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When you drive an electric motor forward with voltage +V, part of the electrical energy is dissipated as heat in the motor winding and the rest of it (I am overlooking other kinds of losses) goes into the kinetic energy of the rotor.

However, when you drive this previously-forward-driven motor in the reverse direction (applying -V this time across the windings using a H-bridge, the supply is still providing electrical power), some of the electrical energy is also dissipated in the motor windings but what happens to the remaining mechanical power/energy provided? The kinetic energy of the rotor is decreasing, so what are the both of them transformed into?

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when you switch the motor polarity while the armature is spinning, the motor becomes a generator and it produces a significant surge of current that tries to run backwards out of the motor and into the power source. That current surge, times the voltage at each instant of the process, yields power that is leaving the system and flowing towards the source.

When the armature finally comes to a stop, so does the current leaving the motor, which is replaced by current flowing into the motor from the source again, and the armature begins spinning in the opposite direction.

This effect is used in electric cars to slow them down and recapture the kinetic energy not only of the armature but of the entire car; in this context the concept is known as dynamic or regenerative braking.

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It depends a bit on the design of the electric motor, but many electric motors act as generators when they run backwards. So, the kinetic energy could basically produce a back EMF that feeds power into the battery or power grid that would otherwise power the motor.

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Voltage is only one component of power (energy per unit time)

P = V*I

Applying a voltage to a motor does not necessarily mean you are applying power. If the current is zero there is no power, and if the current is negative, it is actually negative power -> removing energy from the rotor.

The current will be determined by the resistance in the windings, and the back-emf of the motor (if it is rotating).

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