AC motors work by using a rotating magnetic field created by the alternating current in the stator windings to induce a magnetic field in the rotor windings. As the rotor's magnetic field attempts to stay in sync with the stator's magnetic field by mutual attraction, it rotates.
When the electricity is cut off from the motor, it spins freely until friction slows it to a stop. Large motors, however, have a lot of rotational inertia. Large motors driving large loads have even more. It can take a very long time for such motors to stop spinning. In many applications, it would be beneficial to bring the motor to a halt faster through some means of braking.
Of course, a motor can be stopped with a friction brake. This is simple and efficient, but it causes wear and tear on both the motor and the brake. A more elegant solution is to use DC injection braking. This is done by applying a direct current to a motor that operates on alternating current, which produces a stationary magnetic field rather than one which rotates.
In this case, the still spinning rotor will be magnetically drawn not to a rotating magnetic field which it will try to follow, but rather to a stationary magnetic field which will try to hold it in one position. This slows and eventually halts the rotor completely. As long as the DC voltage is applied to the stator, the rotor will be held in position and resistant to any attempt to spin it. The higher the voltage that is applied, the stronger the resistance will be to being turned out of position.
Some variable frequency drives come equipped with DC injection braking as a standard feature. This is easy to do since the operation of a VFD involves converting AC to DC and back to AC again. To inject DC into the motor, the drive simply skips the DC to AC conversion.
While the rotor is spinning, the stationary magnetic field is doing work. Once the rotor has stopped, the magnetic field is no longer doing work, and the current in the motor's windings can flow freely, impeded only by the resistance of the windings themselves. If the voltage is too high or the winding resistance too low, this creates very high and potentially damaging currents in the motor windings because they were not designed to handle DC power. It is best to stop applying the DC injection braking as soon as the motor has come to a complete stop.
Most DC injection braking systems have the ability to adjust the length of time they will operate and the maximum torque they will apply. They generally begin braking when they detect that the motor is no longer receiving its "run" command and come equipped with hardware to prevent the motor from receiving another "run" command until the braking is finished.