Running alternator as a motor for redundant starter motor and EOC

[saand]

Hi All,
I have been toying with the idea of turning my engine off when im at lights or in neutral however i don't want to risk the car not restarting.
Ideally id like a 2nd starter motor. The alternator is already connected up so i am wondering if the alternator can be driven as a motor so that i can restart my car using the alternator.
If i have a 2nd battery it will give me a redundant car starting system and the alternator wont wear through the brushless like the starter motor.

I am wondering if anyone has ever tried to run their alternator as a motor.

I understand i will need to make 3 phase at high current and sense the alternator speed so i can sink the 3 phase to the alternator position.
I am interested to know if anyone has done this or knows if the alternator has enough torque run under 12v at all RPMs to spin up the belt and therefore kick over the engine.

[dcb]

I've looked into it a bit, it needs to have some position sensors added, isn't very efficient. One of the reasons I switched to stick was for redundant starting ability. I imagine a 91 626 transmission and clutch pedal bits and?? are pretty affordable from the boneyard.

You might just go ahead and get your starter rebuilt from a local shop as preventative maintenance, and carry a jumper and figure out how to bypass the key starter switch in case it craps out too.

Another option might be a pully and a bit of rope on the crankshaft? or??? That might be worth an experiment or two.

http://www.youtube.com/watch?v=PI46qVW2WA4

[Ryland]

Your alternator will not work as a starter, the belt is not strong enough, you not only would need to know at what rotation the alternator is in it's rotation to give it the correct a/c but you will need a variable a/c motor speed controller and the alternator is designed to put out 60 amps peek your starter can draw 5 times that, if you compare the size of wire in the alternator and the starter you will see that the starter is much heavier.
The better option, like has already been stated, is to make sure that your starter and battery and cables are all in good shape because stuff like this does not just die on a whim.
one tool to think about would be a scan gauge so you can keep track of your battery voltage, as a battery that is near the end of it's life will kill a starter much quicker, then call around to auto part stores and find your self a starter rebuild kit, it's going to be 1/10 the price of a rebuilt starter and is going to include all of the parts that go in to a rebuilt starter but you are most likely going to find that unless your car has a ton of miles on it that the starter is just fine, battery cable ends are the next weak spot, if they look coorded then replace the end or the whole cable.

[jamesqf]

Originally Posted by Ryland

...because stuff like this does not just die on a whim.

There speaks someone who's obviously never owned a British sports car!

All hail Lucas, Lord of Darkness!

[EdKiefer]

Also a starter motor is geared way down so it has the power,TQ to turn the engine few 100 rpm. Alternator is not able to do this .As mention above amp usage is completely different, much higher on starter motor .

[Weather Spotter]

Drive a stick shift
It thought I saw that on the 100+ tips page

[saand]

Thanks for the replies, looks like this idea has a few problems.

mainly
- the alternator belt size which might be overstressed by trying to start the engine, this belt could be changed to a larger one
- the power torque curve of the alternator, this one is an idea killer and i can only get around it by changing the alternator to a larger brushless DC motor

I do like the idea of the rope and pully, i wouldn't have thought it would be possible to hand start an engine like this.

When i get time i might try getting a 2nd alternator which i can break open to get access to the separate phases. Then i will be able to test the concept by putting DC levels on the phases to "step" the motor. If it turns the engine in this configuration then the concept has some chance.
If that doesn't work and i get particularly keen i might look at using a different motor/alternator with higher current.
Note: I may be able to get around the lower current issue by running the alternator at 24v, ill just have to keep an eye on the winding temperature

I think ill have to finish up the other mods on my list before i attempt this one.

[Vwbeamer]

Om my old Air cooled VW, I would often run the geerator as a motor to cool the motr between passes at the drag strip.

Couple of thoughts, I don't think you can run an alternator as a motor and the amount of torque it produces would not be enough IMHO to start the car. The starters is geared down much lower than the alternator.

[Old Tele man]

...and, now, the electrial reason why it won't work:

DC = Generator
AC = Alternator

...where are you going to get a source of AC-voltage to feed 'back' into the alternator to turn it into an AC-motor?

...with a generator, you could use the DC-voltage from the battery to "reverse" the generator into a motor; however, batteries don't "store" AC, only DC.

[saand]

aahh, good question
That is the easy part for me, im an elec engineer so i can make up a DC to AC device which will control the motor.
It is fairly involved and time consuming but i know that part is possible. It would be similar to the motor controllers which the electric vehicle people use

The crazy mechanical magic is the trouble for me.