Tractor First Drive!

[David Dymaxion]

I'm converting a 43 year old John Deere garden tractor to electric power. I drove it tonight! It was just off a 12 Volt battery, and there's more to do, but hopefully I'm mowing electrically in a week or two. This tractor is really heavy duty -- the motor and hitch area are 1/4 inch steel, and there are videos of these towing Chevy trucks up hills. With a 400:1 first gear it has thousands of pounds of rear axle torque possible.

This is actually my 2nd tractor. The first one took 200 Amps to just move -- it turns out the tranny was shot. I now call this old piece of junk "the tinfoil tractor." If someone would like the electrical part of it, or a project, send me an email.

The John Deere has a 4 speed tranny. I was very happy to see it would drive up hill in 4th gear at 45 Amps, and up a grassy hill at 80 Amps in 4th gear. It idles at 13 Amps. In lower gears it runs at about 30 Amps. Going down hill the current draw drops to about 1/2. For fun I pushed it while someone else was driving. The Amps went negative, meaning I was getting regenerative braking!

It has no controller, just a shunt motor. Shunt motors are naturally constant speed, and were used for elevators and conveyor belts. If pushed faster than key speed a shunt motor naturally regens. Since the tractor was designed to work with a constant speed gas motor, this is a perfect fit for this application and saves the cost of a controller.

[Kelly Hales]

Cool Dave!

Since I can't really afford converting my old Datsun yet, I've been wanting to do a yard tractor too. I'd like to find one with a bad gas motor cheap, just haven't really looked yet. How much voltage are you going to run when finished?  Where did you find your "glider" lol?

Sent from my iPhone

--
--
http://groups.google.com/group/UtahEVinterest
 
---
You received this message because you are subscribed to the Google Groups "UtahEVInterest" group.
To unsubscribe from this group and stop receiving emails from it, send an email to utahEVinteres...@googlegroups.com.
For more options, visit https://groups.google.com/groups/opt_out.
 
 

[David Dymaxion]

I'm planning to run 48 V, as the motor is rated for 36 to 48 V.

You can have my old tractor if you want it -- send me an email if you want it. If I were to continue playing with it, I'd gut or throw out the transaxle, and figure out a belt drive to each rear wheel or something. You'd have the facilities to do that at BYU.

---

From: Kelly Hales <kelly...@gmail.com>
To: "utahEVi...@googlegroups.com" <utahEVi...@googlegroups.com>
Sent: Monday, June 3, 2013 8:36 AM
Subject: Re: [UtahEVinterest] Tractor First Drive!

[Kelly Hales]

Sure I would love to have it!  I'll call you about it later this week. Pretty sure I still have your number. 

Thanks Dave!

Sent from my iPhone

[David Dymaxion]

Rather than build batteries boxes, I just threw the batteries onto our wagon and did the wiring instead. This way we could verify we really wanted 48V. I'm also hoping to slip in a lawn mowing soon by towing the batteries on the wagon. It was really nice to just twist the key to start, rather than quickly connecting and disconnecting arcy sparky jumper cables!

At 24V all worked well. At 36V we blew the automotive 175 Amp fusible link, even though we were getting current draws of only around 20 to 35 Amps. It was probably a mistake to put a 12V automotive fuse in a 36V system. We put in a big 200A, 72V rated fuse and everything was fine.

On 48V nominal the motor spins 3300 rpm. That's about perfect, the gas motor was nominally 3600 rpm.

As I expected, the Amp draws did not go up significantly with higher voltage, even though the tractor was much faster.

In 4th gear, it is a pretty honest run to keep up with it. It is way faster than my old lawn tractor! It was also nice, though, to put it into first and crawl up the curb. 4th gear uphill up our street at a running pace was about 50 Amps. Downhill was about 17A. The regen is nice, the tractor holds a nice steady speed downhill.

There is more work to do:

o  The clutch is dragging. I had adjusted it to get belts on and off, so it probably just needs adjustment.

o  I used the starter relay to switch power to the field. It gets hot, apparently it is only designed to be used briefly to start the motor. I can either replace it with a real contactor, or put on a coil economizer. A coil economizer is a simple circuit with a capacitor and resistor that lets the relay get a full kick at first, to slam the contacts home, but them lessens the current to hold the state.

o  I calculated the motor should draw 124A at 48V to make its rated 8 hp. We never ran over 1/2 that. I thought the motor was hotter than I expected after some testing. However, we were spinning the motor at 1/2 speed on 24V for much of the testing. Slower motor = slower internal fan speed = less cooling. We'll try 48V and high fan speeds the whole time next test.

o  I'm going to add a reed switch that will guarantee the field has current before switching on the armature. If the field ever breaks connection, the motor could quickly overspeed and/or overcurrent. This will also let the high inductance field build up some current before switching on the armature. This will prevent the very brief current surge and higher rpm if you switch them on together (the field builds up current much slower than the armature in a sepex motor, so the motor draws a brief surge of current and spins fast as the field catches up a fraction of a second later).

o  I need to still do battery boxes.

On a personal note, my son and daughter have been helping me with this project. It has been great fun!