B7800 Question

[Dave M7040]

rScotty

I am confident this tractor has a dynamo.

The B7800 is actually a newer tractor as things go since it has a no charge warning light. This model never had a true DC generator with brushes.

The old B models (5000, 7100) did not have a no charge warning light and the 5,000's actually had three wires from the dynamo not two.

One pair of the three wires went straight to the headlight switch as AC and were never rectified to DC. The other pair of the three were rectified to charge the battery.

Owners who switched to LED bulbs immediately had problems that were very confusing. The LED bulbs would not operate on AC.

The L series tractors are the ones found with a variety of charging systems from brush style generators, to low output North American style 3 phase alternators with mechanical regulators.

Just a rambling bit of Kubota history........

Dave M7040

 

[rScotty]

rScotty

I am confident this tractor has a dynamo.

The B7800 is actually a newer tractor as things go since it has a no charge warning light. This model never had a true DC generator with brushes.

The old B models (5000, 7100) did not have a no charge warning light and the 5,000's actually had three wires from the dynamo not two.

One pair of the three wires went straight to the headlight switch as AC and were never rectified to DC. The other pair of the three were rectified to charge the battery.

Owners who switched to LED bulbs immediately had problems that were very confusing. The LED bulbs would not operate on AC.

The L series tractors are the ones found with a variety of charging systems from brush style generators, to low output North American style 3 phase alternators with mechanical regulators.

Just a rambling bit of Kubota history........

Dave M7040

Well, that's interesting. Thanks for the Kubota history. I enjoy hearing about the differences between the early Kubota's and Yanmars - two very different design philosophies competing head to head for a relatively small market.

Not surprisingly, they came up with variations on how to build a generating system. Here's an interesting question: How many times do you think you could start that B7800 on a normal group 24 automotive replacement battery if you didn't use the battery for lights for anything else - and only used it for normal starts?

Without any thing to back me up at all, just purely guessing, I'd guess you could start it every morning for weeks, maybe a month.... What do you think? If so, that's almost worth just gluing a flexible 12 volt solar cell to the hood and calling it "done".

I think that what you are calling a dynamo in auld "Brit Speak" is what we tend to call an "Alternator" in the states. If so, it will have a diode plate with 4 diodes for rectifying the AC to DC. Somehow the naming convention didn't ever extend to giving us a clue as to whether the plate containing the diodes was located hidden away inside the alternator body or somewhere remotely & more accessible. For awhile some low power alternators even tried to put the rectifier circuit into the body of the voltage regulator - although that wasn't optimal for heat dissipation.

Those old Kubotas with 3 wire output, must have had real flickery lights & RPM sensitive, too. Were they able to also turn on the lights without the engine running? That is a handy feature of DC lighting for putting the tractor away in the evening. AC lighting seems like a hard way to solve a simple problem. I wonder if the coils were centertapped or conventional?
AC lighting doesn't make much sense to me. I don't see the advantage. But having had old motorbikes with AC lighting, I've experienced the disadvantages.

High peak to peak AC voltages from the alternator (in the 40 volt peak or even higher range) are usually the result of designing to compensate the magnetic field into a narrow area within a small coil. That is done in order to use much less expensive low-output permanent magnets and cheaper coils back in the days when poor old Alnico was the best permanent magnet available. Mechanically that design philosophy is identifiable within the alternator rotor by the toothed or triangular shape of the metal that focuses the magnetic field across the coils.

The other way to solve the problem of generating output without such high peak to peak voltages is to use a broadly focused magnetic field from a more expensive magnetic substrate to interact with a larger coil for a longer time (dB/dt). However, this second way - although better from an operational standpoint - does require more copper in the widing of larger coils -i.e. yet another expense. The advantage to doing it that way is to generate the same output wattage at around half the peak voltage.

And now I'm doing the rambling and the OP is no closer to getting his problem solved...
rScotty

 

[Dave M7040]

rScotty

My first exposed to dynamo's was on a bicycle with a lighting system rubbing against the front tire. The faster you pedaled the brighter the light. Stop and the light went out.

3 wire dynamo ..... non running engine means no lights. Brightness increased with rpm.

I think the AC lighting was because the losses in the rectification process were eating up a significant portion of the dynamo output which was not great in the early days.

After all, the rectifier is a finned aluminum body just to dump the heat produced in the rectification.

A friend with an 1980 Suzuki 1100 motorcycle with a fairing put on a switch to cut off the headlights to aid his battery condition by allowing more current to flow to the battery.

What he failed to appreciate was that the alternator/dynamo was a uncontrolled output device with the output only being changed by rpms. When the battery did not accept any more amps because it was full and with the headlights cut off, now the rectifier had to dissipate the total output of the dynamo. First. came the smell of electrical failure followed by a charge warning light turning on. It was an expensive lesson.

Your question regarding the time the tractor could operate without being charged by the dynamo.

A Group 24 battery is rated to 70 to 85 amp hours.

Using the 70 figure and converting it to amp minutes I get 420 amp minutes.

The glow plugs are the big part of the answer. Assuming they draw 30 amps for 1 minute, 420 divided by 30 says you could do a glow plug cycle 14 times neglecting the starter draw.

If the engine would start immediately without glow plugs, say 4 seconds of cranking at 150 amps. This is 600 amp seconds.

The Group 24 battery would have 420 amp minutes x 60 seconds equals 25,200 amp seconds

25,200 amp seconds divided by 600 amp seconds equals 42.

The engine could be cranked 42 times at best.

Your point that the tractor could operate for some time without a charging system is valid.

Solar charging during a Colorado winter... that would be an interesting experiment.

Dave M7040

 

[GKChesterton]

Guys, thank you for all the help and advice. It took some time to work through the various steps because I never had much time to work on the tractor during hunting season. Narrowed the issue down to the Regulator - as you predicted - which I replaced this week and I am getting good voltage at the battery now. Really appreciate the insight and advice. One word of note, replacing the regulator was not as difficult as I expected. Dash removed fairly easily and I did not have to pull the steering wheel off. The only tricky part was getting enough space to unscrew the tachometer cable.