DC motor & alternator in one unit?

I used something like that to start the engine on my boat. It came off an old "Kohler" single cylinder petrol engine that was used to drive the pump on a milk tanker. (where I'm from tankers pick the fresh milk up from the farms & carry it to the dairy plants). It was a DC starter that ran as a generator once the engine was running. It wasn't an alternator though - just a DC generator. You can get units like that still though I'm not sure where you'd go. Try a motor rewinding firm for a start.

Unless you’re building something that’s going to be used as a prototype for production rather than just a science fair project you’re not going to want to involve yourselves with complicated electronics to control the motor / generator. I know a chap that electrified a ’72 VW beetle & used thyristor control etc. The electronics cost him more than the batteries & motor put together. The motor you get will most probably be a shunt motor, that is the field coils are in parallel with the armature so for charging the batteries a simple voltage regulator should suffice & give you about 15 volts from the generator to force charge into the batteries. Though it will help it’s never going to compensate for the charge you spent climbing the hill. For controlling the motor you won’t be able to control the current going into the armature because it will be too high especially when the motor is just starting up. You’ll need to control the field current & you can do that with a variable resistor (which will waste a little power as heat) or a bank of resistors arranged so you can switch them in or out. You might get lucky & find what’s called a series motor which has the field coils in series with the armature such that armature & field current are the same. The great thing with these is that although when they are not coupled to a load they speed up & run to destruction, when they are coupled to a load they apply maximum torque at start-up & as they speed up the back-EMF caused by the armature rotating within the field limits the current going into the motor - in effect self regulating. Diesel-Electric locomotives sometimes use this arrangement. Conversely when used as a generator & there is a lot of torque available to drive it as when coasting down a steep hill the armature will generate a lot of current which will strengthen the field, apply a stronger braking effect & more voltage to charge the battery. Use a big battery though as it should be better able to absorb a heavy charge when applied to it & can cope with a heavy discharge. You can also get compound motors which are a combination of series & shunt wound motors. They’re often used in applications where a heavy starting torque is required or where the load is unstable, e.g., big ones are used on winches on big trawlers.

Any DC motor can serve as a generator (a generator is what you need, not an alternator) to recharge the battery, as is done in hybrid autos today.

So yes, this is possible. The problem is the complicated electronics to direct the flow of energy to or from the battery, to charge (and not over charge) the battery correctly, or to draw the correct amount of current from the battery. Hopefully someone has already solved this problem and you can find the electronics on the internet somewhere...

I'll make a search later if I have time...

.

Don is correct about the voltage/power relationship. But a car alternator is not DC. Hence the name alternator. The output is AC and is converted by rectifiers to DC.