Alternator or Dynamo?

[5030]

Electrical 101:

A dynamo has either a permanent magnets in the armature and a wound field or vice versa. As the magnets revolve they induce DC current in the windings which will charge your battery and run your lights but not much more. I remember my first Kubota had a dynamo about 20 years ago and if the tractor is still running, I would assume the dynamo is still working fine. If I remember correctly, the little dynamo put out around 3 amps which was fine for the little headlamps and keeping the battery charged. You don't need a votage regulator because it's wound for 12 volts and 3 amps or so won't biol your battery dry either. My little Kubota could be had with an acessory alternator too, but why have an alternator. The Dynamo is maintenance free.

My present Kubota's have alternators. The M9 has a 55 amp and the 105 has a 70 amp. Alternators are way more complicated. Alternators, as the name implies produce AC current. The AC has to be rectified to DC and the voltage has to be regulated to not more than 14.5 volts. An alternators forte' is output. A 55 amp alternator outputs 55 amps at it's rated rpm, usually more than 1000 but less than 3600. Same hold true for a 70 amp or a 105 amp. On bigger tractors you need alternators for the output, output to run AC units, cab mounted halogen lights, flashers, headlights and what have you.

 

[N80]

As far as the L Series goes, the L4400 has an alternator standard. The L2800 and L3400 have dynamos.

 

[Volfandt]

According to my BX23 WSM, a dynamo puts out AC voltage just like a std alternator which is both rectified and regulated by the outboard rectifier/regulator module.
At PTO rated RPM it can put out a max of 14 to 15 amps.
Not a bad charging solution for the basic electrical load on SCUTs & CUTs but lacking for the more demanding loads of additional lights, fans, radios etc.

 

[Bob_Young]

Well, permanent magnets on the rotor would make sense. They would replace the rotating field windings of the alternator, the slip rings/brushes for getting juice to the rotating field, and the possibility of opens in the field circuit. They would also eliminate the possibility of using field current to regulate output. Permanent magnets are much better & stronger these days, so it would seem workable; for awhile at least.

That would mean the power would be generated in the stator windings and it would be AC. If that's the case, Volfandt is correct in saying that the output must still be rectified.

If the permanent magnets were in the stator, the power would be generated in the rotor windings. This would require either slip rings/brushes & a rectifier to obtain DC or commutator/brushes (like the old DC generators). My guess is that a commutator is out of the question due to cost and that a rotating permanent magnet version would be the cheapest to make and most reliable.

There probably is a form of rudimentary regulation built into it, somewhere. Being wound for a low output would simplify the requirements. Maybe just clip the peaks of the AC sine wave at the stator output with some kind of SCR circuit. Whatever it is, is probably built into the dynamo.

Just thinking out loud here, trying to pull together what little I know plus what's been said. Being an engineer, I like to know what principle things operate on.

Besides, if I spring for the alternator kit, there's gotta be some good use for the dynamo that comes off
Bob

 

[Volfandt]

Well, permanent magnets on the rotor would make sense. They would replace the rotating field windings of the alternator, the slip rings/brushes for getting juice to the rotating field, and the possibility of opens in the field circuit. They would also eliminate the possibility of using field current to regulate output. Permanent magnets are much better & stronger these days, so it would seem workable; for awhile at least.

That would mean the power would be generated in the stator windings and it would be AC.

I believe you've hit the nail on the proverbial head
In the drawing and parts breakdown of the dynamo in my BX23 WSM, it appears that the stator is stationary and has the pole shoes with the induction windings wrapped around them. Since it is stationary the two wires that output to the regulator/rectifier do not use nor need brushes, a commutator or slip rings. The rotor on the other hand constitutes the shell of the unil and has NO electrical connection to anything so it must contain the permanent magnets. There is no additonal circuitry located in the dynamo itself, just the windings that wrap around the pole shoes.

The outboard rectifier / regulator is broken down somewhat in the schematic also and if my memory serves me correctly it contains a bridge rectifier, several reverse current / blocking diodes, a regulating SCR and it's associted circuitry. All the "smarts" of the chargeing system reside in the outboard Reg/Rect.

The dynamo seems to be bass-ackward from a std alternator's construction but is simplicity at it's finest
I doubt that the dynamo charging system could maintain a full 14 to 15 amp output continuosly but for the normal demands placed upon it by a small diesel tractor it should last quite a while. I added 2 35 watt ROPS lights to the BX23's std lighting load just after I bought it alittle over 3 yrs ago and so far with the limited amount of time I've run the tractor at night I've yet to have a problem with charging. That said I'd be upgrading to the 40amp alternator kit should I ever have the need to run brighter lights for longer periods of time.

 

[kwolfe]

Yes, the dynamo does not put out enough current to handle lots of lights. I have a BX1500 with the standard dynamo. I have 2 35w floods hooked up for when I need them. If I have them and the headlights on, I am close to maxing out the dynamo. I could have probably gotten away with two 55w and no headlights, but for the work I do, I would rather not take the chance.

 

[hagman]

I didn't realize that my dynamo question would get so much attention. That said, I have another question. I am assuming (yes, I know what that stands for) that if an alternator is added that a voltage regulator must be added also. So is the regulator external or internal and does the switch over require a different wiring harness? Ace Power feel free to chime in on this one as you even had the Kubota part number for the kit. Does Ace or anyone else know what comes with the kit and is it all that's required for adding an alternator?

Hagman

 

[Taiser]

The kit, if I remember comes with everything, including a new harness, which you are suppose to take the dash apart to install. But I think I saw a thread earlier that showed you did not have to do that and it could be installed without ripping the dash and the cover under it apart. Depends how creative you are I guess.

You also have to replace the oil dipstick tube but some members have said they managed to bend the old one instead so that it can work. Apparently you have to remove the side panel after installing the alt. to check the oil. Don't think I'm crazy about that idea.

 

[Northland]

I have a B7800 with a dynamo-as I have now learned. I upgraded the bulbs on my headights to 50 watt halogens whichg made a great difference in visibility. I then added a 50 watt halogen 4x6 tractor light from TSC to the rear aux worklight wire. After running the the lights for several hours one night my battery was dead the next day-oops! So, I added a switch to the rear light an only use it when needed. This set-up is OK for now but I will upgrade to the alternator at some point.