Author Topic: Help on seeing if this will work  (Read 840 times)

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Offline troop231Topic starter

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Help on seeing if this will work
« on: May 02, 2013, 07:28:20 PM »
First off, I'm new here, but hopefully I can get some much needed help. I wasn't sure if this should've gone under the mechanical section or not since it's electrical related as well.

My project has a small lawn mower gas engine that is rated at 4.5 ft. lbs. of torque at ~3200 rpm. I want to use a 24 Volt 100 Amp alternator driven by the engine to keep two 12 Volt 5 Amp hour batteries topped off for my two 350 Watt 24 Volt geared scooter motors that have a rated current of 19.5 amps each (1.8 amps no load)

The alternator is self exciting and is rectified for DC output, so it will be connected directly to the two 12 Volt batteries which are in series.

My question(s) are will the engine be too bogged down with the load of the alternator only if it's keeping the small 5Ah batteries topped off? Also, does this seem technically feasible and correct? I definitely don't want to order the alternator and parts if I'm missing a key concept.

Thank you so much in advance!
« Last Edit: May 02, 2013, 07:30:54 PM by troop231 »

Offline jkerns

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Re: Help on seeing if this will work
« Reply #1 on: May 02, 2013, 08:48:43 PM »
The motor won't deliver the power requred to deliver the full 100 amps from that alternator.

But, the alternator won't deliver the full output unless the batteries are somewhat discharged, and it will keep up with your electric motors.
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Offline jwatte

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Re: Help on seeing if this will work
« Reply #2 on: May 02, 2013, 08:56:29 PM »
I think that alternator is way overkill for "topping off" those batteries. And, while it may be too big for that motor when run at full juice, the resistance of the alternator is proportional to the load on the alternator, plus losses, so it'll probably run. You might want to look at a smaller one, though.

Also, if you're using SLA batteries, that will probably work well enough, as SLA aren't particularly picky about how you charge them. If your batteries are LiPo or similar, then keeping them topped off is actually a good way of killing them, and charging them with a rectified sine wave is another good way of killing them :-/

Offline troop231Topic starter

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Re: Help on seeing if this will work
« Reply #3 on: May 02, 2013, 09:06:25 PM »
I think that alternator is way overkill for "topping off" those batteries. And, while it may be too big for that motor when run at full juice, the resistance of the alternator is proportional to the load on the alternator, plus losses, so it'll probably run. You might want to look at a smaller one, though.

Also, if you're using SLA batteries, that will probably work well enough, as SLA aren't particularly picky about how you charge them. If your batteries are LiPo or similar, then keeping them topped off is actually a good way of killing them, and charging them with a rectified sine wave is another good way of killing them :-/

Thank you for the helpful information! What amperage alternator would you recommend I use instead of 100 amps? And yes, I'll be using SLA batteries. Quick question, could I go smaller than 5 amp hours or do I need to go even bigger since the alternator will be keeping them full basically while my motors are being used. Should I just stick with 5 amp hours like I had planned?

Thank you all again!

Offline Duane Degn

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Re: Help on seeing if this will work
« Reply #4 on: May 02, 2013, 09:31:20 PM »
You ought to take a look at an article by Ken Gracey in Robot magazine. He used a gas motor and generator to power the electronics (including electric motors) on his robot. It seems like there are similarities between your project and Ken's.

There's a pdf of the article in post #2 of this thread.

http://forums.parallax.com/showthread.php/96081-Great-Article-on-the-Hybrid?p=663845&viewfull=1#post663845

Offline troop231Topic starter

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Re: Help on seeing if this will work
« Reply #5 on: May 02, 2013, 09:50:21 PM »
You ought to take a look at an article by Ken Gracey in Robot magazine. He used a gas motor and generator to power the electronics (including electric motors) on his robot. It seems like there are similarities between your project and Ken's.

There's a pdf of the article in post #2 of this thread.

http://forums.parallax.com/showthread.php/96081-Great-Article-on-the-Hybrid?p=663845&viewfull=1#post663845


Wow thank you! That was a good read and a gold mine of information! He used a 12 amp hour battery, does this mean my idea for two 5 amp hour batteries in series are too small of capacity?

Thank you all.

Offline jwatte

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Re: Help on seeing if this will work
« Reply #6 on: May 02, 2013, 10:23:08 PM »
With 5 Ah of battery, and at full stall current (20 Amps each) you can run for seven or eight minutes after the gas runs out. That seems like quite sufficient buffer.
Also, your motors won't actually be drawing the full stall current most of the time. Unless you're planning to do something truly heinous, which would probably end up burning out the motors before you ended up wearing down your batteries.

So, if stall current is 20 amps, times two, then a 50A alternator ought to be able to sustain the motors for as long as you have gas. If your run current is more like 5A (1/4 of stall; a good design target) then a 20-25A alternator ought to be quite enough. And easier for your engine to drive :-)

Offline troop231Topic starter

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Re: Help on seeing if this will work
« Reply #7 on: May 03, 2013, 09:39:21 AM »
With 5 Ah of battery, and at full stall current (20 Amps each) you can run for seven or eight minutes after the gas runs out. That seems like quite sufficient buffer.
Also, your motors won't actually be drawing the full stall current most of the time. Unless you're planning to do something truly heinous, which would probably end up burning out the motors before you ended up wearing down your batteries.

So, if stall current is 20 amps, times two, then a 50A alternator ought to be able to sustain the motors for as long as you have gas. If your run current is more like 5A (1/4 of stall; a good design target) then a 20-25A alternator ought to be quite enough. And easier for your engine to drive :-)


Other people that are using the motors I'll be using claim that when the rotor is locked, currents can approach around 100 amps! I guess this would happen if the robot ran into a stationary object like a fence?

My last question (for now) in the .pdf article linked above, he talked about creating a voltage regulator so the alternator doesn't fry or possibly explode the batteries. His system is 12 volts though, so I'm not sure where to begin this for my 24 volt system. Here is a pic of the schematic from the .pdf.



Thank you again everyone.

Offline jwatte

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Re: Help on seeing if this will work
« Reply #8 on: May 03, 2013, 11:47:52 AM »
Quote
when the rotor is locked, currents can approach around 100 amps!

If that is the case, then yes, you need a beefy alternator! When you said "no load current 1.8 A, rated current 19A" I thought the "rated current" was the "stall current," but from what you say, it's actually the "holding load current." Thus, scale the math I did by a factor 5 :-)

Convenient formulas:
horsepower == footpounds * rpm / 5252
horsepower == kilowatts / 0.75

Your alternator is approximately 3 horse-power, and your motor is also approximately 3 horse-power, so there's some risk of stalling out at full load. It's unlikely your batteries will ever be depleted enough to draw the full load on the alternator. However, the voltage regulator may actually generate significant load even when the batteries are full, although that 16 Ohm resistor seems to limit it to 1A, which I'd think isn't big enough to worry about in this system.

Offline troop231Topic starter

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Re: Help on seeing if this will work
« Reply #9 on: May 03, 2013, 12:32:07 PM »
Quote
when the rotor is locked, currents can approach around 100 amps!

If that is the case, then yes, you need a beefy alternator! When you said "no load current 1.8 A, rated current 19A" I thought the "rated current" was the "stall current," but from what you say, it's actually the "holding load current." Thus, scale the math I did by a factor 5 :-)

Convenient formulas:
horsepower == footpounds * rpm / 5252
horsepower == kilowatts / 0.75

Your alternator is approximately 3 horse-power, and your motor is also approximately 3 horse-power, so there's some risk of stalling out at full load. It's unlikely your batteries will ever be depleted enough to draw the full load on the alternator. However, the voltage regulator may actually generate significant load even when the batteries are full, although that 16 Ohm resistor seems to limit it to 1A, which I'd think isn't big enough to worry about in this system.

Your help has been greatly appreciated. I doubt I will ever stall the motors as I've never stalled my 24 Volt wheelchair motors before and im using a 25 Amp Sabertooth controller on that project for several months with no issues.

I'm thinking about getting a 40 Amp 24 Volt alternator for this new project. Is there anything I should or need to do differently with the battery charging circuit shown above? It would be nice if I didn't need a charging circuit, but safety and longevity are important to me.

Thank you.
« Last Edit: May 03, 2013, 12:35:51 PM by troop231 »

Offline jwatte

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Re: Help on seeing if this will work
« Reply #10 on: May 03, 2013, 12:48:01 PM »
I'm not great with car electronics. If I had an AC alternator, I'd run it through a full-wave rectifier, and if I needed to control the charge of a battery, I'd use either a linear voltage regulator as a current limiter, or a PWM regulator. That circuit you posted works differently, so I'm hoping someone with a better knowledge of that side of things can chime in!

Offline jkerns

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Re: Help on seeing if this will work
« Reply #11 on: May 03, 2013, 01:17:02 PM »
I'm not great with car electronics. If I had an AC alternator, I'd run it through a full-wave rectifier, and if I needed to control the charge of a battery, I'd use either a linear voltage regulator as a current limiter, or a PWM regulator. That circuit you posted works differently, so I'm hoping someone with a better knowledge of that side of things can chime in!

An automotive alternator typically has a full wave rectifier built in. Some have built in regulators as well.

You typically control the output of the alternator by controling the current to the field - Increasing field voltage / current increases the alternator output.  I didn't try to dig up the data sheet on the chip in the schematic to see what pins are what and how the "set point" of the regulator circuit is set - but the idea is to control the field current based on the alternator output voltage. I would assume that the circuit posted could be modified.

You could also limit the field current to limit the alternator output and avoid overloading the gasoline motor.
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Offline troop231Topic starter

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Re: Help on seeing if this will work
« Reply #12 on: May 04, 2013, 01:12:07 PM »
I think I need help designing a circuit to keep my batteries charged so they don't get depleted when I'm driving the robot.

Offline jwatte

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Re: Help on seeing if this will work
« Reply #13 on: May 05, 2013, 08:59:07 PM »
I think the circuit above, and a 40A or better alternator, would do that just fine for you!

Offline troop231Topic starter

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Re: Help on seeing if this will work
« Reply #14 on: May 06, 2013, 07:41:27 PM »
I think the circuit above, and a 40A or better alternator, would do that just fine for you!

Thank you for replying. Isn't the circuit above for a 12V setup though? I need double for 24V (2 batteries in series)

Offline jwatte

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Re: Help on seeing if this will work
« Reply #15 on: May 06, 2013, 09:06:45 PM »
The LM723 is rated for up to 40 Volts, so as long as your alternator doesn't put out more than that, adjusting the regulator voltage resistive divider is all you need to do. Here's a link to the data sheet:
http://www.ti.com/product/lm723
The necessary adjustments can be found towards the end of the data sheet.

Offline troop231Topic starter

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Re: Help on seeing if this will work
« Reply #16 on: May 07, 2013, 08:38:48 AM »
I'm thinking about ditching the alternator idea, since it will take about ~1 HP away from the engine, which will be harder to cut grass. I'll probably just use two 12V 22Ah batteries in series now.

Offline jwatte

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Re: Help on seeing if this will work
« Reply #17 on: May 07, 2013, 04:01:37 PM »
Quote
I'm thinking about ditching the alternator idea, since it will take about ~1 HP away from the engine

If you drive the knives with electric motors, you won't have the loss of torque at low RPMs that engines do, and thus your blades won't stall out nearly as easily as with an engine. You could say that ALL that the engine does is run the batteries, and make both propulsion and blades run by electric motors, and probably come out ahead, as the engine can be run at its optimal RPM, and won't stall out if the blades cut into thicker grass.

 


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