VFD Kinetic Buffering and Flying restart

Voltage Loss ride through with flying restart:
In this method, when the voltage sag causes the variable frequency drive to reach its undervoltage trip level, the VFD drive will shut off the inverter section and thus remove power from the motor instead of tripping. The motor will coast down during the duration of the sag and, as soon as the voltage recovers, the VFD will start into the still-spinning motor and ramp up to set speed. How much the motor speed will drop depends on the inertia of the load and the duration of the sag.
You have to configure the VFD for flying restart. During low input voltage the inverter section is cut off to maintain the DC bus voltage. If the voltage restores before the DC bus voltage goes below the tripping value, the inverter is again put on but the driven load speed has already reduced due to brief period of no voltage at the motor terminal. Flying restart feature enables the variable frequency drive to restart the Motor at the same speed at which the motor is operating thus preventing any high current. So it is basically catching a spinning motor. Without flying restart high current will be observed once the inverter section is put ON. Flying restart feature is also helpful if you want to restart a motor which is already spinning.

Kinetic backup
This option, which is also provided by some variable frequency drive manufacturers, uses the energy stored in the mechanical load to keep the DC bus voltage from dropping down to the trip level. This is accomplished by running the inverter section during a voltage sag at a frequency slightly below the motor frequency, causing the motor to act as a generator. Similar to the flying restart option, the motor speed will drop while it is acting as a generator, however the advantage is that the motor is never disconnected from the drive. This option works best for those high-inertia loads.
Kinetic buffering is a feature to prevent the variable frequency drive from tripping during voltage sags. If the VFD trips due to DC bus undervoltage there is no need for kinetic buffering.