Flying start on VSD when the motor is not 'flying'

[ozmosis]

Has anybody any practical experience of the flying start function on VSD's (catching a spinning motor) when the motor is not spinning? It's not a trick question, I am finding a number of strange characteristics when the function is activated but the motor is not actually spinning, especially when the load is a low inertia (axial fan type). When the motor is spinning and the VSD starts into this load it catches it no problem but if the motor has come to a standstill and then starts, we get the occaisional overcurrent trip. I just wondered if anybody out there has encountered this.
It sounds a bit obvious but the function of flying start is generally tested on a spinning motor. I'm wondering if we have missed something somewhere or if others are like this too.

 

[jbartos]

Suggestion: The motor restart on the fly via VSD has its boundary conditions at which it should work automatically. Contact the VSD tech support. There may be some VSD malfunction or setting to be enabled.

 

[aolalde]

In my opinion an encoder feed-back the speed information to your VFD. With that information the driver should know when the flying start function should be inhibited.

 

[jbartos]

Suggestion: The more specific posting will result in the more specific responses. Therefore, it will be helpful to post the VSD name plate data.

 

[ozmosis]

The drive is a Danfoss VLT2800 7.5kW. We have since done something we always advise our customers to do and that is 'read the manual' in more detail(or words to that effect) and found a passage indicating that "...Too low inertia will lead to load acceleration, which can be dangerous or prevent correct catching of a rotating motor". I think this is the problem. The injected DC current to find the correct speed is driving the fan quickly during it's search process causing the OC trip.
Thanks for your input.

 

[jbartos]

Comment on the previous posting: A VSD design does not necessarily cover all motors. This is where DANFOSS is covered by the mentioned statement in the manual. First apparent solution without much knowledge about Danfoss VLT2800 7.5kW would be to add a flywheel thus increase the inertia. To modify the VSD internal is the VSD manufacturer task or option.