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When a motor is coasting to standstill, if I apply a DC voltage across the windings I will brake that motor... We'll at least that's what I've read.

Now here's the thing. With reverse current braking I literally reverse the direction of the motor until it's at 0rpm,and then I disconnect the supply. Which is understandable. But when it comes to DC braking, I just apply the voltage across a winding, without taking current direction into consideration, and the rotor will stop. How does this happen? In fact, why does it stop to begin with?

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    \$\begingroup\$ One clue: DC braking is a technique applied to AC motors. en.wikipedia.org/wiki/DC_injection_braking. AC motors expect the current to be changing sinusoidally at a fixed frequency. It doesn't take much imagination to deduce what would happen if you apply a steady DC current to such a motor. \$\endgroup\$ Jul 9 '19 at 17:17
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Induction motor DC injection braking is like eddy-current braking. DC injection braking is accomplished by disconnecting the AC supply and applying a DC voltage to the stator windings. DC current in the stator windings produces a stationary magnetic field. The motion of the rotor bars through the stationary field produces currents in the rotor that have a resulting magnetic field that opposes the motion that produces it. That is best explained in the Wikipedia eddy current article and the Wikipedia eddy current brake article.

The torque produced by DC injection braking is moderate to high at high speed, increases to a peak at a lower speed then drops to zero at zero speed. DC injection braking will not hold the rotor stationary. It provides only minimal resistance to slow rotation.

See also: How much DC voltage is needed to brake a 3 triphasic motor?

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