- Tractors use a hand throttle, you set it at a particular position and then leave it there. This is so that you can essentially set an RPM to suit what you're doing, for instance driving an implement on the PTO. A ground speed is selected by changing gears, so tractors have lots of gears. You can change gears while keeping the RPM more or less constant.
- I'm looking to convert a tractor rated at about 40 kW. Unlike cars, tractors are sometimes used at a continuous high power. So either the motor I use should be rated at 40 kW continuous, or I need to de-rate the tractor specifications.
- The brakes on a tractor are normally not very good. The low gearing means that engine braking is extremely significant, and is normally the only braking required to avoid runaway when driving down hills. However, on some propertiess (e.g. here) there are no hills, so this is only required when driving the tractor on the road once in a blue moon.
An AC induction motor would tick all of the boxes, but they are expensive!
My question, then, is how about a parallel (shunt) wound DC motor? The DC motors that I've worked with for EV conversions were provided with the connections so that you can wire them in series or shunt. But I have not used one in shunt mode, and I have some questions about how it would work.
Matching up with my numbered points above:
- Shunt DC motors apparently are speed controlled. So on the face of it, this looks like a good solution for the way that tractors are driven. Set the throttle lever to a position and the controller will deliver torque until the requested motor speed is reached. Push in the clutch (take the load off) and the motor speed will remain constant.
- I assume that motor power ratings are the same whether wired as series or shunt.
- What happens when a shunt DC motor is pushed faster than the requested speed? Does it just 'coast' faster and faster like a series motor? Or is there some kind of engine braking effect? If there is engine braking, where does the power go? If there isn't engine braking, is there another way of causing this effect (e.g. shorting the terminals). This situation is rare enough that regenerative braking (putting power back into the batteries) is not required.