Tractor Electrical Systems

Electrical systems have been an important part of nearly every tractor built after steam engines. From the most basic magneto systems to today's complex computerized tractors, the electrical systems use in tractors have evolved as much as the tractors themselves.

Starting as early as the first gas powered tractor, tractors were already using electricity in the most basic electrical system that tractors have implemented. The magneto is an ignition system that is a self contained generator, ignition coil, and distributor in one package. All gas powered engines are ignited with a spark. This spark requires electricity. I wont get into the specifics of the magneto here, but they are well worth noting due to their simplicity, importance, and how long they were used. The most simple magneto system simply operates as long as the engine is spinning. An example would be the Wico X magneto found on my unstyled John Deere B. there's no buttons, no switches, only 2 spark plug wires. You cant shut the tractor off by turning the ignition off unless you physically pull the spark plug wires out. One step above this system was a magneto with a kill button like the one on my John Deere L. The button simply connects the ignition coil to ground and prevents the magneto from making a spark. This still requires getting off the tractor to shut it off. Which is where the next upgrade comes in, a switch by the operator. You'll find this setup on my Farmall B. It works just like the button, but its one wire that goes to where the operator sits and connects to a switch. Magnetos are great because of their simplicity. A properly tuned tractor with a magneto starts easily even without a battery. And isn't prone to stalling because of a failing battery. Although many tractors have had additional electrical systems while using a magneto, many tractors that used a magneto didn’t have any lights or electric start because the magneto does not provide any power that it doesn’t use itself. Magnetos began to fade in popularity around the 1940s when electrical systems were beginning to get more advanced and electric start was beginning to become more common and affordable. Magnetos definitely aren't a thing of the past though. Although tractors had advanced to battery ignition many small engines continue to use magnetos even today. With the introduction of battery ignition though magnetos became the more expensive and more complex option.

The addition of a generator and a battery dramatically improved the functionality of the tractor. Later on in the 1920s and 1930s generators were sometimes an option. Lighting systems became possible, electric starters became practical, and ignition systems got simpler. These early electrical systems were typically 6 volts and used positive ground. Farmers could work into the night with headlights, starting the engine became much easier and much safer with electric start as well. And with electric start it became practical for engines to get bigger. The early generator systems were very crude but ingenious too. Lights and batteries don’t require very clean power. They tolerate voltage dips and spikes very well. Because of this it was easy to implement a system that didn’t need any regulators. This was the beginning of the era of the 3 brush generator. Ill cover how they work shortly. The only additional device needed for a 3 brush generator to work was a cutout relay that disconnected the generator from the battery when the ignition was turned off. Otherwise the battery would back-feed through the generator when the engine wasn’t running. Regulating the generator was handled by the light switch. And ill get into this shortly as well. Battery ignition systems became common. The ignition system was stripped back to being an ignition coil that didn’t have any moving parts, a distributor that barely had any wear parts, and a set of points that controlled the ignition timing. It did away with the entire generator part of the magneto which was sometimes known to cause problems. The biggest drawback to the 6v system was its weakness to dirty connections. Tractors of the time could start even when cranking slowly, but at 6v there wasn’t much tolerance for resistance between connections. 8V batteries were sometimes used to offer a band-aid to the problem. 6V systems were eventually replaced by 12v. And generally it was pretty easy to convert a 6v tractor to 12v. The generator didn’t care what voltage it was putting out as long as it was spinning fast enough and it was being regulated correctly. Most 6v starters were very tolerant of 12v, and ignition systems only needed a resistor to work with 12v safely. Even the transition to negative ground was usually easy. Most starters turn the same direction regardless of the battery polarity, generators could easily be swapped to reverse polarity, and ignition systems only needed a few wires moved around.

The 3 brush generator was a “self regulating” generator that used 3 brushes inside the generator. The first brush is connected to the case of the generator, this is the ground brush, the second brush was on the commutator directly opposite, this was the brush that connected to the output terminal. The third brush did the regulating. This is called the field brush. And it is connected to the field coils inside the generator then the field coil connects to the field terminal. You can learn a lot about the internals of a generator by looking up how a brushed dc motor works. Its almost the same except for the field brush. The basics though are that the armature, which is the coils of wire that spin inside the generator, create an electrical current when they pass through a magnetic field. In a motor this magnetic field is created by magnets on the side of the motor. In a generator this electric field is created by electromagnets that we call the field coils. Using electromagnets dramatically improves the power output compared to using permanent magnets. But electromagnets require electricity, and this is where the 3^rd field brush comes in. it provides power to the field coils. More power to the coils means more power from the generator. This is controlled in two ways. Typically the field brush is adjustable. Move it closer to the main brush for more maximum power, move it away for less maximum power. This adjustment cant be done in the field though, and this is where the field terminal comes in. by controlling the connection to ground you can regulate how strong the field magnets get. More resistance reduces output, less resistance increases output. The resistance limits the amperage that can flow through the coil, and amperage directly controls the strength of an electromagnet. The light switch contains a number of resistors and as they are connected one by one the resistance on the field terminal drops which increases the generators output when you turn the lights on. This is “self regulation” in the crudest way. At an idle the field coils aren't getting enough power to generate electricity, high rpm for extended periods would create too much power and slowly damage the battery unless there was manual intervention. The best setting you could get with a 3 brush generator was a slight overcharge, any less and you were running your battery dead or at least not charging. And this also depended on engine rpm. Just by revving the engine form idle to working speed you could transition from running the battery dead to overcharging the battery. Charging currents were generally pretty low with these generators. Often only 5-10 amps max which helped limit the amount of overcharging but it could also lead to long times required to recharge the battery. I mentioned previously that these generators could be used in positive or negative ground configurations. This is thanks to the field coils being powered by the generator itself. Since the coils are powered by the generator, if it stops spinning it loses its magnetic field and it cant create power even if you start spinning it again since there's no longer a magnetic field to move the armature through. This is where a process called “polarizing the generator” comes in. by shorting the field terminal to the battery terminal on the cutout relay it forces the field coils on at full power. This magnetizes the iron core inside the coils much like how magnetizing a screwdriver tip works. The iron core will usually keep this magnetic field for a long time and this process doesn’t have to be repeated unless the generator is serviced or left unused for a long time. This provides just enough magnetic field that the generator can power the field coils next time it starts spinning though. The important part is that it can be polarized in either direction and it will change its polarity just by doing this. There are a lot of other more detailed nuances about 3 brush generators that I could cover, but that's probably best left for another time. 3 brush generators faded out of popularity when the 2 brush generator came on scene in the early 1940s.

As electrical demands in tractors grew something better than the 3 brush generator was needed. Something that could be regulated better, and that provided more power. The 2 brush generator was the answer. The 2 brush generator was controlled by a mechanical voltage regulator. This consisted of 3 separate regulators inside the same box. The first was a cutout relay. It was needed for the same reason a 3 brush generator needed one. The other two coils controlled the field terminal. The field coils were now powered by the battery though. This gave them full strength regardless of rpm. Tractors could now charge at lower rpm than before and at higher rpm. And the mechanical voltage regulator could keep the system happy even with varying loads by varying the current flowing through the field still. One coil is the cutout relay. The second is voltage controlled, and the third is controlled by amperage. When voltage or amperage would rise too high the associated coil would open a contact and disconnect the field from ground. Current was then forced to bypass through a resistor that would gradually reduce the current through the field coils, reducing the generator output, until the voltage and amperage dropped and the coil connected the field to ground again. This process could happen tens to hundreds of times per second. The result was a battery that didn’t get over charged despite the load on the system or the engine’s rpm. And it could provide plenty of power, sometimes 40 to 60 amps. It also gave freedom to add to and change the electrical system without re-tuning the generator. Another benefit to 2 brush generators and the fact that the field coils are powered by the battery is that the generator can generate significant torque if power is applied to it. With some minor design changes to allow the generator to tolerate high current for a short period the starter generator was created. Starter generators were mostly limited to small engines, but it provided a cost effective way to add electric start to many small tools. Typically a starter solenoid would apply battery power directly to the generator which would turn the engine over. Then once the engine started it would immediately start generating power to recharge the battery. As always though, technology continued to advance, and around the 1960s there was a new device that was better yet.

Alternators using semiconductors started appearing tin the 1960s. Alternators quickly became popular for their low maintenance, high output, and clean power output. Some generators only needed one terminal connected directly to the battery. The regulators were solid state and built into the alternator a lot of times. Many older tractors were converted to use alternators. Around this time 12v and negative ground became standard too. Mostly because most alternators were made for this configuration. The clean power output was becoming more important on tractors as more sensitive equipment, such as radios, were finding their way into tractors as well. Although alternators still provide power to the electrical system, they operate very differently than generators. Alternators create an alternating current similar to what you would find in a building. Then semiconductors are used to convert the AC current into the direct DC current that a 12v system needs. The difference in the design of the brushes of an alternator meant that there was much less electrical noise generated by the alternator. This was important to radios that would pick up electrical noise in the form of static, and it became critical in more recent years as tractors began incorporating computers to control various systems. The alternator continues to be the favored device for generating electricity on tractors.