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Need help with motor control

Thesecret20111

Apr 11, 2014
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Hi guys,

I've just finished building a robot with two movement motors and a reversing circuit (for when it bumps into walls). The problem I'm having is that when I switch it on only one motor spins. When I forcibly stop the motor that's spinning the other one starts turning and this one remains stopped. Only one motor is ever spinning.. What's happening and how can I fix this issue?

Thanks,
Thesecret20111
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Have you wired both motors in series?
 

KrisBlueNZ

Sadly passed away in 2015
Nov 28, 2011
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Hello and welcome to Electronics Point.

Unfortunately, my crystal ball is away for repolishing, so you're going to have to post a schematic.

Edit: Try connecting them in parallel instead of in series.
 

(*steve*)

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If the motors are in series then this is the expected behaviour.

A motor required more current to start than it does to run.

When both motors are stopped, both will get effectively almost the full current. The first one that starts turning will begin to reduce the current from the other. So whichever one starts first prevents the other from starting.

When you stall the moving one, you prevent it from starting and the other one can start. When you release your fingers there is now insufficient power to start the motor you held stalled.

This behaviour is most pronounced if you try to run a pair of (say) 12V motors from a 12V source.

As Kris has said, putting them in parallel is the best solution, but if you do, make sure that you're not running each of the two (say) 6V motors from 12V.
 

Thesecret20111

Apr 11, 2014
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@KrisBlueNZ
Thanks for your welcome :) Unfortunately I don't actually have a schematic as I kind of made the design up as I went along.

@Steve
Ooooh very nice explanation thanks so much! I can defiantly try one of these solutions tomorrow. Just out of curiosity is there any simple circuit you can use to split the current between both motor equally?
 

(*steve*)

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Just out of curiosity is there any simple circuit you can use to split the current between both motor equally?

You do this by placing them in series. That way they will get exactly the same current.

As you've seen, that's *exactly* what you don't want.

You may have made it up as you went along, but that doesn't stop you drawing a schematic. We want to know what is connected to what and how.
 
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