Lawn tractor conversion advice needed

Lawn tractor conversion advice needed

I have a John Deere lawn tractor with a variable transmission. I purchased a manta II permanent magnet DC electric motor and 4 deep cycle 12 V batteries. I thought that this would be an easy set up because I did not need a controller. The gasoline motor I am replacing runs at full speed all the time and operating the transmission determins the speed of the lawn tractor. When I read the small print though that came with the electric motor it said never to let it spin above 4000 RPM. Further reading also confirmed that permanent magnet motors should not run at full speed unless there is a load attached to it.

Now I am confused. What kind of circuitry do I need to make this work? Do I need a controller after all? In my initial ideas I thought I would have 2 solenoids to start the motor. One which switched the motor on with 24 V and the other solenoid would engage a second later for the full voltage because I did not want to harm the motor or the battery by having too many amps go into the motor when it starts with a full 48 V.

I guess I could make an electronic circuit were the RPMs would be measured and then a transistor which needed to be able to handle about 300 Amps would pulse width modulate the 48 V so the motor would not exceed a certain RPM. If there is a load attached then the RPMs would go down and my circuit would give the full 48 V. This of course essentially is a controller.

Any suggestions on how to do this are much appreciated. Is there really an easy way to do this without any controller without harming the motor or is that just a myth and I do need a controller or somehow built my own circuitry to prevent the motor from spinning up unloaded. Thank you very much for your input

What type of transmission? Cvt or hydro?

Will you be mowing or just driving?

Lawn tractors with hydro transaxles are set up to run the motor at 3600
rpm. A Curtis controller with a tach input will do nicely. Contact Tony at FSIP

HOWEVER: Hydro Tractors are NOT very efficient.

I would Suggest a manual transmission tractor with a motor for the deck and a motor for the drive. This eliminates the need for controllers.

This has been done quite a few times already.

Go Here:

Tractor Forum : MyTractorForum.Com Tractor Forums

and here


Thanks for your reply but I already have the tractor. Also, I figured out that at 48V I am well below the rated maximum speed. So I don’t need a controller, I think.

However, the tech specs for this motor say " MOTORS MUST BE UNDER A HEAVY WORK LOAD AT ALL TIMES!"

That’s what worries me. Can anyone say what this means?

Also, if 24V is too much for a gentle start of course I could start it with just one 12V battery and then use a solenoid to add the other 3 batteries for a full 48V.

Thank you

Try it and see. The hydro will load it some.
Start with 12v and measure amp draw and rpm. From that you can figure approximate hp and torque delivered.

Do not use a belt or any coupling that can fail, or motor will overspeed and explode.

I suppose a switch on belt tensioner might be ok if there is no danger to life and limb.

I like building without controller. My mini-rail for kids uses 2 motors connected series or parallel.

In your case a shunt motor would be the safe choice. Pm next, and series motor bad idea.