Need help with 12 amp mower motor

[dabw0001]

I have been searching on these forums and google for about 3 days now and have yet to find a complete answer, plus I have no idea where to start.

Currently have a Black & Decker MM875, 12 amp corded mower. I want to convert this into a cordless mower with 2 12v 115mah deep cycle marine batteries.

First question is, 24v enough to power a 12 amp motor? Sorry, that's all the specs I could find on this motor. Can I just use one 12v batteries to eliminate overkill? I have less than half an acre of grass to mow, usually takes me 45min to 1hr.

Secondly, if the batteries are sufficient to power the motor, how do I control the motor? What do I need between the motor and battery? I don't really care for the speed control as long as it spins as fast as it did on AC. I would like to be able to control it via R/C as this is going to be a R/C mower project.

Lastly, will a DC-AC power inverter be more cost effective/easier?

Thank you all for any input, I greatly need every bit of help I can get.

 

[JimB]

Presumably this mower plugs into the mains for its power, what is your mains voltage? You do not state your location.

So, the mains is either 110 or 240v, lets assume 110v for now.

The power of the motor is 110 x 12 = 1320 watts.

To get 1320 watts from a 24v battery, you will need 1320/24 = 55amps, but it is a 110volt motor, it will hardly work on 24v.

If you wnt a mower which is independant of the mains, get one with a petrol engine.

JimB

 

[Grossel]

Or, if you're really good to welder things together, you may pull out the motor that is installed. Go get a car wreck and pull out the electric start motor and use this.
Doing this, you have to take the chance that your mower turns useless.

 

[cr0sh]

Or, if you're really good to welder things together, you may pull out the motor that is installed. Go get a car wreck and pull out the electric start motor and use this.
Doing this, you have to take the chance that your mower turns useless.

Starter motors may or may not work well in this application; as they are an "intermittent duty" device, whereas lawn mowing is a "continuous duty" task, it would all depend on the duty cycle rating of the particular starter motor. Then there's the fact that starter motors are used in a horizontal rather than vertical position (does it have thrust bearings? doubt it).

Likely, the original motor on the mower is a series (or possibly/probably shunt) wound (or maybe hybrid) motor, wound for the AC voltage level (110V? What country are you in?). What would probably work better would be to have the motor re-wound for 24 VDC. I am not sure what that would cost, though...

 

[dabw0001]

sorry for not clarifying earlier, I live in US. I remember reading in more than one place that the motor in this particular mower is dc or pmdc connected to some kind of converter. can anyone else verify this?

 

[Grossel]

What would probably work better would be to have the motor re-wound for 24 VDC. I am not sure what that would cost, though...

Oh no. Once a dome I tried rewinding a dc motor for a grass trimmer to work at lower voltage levels. That project failed big time

Rewinding a motor would demand extensive skills in doing that. Remember that rotor should be physically balanced too. If not it will shake heavily and probably fail after not long time.

 

[cr0sh]

Oh no. Once a dome I tried rewinding a dc motor for a grass trimmer to work at lower voltage levels. That project failed big time

Rewinding a motor would demand extensive skills in doing that. Remember that rotor should be physically balanced too. If not it will shake heavily and probably fail after not long time.

Right - I guess I wasn't clear enough; I was meaning to imply that he should take it to a professional motor repair/rewinding shop and have it done.

If it is a permanent magnet DC motor, then the "converter" might be as simple as a bridge rectifier of some sort - still, you would be stuck with a DC motor needing around 100 VDC...

 

[dabw0001]

so I'm guessing the consensus is a dc-ac 1500w inverter and call it a day?

 

[ronv]

It's going to take a couple hundred dollars worth of deep cycle battery to supply 1500 watts for 45 minutes or so.

 

[picbits]

As above - purchase a Petrol Mower.

When you average out the running costs + building cost + maintenance cost (replace the Batteries evey 500 charges or so) its probably cheaper in the long run to buy a petrol mower.

 

[waxman]

Converting 24 VDC to 120 VAC could be done with an inverter, but why when DC motors for grass cutting service are already available. I've seen and used spinning aluminum discs with small blades that require very little 12 or 24 volt power.
Forget gas engines. Not cool. (Unless you want cheap and dirty)
The project direction depends on how much you care to do on your own to build a mower vs. modifying existing mowing platforms.

 

[rfs100]

As above - purchase a Petrol Mower.

When you average out the running costs + building cost + maintenance cost (replace the Batteries evey 500 charges or so) its probably cheaper in the long run to buy a petrol mower.

And Petrol motors (at least in this country) are a dime a dozen. Hell even I have one Ive been trying to get rid of, Ive become tired of buying Gas, and mixing it with Oil....from now on, its electric all the way

Recently, the magazine (I think its called:Make) had a major project artile (on the front cover as well) for a Robotic (or remote contolled) lawnmower, you should look it up.

There are also MANY MANY videos on U-Tube that show various robotic lawnmowers in action.

http://www.youtube.com/watch?v=zuXqg4q3cQ8

**broken link removed**

http://www.engadget.com/2007/02/13/build-your-own-rc-lawnmower-intimidate-neighborhood-dogs/

 

[Oznog]

It's gonna be a DC "universal" motor with a fullwave bridge rectifier on the AC.
Generally, its speed is proportional to voltage, so you need 10x 12v batteries in series here to make 120VDC.

12A=2400W is a LOT of power to try to get from small lead-acid batts. However, the power draw's not just automatically 12A. The power draw will be far, far lower when just running there. 12A doesn't mean much other than a claim that the motor won't overheat until it exceeds 12A of load for an extended period. I've no idea if that's likely, nor what the actual draw is to mow "normally".

A DC/DC is probably not gonna fly, because the power levels are too high. You'd be probably asking too much of a 24v battery. Like a 20AH batt, you can't ask for 40A, that'll not only run it down really fast, it'll permanently damage the plates inside just about any SLA if you keep asking for that current for an extended period.

There was the Neuton cordless mower. Probably 24v... probably a smaller cutting width and limitations on how much current it will actually draw if you bog it down.