Normally the power in a variable frequency drive system flows from the drive to the motor and then to the load. However, sometimes the energy can flow in reverse. This is called regeneration. If a crane is lowering a load, or if a conveyer is going downhill, then regeneration can occur. If you try to decelerate a high inertia load such as a fan, then regeneration can also happen. When the energy goes back to the drive, it cannot go back to the supply – the rectifier diodes block this, so the DC link capacitor absorbs the energy as a rising voltage. If the voltage gets too high the drive will detect this and trip on ‘overvoltage’. When this happens the drive stops supplying magnetising current to the motor, so the motor no longer regenerates, but coasts to a stop in its own time.