VFD and voltage decay. 2

[slavag]

Dear All.
We have provide fast transfer ( break before make, main-tie-main) on the MV bus with all MV motors with VFD.
Motors w/o VFD with break of suplly start work as generators and we can use high speed busbar transfer systems with dynamic synchronisation. What happend with VFD? As far, as I understand ,motor isn't back energy to bus via VFD.
What happend with motors ( VFD eqp), if supply back within 200-250ms.
Thanks in advance.
Best Regards.
Slava

 

[lansford]

Most but not all VFDs do not regerate back to the source. The exception are mostly on excavator drives (electric power shovels and draglines, I believe)
Some of the better VFDs can go and "pick a motor back up and catch it" after momentary power interruptions.
Best bet is to check with the VFD manufacturer.
JIM

 

[slavag]

Jim,
Thanks a lot.
Of course we check it with VFD manufacturer.
And will provide timing, ONLY according to manufacturer's recommendation.
I would like learn more for myself about this issue.
Maybe someone have graphs,theoretical documents, links...
Best Regards.
Slava

 

[CJCPE]

I don’t recall seeing very much about this subject published anywhere, but I might be able to provide a little additional information.

You already know that most VFDs have an input power rectifier that can not return power to the source. However, the motor is inherently capable of regenerating energy to the VFD under some operating conditions. Regenerated energy is seen as a reverse current in the DC bus that charges the DC capacitors. If that energy is not limited, the capacitor voltage will rise to an unacceptable voltage. Therefore, the VFD must have designed provisions that will limit regenerated energy.

To limit regenerated energy when power fails, the VFD detects the power failure and executes an orderly shutdown. The VFD control circuitry must have sufficient power outage ride thru to allow it to function until the shutdown is complete. Either the DC bus capacitors or control power supply capacitors can provide the ride thru. One strategy to limit regenerated energy is to detect reverse DC current or rising DC bus voltage and increase the output frequency to reduce the motor slip to zero and thus stop regeneration. At the same time, the output voltage can be ramped down to remove the motor magnetizing voltage.

If the VFD has regenerative capability, it still would not be allowed to regenerate during a power failure because that would be undesirable and because the regenerative bridge can not reliably synchronize with a failing source.

One way to start a VFD into a rotating motor is to start at maximum frequency at a reduced voltage, ramp up the voltage until regeneration is detected, reduce the frequency until the proper operating frequency is found, adjust the voltage for proper motor operation and then increase the frequency to accelerate the motor to the desired operating speed. I think that is a reasonable description, but the actual implementation may be a bit more sophisticated, especially in a vector or sensorless vector drive.

Most VFD manufacturers provide a pretty complete list of product capabilities in their sales literature. Starting into a rotating motor may be called “fly catcher,” “coasting restart,” or something like that.

 

[slavag]

CJCPE.
Thanks a lot.
Your information is very helpful.
Now, we know : what we have loocking for and waht we have ask.
Best Regards.
Slava