VFD braking with DC current 2

[fab1961]

Good morning everybody,
I would like to understand better (from the theorical point of view) what is the physics
of braking with DC. Can you suggest me something to read?
Thank you

 

[LionelHutz]

I don't have anything for detailed data, but you can more or less consider it as trying to run the motor at 0Hz while only using 2 of the terminals for input power.

 

[jraef]

To expand on that a little, an AC induction motor spins because there are rotating magnetic fields in the stator (stationary part) and they induce current to flow in the rotor (rotating part), which creates magnetic fields there at the same time. These opposing magnetic fields repel each other making the rotor spin. If you apply DC to one set of windings in the stator, the magnetic field that creates in the stator is no longer rotating. The induced magnetic field in the rotor then tries to align itself with it, binging it to a stop (the zero Hz Lionel is referring to).


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[electricuwe]

Braking with DC was quite common prior to the broad introduction of VFD. Today with a VFD usually regenerative braking is preferred, either feeding the energy back to the grid via an Active Frontend, storing it locally or dissipating it via a brake chopper.

However, if none of this options is available or the brake chopper is as it's temperature limit, some VFD are also capable of DC-braking.

The disadvantages of DC braking are:
- variable torque according to motor characteristics
- energy is dissipated mainly within the motor

 

[jraef]

Not just mainly, ALL of the kinetic energy in the rotating load is converted to heat in the motor.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden