# How Do I Reverse The Polarity Of DC Motor Without Burning It?

Discussion in 'Homework Help' started by Deccoyi, May 22, 2015.

1. ### Deccoyi Thread Starter New Member

May 22, 2015
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Lets say I am gonna drive a DC motor and suddenly chane its polairty. If I do that motor will produce high power and burn.
Everyone says first stop the motor and reverse it. But I already know that I have to change it while its going.

Is there a way to protect the motor ?

ps: I'm gonna use L293D to drive it.
I searched a lot about it but I couldn't find any satisfying answers. (my english is not so great so maybe i did find it but couldn't understand )

2. ### DerStrom8 Well-Known Member

Feb 20, 2011
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Motors cannot instantaneously change direction. Think of it this way: You're running down the road as fast as you can when you realize you forgot something at home. Can you instantly start running back the way you came? No, you have to take time to slow down and stop before turning around and getting back up to speed in the other direction.

Stepper motors might be an option because they only turn a fraction of a circle per signal pulse, so they already stop after each increment. It would make it much easier to switch direction quickly, though they require a lot of extra control circuitry and will be difficult to manipulate without a microcontroller.

3. ### kubeek AAC Fanatic!

Sep 20, 2005
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You would have to limit the current somehow. But anyway, reversing the motor while running is technically the same thing as stopping it and then going in the other direction - you see the motor will be for a brief moment stationary in both cases, but simply changing the polarity will cause huge current to flow and will likely destroy the motor in a short time.

You need to decide how fast do you need for the direction to change. Then calculate the stop and start time accordingly to stay within recommended limits.

4. ### Deccoyi Thread Starter New Member

May 22, 2015
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It is not that critical circuit. It is my homework. So speed doesn't matter. Can you give me an example circuit ?

I'll use 12V dc motor.

5. ### MikeML AAC Fanatic!

Oct 2, 2009
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Since it is homework, then it should have been posted in the homework forum.
Since it is homework, you get to research the question, make up a proposed solution, then we will critique it...

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6. ### pwdixon Member

Oct 11, 2012
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The motor will look like a short at reversal, I tried this very thing a on a model trainset when I was a kid, I was using PWM going from a positive to a negative voltage and when the train was stationary the motor got very hot. You could just limit the current into the drive and accept that the motor will push the drive tinto limit on reversal, but I doubt that it will make the reversal any quicker and it might be just easier to turn off then reverse.

7. ### Deccoyi Thread Starter New Member

May 22, 2015
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I posted here because I saw a topic similar to this in this forum. And I couldn't find anything that can give me an idea for solution.

Jul 18, 2013
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A common method for rapid reversal is to use a braking resisitor, this opens the motor conductors and the resistor is placed across the motor, dissipating the regenerated voltage and in the process braking the motor.
A short pause for braking is still required however before applying reverse power to the motor.
Max.

9. ### Deccoyi Thread Starter New Member

May 22, 2015
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How do I Suppress the back emf both ways. Everyone on the internet says freewheeling diode. But It work only one direction.

Jul 18, 2013
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The braking resistor!!
Max.

11. ### Deccoyi Thread Starter New Member

May 22, 2015
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If I could stop and switch the polarity I could use it. But polarity suddenly changes.

12. ### ian field Distinguished Member

Oct 27, 2012
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When I was a kid with a toy trainset, one of my favourite tricks was suddenly reversing it to cause wheel spins.

Some of the steam locomotives had reasonably authentic looking linkages on the wheels - I broke a few of those, but all the electric motors survived.

13. ### kubeek AAC Fanatic!

Sep 20, 2005
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You could come up with some circuit that will wait some time after the polarity has swapped before it applies power to the motor in the other direction.

14. ### alfacliff Well-Known Member

Dec 13, 2013
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its called a plugging switch, it prevents reversal before the motor comes to a stop.

Jul 18, 2013
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Plugging traditionally on a 3 phase motor is instant power reversal.
Max.

16. ### ian field Distinguished Member

Oct 27, 2012
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Years ago someone asked me to repair the control pad for an electric winch.

Probably most people know you can reverse a motor with a cross-wired 2-pole changeover switch, the winch pad was a little different - each of the 2 buttons controlled one pole of the switch independently of the other.

Each changeover switch was wired with the NO contact to +V and the NC contact to 0V. The motor was strung between the two moving contacts, with neither button pressed; both ends of the motor were connected to 0V, also if both buttons were pressed, both ends of the motor were on +V - either of those conditions shorted the motor and provided motor braking.

With only one of the buttons pressed, that end of the motor was connected to +V so the motor would run in the appropriate direction for the button pressed. This would be simple to duplicate with a pair of single pole changeover relays, its probably do-able with transistor or MOSFET full-bridge drive - but I'm not so sure how well the motor braking would work.

17. ### kubeek AAC Fanatic!

Sep 20, 2005
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Simple and idiot-proof at the same time.

18. ### Alec_t AAC Fanatic!

Sep 17, 2013
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If only .... .

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19. ### ian field Distinguished Member

Oct 27, 2012
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The bloke who took it apart and couldn't put it back together was a bit offended when I mentioned how simple the switching arrangement was.

You don't neccessarily need to be an idiot to inadvertently press both buttons at once, the winch was for hauling broken cars onto a recovery truck - vehicle recovery doesn't always go smoothly, particularly in pitch darkness and filthy weather. Just as the car starts to topple sideways off the truck, its an instinctive reaction to press everything at once trying to stop it.

Then there's always meddling kids that sneak up while you're not looking.

20. ### ian field Distinguished Member

Oct 27, 2012
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At the time - the bloke that took it to peices paid my wages, so I tried to be as tactfull as I could manage.

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