would be appreciated.

Ash

 

[halcyon]

The best bet is to take it off and get your local LSUK to check it.
You may have lost the mag in the armature, if the cable hit the body when it broke it would have caused a short and it may have damaged the regulator
If the reg is damaged you do not want to know the price of a new one !!!!.
May be easier to use the dynastart as a starter, and fit a split charge relay and use the alternator to charge the engine battery.

Brian

 

[halcyon]

When the power lead broke off, it may have damaged the charge coils, or lost coil mag, but I would have though the starter usage would have put this back.

What size cable was it ? I am just wiring our MD6A up and have 25 sq mm cables for main feed to dynastart.

Brian

 

[Kylora]

Hi Brian

Thanks for the ( LSUK ) link.

In my initial checks, I seemed to be getting higher volts (about 4 ) at the D+ terminal of the regulator. The wire may have been hanging on by a single stand and only broke off completely when I put my hand down behind the dynastart when the engine was stopped.

I can temporarily bypass the function of the regulator by shorting the DF wire to earth but still no volts.

I also vaguely remember that you could change the polarity of a car dynamo from positive to negative earth. I seem to remember that you had to flash the positive battery terminal on to the DF terminal - to remagnetise things with the opposite polarity ?

I'll probably follow your plan of using the dynastart only as a starter and linking battery to alternator - split charge relay or even just a diode - the engine always starts easily so doesn't use up many AHrs.

I'm hoping to replace the MD6A with a Beta before next season, so certainly don't want to spend a lot of money on the Volvo.

BTW - websites for old 3 wheelers have good info on dynastarts - see

http://www.isettaofsc.homestead.com/Dwnloads.html

for example.

Ash

 

[Kylora]

Hi

Just found your second post.

The start current cable, 30h to big terminal, is certainly hefty.

The output cable, D+ to D+/61, is probably about 4mm.

I also feel that I've somehow lost/ reversed the magnetism. I've removed and insulated the D+, and DF cables from the regulator. I'll recheck after using unit as a starter a few times.

I wonder if I can isolate DF cable from earth - apply 12 volts to D+ - and measure voltage at DF to see if I have continuity through shunt coils.

I assume that if the unit works as a starter, then the brushes and armature must be OK but I may be mistaken.

Ash

 

[stevebirch2002]

Hi Ashley. If the Dynastart isint charging then I expect teh charging coil has burnt out. You can still use the starting part of the DYnastart and charge both batteries via the alternator via a switch system or split diode. Many Vegas have done this when the Dynastart charge coils burn out as tehy are expensive to rewind. Cheers

 

[William_H]

I think the starter section uses the same field coils or at least the same iron for the magnet so use of the starter should reinforce the residual magnetic field each time it starts. The brushes may however be different to different windings on the armature due to the need for higher voltage and less current. I might expect then that your problem could be worn brushes.
I doubt the field coil would burn out even though it has been running with full field current trying to bring the output voltage up (and failing cos it is not connected) I reckon a burnt out field coil would be obvious from smell although a continuity check may be worth while. Are you bringing the engine up to reasonable revs when testing? It is likely the dynastart won't charge at idle speed.

good luck.....olewill

 

[Kylora]

Hi

Thanks to everyone for their advice.

I did have a quick look under the cover for the brushes but didn't poke about too much as I don't want to lose the starter facility. The brushes look a good length, and I couldn't see anything obviously wrong. I could see the heavy starter coil connection to one of the brushes - on the same connection is a much smaller braided wire - I assume that this is the positive end of the generator coil. What I found strange was that it was uninsulated - I wondered if I had burnt the insulation off.

I'll do further checks and report back.

Ash

 

[VicS]

I don't know anything about Dynastarts so have kept away from this one but I will point out that what you said in your opening remarks when you first posed the question is not strictly true. You do not damage an alternator by running it disconnected but you will wreck the diodes if you actually disconnect it while it is running. Because dynamos do not have diodes you cannot damage those in the same way.

 

[bluedragon]

If the regulator has burnt out, then LSUK do have an alternative for about £50 and it's way better than the original Bosch model. I can get a full 12A out of mine now with the new reg. If you haven't done so yet, take the Dynastart to LSUK. They will test it on the spot free of charge. A completely reconditioned exchange unit will cost something like £275.