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I am studying DC motors and I know that what the direction of the rotation will be given by the left hand rule.

enter image description here enter image description here

If I compare the right and left hand rule, if I point the thumb and the first finger to the same directions with both hands, only my second fingers will be in opposite directions (the current).

enter image description here

So I am wondering if I suddenly cut the current of the motor, then it will still be in motion due to inertia, then the right hand rule apply and a current will be induced in the opposite direction of the current that I was supplying. Does this mean that I need to protect my circuit against reverse current? Or am I getting everything wrong?

Why this adafruit circuit doesn't protect against reverse current? It does the opposite, it protects against inducted current in the same direction:

enter image description here

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2 Answers 2

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Yes, indeed.

That's why flyback diodes are used to protect the transistors used for switching current off or on to electric motors.

Flyback diodes in motor

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    \$\begingroup\$ Actually diodes are used if the current stays in the same direction. I am reading more and I think the motor works as an inductor, and all the drawings of the left and right hand rules are wrong, because in the place of the current is actually voltage, so the voltage is the one that inverses (as the motor will work as a generator), not the current (at least I think, I am not sure). If you see this circuit: learn.adafruit.com/adafruit-arduino-lesson-13-dc-motors/… the diode only makes sense if the motor keeps the current in the same direction when the transistor is off \$\endgroup\$
    – Lilás
    Feb 23, 2017 at 21:04
  • \$\begingroup\$ reading more I find out that your answer is true (we will have a reverse current) if we are working with a permanent magnet, if the magnet field is generated by electromagnet, then it will be an inductor which will keep the current on the original direction, the current caused by the inductor is "stronger" then the reverse current \$\endgroup\$
    – Lilás
    Feb 24, 2017 at 12:51
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I think I found an answer for all my doubts and I am sharing it here:

According to this video: https://www.youtube.com/watch?v=5mf4NmmLWnE when the the battery to the motor is on, and the armature is spinning we have both rules (right and left hand) acting simultaneously.

The right hand rule "generates a current" (actually a voltage) in the opposite direction of the applied voltage by the battery, but it is weaker then the battery and the resultant voltage is positive (thus keeping the current going forward and not reverse). This is called back emf.

Here is another great explanation about back emf: https://www.youtube.com/watch?v=5XcWJa9JBuY

If the motor uses a permanent magnet, then if we cut the applied voltage, the motor will continue to spin due to inertia and only the back emf will remain, thus the back (negative) voltage will generate a current in the opposite direction and our circuit needs a protection.

But if we are using a DC motor that uses electromagnet instead of permanent magnet (as the motor used by Adafruit in the question), then the motor basically have an inductor inside it. This inductor (the electromagnet) can be connect in parallel (shunt motor) or in series as explained here: https://youtu.be/LAtPHANEfQo?t=158

As the inductor tends to keep the same current flow, when you turn off the applied voltage this inductor will try to keep the current in the same direction acting as a generator. And this inductor is usually stronger then the back emf, thus not causing a reverse current.

So in the schematic presented by Adafruit, the diode only protects against the inductance inside the DC motor. It doesn't protect (and it doesn't really need to protect) the circuit against reverse current unless some mean person spins the motor while it is off.

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