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Interview Question :: How is electrical energy generated in Dynamic Braking when using AC induction motors

Locomotives that use Dynamic Braking with DC traction motors excite the field coils of the motors and turn them into energy producing generators. AC induction motors don't have that option since there are no windings on the rotating armature and there are no slip rings. When used as a traction motor, there are short circuited windings on the rotating armature that produce magnetic fields when rotated in magntic flux fields produced by three phase rotating fields in the outer windings. It would seem that for these traction motors to produce electricity that they must have additional windings on the rotating armature or permanent magnetics installed. Other articles on AC locomotive dynamic braking say that an induction motor is used with no slip rings. If so then how can the traction motor be turned into an alternator?

Answers to "How is electrical energy generated in Dynamic Braking when using AC induction motors"

RE: How is electrical energy generated in Dynamic Braking when using AC induction motors?
Induction motors that are controlled using variable frequency drives VFDs), become generators and produce braking torque when the applied frequency is reduced. The VFD continues to provide magnetizing current, but the phase relationship between the current and voltage changes such that net power is returned from the motor to the VFD. The VFD can be equipped with a converter that will return power to the source in the same way that a DC converter returns energy to the source. It is difficult to explain how this works without diagrams. I will try to find some reference material on line and post links. Edit 1 The speed vs. torque curve for a typical induction motor is shown here: http://i13.tinypic.com/730cfhy.jpg In normal operation, the motor accelerates quickly through the orange part of the curve when it starts and operates in the green part of the curve. Note that the motor only produces driving torque when it is operating at less that 100% (synchronous) speed. If the load were to make the motor operate above 100% speed, such as could occur when an electric locomotive is going down hill, the motor operation moves into the red part of the curve. As operation moves from the green part to the red part of the curve, the torque produced by the motor drops to zero and becomes negative. When the motor is producing negative torque, it is acting as an induction generator. It is braking the load and generating electric power with the energy received from the load. Edit 2 The Wikipedia article, http://en.wikipedia.org/wiki/Power_facto... explains that when AC voltage and current waveforms are in phase with each other, the instantaneous value of the power is always positive. When the current and voltage are not in phase, the power is negative during part of the cycle and positive during part of the cycle. The net power transferred to the load is less than the RMS load current multiplied by the RMS load voltage. In an induction motor, that means that, in effect, part of the current is being used to generate the magnetic fields in the motor and the rest is being used to transfer power. When the motor load is reduced to zero, the current that is being used to generate the magnetic field remains relatively constant, while the current due to the load is reduced to zero. A small percentage of the current is also caused by losses in the motor resulting in heat. When the motor torque is reduced to zero, the current is reduced something like one third of the full load value, but the phase relationship changes so that very little power is transferred to the motor. When the load torque becomes negative, the current increases and the phase relationship changes so that power is transferred from the motor to the source. This all happens due to the inherent characteristics of an induction motor, no control action is necessary. However, with a variable frequency control unit, the motor’s torque vs. speed curve can be adjusted to make the crossing point from positive to negative torque occur at any speed. That makes it possible to provide braking or accelerating torque as desired.
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RE: How is electrical energy generated in Dynamic Braking when using AC induction motors?
A low voltage DC is applied to winging after motor is stopped which creates the magnetic Field applied on rotor to stop it
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RE: How is electrical energy generated in Dynamic Braking when using AC induction motors?
Are you confusing Dynamic Braking and regenerative Braking? --- Think when an induction motor is driven by shaft, rather than electric, it will work as generator, because there's a small amount of residual magnetism that remains in the field coils (electromagnets). Think the tiny voltage that's generated builds on itself, and will eventually power the electromagnets up. --- This has a bit on "Reversing and Dynamic Braking of Single-Phase Induction Motors" maybe it'll help http://www.homemetalshopclub.org/news/se...
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