So you are saying that this "anti windmilling feature" is similar to DC injection braking except it injects the the DC when the run command is given? Does DC injection work by simply injecting a DC current into the motor thus creating a steady DC field in the stator and lock up the rotor or prevent it from spinning?
Yes it is a DC injection scheme, so that creates a non-rotating magnetic field in the stator, which in turn creates a magnetic field in the rotor that is
rotating counter to the direction the rotor is spinning, which is what brings it to a stop.
So you are saying that without a physical sensor such as a tach the drive can only detect speed with power is applied to the motor? I guess then there is not enough residual magnetism in the motor when it is stopped to create the field necessary for counting the rotor bars as you mentioned?
Detection via residual magnetism is a horse of a different color; it's something you do by applying a meter. The VFD is not "looking" at anything when it is in the off state, it's only connection to the motor is the transistors and if you turn them on to try to see anything, they will be emitting as well. So once it is on, it is far more reliable for it to energize the windings so as to count pulsations from the rotor bars than rely upon residual magnetism, which by the way varies greatly from motor to motor and in fact, some motors have nearly zero.
If the point of the DC injection braking is to lock the motor then how would this DC injected field be used to count the rotor bars and thus the speed if the motor is supposed to stop when "injected"?
The DC injection is just setting up that stationary field, primarily for the purpose of stopping the motor but since it is already doing that, monitoring for current pulsations as the rotor bars cut the lines of force is just a simple and reliable way to watch the rotor speed. It doesn't interfere with the braking action.
I know a motor that has power removed and then has power re-applied while there is still a field in the motor will cause the motor generated voltage to be out of sync with the source voltage and thus cause a huge current spike when power is re-applied to this spinning motor.
However what about a motor that is spinning at some speed, who's field has totally decayed? Are you saying that applying power to this motor at an other frequency than its spinning at will not cause a current transient? Can you explain what happens from a torque and current standpoint when this occurs?
We have another drive that seems to trip when a similar instance occurs.
Of course there is still a current transient when power is applied, at any speed. Think about it, when you apply power to a motor that is standing still, you STILL have a difference in the applied frequency and the motor frequency, it just so happens that the motor frequency is 0!
But I think I understand what you are asking. To get it clear, realize first that there are two "transients" that occur when you start a motor; Inrush and Starting current. Inrush is only the very brief spike in current that is a result of establishing the magnetic field in the stator and rotor, before the back-emf inhibits it. The magnitude is limited only by the resistance of the wire itself, the magnetic permeability of the laminated steel and the air gap between the stator and rotor. It decays very quickly, usually in less than 2 cycles. This is what does not change, regardless of whether the rotor is spinning or not. Starting current is the amount of current, OVER TIME, that is pulled as a result of the high slip that occurs between the stator rotating field and the rotor's rotating field. So if the rotor is already spinning, there is less slip differential and that current magnitude ends up being slightly lower and lasts less time because it takes less time to get to full speed.
So to put that together, when you energize into a spinning motor (assuming it's own field had decayed), there will still be "Inrush" as the magnetic fields are established, which will be just as high as if the motor were standing still. Once those magnetic fields are established, you start dealing with slip differential. So a motor at standstill is at 100% slip, you get locked rotor current magnitude, which decays as the motor speeds up and the slip differential decreases. If the motor is already moving at lets say 80% speed, you get less slip differential and therefore less magnitude of the current transient. That curve is very steep however, so anything less than about 60% speed is not going to show much difference in magnitude, just duration.
But VFDs make it more interesting because the applied frequency of the power coming from the drive is not the design frequency of the motor, so the slip differential is relative now. In other words, if the drive can determine what speed the motor is running at, it can find a reasonable output frequency to apply and keep the current at absolute minimal levels. If, however, the VFD does not have that ability to "find" the rotor speed, then it's just a shot in the dark. If your drive is having troubles, either it is an older design that can't monitor rotor speed and do a flying restart, or maybe you just don't have the feature enabled.
And by the way, torque follows current, so what happens in the above is essentially happening with regards to torque as well.
OK, my fingers hurt now...
