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