My Science Fair Project

[Lectroid]

This is an oddball question for here but it's basic DC electricity, and someone might have thoughts.

A few years ago I got a self-propelled 60V cordless lawn mower and later I bought a rear axle assembly for it cheap on eBay, so now I have enough to start the Self-Propelled Garden Wagon project, using a little steel cart from Amazon.   
https://www.amazon.com/Removable-Utility-Rotating-Capacity-Outdoor/dp/B0D1GD5RRW/ref=sr_1_2_sspa

I need a motor speed control to power the drive motor for the wheels, control the cart speed, maybe a Reverse mode.  It's a brushed motor but I have no specs/docs, no idea of its stall current, etc.  On a DC power supply, its speed changes readily with the voltage.

1. Can I can use a PWM motor controller?  Amazon has cheap ones for under $20 that say they can handle 60A at 60VDC.  Is this legit?  Would something like this work?   https://www.amazon.com/Adjustable-Digital-display-Controller-regulation/dp/B0991XYV4F/ref=sr_1_2_sspa
If not, what should I be looking for?

I want the figure out how much more current the drive motor will draw once I (over)load the cart with, say, triple the weight of the lawn mower that it was designed to push?  Like 3 bags of topsoil.  I know these are vague questions--but even ballpark guesses would help here.  I know zip about motors although I bet Ohm's Law applies to them.

When I mow my slightly up-and-down sloped lawn using the self-propelling feature, a 5Ah battery will last no more than 30 minutes.  So over that half an hour, the (blade motor+drive motor) together must have been pulling (at least) an average of 10 amps from the battery. Right?

And since the blade motor is brushless and the blade speed is fairly constant once it spins up, I suspect it uses less current and that the drive motor probably draws more than half the total current used.  So let's say it draws three-quarters of the total: 7.5A out of the 10A total used.   To be conservative, I'll rate it at a maximum current draw of 10A--when performing its intended job: pushing a 40 lb. lawn mower around the yard.

That's the ballpark as I see it.  Does that make sense to anyone else?   If so, it looks to me like a 60V PWM power supply with a 40A - 50A rating might be plenty.

Thanks in advance.

[Carlsoti]

In the past, I'd ask this question of the gurus that used to post at RCGroups, but it's honestly been a decade since I've had my head in that space and I'm not sure the same folks prowl around there anymore.

[shooter]

^^^  rc or mobile robotic platforms is where I'd start my surf.


maybe find an electric troll-motor, usually they are variable speed.  add in some gear n pully physics math and I would suspect you could use the troll motor with a handful of X:1 step down gears/pullies to get a "walking speed" base at <2AH no load, 5AH full load.


I built a robotic base to patrol my swim pool, carried a deep-cell marine battery, 2 12vdc drive motors, gear boxes, sensors n processors.  I used a simple pic-chip, i/o module, etal, wrote some code, did a proof-of-concept proto-type, then life happened  circa '88.

[Lectroid]

@ shooter, carisoti,

Thanks for thinking of the robotics world, I have spent many hours there. 

But the transmission isn't a problem for this project because I'm using the lawn mower's complete rear axle assembly, out to the wheels. It's literally a bolt-on deal mechanically, just moving the lawn mower's entire drive train to the garden cart.

I still need some way to control the motor speed.

1.  Will a PWM controller (like the one in the link) work for this?   Can anyone confirm this?
2.  Does my estimate/thought experiment of the motor's current draw make sense?

Those are the questions I most need answers to.   

Thanks again.        

[Carlsoti]

FWIW, I mostly played with sailplanes and electric parkflyers and did a short stint at a UAV manufacturer, but I knew a few folks that were into cars/trucks/boats, etc. The thing that surprised me was how "overbuilt" their electronics were compared to planes. IIRC, it has something to do with the back-emf from driving a brushless motor against a high-friction medium like water or pavement that spikes the load considerably more than a flexy plastic prop paddling through the air. I'd expect, at minimum, a 100A ESC on something like that, but that's just guessing, not running the numbers at all.