# Controller for a starter motor



## sylar (Nov 12, 2007)

i am building an EV mower (starter project, can't quite ready to take the plunge yet), and I am considering taking the route from this guy.

http://www.docdockdocuments.com/conversion/Conversion94.htm

he advocates using an old car starter motor, which would be more powerful (and cheaper) than needed. i'd like to take his advice and attach a controller to regulate the motor a bit. can anyone recommend one?

also, our backyard has quite a lot of trees and tree chunks in the grass. the gas mower just stops (although violently) when the blade gets lodged on a branch. what will happen to a DC motor?

thanks in advance.


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## mattW (Sep 14, 2007)

*Short Answer: *If the motor is not allowed to turn, the current through it will increase and burn out the motor. 
*Long Answer:* When the motor is spinning there is an induced voltage in the opposite direction to the existing voltage so even if you have 12V battery across the motor at top speed the voltage the motor 'sees' is probably only about 5V. This is good because the only thing limiting the current in the motor is its internal resistance,as a random guess lets say its 2 ohms (giving a current of 2.5A at full speed). When the motor isn't allowed to turn there is no back voltage induced so the motor gets the full 12V and therefore 6A of current. That is a 476% increase in the power across the motor and will make it heat up pretty quick until it burns out. 

Don't know about the controller though.


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## Mr. Sharkey (Jul 26, 2007)

I can tell you what happens when I mow into branches with my Electrak mower, shavings! About the only thing I fear when mowing is hitting large hidden rocks, or fence wire. Either will stop the motor so violently that the ceramic permenant magnets either are dislodged from the casing, or they shatter altogther. Having some sort of a clutch arrangement would be a good safeguard.

As for controllers, you have several options, resistive (wasteful), PWM (very efficient, full mtor torque, but more difficult to implement), or simply run the motor at full voltage and put a well chosen circuit breaker in the circuit, one that can be reset if the mower motor is overloaded.

There are actually some one-chip PWM solutions, but they will all rely on external pass semiconductors, either IGBT's or MOSFET's to control the actual motor current. Unless you are experienced with electronics, I won't go any farther....


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## sylar (Nov 12, 2007)

ok, thanks, i think i got the hang of it. i don't think i'll go for the controller in the first iteration, and just try to make do with a fuze. most plans online seem to use a 50A one. this is another reason to keep construction cheap, so it's easily replaceable. 

one more question. i plan on using a scrap car battery (open to suggestions here). what would be the cheapest charging solution? i'd like something i can plug in and forget, possibly with a meter and auto-shutoff when fully charged. i welcome something that's not a self-contained unit, so i can design and manufacture a nice charging station for the battery


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## dbgtwill1 (Dec 11, 2007)

i would go for the battery tender & tender jr iv used it and works pretty well just set and forget cant rember price should be ok


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## Noel Warren (Feb 4, 2017)

I think that if you use a fan control speed from a vehicle, this might work but however I am experimenting soon with this one myself and don't yet know if this could do the trick for sure. Maybe someone else could help us on this one. I am guessing that a fan speed control drops voltage in each stage of the clicks at the control switch I have chosen from a Ford Au and I have to figure out if I can hook this up successfully or not.


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## Noel Warren (Feb 4, 2017)

sylar said:


> ok, thanks, i think i got the hang of it. i don't think i'll go for the controller in the first iteration, and just try to make do with a fuze. most plans online seem to use a 50A one. this is another reason to keep construction cheap, so it's easily replaceable.
> 
> one more question. i plan on using a scrap car battery (open to suggestions here). what would be the cheapest charging solution? i'd like something i can plug in and forget, possibly with a meter and auto-shutoff when fully charged. i welcome something that's not a self-contained unit, so i can design and manufacture a nice charging station for the battery


See if you can get a hold of a 12v childs scooter battery since they are compact and light weight and might do the job you want. however I am not a qualified electrician in anyway so I experiment a lot myself depending on my requirements. All the best with your venture.


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## piotrsko (Dec 9, 2007)

Most car starters make 1.5 to 2.0 hp. Your current mower should be rated at 4-6 hp perhaps more.

Car starters are terribly inefficient, and are very short duty cycle, about 1minute at rated hp.

Ah rating of battery indicates how long motor will run. Bigger is better


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