Need help with choosing and alternator for a robot

[Oscilloscope]

Hey guys! First post. I've been lurking around for a while though :grin:

I dislike mowing my lawn because here where I live, it's either always too damp when it's not excruciatingly hot and humid to do it, or it's nice and dry but breathing outside is like breathing under water. Plus I have asthma, which makes it worse.

So I mounted two 400w wheelchair motors to the mower along with two 400w full-bridges, an RC receiver and controller. It's working just fine, except that the two 18v power tool batteries I use right now drain too quickly and the result is that I have to finish mowing the lawn by hand. I could purchase and extra two power tool batteries, but I thought of a "better" way to do it.

I decided to add an electric starter, an alternator and two 12v batteries (from this point forward referred to as "the battery"), to the mower. I had 2 choices:

1) a deep cycle battery (recharged by and induction charger when the robot goes back "home")
2) a sealed lead acid battery (constantly recharged by an alternator being spun by the mower's engine)

In the end I chose the sealed lead acid over the deep cycle because the deep cycle ones have a limited number or recharges before they have to be replaced (it's around 300 cycles, but given I'll eventually use this same frame to mount to my snow blower, it'll be sooner rather than later).

So I needed a 24 volt alternator, which I have someone who's going to rewire an old car alternator so that it puts out the 24 volts at the mowers running RPM (I removed the governor by the way to compensate for the extra work it'll have to do to both mow the lawn AND spin the alternator). The question he has before he builds this alternator for me is how much current will it have to output. And I'm having doubts as to what that needs to be.

The battery will provide the juice to run the starter, then the engine starts and the alternator will be used to both recharge the battery AND supply the power to the two DC motors.

Now the motors run at around 17 amps (average), but it can peak at well 2 or 3 times that current depending on breaking and/or reversing directions. At the times when the motors need more than what the alternator can put out, the battery will supply the rest, but I want to make sure that by the time the mowing is done, the battery is fully recharged instead of partly drained due to those occasional high current needs of the motors.

If the alternator is putting out too much current, it can both wreck the battery and fry it's own regulator because that current has to go somewhere when it's neither being used by the motors nor to recharge the battery.

So the question is, what's a suitable current rating for the alternator?

Any suggestions?

Thanks in advance.


-Mario

 

[chuckey]

"If the alternator is putting out too much current, it can both wreck the battery and fry it's own regulator because that current has to go somewhere when it's neither being used by the motors nor to recharge the battery."
No this is wrong, alternators work with a voltage stabilizer, so their output voltage is stabilized. This is something you will have to build as if the 12V alternator has a regulator built in - most do, it will be set to 12V.
First of all the starter current, Err, 100 A? 200 A?, no how long does your starter run for? 10 -20 seconds, so its used about 200 X 20/3600 Ampere hours ~1, in a hour your electric motors will have used 17 Ampere hours, so disregard the starter current, though make sure the wiring is up to it!
Back to the alternator, the armature has a winding on it that is fed with up to 10 amps to get a rotating magnetic field, if the voltage output is too high this current is reduced. Depending on the design of the alternator, it would be easier to use a smaller pulley on, so it runs faster and just attend to the regulator to get your output voltage. As a rough guide in cars you can get full charge (> 12V) at 1500 - 2000 RPM, so if the alternator was running at 3000-4000, it could generate 24V
Frank

 

[Oscilloscope]

Excellent, a standard off the shelf 24v 20/25amp alternator regulator should do them.

Thanks for the quick reply.


-Mario

 

[chuckey]

Just to clarify, perhaps you should check. I mean't the cars engine was going at 1500-2000 RPM, the alternator is normally geared up from the engine, by a factor of 2? so the alternator will be actually running at 4-6000 RPM normally. So need this sort of revs ++ .
Frank

 

[Oscilloscope]

I'll double check with a tachometer, but from the research I've done, off the shelf mowers in the use have a max blade tip speed of 19,000 feet/sec. For my 21" mower that's 3455 RPM, fine for the alternator.