In my circuit my motor terminals are connected to the output of a VNH5019A motor driver https://www.pololu.com/product/1451. When the circuit is no longer powered I want to allow the user to freely rotate the motor (a dc motor) output shaft (which in my application has a load attached to it that I want the user to be able to move if power has disappeared). Currently if I rotate the motor fast in one direction and then suddenly change direction I can feel a "resistance" to the movement in the new direction. I assume this is because the motor is generating a voltage which opposes the movement in the new direction. Can anyone explain to me what is happening and suggest a modification to my circuit that would prevent this from happening?

  • \$\begingroup\$ You will always feel a "resistance" because of the moving mass. It's not possible to drop the inertia to zero in zero time. \$\endgroup\$
    – Janka
    Sep 13, 2017 at 11:57
  • \$\begingroup\$ Hi @Janka, thanks for your reply. I don't think the resistance I'm feeling is as a result of the moving mass as it immediately goes away if (whilst no power is connected) I briefly short the terminals of the motor. That's why I think it's due to the motor building up a voltage which opposes the movement in the new direction. \$\endgroup\$ Sep 13, 2017 at 12:02
  • \$\begingroup\$ Do you feel the same if the motor is not connected with leads not touching each other? \$\endgroup\$ Sep 13, 2017 at 12:10
  • \$\begingroup\$ Hi @Marko, yes even if I disconnect the motor from my circuit and with no leads touching each other and if I just have a lever (so effectively no mass) on the output shaft which I move quickly in one direction and then suddenly in the opposite direction I feel a large resistance - as if the motor is breaking. I don't seem to feel this if I only move the motor slowly in one direction and then slowly change direction. \$\endgroup\$ Sep 13, 2017 at 12:17
  • \$\begingroup\$ In such case, Janka was right. It's so called moment of inertia. \$\endgroup\$ Sep 13, 2017 at 12:20

1 Answer 1


The reason you are noticing it is worse at higher speeds will be because of the fly-wheel effect. The energy stored in the rotating core if proportional to the square of the angular velocity... So twice as fast = four times the energy.

enter image description here Momentarily shorting the leads will cause the motor to brake rapidly dissipating much of that energy as heat in the coil.

  • \$\begingroup\$ Hi @Trevor do you know of any circuit modification I can make to remove its effect? \$\endgroup\$ Sep 13, 2017 at 15:24
  • \$\begingroup\$ @user1242670 it's a mechanical effect so no. If you want it to stop and reverse more quickly you wold need to use your circuitry to actively brake it.. i.e apply a reverse voltage using a circuit capable of handling the back currents. \$\endgroup\$
    – Trevor_G
    Sep 13, 2017 at 15:28
  • \$\begingroup\$ @user1242670 or use a motor that has a lower moment of inertia. \$\endgroup\$
    – Trevor_G
    Sep 13, 2017 at 15:37

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